UNIVERSITYOF NOVA GORICA GRADUATE SCHOOL FLOODPLAIN FORESTS OF SOČA RIVER BETWEEN KOBARID AND CONFLUENCE WITH RIVER TOLMINKA: CURRENT SITUATION AND DE

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1 UNIVERSITYOF NOVA GORICA GRADUATE SCHOOL FLOODPLAIN FORESTS OF SOČA RIVER BETWEEN KOBARID AND CONFLUENCE WITH RIVER TOLMINKA: CURRENT SITUATION AND DEVELOPMENT MASTER'S THESIS Janez PAGON Mentor: Assoc. Prof. Marko Debeljak Nova Gorica, 2016

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3 UNIVERZA V NOVI GORICI FAKULTETA ZA PODIPLOMSKI ŠTUDIJ OBREČNI GOZDOVI SOČE MED KOBARIDOM IN SOTOČJEM S TOLMINKO: STANJE IN RAZVOJ MAGISTRSKO DELO Janez PAGON Mentor: izr. prof. dr. Marko Debeljak Nova Gorica, 2016

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5 ABSTRACT In its upper course the Soča River is one of the best preserved Alpine rivers. Despite that, there have been few studies of its riverine forests, which are an integral part of the river. The research in this thesis encompasses part of the river basin between the towns of Kobarid and the confluence with the Tolminka River. It was found that the riverine forests in the study area are not a homogeneous habitat. Rather, they consist of numerous phytosociological communities with a syn-dynamic connection. The majority of riverine forests in the study area is younger than 50 years old and were established either by the overgrowing of agricultural land or from degraded forests. Accordingly, the forest structure is characterised mainly by uneven-aged stands or younger structural stages, which are frequently returned to their earlier stages due to human impacts. The majority of these forests have low quality prospects. Based on their vegetation and stand characteristics, the riverine forests under study were classified into six stand types. A comparison of the functions and characteristics of particular forest types revealed similarities between the stand types of initial willow, white willow and grey alder. Forests in this cluster are under the influence of soil water and high floodwaters. Their most pronounced function is ecological. In their growing stock, there is a large share of deciduous softwood. Forests in this cluster cover 62% of all riverine forests in the study area. Silvicultural measures are rare in these forests and are less important for the existence of the stands than environmental features like a high level of soil water and minimised human impact on their habitat and the river course. An important characteristic of the second cluster, which consists of pioneer forest on higher-lying river terraces and the riparian corridor stand type is a lack of contact with soil water. In the tree layer of the growing stock in this cluster, species appearing on drier riverine-forest sites prevail (European ash, small-leaved lime, European hornbeam, sessile oak). These forest stands have a greater timber industry potential; therefore, modest measures and limited human impact (especially that which leads to a change in land use like forest grazing and clearing) are recommended. The stands in this cluster cover 30% of the analysed riverine forest area. The third cluster of riverine forests consists of anthropogenic riverine forest stands where the human impact is so significant that they mainly perform a social function. The stand type of this cluster represents 8% of the riverine forests in the study area, and it is suggested that they should be managed as parkland and not as forest areas. Directing visitors to these areas would unburden other, more preserved stand types from the pressures created by human activities (tourism, recreation, I

6 construction work). Growth and increment analyses of the most common tree species in the riverine forests in the study area show that mainly olive and white willow, followed by grey alder, have substantial current height (both willow types up to 1.14 m/year), diametric (willow up to 1.4 cm/year) and volumetric increments. However, their rapid growth makes these subjects prone to rapid ageing. The region of northern Primorska was inhabited early in history. Accordingly, there was great pressure imposed on riverine forests already in the past. The analysis of old maps revealed that up to the beginning of the 20 th century, the riverine forest area within the scope of analysis was under 50 ha. Within the process of the overgrowing of agricultural land in Slovenia, a process that is currently still underway, the aforementioned riverine forest area increased to ha. This thesis shows that the study area is not an unspoiled wilderness. On contrary, just as in the past, it is a crossroads of numerous and frequently conflicting interests. The riverine forest situation on the Soča River in the area between the town of Kobarid and the confluence with the Tolminka River may improve if a management plan that takes into account and balances the many wishes of various communities and professionals is adopted and implemented. With proper management, all of the potential of these stands for offering ecosystem services in the landscape can be used, and their undisputable significance will be retained. Keywords: the Soča river, riverine forests, successional stage, forest stand. II

7 POVZETEK Soča je v zgornjem toka do sotočja z reko Tolminko, ena izmed najbolje ohranjenih alpskih rek. Kljub temu je bilo v preteklosti narejenih zelo malo celovitih raziskav gozdov obrežnega pasu, ki so z reko neločljivo povezani. V našo raziskavo smo vključili del porečja med Kobaridom in Tolminom. Ugotovili smo, da obrečni gozdovi preučevanega območja niso homogeno rastišče. Sestavlja jih več fitocenoloških združb, ki so sindinamsko povezane. Velika večina obrečnih gozdov preučevanega področja je mlajša od 50 let in je nastala z zaraščanjem kmetijskih površin ali iz degradiranih gozdov. Posledica tega je med drugim tudi struktura gozdov, saj prevladujejo raznomerni gozdovi ali mlajši razvojni stadiji, te pa človek s svojimi posegi pogosto vrača v začetne razvojne faze. Z redkimi izjemami je tudi zasnova gozdov slaba. Glede na vegetacijske in sestojne lastnosti smo obrečne gozdove preučevanega območja razdelili na šest sestojnih tipov. Primerjava funkcij in lastnosti gozdov posameznih tipov pokaže, da so si med seboj podobni sestojni tipi inicialno vrbovje, belo vrbovje in sivo jelševje. Gozdovi te skupine so pod vplivom talne vode in visokih poplavnih vod. Med funkcijami prevladujejo večinoma ekološke vloge gozda. Delež mehkih listavcev v lesni zalogi je velik, skupaj pa obsegajo gozdovi teh kategorij 62% površine vseh obrečnih gozdov na preučevanem območju. Bolj kot gozdnogojitveni posegi, ki so v teh gozdovih dokaj redki, so za te sestoje pomembne lastnosti okolja, ki omogočajo njihov obstoj (visoka nivo talne vode, odsotnost človeških posegov v rastišče in tok reke). Pomembna lastnost druge skupine, v katero uvrščamo sestojna tipa pionirski gozdovi višjih rečnih teras in obvodni koridorji, je pomanjkanje stika s talno vodo. V drevesni plasti teh tipov prevladujejo vrste, ki se pojavljajo na bolj sušnih rastiščih obrečnih gozdov (veliki jesen, lipovec, beli gaber, graden). Ker imajo ti gozdovi večji lesno proizvodni potencial je predvsem pomembna zmernost ukrepov in omejitev človekovih posegov, predvsem takih, ki imajo za cilj spremembo rabe zemljišča (paša v gozdu, krčitve). Ta razred obsega 30% površine analiziranih gozdov. V tretjo skupino obrečnih gozdov smo uvrstili antropomorfne gozdove v katerih so človeški vplivi že tako veliki, da ti gozdovi opravljajo predvsem socialne funkcije. Ta sestojni tip gozda, ki predstavlja 8% površine obrečnih gozdov, bi bilo smiselno upravljati kot park in ne kot gozdno površino. Usmerjanje obiskovalcev v to področje bi ostale, bolj ohranjene sestojne tipe razbremenilo pritiska človeških dejavnosti (turizem, rekreacija, gradbeni posegi). Na osnovi rezultatov analize rasti in prirastka najpogostejših drevesnih vrst obrečnih gozdov preučevanega območja lahko zaključimo, da imajo predvsem siva in bela vrba, nekoliko manj III

8 pa tudi siva jelša, zelo velik višinski (vrba do 1,14 m/leto), debelinski (vrba do 1,4 cm/leto) in volumski prirastek. Prav zaradi hitre rasti pa se osebki te vrste tudi hitro starajo. Ker je bilo območje severne Primorske poseljeno razmeroma zgodaj, je bil tudi pritisk na obrečne gozdove že v preteklosti zelo velik. Analiza starih kart je pokazala, da je bila do začetka dvajsetega stoletja površina obrečnih gozdov na analiziranem območju pod 50 ha. V procesu zaraščanja kmetijskih površin v Sloveniji, ki traja še danes, se je ta površina povečala na 229,16 ha. Naloga je pokazala, da preučevano območje ni neokrnjena divjina temveč tako kot skozi celotno zgodovino tudi danes ostaja stičišče mnogoterih, pogosto tudi nasprotujočih si interesov. Izboljšanje stanja obrečnih gozdov na področju med Kobaridom in sotočjem z Tolminko lahko dosežemo le z upravljalskim načrtom, ki bo uskladil želje skupnosti in različnih strok. Tako bodo ti sestoji s primernim gospodarjenjem v prihodnosti lahko izkoristili ves potencial za nudenje ekosistemskih storitev v krajini in obdržali pomen ki ga nedvomno imajo. Ključne beside: Soča, obrečni gozdovi, sukcesijski stadij, gozdni sestoj. IV

9 TABLE OF CONTENTS ABSTRACT...I POVZETEK... III TABLE OF CONTENTS... V FIGURES... VIII TABLES... X 1. INTRODUCTION PROBLEM UNDER STUDY OBJECTIVE OBJECT OF STUDY Soča River basin Riverine belt of the Soča River between the town of Kobarid and the confluence with the Tolminka River Scope Hydrological description Meteorology Morphologic and orographic description Geological and pedological characteristics Vegetation Forest management in the past and present METHODS Description of stand types in riverine forests Forest functions Ecosystem services Dendrometric analysis Development of riverine forest land in the study area, V

10 5.4. Land use in the study area DATA RESULTS Description of stand types in riverine forests Initial willow stands General Stand description Forest functions Initial willow stand management guidelines White willow stands General Stand description Forest functions Ecosystem services White willow stand management guidelines Grey alder stands General Stand description Forest functions Ecosystem services: Grey alder stand management guidelines Pioneer forests on higher-lying river terraces General Stand description Forest functions Management guidelines for pioneer forests on higher-lying river terraces Riparian corridor stands VI

11 General Stand description Forest functions Ecosystem services Riparian corridor management guidelines Anthropogenic riverine forest stands General Stand description Forest functions Ecosystem services Anthropogenic riverine forest stand management guidelines Stand type comparison Dynamics of the growth of riverine forest tree species dendrological analysis Growth in height Diametric growth Volumetric growth The development of riverine forest land in the study area Land use in the study area DISCUSSION AND CONCLUSION SOURCES VII

12 FIGURES Figure 4.1: Daily flow rate of the Soča River at Kobarid... 8 Figure 4.2: The characteristic turquoise, emerald-green colour of the Soča River Figure 4.3: The Soča river basin Figure 4.4: Average annual persistence of high waters of the Soča River Figure 4.5: Average annual persistence of high flow rate of the Soča River Figure 5.1: Felling sites whose trees were used for dendrometric analysis Figure 7.1: Olive willow stand on a blind branch of the Soča River Figure 7.2: Extent and areas of initial willow stand type Figure 7.3: Values of forest functions in terms of initial willow stand type Figure 7.4: White willow stand at Grofov ribnik Figure 7.5: Extent and areas of white willow stand type Figures 7.6, 7.7: LIDAR scan (left) of natural relief of the lowest-lying river terraces Figure 7.8: White willow stands near the town of Tolmin during flooding in Figure 7.9: Values of forest functions in terms of white willow stand type Figure 7.10: Gray alder stand at the Tolmin aggregate extraction site Figure 7.11: Extent and areas of grey alder stand type Figure 7.12: Values of forest functions in terms of grey alder stand type Figure 7.13: Pioneer forest stands on higher-lying river terraces Figure 7.14: Extent and areas of the pioneer forests stand type Figure 7.15: Values of forest functions in terms of pioneer forest stand type Figure 7.16: Riparian corridor close to the village of Volarje Figure 7.17: Extent and areas of the riparian corridor stand type Figures 7.18, 7.19: LIDAR scan (left) and orthophoto (right) of an eroded riverbank Figure 7.20: Values of forest functions in terms of the riparian corridor stand type Figure 7.21: Bathing area on the Soča River below the village of Volče VIII

13 Figure 7.22: Extent and areas of the anthropogenic riverine forest stand type Figures 7.23, 7.24: Relief characteristics of natural white willow stands Figure 7.25: Values of forest functions in terms of the anthropogenic riverine forest stands.. 72 Figure 7.26: Dendrogram of riverine forest stand type functions in the analysed area Figure 7.27: Growth in height of tree species in riverine forest stands Figure 7.28: Current height increment of tree species in riverine forest stands Figure 7.29: Diametric growth of tree species in riverine forest stands Figure 7.30: Current diametric increment of tree species in riverine forest stands Figure 7.31: Volumetric growth of tree species in riverine forest stands Figure 7.32: Current volumetric growth increment of tree species in riverine forest stands Figure 7.33: Riverine forest area through time Figure 7.34: Development of relative forest share Figure 7.35: Riverine forest area as of Figure 7.36: Riverine forest area as of Figure 7.37: Riverine forest area as of Figure 7.38: Riverine forest area as of Figure 7.39: Riverine forest area as of Figure 7.40: Riverine forest area as of Figure 7.41: Land use in the study area IX

14 TABLES Table 5.1:Stand types in the study area Table 5.2: Maps and data layers forming the basis for creating maps Table 5.3: Definition of land use categories Table 6.1: Maps used for analysing the riverine forest area Table 7.1: Natural value in the forest habitat of the initial willow stand type Table 7.2: Natural value in the forest habitat of the white willow stand type Table 7.3: Natural value in the forest habitat of the grey alder stand type Table 7.4: Parameter values of regression curves for growth in height of tree species Table 7.5: Parameter values of regression curves for diametric growth of tree species Table 7.6: Parameter values of regression curves for volumetric growth of tree species X

15 1. INTRODUCTION The riverine-forest group consists mainly of lowland woods located within the area of larger streams and under the direct influence of water. River dynamics exert an influence on riverine forests via a high level of soil, water and surface flooding. Riverine forest vegetation must adapt to two extremes in order to thrive: extreme flooding and extended dry periods. Riverine forest stands consist of moisture-loving trees and shrubs, namely: olive, purple, white, crack and grey willow (Salix eleagnos, S. purpurea, S. alba, S. fragilis, S. cinerea), black and white poplar (Populus nigra, P. alba), speckled and black alder (Alnus incana, A. glutinosa), European white-elm (Ulmus laevis), European and narrow-leaved ash (Fraxinus excelsior, F. angustifolia), small-leaved lime (Tilia cordata), and English oak (Quercus robur). Their communities are called riverine forests and woodlands. Riverine forests of deciduous softwood (willows, poplars and alders) mainly consist of stands in younger structural stages, characterised by successional series from overgrowing gravel sites to higher (older) river terraces (Dakskobler et al., 2013). Riverine forests of deciduous hardwood are called woodlands and consist mainly of English oak, European white-elm and European and narrowleaved ash. They are found in older successional stages, usually located away from riverbanks and thriving on more developed alluvial soils (Dakskobler et al., 2013). Hydromorphic soil plays an important role in the development of riverine forests and woodlands. Dakskobler et al. (2013) state that vegetation dynamics or the successive replacement of communities, and in places also zonation from the lowest (youngest) to the higher (older) river terraces, takes place in riparian areas only in the absence of excessive human intervention and proceeds from very unstable pioneer stages (willow shrubs) via stands of grey alder, ash and European white-elm to already more developed, stable, established and commercially interesting communities of woodlands, which are still subject to the influence of soil water or floodwaters. During flooding, the river dynamics can make the riverine vegetation regress to an earlier or even initial stage of succession (during flooding, the river partially washes away gravel sites). 1

16 Klimo and Hager (2001) state that the main characteristics and functions of riverine forests are: A high production level of forest sites; A high level of biodiversity based on the high variability of forest sites; Emphasised land- and stand-protection functions and hydrological function; A high number of nature reserves; Emphasised recreational and aesthetic functions; An important role in the water cycle and retention of floodwaters. Floodplain and swamp forests in Slovenia are distributed especially in the floodplain areas of all major rivers (along the Dragonja, Reka, Vipava, Nadiža, Soča, Idrijca, Sava, Sora, Ljubljanica, Kamniška Bistrica, Savinja, Krka, Mirna, Kolpa, Sotla, Dravinja, Drava, Mura and their tributaries), as well as, at least in spots, along sinking streams in karst fields (such as the Cerknica and Planina fields along the Pivka). In general, riverine forests are not managed well in Slovenia; therefore, the majority of them are not nurtured. Carr (black alder forest) and woodland seem to be an exception to the above these are commercially important forests with high yields (Dakskobler et al., 2013). Riverine forests are characterised by an important protective role as they represent an important factor in the regulation of the water balance and, where still preserved, they serve as flood protection for settlements. In addition, they provide habitat for some rare, protected and/or highly important species in Slovenia, such as the German false tamarisk (Myricaria germanica), which is, according to Müller (1995), one of the typical species of the preserved upper portions of Alpine river basins. Woodlands as well as other floodplain, swamp and riverine forests provide a refuge for cavity-nesting and other birds as well as amphibians (Dakskobler et al., 2013). Most of these floodplain, swamp and riverine forests have a very important natural-protection role and are thus classified as important European habitat types (Natura 2000), and for their protective function they have been declared as protected forest. A comparison of the upper course of the Soča River (from its source to the confluence with the Tolminka River) with the rarely preserved upper courses of Alpine rivers in Italy, Austria and Germany shows similar characteristics. Among the characteristics of preserved Alpine rivers, Müller (1995) mentions the presence of riverine forests of deciduous softwood, especially pioneer stands of olive willow shrubs, which also overgrow vast gravel sites along 2

17 the Soča River. Preserved Alpine river basins contain, as a result of the river dynamics, a mosaic of plant communities of various age, successional and structural stages (Müller, 1995). In the case of the Soča River, the aforementioned spectrum of vegetation exists, but it is partly the result of human impacts (pioneer stands on abandoned gravel depots). When comparing the riverine forests of the Soča River basin, it is impossible to avoid the analysis provided by Dakskobler (2007). He points out the great similarities between the floristic composition of the riverine forests of the Nadiža River and along the Soča River in the section between the towns of Kobarid and Tolmin. Another characteristic of the riverine forests of the Soča River in the aforementioned area is the largest share of invasive alien species in the Posočje region as well as the diversity of phytosociological communities. The riverine forests located between the villages of Čezsoča and Srpenica and along the Bača River have a similar floristic composition to the riverine forests of the Nadiža River and along the Soča River in the section between the towns of Kobarid and Tolmin (Dakskobler, 2007). After centuries of river regulation, European watercourse guidelines have been changing recently in a way that stresses the sustainable management of river basins. Despite the European trend of favouring eco-remediation of river basins, it needs to be pointed out that currently, there are only a few sections of rivers where the conditions allow natural succession toward the overgrowing of gravel sites (Kollmann, 1999). This is especially true for pioneer vegetation on the gravel sites of Alpine rivers, where these sensitive habitats have almost entirely disappeared (Muller, 1995). 3

18 2. PROBLEM UNDER STUDY Riverine forests in the area between the town of Kobarid and the confluence with the Tolminka River were mainly established on former agricultural land or developed from degraded forests. Due to the aforementioned origin and the fact that the current forest area is still limited ( ha in total), their management is mainly unplanned and left to the interests of various sectors, such as construction (gravel exploitation, hydro-technical measures), tourism and farming. In this area, forestry is less active in forest management and silviculture more significant is its natural protection role as it acts mainly as a caretaker in terms of protecting the Soča River basin in various ways. The riverine forests of the Soča River are included in the European network Natura 2000 and have been declared as protected forest and natural value of national significance. In order to balance the numerous interests in this region, the riverine forests in the study area should be delineated and appropriately described. The riverine forests of the Soča River are not described in any foreign literature which includes integral studies of European floodplain forests.čater M. and Kutnar L., in the book Floodplain forests of the temperate zone of Europe (Klimo et al., 2008), limit their scope of study on riverine forests to the region of Prekmurje and the Krško-Brežice basin. Čater M., Kutnar L. and Acetto M., in the book Floodplain forests in Europe:current situation and perspectives (Klimo in Hager, 2001), are no different, describing the riverine forests only in the aforementioned regions.kollmann et al.(1999), in their article Interactions between vegetation development and island formation in the Alpine river Tagliamento, analyse the dynamics of gravel-site and island formation in Alpine rivers and study the early succession of these newly established land areas.initial successional stages on gravel sites of the Soča River between the towns of Bovec and Tolmin were studied by Čušin and Šilc (2006). Their study is published in the article Vegetation development on gravel sites of the Soča River between the towns of Bovec and Tolmin.In terms of describing the initial willow stand type, the article Razvoj vegetacije na prodiščih reke Idrijce v zahodni Sloveniji (Eng.: Development of vegetation on gravel sites of the Idrijca River in western Slovenia) (Dakskobler, 2010) is important.the riverine forests of this river are also described in the article Fitocenološka in floristična analiza obrečnih gozdov v Posočju (zahodna Slovenija) (Eng.: Phytosociological and floristic analysis of riverine forests in the Posočje region 4

19 (western Slovenia)) (Dakskobler, 2007).In this article, the author describes and compares in floristic terms the riverine forests along the Soča River, namely between the lower section of the Trenta Valley and the village of Soča and at the village of Plave, and along the Lepenica, Idrija (Ital. Judrio), Bača, Kanomljica and Trebušica rivers in western Slovenia.He classifies the stands of these riverine forests into seven phytosociological communities.this article is important in terms of vegetation, because the author fits the analysed area into the broader context of riverine forests of western Slovenia.Dakskobler et al. (2013), in the book Poplavni, močvirni in obrežni gozdovi (Eng.: Floodplain woods, swamp woods and riverine forests), describe types of riverine forest stands in Slovenia and elaborate on them in terms of vegetation and in terms of their extent in Slovenia. In addition, the authors of the aforementioned book describe the geological, pedological and phyto-geographical features of these types of forest stands; the relief and climate characteristics of the sites on which they thrive; and their characteristics in terms of forest management, silviculture and natural protection.the riverine forestsof the upper basin of the Soča River were also analysed by Dakskobler et al. (2004) in the article Riverine forests in the upper Soča Valley (Julian Alps, western Slovenia).Here, the authors do a vegetation analysis and describe the successional development of the vegetation in this region. Analysis of the available literature and other sources reveals that the riverine forests of the Soča River in the area between the town of Kobarid and the confluence with the Tolminka River have not yet been studied integrally.the existing studies are focused mainly on succession processes or analyse the vegetation aspect of riverine forests.their scope, structure and development are yet to be studied.in order to achieve successful management of these forests and to successfully direct the interests of various sectors such as nature-protection, farming, forestry, the construction industry and tourism, knowledge of riverine forests and their characteristics is of vital importance. 5

20 3. OBJECTIVE This thesis aims at delineating the scope and area of riverine forests along the Soča River in the section between the town of Kobarid and the confluence with the Tolminka River and at describing their structure and development over time. An inventory of their current structure should serve as a basis for further study and management of this sensitive ecosystem.the thesis shall result in a comprehensive and detailed analyses of the riverine forests located between the towns of Kobarid and Tolmin. First, the basin of the Soča River is described in its entirety and its riparian forests are fitted into the broader context of riverine forests by means of Slovenian and foreign sources. Then, via a hydrological and geomorphological description of the Soča River basin, and a portrayal of its vegetation, the most preserved upper course of the river to the confluence with the Tolminka River is extracted for the purposes of this study. This part is further divided in terms of vegetation and morphology into three distinct sections. Finally, the lower section of the upper course of the Soča River, namely the area between the town of Kobarid (where the river exits a gorge) and the confluence with the Tolminka River, is analytically defined in scope with the aid of a map of seldom-flooded areas (Opozorilna, 2011). The objective of this part of the study is to precisely delineate the study area of the research. Within the scope of this research, the riverine forests are then analysed in terms of area over time and in terms of their current structure.the riverine forests under study are classified into six stand types.for each stand type, a general description is given first, followed by a stand description, forest functions, its potential for ecosystem services, and management guidelines.the objective of this part of the study is to identify, delineate and describe the stand types, and further to present them on maps and define their similarities via cluster analysis methods.in addition, growth dynamics in terms of height, diameter and volume of the six typical tree species of riverine forests are presented by growth and increment curves for each tree species. An analysis of land change and development of riverine forests over time is provided by means of digitisation of archival maps.analyses and interpretation of maps and data on land use serve to evaluate the importance and the area of the forests within the scope of this study 6

21 over time.the objective of this part is to present the development of riverine forest dynamics in terms of time and area by means of six archival maps depicting the development of the riverine forest area in the past and a map of current land use. 4. OBJECT OF STUDY In chapter 4.1, the hydrological, meteorological, pedological and geomorphological characteristics of the entire Soča River basin are described, and the basin (the Posočje region) is divided into upper, middle and lower parts. In the upper part, the basin between the town of Kobarid and the confluence with the Tolminka River is taken under scrutiny and is described in detail in chapter Soča River basin The Soča River has its source in the Trenta Valley, in the heart of the Julian Alps. It has a length of 138 km and flows mainly in Slovenia (96 km). There is an altitude difference of 820 m between its source and the site where it enters Italy (Jarc et al., 2002). The Soča River is one of the Alps cleanest waters, especially in the section from its source to the town of Tolmin. Downstream from Tolmin, the river changes significantly due to hydro-electric power plants, extensive farming, transport (railway and main road) and industry. Therefore, it is degraded ecologically. Its average annual rate of flow at the Kobarid measuring station is 34.3 m 3 /s (Figure 4.1). The highest characteristic rate flow exceeds 500 m 3 /s ( m 3 /s, on ) and the lowest is under 10 m 3 /s (4.59 m 3 /s, on ) (Površinske, 2014). This characteristic feature of the river, namely its possibility to occasionally contain catastrophically high water, has shaped the shoreline areas and eco-systems along the river. The torrential Soča River carries a great amount of gravel along its course and therefore creates a variety of natural phenomena, making a special mark on the landscape it runs through. Its characteristic colour changes with the flow. Its remarkable emerald-green colour can be observed only during the average or low rate of flow (Rojšek, 1991) (Figure 4.2). 7

22 Settlement in the Upper Soča Valley has had a long history. Archaeological studies have shown that people were living there already in 50,000 B.C. This region was a transit zone from time immemorial and an ancient amber-trade route, leading via this region to Aquileia, has left a cultural mark on the landscape (Jarc et al., 2002). Population density exerted a significant influence on riverine forests in the past in terms of exploitation that exceeded their natural potential. Figure 4.1: Daily flow rate of the Soča River at Kobarid, (Površinske, 2014). The Posočje region is divided into upper, middle and lower parts, and further into smaller units (Jarc et al., 2002). The Upper Soča region consists of the highlands of Julian Alps, including the highest peak of Triglav (2864m) and deep, high-walled valleys. The Soča River comes to the surface as a karst spring. Where the river runs through steep, uneven terrain, at some places its course is narrowed into deep gorges (the Great Soča Gorge) and at some places the river spreads widely onto plains (the Bovec Basin, the area between Kobarid and the confluence with the Tolminka River), creating vast gravel sites. The waters of the Upper Soča River Basin and its less abundant tributary Koritnica have to struggle through terminal moraines, vast glacial boulders and gorges, and have to bypass a number of river islets to reach the Bovec Basin at Jablanca. Here, at the Čezsoča gravels, the Soča River widens up to 150 m, while floodwaters can reach up to 600 m in width (Rojšek, 1991). At the Boka 8

23 Waterfall the river changes its flow direction and at the meander the stream of Učja flows in. Ravines between the village of Srpenica and the town of Kobarid, established on land originating from detritus, morainic dams and rockfalls, are characterised by large rocks in the riverbed with rapids and deep pools around them. In the area between the town of Kobarid and the confluence with the Tolminka River, the Soča River does not have any significant steep falls. Here, it meanders and accumulates an enormous quantity of gravel on the one hand and washes away the banks and precious fertile soil on the other. Among its right tributaries, the Idrijca and Kamnica should be mentioned, and among its left tributaries the Ročica, Volarja, Tolminka and Godiča. Figure 4.2: The characteristic turquoise, emerald-green colour of the Soča River at Velika korita (Great Soča Gorge). The middle part of the Soča region is spread over very diverse Alpine foothills and high karst.in the region between the confluence with the Tolminka River (here the upper Soča region ends) and the Podselo dam, the Soča River is captured in an artificial lake which serves as the reservoir for the Doblar hydro-electric power plant. Downstream from the town of Most 9

24 na Soči, the river gouged out of karst terrain a 500 m-deep and m-wide glen. The glen gets wider at its end and transitions into the small basin of Avče; it narrows again into a valley at Ajba; and as it flows past the hamlet of Plave, it narrows further into a ravine between Sabotin Hill and Kuk-Skalnica Ridge (Rojšek, 1991). The last gorge of the Soča River can be found near the town of Kanal ob Soči. Downstream from Solkan, the Soča River flows through the plains of Gorica. This is a section where some small tributaries flow into the river, the bottom of the valley is cultivated, and the slopes are overgrown with forest. The middle course of the Soča River is characterised by an enlarged forest share, mainly on the account of abandoned pastures and mown grasslands (Jarc et al., 2002). In the area between the towns of Kanal ob Soči and Plave, the valley is highly industrialised and consequently highly polluted. An additional disturbance in the environment is presented by dams for the hydro-electric power plants of Plave and Solkan. The lower part of the Posočje region encompasses the vast plains of the rivers Ter (Ital. Torre), Nadiža (Ital. Natisone) and Soča (Ital. Isonzo), all the way down to the Adriatic Sea. The lower course of the Soča River starts at the end of the ravine at Solkan and flows almost entirely in Italy. On the plains of Gorica, the river gouged out its own detritus and formed a channel up to 55 m deep. At Sovodnje (Ital. Savogna d Isonzo), the Vipava River, which is regulated throughout its course, flows into the Soča River. The plains adjacent to the lower part of the Soča River are criss-crossed with canals and dry channels for torrential waters. During times of heavy rain, they become filled by the waters of the river Torre (a tributary of the Soča River) and its tributaries Natisone and Judrio the waters then flow into the Adriatic Sea via the Soča River and several branches in its delta (Rojšek, 1991) (Figure 4.3). Almost all the plains of the lower course of the Soča River are cultivated. Irrigation of fields and orchards results in a temporarily dry river channel. Vast areas are drying out also due to poplar (Populus sp.) plantations. 10

25 Figure 4.3: The Soča river basin (the study area is marked in red). Most rocks in the Upper Soča Valley consist of carbonate sediments from the Mesozoic; the middle part of the Posočje region consists of similar rocks, an exception being the extremely complicated hill formation of Idrijsko-cerkljansko hribovje; the lower part of the Posočje region, however, is totally different, because of its young age. Here, the oldest (some tens of 11

26 billions of years) formation is flysch (Goriška brda and Vipava), while parts of the plains are still in the process of formation via river deposits (Jarc et al., 2002). The Soča Valley was shaped by rapid geomorphologic processes: on the 140 km-long journey from its source to its termination at the seaside, the river turns its course 90 degrees four times (Rojšek, 1991). The middle part of the Soča Valley, between Kobarid and Sela pri Volčah, was first transformed by the Soča glacier and then by karst processes and erosion. Due to rockfalls and slope transformation, the glaciated U-shaped section of the valley is currently possible to see only near the village of Srpenica. In addition, the landscape in the Posočje region was strongly affected and transformed by water and its power, which is obvious in the numerous gorges. The deepest gorges are some 100 m deep and the widest 60 m wide (Jarc et al., 2002). The climatic conditions are warm and sub-mediterranean in the coastal area and cold, harsh and Alpine high in the mountains. The influence of the Adriatic Sea stretches up to the headwater areas of the Soča River and is also reflected in its flora many karst and sub-mediterranean species thrive here. One can even find pure Mediterranean species in the Alps and in the middle part of the Posočje region, such as the southern maidenhair fern (Adiantum capillisveneris L.) which thrives at Avče and in Baška grapa (Jarc et al., 2002). The Mediterranean influence is not the only impact made by the proximity of the seaside; there is also intense precipitation, characterised by heavy downpours and large quantities of snow. The consequence of the downpours is torrential run-off from streams and rivers. The Soča River is a typical torrent, and evidently its flow rate at Nova Gorica can increase from 12 to 3000 m 3 /s in only a couple of hours (Rojšek, 1991). 4.2 Riverine belt of the Soča River between the town of Kobarid and the confluence with the Tolminka River The entire basin of the Soča River is too large to study in its entirety; therefore, this analysis focuses on the area between the town of Kobarid and the confluence with the Tolminka River, where in the upper course of the Soča River the largest dense stands of riverine forest grow. The overall size of the study area is ha of which ha is overgrown with forest. The stands in the study area are delineated in terms of location and the hydrological characteristics of this part of the Soča River are described, as are the climatic, geological and pedological structure and successional development of the riverine forests in the study area. 12

27 4.2.1 Scope The Soča River is one of the best preserved Alpine rivers, above all its upper course: from its source in the Trenta Valley to the confluence with the Tolminka River at the town of Tolmin. This part of the river is protected as a natural monument. In addition to the river, also protected are the gravel sites, willow stands and riverine forests which overgrow the youngest river terraces that are up to a few metres above the present river level. Dakskobler et al. (2004) divides the Upper Soča Valley from Trenta to Most na Soči into three sections. The section stretching from its source in the Trenta Valley to the confluence with the Koritnica River at Bovec, is mostly Alpine in nature. In this section the Soča River runs mostly down a narrow valley, on entirely limestone terrain. The only larger gravel sites are near the village of Soča, downstream from the confluence with the Lepenica, at about 430 m a.s.l., which are at least partly overgrown with forest stands of olive willow and spruce. The second section of the Upper Soča Valley is between the confluence with the Koritnica River and the ravine at Trnovo ob Soči. Large gravel sites in this section are in the vicinity of the village of Čezsoča, downstream from the confluence of the Koritnica and Slatenik rivers. Here, limestone gravel prevails, although the Koritnica and Slatenik rivers carry some clay and marl gravel as well. The next extensive gravel sites are located at the confluence with the stream of Učja, and between the villages of Žaga and Srpenica, where the torrent of Sušec flows into the Soča River. Along the river banks, the gravel sites are overgrown with initial willow stands; however, away from the river, they are gradually replaced by a mixed riparian forest of olive willow, grey alder and other deciduous trees. On slightly more developed soil away from the river, which is more rarely exposed to flooding, mixed stands of Scotch pine, spruce and deciduous trees grow. The third section of the Upper Soča Valley starts at the town of Kobarid, where the Soča exits the gorge between Stol and the Polovnik ridge. In this section the river flows over a wide plain with more or less extensive gravel sites on both banks all the way to the confluence with the Tolminka River (at about 160 m a.s.l.). The river terraces in this section are mostly transformed into cultivated meadows; riverine forest stands are preserved only in seldom 13

28 flooded areas. It is the lower section of the upper course of the Soča River and the riparian area of seldom flooded land that fall within the scope of this study Hydrological description Within the research area the Soča has a snow regimen. The height of the water level is above average between April and July, and reaches its peak in May. Autumn high waters with their peak in November are short-term but largely exceed the average yearly flow rate of the river (at Kobarid it is 34.3 m 3 /s) (Površinske, 2014). Figure 4.4: Average annual persistence of high waters of the Soča River (measurement station: Kobarid I), (Površinske, 2014). Near the town of Kobarid the water level of the river can be elevated up to 5 m during floods, but the waters of this height are short-term. During the last five decades ( ), there were only six days per year observed when the water level of the river was at least two metres higher than the referential level (Figure 4.4)(Površinske, 2014). Periods of extremely high flow are rather short (Figure 4.5). Again, on average, a flow rate exceeding 100 m 3 /s was measured only six days per year during the last five decades. 14

29 Figure 4.5: Average annual persistence of high flow rate of the Soča River (measurement station: Kobarid I), (Površinske, 2014). Summer low water is limited to August, while winter low water lasts from December until March. The Soča Valley is a highly torrential area up to the confluence with the Tolminka River. Some studies show that 14,500 m 3 of material per square kilometre is dislodged from this region every year (Pavlič, 1995, as cited in Dakskobler et al., 2004) Meteorology The climate in the upper course of the Soča River is mostly Alpine, but there is a considerable sub-mediterranean impact via the Soča River. A warm and moist climate with sub- Mediterranean impact creates favourable conditions for forest growth (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012 and Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009). Average annual rainfall in the period was 2699 mm in the town of Kobarid (263 m a.s.l.) and 2243 mm in the town of Tolmin (180 m a.s.l.) (B. Zupančič, 1995, as cited in Dakskobler et al., 2004). Precipitation is evenly distributed throughout the year, with a peak in the autumn months. In winter, snowy precipitation may be abundant at higher altitudes, but snow cover lingers only in the mountains, whereas it melts quickly in the valley and on sunfacing slopes. Fog is more common in the autumn; there are foggy days per year. The average yearly temperature in Tolmin in the period was 10.6 C (interpolated value), the coldest month was January (0.6 C), and the warmest month was July (20.0 C) 15

30 (Mekinda-Majaron, 1995, as cited in Dakskobler et al., 2004). Temperatures here are lower than those recorded in typical sub-mediterranean regions, but they are higher than those recorded in Alpine or pre-alpine regions. The growing season lasts around five to six months: from mid-april to mid-october. The Kobarid phenological station shows that during the average growing season of small-leaved lime (Tilia cordata) was 174 days, and of European ash (Fraxinus excelsior) 164 days (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012 and Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009) Morphologic and orographic description. Characteristic of the Soča Valley is its wide bottom with various terraces shaped by the river. Riverine forests are mainly established on regulated sites and the lowest-lying river terraces. The terraces are flat, and in some places their edges are partly wavy and criss-crossed with dikes Geological and pedological characteristics The riverine soil is highly influenced by the river dynamics, namely alluvial deposits and erosion.in geological terms, the parent material of the land closest to the river consists of gravel and sand, rarely of glacial material or alluvial fans created by torrential rainfalls. The soil is undeveloped and alluvial (fluvisoil), with no horizons. There are distinct layers of sediments and although there are large quantities of detritus, yearly sedimentation impedes the process of decomposition leading to humus. The soil of stands that overgrow gravel sites and alluvium next to the river is explicitly gravelly and sandy, shallow, and is not gleyed, while the soil of white willow (Salicetum albae) stand habitat is of the argillaceous type, medium deep to deep, and partly gleyed (Dakskobler et al., 2013). The parent material of areas where grey alder (Alnetum incanae) communities grow consists of older gravel alluvium (conglomerate, gravel, sand) and in some places alluvial fans can be found. The soil is undeveloped, alluvial with no horizons, shallow to moderately deep, and 16

31 under the influence of water (non-gleyed to moderately gleyed). Detritus decomposition is impeded, but some developed soil rich in humus can be found away from the watercourse. On river terraces that cannot be reached by the Soča River even at its highest water level, the predominant soil is alluvial and of varying deepness and granulation. It presents a transition between fluvisoil and automorphic soil. The parent material consists of various gravels: the majority are clay and sandstone, but there is also some limestone Vegetation The successional development of the studied riverine forest is taken from Dakskobler et al. (2004). The riverine forests within the study area grow on the gravel sites and river terraces of the upper course of the Soča River on initial, undeveloped, alluvial soil (vegetation map of the study area is in Appendix D). The analysis includes riverine willow stands, stands with dominating white willow near the town of Tolmin, and mixed riparian forests of olive willow, grey alder, European ash and other deciduous trees, which show a syn-dynamic (successional) connection with forest stands of oak species and European hornbeam. These were cleared long ago for agricultural land (fields and later cultivated meadows) in this part of the Posočje region. In general, these are young stands that were formed during the last five decades on former pasturelands or lands where willow withes were collected (in the vicinity of the town of Tolmin). Stands thriving on frequently flooded alluvial soil near the town of Tolmin are classified into the broader category of white willow (Salicetum albae Issler 1926), which is further divided into two sub-associations. Stands found in concavities where water is retained for a longer period are classified into the sub-association Salicetum albae Issler 1926 myosotidetosum I. Kárpáti ex Soó However, currently thriving along the Soča River are mostly those stands of white willow which, regarding their floristic composition and ecology, are a link in the syn-dynamic development from white willow stands to grey alder stands. Human impact on this development is rather strong, due to various interventions (e.g. regular gravel excavation and deepening of the river channel) which cause a drop in the soil water level and 17

32 are the cause of a lower frequency and duration of flooding. This is the reason why the soil in these stands, at least in comparison with the soil in a typical white willow stand, is drier and more developed. Stands with prevailing grey alder (Alnus Incana) and olive willow (Salix eleagnos) with European ash (Fraxinus excelsior) mixed in, sometimes in a larger share, and in some places small-leaved lime (Tilia cordata), are classified into the macro-association Alnetum incanae s. lat.. In terms of vegetation, this association is divided into two sub-associations: typical (i.e. typicum) and the sub-association caricetosum albae. The typical sub-association is further subdivided into the more initial variant (var. where olive willow (Salix eleagnos) still usually prevails in the tree layer, and the more developed form (var. typica), with grey alder predominating in the canopy. Stands thriving on sites which are only rarely (occasionally) flooded, where the soil water level is lower and hydromorphic soil becomes automorphic, have an abundant presence of characteristic species, differential species and constant companions of forests of the alliance Alnion incanae. Therefore, they are temporarily still classified in the association Alnetum incanae s. lat. as the specific sub-association caricetosum albae. In this sub-association, small-leaved lime (Tilia cordata) is most abundant in the tree layer, European ash is rather common, olive willow and grey alder grow in places, and English oak (Quercus robur) occurs in a few locations near Tolmin. European hornbeam is regularly present here, but only individually. Even regarding its floristic composition this community seems to be a transitional one between forests of the alliance Alnion incanae and forests of the alliance Erythronio-Carpinion. These stands show particular similarities with stands of the association Carici albae-carpinetum betuli Čušin There are two variants. In the more initial (typical) variant, olive willow and grey alder are still common in the tree layer. Stands of this variant connect stands of the sub-association caricetosum albae with stands of the typical sub-association. In stands of the more developed form on automorphic soil, small-leaved lime prevails in the tree layer; however, European hornbeam is relatively common in this layer as well. Among all of the studied stands, these are the most similar to stands of European hornbeam communities, mainly to stands of the association Carici albae-carpinetum betuli,described by Čušin (2002) in the nearby region of 18

33 Breginjski kot. Some of the stands of this variant could probably also be included in this association as an alternative (Dakskobler et al., 2004). Based on Dakskobler et al. (2004) and the map of forest communities of the forest management plan for Kobarid forest management unit (Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009), these stands were categorised as a separate stand type, namely pioneer forests on higher-lying river terraces (Chapter 7.1.4). The successional stages in riverine forests are long-term; therefore, within them, overlapping cyclical development of the forest via structural stages occurs (Chapter 5.1). These stages depend mainly on stand age and human impact, which can change the natural structure of some stands (selective thinning in stands of greater quality changes uneven-aged stands into even-aged) or reverse their development stands return to their previous structural stage (e.g. shrubs in abandoned sandpits) Forest management in the past and present There is a direct link between the current structure and extent of riverine forests and forest management in the past, specifically as regards the manner and intensiveness of agricultural land use. According to archaeological findings, the Upper Soča Valley was populated early in history; however, it can be speculated that at that time no significant exploitation of the forest occurred, nor were forests subject to great interventions. However, in the Middle Ages, impacts on the forest increased, mainly due to densely populated areas, farming and pasturing, and consequently at the end of the Middle Ages, the forest cover was only 25% of the area (the current share is 65%). According to the above, already in 1628 the Austrian government prohibited land- and forest-clearing, which at that time was moving apace. Despite the aforementioned prohibition, almost all accessible forests were changed into agricultural land and the rest of them served for grazing, collecting of woody detritus and trimming of branchwood. Timber-floating was also a common practice along the Soča River. In order to stop the timber and collect it at the town of Tolmin, a special rake-like device was built. Intensive forest management in this area lead to the situation of being among the first areas in Slovenia 19

34 which had to make forest management plans in order to achieve a sustainable forest yield (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012 and Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009). The second half of the 19 th century was marked by the expansion of industrialisation, leading to an agrarian crisis. Lower-quality agricultural lands were gradually abandoned, and a process of overgrowing started, a process currently still ongoing. The consequences of such riverine forest management practices include the following: a decrease in the quality and quantity of growing stock; large changes in the natural structure and proportion of tree species; and an increased share of shrubs and pioneer forest stands on abandoned agricultural sites.apparently, the current riverine forest stands are the offspring of these destroyed and degraded forests. The management of riverine forests after World War II is characterised by a planned approach (forest management plans), low-intensity impacts (which is mainly due to a lack of interest on the part of the owners), and inter-sectoral balancing as a basis for fulfilling a variety of public interests. 20

35 5. METHODS In this chapter, the methods used to analyse the riverine forests of the study area are described. In particular, the methods for determining the scope of the analysed area and a description of stand types and their classification are identified. Additionally, the methodology that was used to determine, describe and evaluate the forest s function and its potential for eco-system services is described. A dendrometric analysis of growth and increment is carried out. Finally, analyses and interpretation of maps are used to depict development of the riverine forest area and current soil use in the study area. 5.1 Description of stand types in riverine forests With the aid of orthophoto records of the study area, and the GIS program package Mastersoft MPX Pro 7.1, the map of riverine forest stands was created by a combination of various data layers. The study area (chapter 4.2) was defined based on the map (layer) of rarely flooded areas issued by the Slovenian Environment Agency (Opozorilna, 2011). The map was examined visually, and some fragments were updated based on phytosociological inventories compiled by Dr. Igor Dakskobler (Dakskobler, 2007). The forest areas were then extracted with the aid of the map (layer) of forest edges issued by the Slovenian Forest Service (Odlok, 2012) and finally the distinct forest stands were separated based on visual examination. Such a map of stands at the scale of 1:10,000 was used as a basis for fieldwork. Throughout the fieldwork, in the process of describing the stands, special attention was given to: delimitation of stands, the forest s structural stage, proportion of tree species in the growing stock, crown closure, stand quality prospects, and other particularities influencing the growth and development of the forest (site characteristics, anti-flood facilities, gravel exploitation, etc.). Fieldwork preparations included the sample-plot grid concept, but the high level of heterogeneity of the riverine forest found on site made it impossible. Therefore the growingstock shares of particular tree species were only estimated. This is normal practice in describing stands for forest management plans, where the share of tree species in the growing stock is not determined but is partly corrected via permanent sample plots. 21

36 The structural stages of an even-aged forest were defined based on the Rules on Forest Management Plans and Game Management Plans (Pravilnik, 2010) as follows: sawlog (breast-height diameter exceeding 30 cm), pole (breast-height diameter between 10 and 30 cm), sapling (young, not yet measured trees with significant stratification), and seedling. In addition, the following structural stages were used in descriptions: shrubs, uneven-aged forest, and coppice. Within the process of describing stands based on fieldwork, the forest structure was estimated, which is normal practice when describing stands within the procedures of compiling the forest management plans at the Slovenian Forest Service. A more precise determination of forest structure via sample plots and diametric distribution would not be suitable due to the small diameters and extreme heterogeneity. The inventory of various stands that we obtained via fieldwork served as a basis for further analyses, namely the forest stands were clustered into six stand types (Table 5.1) according to their characteristics and the results were compared with the vegetation map of the forest management plan of the forest management units of Tolmin (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012) and Kobarid (Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009) The aforementioned procedure resulted in a map and description of individual stand types in the study area. Table 5.1:Stand types in the study area. STAND TYPE AREA (ha) RELATIVE S HARE (%) Initial willow stands White willow stands Grey alder st ands Pioneer forest on higher-lying river terraces Riparian corridor stands Anthropogenic riverine forest stands Total Classification of stand types into categories was carried out by applying hierarchical clustering analysis (HCA) via the SPSS Win 9.0 package. The criterion upon which the clustering was done was the value of forest functions of a particular stand type (the value was determined in terms of how the function influences forest management practices). 22

37 5.1.1 Forest functions Forest functions were described, classified and given a value according to the methodology used by the Slovenian Forest Service (Zakon, 1993 and Pravilnik o načrtih, 2010). The Slovenian Forest Act (Zakon, 1993) classifies forest functions into three groups, namely: Ecological: function of protecting forest lands and stands, hydrological function, biodiversity-preservation function and climate function; Social: protection function protection of buildings, recreational, tourist, informative, research, hygiene/health protection, natural-value protection, cultural-heritage protection, defence and aesthetic function. Production: timber-industry function, function of acquiring other forest products, and hunting-management function. Forest-function values were determined in accordance with the Rules on Forest Management Plans and Game Management Plans (Pravilnik o nacrtih, 2010) as follows: Value 1: function determines forest management practices; Value 2: function exerts an important influence on forest management practices; Value 3: function only partly influences forest management practices. Based on the description of functions and criteria given by the Rules on Forest Management Plans and Game Management Plans (Pravilnik o nacrtih, 2010), the forest-function values for each stand type were determined and a radar chart was plotted for each stand type Ecosystem services The ecosystem-services potential for each stand type was determined upon field inspection. The typology of ecosystem services was taken from Danev (2013). 5.2 Dendrometric analysis Dendrometric analysis (Kotar, 2005) was used to describe dynamics of the growth of riparian forest stands. Nine locations were chosen for felling sites whose trees underwent dendrometric analysis. Felling locations were chosen according to the following criteria: 23

38 dense stands of older structural stages in riverine forests of all stand types. Knowing that the larger the sample size, the smaller the random error, the dendrometric analysis was carried out in a sample of 45 trees, representatives of the seven most common tree species (tree data are in Appendix A, Figure 5.1). In each location, tree species forming the stand s canopy and fitting into the category of dominant and co-dominant trees according to the Kreft classification were included in the analysis. The crowns of these trees were widely-spaced during their growth, and therefore their increment depended mainly on the site and the characteristics of the tree species. Figure 5.1: Felling sites whose trees were used for dendrometric analysis (green dots). Kotar (2005) states that according to his experience, individuals may show some differences, but also a number of common characteristics, and therefore it is also rational to use some average values characterising the population per se. Trees were felled and cut into sections and the samples thus obtained were marked according to dendrometric analysis methods (Kotar, 2005). Data analysis was carried out at the University of Ljubljana, Biotechnical Faculty, Department of Forestry and Renewable Resources by applying the SPSS Win 9.0 package. In all analyses, the data were fitted by applying nonlinear regression. Growth curves are more reliable when they are constructed on the basis of a larger number of trees; therefore, the calculation of growth curves is identical to 24

39 the calculation of regression curves where the scattered data obtained by analysing a larger number of trees are fitted to the regression line (Kotar, 2005). The Chapman-Richards function was applied to fit the regression line when analysing height and diametric growth (1) (Kotar, 2005), and the exponential function when volumetric growth was analysed. YY(tt) = aa(1 ee bbbb ) cc (1) Y(t) =growth parameter (height/diameter) in time (t) t = time a, b, c = coefficient s The characteristics of tree height, diameter and volume in terms of growth and increment were presented by growth and increment curves for each tree species. In order to plot the increment curves, the current annual growth increment for height, diameter and volume parameters was calculated from values based on five-year growth. CCCCCC = YY(tt) YY(tt 5) 5 CAI = current annual increment Y(t) =paramet er value in time (t) Y(t-5) =parameter value in time t minus 5 years (2) Due to the fact that structural stages in riverine forest phytosociological communities depend greatly on succession and changed habitat, their transition is gradual (Chapter 4.2.6) (Dakskobler, 2004). Accordingly, the structure of the tree layer of particular species in various stand types depends on the successional stage. The tree species forming the stand canopy of the typical grey alder stand type can be found in stand types in earlier structural stages (white willow and initial willow stand type) and also in the stand type of the next successional stage (pioneer forests on higher-lying river terraces) (Chapter 4.2.6). Individual trees of particular tree species were felled in various stand types where these species were naturally present in the tree layer and formed the canopy of the stand. Therefore, the growth characteristics of particular tree species were not connected with their classification into particular stand types; rather, they were analysed in the broader context of riverine forests. For a simple comparison of the growth characteristics of various tree species, dendrometric analysis is presented for all of the species together. Due to the fact that the number of felled subjects of olive willow was low and because of the very similar growth and increment 25

40 characteristics of olive and white willow, both species were placed in the same category named after the prevailing white willow species in the analysis. Therefore, the results for the white willow species are also valid for the olive willow species. 5.3 Development of riverine forest land in the study area, Maps depicting the development of the land in the study area were created through the digitisation of maps given in Table 5.2 and data layers provided by Mastersoft MPX Pro 7.1. Table 5.2: Maps and data layers forming the basis for creating maps depicting the development of riverine forest land. MAP NAME YEAR O F ISSUE SOURC E FO RM Slovenija na vojaškem zemljevidu Slovenija na vojaškem zemljevidu (Eng.: Slovenia on military map, Map (appendix in monograph) Carta geografica del littorale 1830 Slovenian Forest Service archive, Tolmin unit Map Catastro forestale Provincia di Slovenian Forest Service archive, 1931 Gorizia, fascicolo 31 Tolmin unit Map Aerial photo records of cyclic Slovenian Forest Service archive, 1958 shooting from 1958 Tolmin unit Aerial photo records Digitalised orthophoto records of Slovenian Forest Service archive, Digitalised orthophoto records 1995 cyclic shooting from 1995 Tolmin unit (data layer) Digitalised orthophoto records of Slovenian Forest Service archive, Digitalised orthophoto records 2011 cyclic shooting from 2011 Tolmin unit (data layer) In order to make a diagram showing the development of riverine forest land in and the relative forest share, the data on the land in the study area were obtained via digitalised maps, whereas the data on relative forest share over time in the municipalities of Tolmin and Kobarid were obtained from the forest management plans of the forest management units of Tolmin (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012) and Kobarid (Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009) the two border municipalities Land use in the study area The map of land use in the study area and the calculation of the area coverage of particular land use were created based on a map (data layer) issued by the Ministry of Agriculture, 26

41 Forestry and Food, namely the Record of Actual Land Use (Evidenca, 2016). The data for the map was collected in the scale of 1:5000 and as such is compatible with the scale adopted in our analysis. By applying Mastersoft MPX Pro 7.1, this map was combined with the map of the study area. The obtained layer was examined and based on field inspection, some errors in terms of land-use classification and delineation of land use were corrected. In addition, the number of categories of land use and their names were changed, resulting in classifications that best fit the actual situation in the study area (Table 5.3). Table 5.3: Definition of land use categories. TYPE O F AC TUAL US E DESCRIPTIO N O F ACTUAL USE Fields and gardens Permanent meadows and pastures Orchards Land in succession Forest Developed land and similar Wet lands Gravel Wat ers Agricultural land, being ploughed or otherwise cultivat ed. Agricultural areas covered by grass, clover and other herbs, being permanently mowed or grazed. Areas planted with a single type of fruit tree, where the production is undertaken using modern technology. Abandoned, overgrown agricultural land. When this land is not in agricultural use for 20 or more years, when the canopy cover exceeds 75%, and the circumference of tree trunks exceeds 10 cm, it is developing into a forest. Land overgrown by forest trees in the form of stands or other forest vegetation that enables any kind of forest function. This category includes all land covered with buildings, roads and other infrastructure used for human activities. Lower-lying lands, frequently flooded, of more or less soggy soil, that are not used for agricultural purposes. Undeveloped land with little or no vegetation. This category includes gravel sites along and in the wat ercourse. Areas covered by surface waters, such as lakes, rivers, streams and dams. 6. DATA When analysing the riverine forests of the Soča River in the area between the town of Kobarid and the confluence with the Tolminka River, the following data were used: Data on the Soča River flow rate and water level at the town of Kobarid (Kobarid measuring station I) for the period from to These data are publicly available on the website of the Ministry of Environment and Spatial Planning Slovenian Environment Agency (Površinske, 2014). Phytosociological inventories in the study area were compiled and published in an article (Dakskobler et al., 2004) by Dr. Igor Dakskobler et.al. 27

42 The data needed to describe the stand types, forest functions and their potential for ecosystem services were obtained during fieldwork in the 2015 growing season (methods are described in Chapter 5.1; the table of codes for the described characteristics of stand types is in Appendix D). The data for dendrometric analysis were obtained during 2013 and 2014 via fieldwork, namely by felling 45 trees of seven tree species (methods are described in Chapter 5.2; the tree data are in Appendix C). The maps used for analysis, interpretation and further processing are listed in table 6.1. The data on the forest area in the municipalities of Tolmin and Kobarid in the past were obtained from the forest management plans of the forest management units of Tolmin and Kobarid (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012 and Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009). The data on natural values in riverine forests were obtained from the forest management plans of the forest management units of Tolmin and Kobarid (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012 and Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009). Table 6.1: Maps used for analysing the riverine forest area. MAP NAME Karta gozdnega roba za leto 2011 (Eng.: Map of forest edges, 2011) Opozorilna karta poplav (Redka območja poplavljanja) (Eng.:Floodwarning map (Seldom-flooded areas)) Karta gozdnih združb na področju gozdnogospodarske enote Tolmin (Eng.: Map of forest communities in the area of the Tolmin forest management unit) Karta gozdnih združb na področju gozdnogospodarske enote Kobarid (Eng.: Map of forest communities in the area of the Kobarid forest management unit) Slovenija na vojaškem zemljevidu (Eng.: Slovenian military map, ) YEAR O F ISSUE SOURC E FO RM 2011 Odlok, 2012 Dat a layer 2011 Opozorilna, 2011 Pravilnik o gozdnogospodarskem 2012 načrtu gozdnogospodarske enote Tolmin, 2012 Pravilnik o gozdnogospodarskem 2009 načrtu gozdnogospodarske enote Kobarid, 2009 Slovenija na vojaškem zemljevidu , 1995 Slovenian Forest Service archive, Carta geografica del littorale 1830 Tolmin unit Dat a layer (publicly available on the int ernet ) Map (Supplement to the forest management plan, Tolmin unit Map (Supplement to the forest management plan, Kobarid unit Map (appendix in monograph) Map continues 28

43 MAP NAME Catastro forestale Provincia di Gorizia, fascicolo 31 Aerial photo records of cyclic shooting from 1958 Digitalised orthophoto records of cyclic shooting from 1995 Digitalised orthophoto records of cyclic shooting from 2011 Evidenca dejanske rabe kmetijskih in gozdnih zemljišč (RABA) (Eng.: Records of actual agricultural and forest land use) YEAR O F ISSUE SOURC E 1931 Slovenian Forest Service archive, Tolmin unit 1958 Slovenian Forest Service archive, Tolmin unit Slovenian Forest Service archive, 1995 Tolmin unit 2011 Slovenian Forest Service archive, Tolmin unit 2016 Evidenca, 2016 FO RM Map Aerial photo records Digitalised orthophoto records (data layer) Continues Digitalised orthophoto records (data layer) Data layer (publicly available on the int ernet ) 7. RESULTS 7.1 Description of stand types in riverine forests The scope of research encompasses six stand types. Each type is analysed in dendrological terms and in terms of its extent. The forest s function is described as is its potential for ecosystem services. Some forest economics and management guidelines for each stand type are given Initial willow stands General The initial willow stand type is comprised of shrubs (i.e. orogenous willow forests) and forests with a gappy crown closure growing on extensive, occasionally flooded gravels along the Soča River. River dynamics, namely alluvial deposits and erosion, exert a crucial influence on these forests. During high torrential waters or floods, their structure may change, and shrubs may even disappear with the changed riverbanks. Harmful human impact on initial willow stands is characterised by gravel excavation and riverbank regulation. Therefore, these sites remain in the pioneer succession stage, which, due to ecological conditions and human impact, never achieve higher successional levels. This is the so-called paraclimax stage. In Europe, the number of streams and rivers which flow more or less naturally (i.e. without 29

44 artificial barriers) is decreasing. Consequently there are fewer and fewer streams and rivers with gravel and sand deposits which enable willow communities to thrive (Dakskobler et al., 2013). In initial willow stands a predominant olive willow (Salicetum eleagni-purpureae Sillinger, 1933) phytosociological community is found where the main species of the upper stand layer are initial olive willow (Salix eleagnos), purple willow (Salix purpurea), grey alder (Alnus incana) and black poplar (Populus nigra). Common shrubs in olive willow stands are alder buckthorn (Frangula alnus), common hazel (Corylus avellana) and guelder rose (Viburnum opulus). These are typical initial communities and therefore include seedlings of various tree species. In initial gravels on Alpine rivers, the herbaceous layer consists mainly of Alpine butterbur (Petasites paradoxus), while in gravels of medium mountain regions the common butterbur (Petasites hybridus) is frequently found and along with it numerous nitrophilous-forest and riverbank-edge species, as well as some weeds and ruderal species (Dakskobler et al., 2013) Stand description Along the Soča River in the Ograjenca region, large, dense areas of initial willow stands are found in overgrown gravels and areas used for temporary gravel excavation and storage (I1 Figure 7.2). Olive willow, a pioneer stand on gravels along the river, is in the successional shrub stage. Away from the riverbank, these stands are of a size that classifies them in the sapling/pole category of pure olive willow characterised by good potential and a high density. Stands are normally under the influence of high floodwaters. In the growing stock, the share of tree layers of older structural stages of initial willow stand is comprised as follows: olive willow 85%, grey alder 10% and European ash 5%. In a dense shrub layer characterised by extensive crown closure, dogwood, common spindle and common maple are found. In the lower layer some individual beech trees occur. Parallel to the river there are anti-flood embankments which enable elevated vegetation sites, which are characterised by forests of uneven-aged forest with stands of grey alder and European ash mixed in. Following a line away from the river to some abandoned sandpits, there are pioneer shrubs of olive willow stand and some individual cases of dominant black poplar (I2 Figure 7.2). Areas which are drier are characterised by smaller patches of grey alder. Their structural stage depends on the stand s age (i.e. the time the area was left to overgrow). 30

45 Figure 7.1: Olive willow stand on a blind branch of the Soča River below the village of Kamno. A cut-off meander of the Soča River located near the village of Kamno is comprised of a gorgeous olive willow stand of diameter cm on the riverbanks (Figure 7.1) and stands of olive willow shrub on a blind branch (I3 Figure 7.2). The predominant olive willow thriving in the tree layer is of good vitality and of normal crown closure. A grey alder community grows in the elevated, middle part of the stand. 31

46 Figure 7.2: Extent (highlighted in yellow) and areas (I1, I2, I3) of initial willow stand type Forest functions Despite the aforementioned economic potential, initial willow stands are important mainly in the area of ecological function. Function of protecting forest lands and stands The initial willow stand with its root system prevents erosion of the riverbank and decreases the amount of alluvial sediments flowing into the watercourse. This function is especially obvious during intensified river flow, when the initial willow stands prevent the river flow from eroding newly established islets of gravel in the river. In cases of extremely intense river flows, it happens that the Soča River washes away the gravel along with willow shrubs. In initial willow stands, plant litter (willow leaves), alluvium and fine gravel sand deposited during floods is accumulated. The majority of this material is washed away by the river during the next flood, and therefore mineralisation and pedogenesis are of limited scope. 32

47 Hydrological function In cases of modestly increased water flow, the Soča River floods the shrubs of initial willow stands along its banks. Orogenous willow forest stands regulate the water regime of the river by impeding the quick drainage of water. This way, water filtration occurs: increased mixing of water contributes to oxygen intake, which enhances its self-cleansing capability. Clearance of orogenous willow forest stands contributes to greater inflow of rainwater. This happens due to decreased transpiration and increased surface runoff taking place as a result of the fact that bare-soil areas are less prone to water infiltration. Biodiversity preservation function Nowadays, orogenous willow forest stands along Alpine rivers are limited to narrow patches of pioneer forest located along riverbanks. Due to the regulation of watercourses (dams, maintained banks), gravel excavation and changing land use, these areas are amongst the most highly endangered habitat types (Dakskobler et al., 2013). Pioneer communities of Alpine river and stream gravels are classified as important European habitat type 3240 Alpine rivers and their ligneous vegetation with Salix eleagnos along their banks (Dakskobler et al., 2013). For the majority of birds, initial willow stands offer nesting habitat and a place of refuge, while larger animals find habitat and safe access to the riverbank there. Climate function Initial willow stands on the Soča River regulate temperature extremes in and next to the watercourse and the felling of riparian shrubs can change the water temperature, which in turn can change the living conditions of organisms in the watercourse (concentration of oxygen, shade). Initial willow stands buffer the influence of the wind in open areas of gravel or abandoned sandpits, decrease wind erosion, and increase soil moisture. 33

48 Tourist function During the summer many bathers visit the gravels of the Soča River. They get shade and shelter from initial willow stands. However, in the case of overuse by visiting bathers, the negative consequences for the forest (litter, campfire sites, etc.) should not be overlooked. Research function As a rare habitat type, initial willow stands are an indispensable element in studies of riparian vegetation and succession on the floodplains of Alpine rivers. Natural-value-protection function The emphasised function of protecting natural values in initial willow stands is embedded in the following classification of natural values: Table 7.1: Natural value in the forest habitat of the initial willow stand type (Source: Forest management plan of the forest management units of Tolmin (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012) and Kobarid (Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009)). NAME SHORT LABEL CATEGO RY S TATUS Soča River with its tributaries up to the confluence with the Idrijca River Soča River with its tributaries up to the confluence with the Idrijca River Hydrological, geomorphic, zoological natural value Natural value of national significance Aesthetic function Initial willow stands have a high aesthetic value in the area. They buffer areas where the gravels meet the forest, have a positive influence on the scenic quality of the landscape, and help in orientation in the landscape. 34

49 Figure 7.3: Values of forest functions in terms of initial willow stand type (value 1: function determines the forest management practices; value 2: function exerts an important influence on forest management practices; value 3: function only partly influences forest management practices) Initial willow stand management guidelines Initial willow stands have numerous functions that we are not conscious of; therefore, they usually don t play an important role when decisions are made on activities which affect the physical environment. Based on current findings in the area of initial willow stand management, the following guidelines should be followed: - Initial willow stands are of no significant economic value; therefore, they demand no forest management measures. - Any measures in their environment should be directed toward the preservation of the conditions in which they thrive. In these terms the relevant level of soil water must be maintained and human impact on river dynamics should be minimised. In addition, gravel excavation should be limited, as should road construction, anti-flood barriers, watercourse regulation, and construction of artificial riverbanks. - Due to the many roles played by initial willow stands, any measures undertaken should be in accordance with the various interests of the discipline along with business and public entities. 35

50 7.1.2 White willow stands General This stand type is comprised of fairly well preserved white willow (Salix alba) stands located in the vicinity of the town of Tolmin. The white willow phytosociological forest community (Salicetum albae Issler 1926) is predominant in this stand type. It mainly occurs on various forms of alluvial deposits (gravel, sand), river regulation sites, and the lowest-lying river terraces of height a.s.l. It thrives in areas located along watercourses, above the margin of the average water level, which puts it under the constant influence of water. Therefore, any lowering of the groundwater due to deepening of the riverbed (gravel excavation) or river regulation causes an irreversible decrease in the scope of these stands, which are usually replaced by the next successional forest stage (i.e. grey alder stands). In fact, the phytosociological community of white willow can be taken as a successional phase leading to grey alder stands. In the tree layer, the predominant white willow (Salix alba) is established and in some cases there are individual crack willow (Salix fragilis) trees mixed in. Other species of the tree layer are not as representative (i.e. their occurrence is not so widespread) and are as follows: grey alder (Alnus Incana), black alder (Alnus glutinosa), European ash (Fraxinus excelsior), white poplar (Populus alba), and black locust (Robinia pseudacacia). In addition, the tree layer is frequently populated by climbing vines like English ivy (Hedera helix), common hop (Humulus lupulus) and old man s beard (Clematis vitalba), while in the bottom part of the tree layer the non-native species box elder (Acer negundo) is found. The shrub layer consists of common spindle (Euonymus europaea), elder (Sambucus nigra), common buckthorn (Rhamnus catharticus), common hazel (Corylus avellana), guelder rose (Viburnum opulus), and occasionally common hawthorn (Crataegus monogyna) and wild privet (Ligustrum vulgare). In addition to the aforementioned species, seedlings of various tree species are found in the shrub layer. They are frequently predecessors of the next succession stages such as: European ash (Fraxinus excelsior), small-leaved lime (Tilia cordata), black alder (Alnus glutinosa), European hornbeam (Carpinus betulus), and black locust (Robinia pseudacacia). 36

51 In the herbaceous layer one can find the native species giant goldenrod (Solidago gigantea), spring snowflake (Leucojum vernum), ground elder (Aegopodium podagraria), large bittercress (Cardamine amara), and lesser celandine (Ranunculus ficaria), and an introduced species, Jerusalem artichoke (Helianthus tuberosus). The white willow stand also contains the protected yellow iris (Iris pseudacorus), spring snowflake (Leucojum vernum) and common snowdrop (Galanthus nivalis) Stand description White willow stands overgrow the lowest-lying river terraces under the influence of floods in the vicinity of Tolmin, namely from the confluence of the Soča and Tolminka rivers to Prod (Eng. Gravel) and to Grofov ribnik (Eng. Count s Pond) below the village of Dolje. These stands are in the sawlog size category. The most highly preserved stands that qualify for protection are located in the following areas: between the St. Ulrich church and the German cemetery (Figures 7.6, 7.7), below the village of Volče, on the right bank of the Soča River, and in the area of Grofov ribnik below the village of Dolje (B1 Figure 7.5, 7.4). There, the share of white willow in the stand growing stock is 90%. The remaining 10% is comprised of grey alder and European ash. The shrub layer here consists of elder, common spindle and common hazel. Trees are mostly of good vitality and due to an abundance of light they are overgrown with English ivy. In regions located upstream from St. Ulrich and within some stands away from the riverbank and in locations where the river used to be regulated by hydro-technical measures (embankments, dams), white willows are prone to decay and as the snags die off, stands with gappy crown closures occur. Under the cover of other trees, it is impossible for the white willow to rejuvenate; therefore, rejuvenation of species of the following successional stages takes place in such gaps: European ash, small-leaved lime, grey alder and black alder. The only successful rejuvenation of a white willow stand can be observed along the road leading from the Volče bridge towards the town of Tolmin, where some clear-felling occurred for the purposes of road construction. White willow stands are highly influenced by gravel exploitation. This activity demands some temporary roads aimed at gravel pickup. There have been many human impacts on white willow stands located on the right bank of the Soča River, such as the establishment of an electric cable network, bathing areas, paint-ball areas, and forestry activity, which altogether has contributed to the degradation of these stands. 37

52 Figure 7.4: White willow stand at Grofov ribnik below the village of Dolje. Some stands, those that are more elevated above the river and are in less touch with the groundwater are in a successional stage leading to grey alder stands (B2 Figure 7.5). This occurs especially in the region below St. Ulrich, up to the road made for gravel exploitation and in the central part of the stands located on the left bank of the Soča River under the Volče bridge. Here, the share of white willow growing stock in the stand decreases on the account of European ash, small-leaved lime and grey alder. In a somewhat smaller share, black poplar, black alder, European hornbeam and beech can be found (with some individuals of extreme dimensions). In the vicinity of the town of Tolmin, some individual understorey trees of box elder (Acer negundo) are mixed in with white willow stands. They are remnants of a nonnative species which was delivered to this area in an attempt to perform amelioration of riparian gravels. 38

53 Figure 7.5: Extent (highlighted in red) and areas (B1, B2) of white willow stand type. Figures 7.6, 7.7: LIDAR scan (left) of natural relief of the lowest-lying river terraces with white willow stands located below the St. Ulrich church near the town of Tolmin. 39

54 Forest functions Since time immemorial, white willow stands have been present in areas inundated by floods. Their most important ecological functions remain unchanged nowadays and only by preserving these functions can the floodplain forests of white willow on the Soča River be preserved. Function of protecting forest lands and stands White willow forests and their wide-spreading root system prevent erosion of the lowest-lying river terraces during floods. A thick layer of shrub and tree overgrowth holds back buoyant material and consequently enables its accumulation, which forms the basis of forest soil. Hydrological function During times of high water, when the Soča River floods the white willow stands (Figure 7.8), they play an important role in regulating the river-water regimen by slowing the flow of floodwaters. Figure 7.8: White willow stands near the town of Tolmin during flooding in

55 This way, water filtration occurs: increased mixing of water contributes to oxygen intake, which enhances its self-cleansing capability. When white willow stands are reduced, their ability to slow the flow of floodwaters is decreased, and consequently decreased transpiration and increased surface runoff take place which enhances water flow into the watercourse. White willow stands absorb some of the organic materials and dissolved minerals flowing from neighbouring agricultural lands. Therefore, they influence the cycle of mineral matter and organic substances and decrease their flow into the Soča River. Biodiversity preservation function White willow forests are classified as priority European habitat type 91E0 Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae) (Dakskobler et al., 2013). For the majority of birds, white willow stands offer nesting habitat and a place of refuge, while larger animals find habitat and safe access to the riverbank there. Climate function Alluvial forests function as a protective belt against wind and therefore decrease wind erosion and increase soil moisture. They increase turbulence and mix the air, which has a favourable impact on forests and neighbouring lands. Recreational function Throughout the year, recreational activities (walking, jogging) are carried out via numerous trails leading through the stands under St. Ulrich. One of the trails is equipped with informative panels for a workout in nature. Tourist function The tourist function is mainly carried out by forests located below the village of Volče, where a mini paint-ball area is based. On the bank of the Soča River located below St. Ulrich, bathing tourism is prominent during the summer. 41

56 Research function As a rare habitat type, white willow stands are an indispensable element in studies of riparian vegetation and succession on the floodplains of Alpine rivers. In the Soča River basin, white willow stands are established only in this area near the town Tolmin, and therefore they are very valuable for various research projects and comparisons with similar habitats in other regions of Slovenia and even in Alpine areas elsewhere. Natural-value-protection function The emphasised function of protecting natural values in white willow stands is embedded in the following classification of natural values: Table 7.2: Natural value in the forest habitat of the white willow stand type (Source: Forest management plan of the forest management units of Tolmin (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012) and Kobarid (Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009)) NAME SHORT LABEL CATEGO RY S TATUS Tolmin below the St. Ulrich cemet ary Floodplain of the Soča River near the town of Tolmin Ecosystemic natural value Natural value of local significance Prodi Occasionally flooded overgrown gravels along the Soča River, west of the village of Dolje Geomorphic, botanical, zoological natural value Nat ural value of local significance Soča River with its tributaries up to the confluence with t he Idrijca River Soča River with its tributaries up to the confluence with the Idrijca Rriver Hydrological, geomorphic, zoological natural value Nat ural value of national significance Aesthetic function White willow stands enjoy a high aesthetic value in the area, because their stands represent the densest complex of riverine forests in the Tolmin basin. They have a positive influence on the landscape s scenic quality and on orientation in the landscape. An example of aesthetic function is a monumental black poplar tree located on the Soča River below the St. Ulrich church. Its diameter measures 157cm and its height 38m. 42

57 Timber industry function White willow stands are primarily of the sawlog size category, but they consist mainly of deciduous softwood which limits their potential for the timber industry. In the past these stands were normally used for fuel, mainly by collecting woody detritus after big floods. Function of acquiring other forest products In the past, people used to graze cattle in stands of white willow and used the withes for basket-making. Hunting-management function This function is emphasised within the riverine forests located below St. Ulrich near the town of Tolmin. There is a maintained clearing in the wood equipped with a hunter s lookout tower. Figure 7.9: Values of forest functions in terms of white willow stand type (value 1: function determines the forest management practices; value 2: function exerts an important influence on forest management practices; value 3: function only partly influences forest management practices). 43

58 Ecosystem services White willow stands have great potential to offer ecosystem services in the domain of regulation, namely by regulating and slowing the influence of extreme events (i.e. regulation of water flow). By slowing the flow of floodwaters, the white willow stands prevent possible damage to lower-lying areas and to some extent relieve the Podselo dam, which is a rather weak spot for floodwaters. Consequently, possible damage further downstream in the area of the Most na Soči reservoir is prevented White willow stand management guidelines In terms of forest economics, white willow stands have little significance. White willow stands are fast-growing and relatively short-lived, and are prone to die-off and decomposition. Therefore, their management needs to be carried out in a protective manner. - Felling is rational only in areas where stands thrive in larger quantities. Because of the somehow depressed rejuvenation of White Willow stands under the cover of other trees, larger felling areas are reasonable. - Any measures taken should be directed toward the preservation of their habitat and the conditions in which they thrive. In order to ensure the relevant level of soil water, human impact on river dynamics should be minimised. Gravel excavation should be limited, as should construction of barriers, watercourse regulation, and construction of artificial riverbanks. - Due to the many functions of riverine forests, any management of white willow stands should be done in accordance with the various interests of the discipline along with forest owners and business and public entities. - Deforestation of riverine white willow stands and their transformation into agricultural land is not acceptable. 44

59 7.1.3 Grey alder stands General The grey alder stand type is characterised by riparian forests with a predominant phytosociological association of grey alder (Alnetum incanae Lüdi 1921 s. lat.) which grows in gravels and on river terraces along the upper and partly on the middle part of the Soča River on initial, undeveloped alluvial soil. These stands are established on vast gravel sites throughout the entire study area and represent a successional stage from shrubs of olive and purple willow (initial willow stands) and white willow stands to pioneer stands of European hornbeam, small-leaved lime and European ash developing on automorphic soil of slightly elevated terraces, away from the floodplain (pioneer forests of higher-lying river terraces). Grey alder stands are in a rather long-standing stage of development in terms of alluvial vegetation; therefore, they generate specific conditions in their habitat, which make them distinct from the rest of the alluvial forests. The tree layer of the Grey alder phytosociological community consists of Grey alder (Alnus Incana) with European ash (Fraxinus excelsior), olive willow (Salix eleagnos) and black alder (Alnus glutinosa)mixed in, and on more elevated dry sites, small-leaved lime(tilia cordata) and European hornbeam (Carpinus betulus) can be found. The shrub layer of grey alder stands is usually dense. The most frequent species found here are: common hazel (Corylus avellana), common spindle (Euonymus europaea), dogwood (Cornus sanguinea), fly honeysuckle (Lonicera xylosteum), February daphne (Daphne mezereum), guelder rose (Viburnum opulus), wayfaring tree (Viburnum lantana), elder (Sambucus nigra), common hawthorn (Crataegus monogyna), alder buckthorn (Frangula alnus), common barberry (Berberis vulgaris) and wild privet (Ligustrum vulgare). In addition, the shrub layer consists of seedlings of various tree species. The herbaceous layer in Alpine riparian deciduous forests is colourful and rich in terms of its variety of species.the predominant species of the dense herbaceous layer are: spring snowflake (Leucojum vernus), common snowdrop (Galanthus nivalis), lesser celandine (Ranunculus ficaria), spring crocus (Crocus napolitanus), herb Paris (Paris quadrifolia), 45

60 three-leaved anemone (Anemone trifolia), five-leaflet bitter-cress (Cardamine pentaphyllos), wild garlic (Allium ursinum), European wild ginger (Asarum europaeum) and yellow archangel (Galeobdolon flavidum) Stand description A large stand of pole-size grey alder is established on a river terrace upstream from the Tolmin aggregate extraction site (J1 Figure 7.11). This is probably the most typical stand of the grey alder phytosociological community within the study region (Figure 7.10). In the tree layer of the growing stock, the predominant grey alder (70%) is established, with white willow (15%), European ash (10%) and small-leaved lime (5%) mixed in. The shrub layer is not too dense. Here, the following species are found: common hazel, elder, common spindle, common barberry, common hawthorn and dogwood. Some years back, part of this stand was fenced off and thinned and at present cattle are still grazing there. The thinning practices in this area enhance the growth of European ash. Uneven-aged forest located below the village of Gaberje is characterised as a pioneer forest on abandoned agricultural and construction (gravel exploitation) sites (J2 Figure 7.11). The forest s trees are of a maximum diameter of 20 cm, with gappy crown closure and low quality prospects. There is no management of this stand. The shrub layer is extremely dense (almost impassable). The pioneer character of the forest is evident in the individual fruit trees, remnants from when this land was used for agriculture in the past. In the direct vicinity, stands with a similar composition are fenced off by owners and with the aim of goat breeding and clearing, these sites are changing into agricultural lands. 46

61 Figure 7.10: Gray alder stand at the Tolmin aggregate extraction site. In the gravels of the Volarja Stream, pole-size grey alder stands consist mainly of European hornbeam (up to 55%) with small-leaved lime, grey alder and beech (J3 Figure 7.11) mixed in. These are stands of good vitality and low density, and are not very intensively managed. On the other side of a high-lying terrace characterised by agricultural land, stands of unevenaged forest grow. Here, the tree layer consists of a predominant olive willow stand of large dimension. Its gappy crown closure allows grey alder trees to thrive in the lower tree layer, while on sites with drier soil, European ash, small-leaved lime trees and/or smaller stands of almost pure grey alder are found. The shrub layer is dense. The stand is not managed and has low quality prospects. In the vicinity of the village of Kamno, there are predominant stands of uneven-aged structures with gappy crown closure (J4 Figure 7.11). In the tree layer of the growing stock, the following species thrive: grey alder (30%), olive willow (30%), European ash (20%), small-leaved lime (15%) and spruce (5%). The shrub layer is not plentiful in terms of its 47

62 variety ofspecies. Common hazel, common spindle and willow can be found here. The majority of stands are not managed. The exceptions are small interventions on agricultural sites. Along the river, there are frequent occurrences of shrub stands with predominant olive willow. Within these stands that are established on nearby structures aimed at river regulation (dams, dykes) or on other elevated sites, there are patches of forest characterised by a large share of grey alder. Above these sites, floodwaters as well as the waters of streams from the hillsides accumulate; therefore, very favourable conditions are created for stands of olive willow to thrive, including a large share in the mature, pole-size category. The stands of small-leaved lime and European ash with good quality prospects occur more frequently away from the river, on somewhat drier soil. Normally, some individual beech trees occur in the lower tree layer. Figure 7.11: Extent (highlighted in blue) and areas (J1, J2, J3, J4, J5) of grey alder stand type. Grey alder stands in the area of the villages of Ladra and Smast are of uneven-aged structure. Some pole-size trees of a maximum diameter of 25cm and some individual trees of larger diameters are found here (J5 Figure 7.11). The forest is partly managed and its stands are of 48

63 good vitality, characterised by low to gappy stand density. On the Ročica Stream, the establishment of Japanese knotweed (Fallopia japonica) is significant. The tree layer growing stock consists of olive willow (50%), grey alder (18%), European ash (15%), European hornbeam (10%), small-leaved lime (5%) and black locust (2%). The edges of agricultural lands are characterised by drier habitat. Here, the grey alder share of the tree layer growing stock can reach 70%. In the centre of the stand established below the village of Ladra, there is a playground, a remnant of the former agricultural areas that are currently largely overgrown by the forest Forest functions Function of protecting forest lands and stands The grey alder forests established closest to the river prevent the erosion of riverine soil during flooding with their wide-spreading root system. A thick layer of shrub and tree overgrowth holds back woody detritus and consequently enables its accumulation, which forms the basis of pedogenetic processes in the riverine soil. In stands located a bit further away from the Soča River, this function diminishes as they lose contact with the riparian dynamics. Hydrological function During times of high water, when the Soča River floods the grey alder stands, they play an important role in regulating the river-water regimen by slowing the flow of floodwaters. This way, water filtration occurs: increased mixing of water contributes to oxygen intake, which enhances its self-cleansing capability. Grey alder stands absorb some of the organic material and dissolved minerals flowing from neighbouring agricultural lands. Therefore, they influence the cycle of mineral matter and organic substances and decrease their flow into the Soča River. 49

64 Biodiversity preservation function Grey alder stands can be classified as European priority habitat type 91E0 Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae) (Dakskobler et al., 2013). For many birds, grey alder stands offer nesting habitat and a place of refuge, while larger animals find habitat and safe access to the riverbank there. On sites where grey alder stands represent the only patch of forest between agricultural land and the river, they play a role as a bio-corridor. Climate function Grey alder stands function as a protective belt against wind and therefore decrease wind erosion and increase soil moisture. They increase turbulence and mix the air, which has a favourable impact on forests and neighbouring lands. Recreational function Throughout the year, recreational activities (walking, jogging) are carried out via numerous trails and roads made for gravel exploitation leading through the stands below the villages of Dolje and Gabrje. The recreational function is more significant within stands located below the village of Ladra, where there is a small sports field in the forest. Tourist function On the bank of the Soča River around the Volče bridge, bathing tourism is prominent during the summer. 50

65 Research function As a rare habitat type, grey alder stands are an indispensable element in studies of riparian vegetation and succession on the floodplains of Alpine rivers. Natural-value-protection function The emphasised function of protecting natural values in grey alder stands is embedded in the following classification of natural values: Table 7.3: Natural value in the forest habitat of the grey alder stand type (Source: Forest management plan of the forest management units of Tolmin (Rules on the Forest Management Plan for the Tolmin Forest Management Unit, 2012) and Kobarid (Rules on the Forest Management Plan for the Kobarid Forest Management Unit, 2009)) NAME SHORT LABEL CATEGO RY S TATUS Soča River with its tributaries up to the confluence with the Idrijca River Soča River with its tributaries up to the confluence with the Idrijca River Hydrological, geomorphic, zoological natural value Natural value of national significance Volarja Stream with tributaries The basin of the Volarja Stream, the left tributary of the Soča River, limestones, Hydrological, geomorphic, geological, zoological Natural value of national flysch and folds of the Volarja natural value significance Aesthetic function Grey alder stands enjoy a high aesthetic value in the area. They act against the monotony of larger areas of agricultural use, have a positive influence on the scenic quality of the landscape, and help in orientation in the landscape. Timber industry function The timber industry potential of grey alder stands is limited due to the large share of deciduous softwood and because younger successional stages are prevalent. A more significant timber industry function is noted in stands thriving on drier vegetation sites, where assortments of precious deciduous wood and spruce of the greatest quality can be grown. 51

66 Figure 7.12: Values of forest functions in terms of grey alder stand type (value 1: function determines forest management practices; value 2: function exerts an important influence on forest management practices; value 3: function only partly influences forest management practices) Ecosystem services: Grey alder stands have great potential to offer ecosystem services in the domain of regulation, namely by regulating and slowing the influence of extreme events (i.e. regulation of water flow). By slowing the flow of floodwaters, grey alder stands prevent possible damage to lower-lying areas. Subsidising this ecosystem service would encourage forest owners to maintain and preserve riparian forests for this aim Grey alder stand management guidelines Successful management of grey alder stands should follow the following guidelines: - In terms of forest economics, those stands of grey alder where the tree layer consists of a greater share of European ash, small-leaved lime, European hornbeam and spruce have a more significant value. These are stands which thrive on drier soil, usually located in a passage transitional area leading to phytosociological communities which thrive on automorphic soil. When properly cared for, these stands are suitable for growing more valuable assortments; 52

67 however, the measures undertaken should be modestly and cautiously employed, in order not to impede their protective role. Mixed-species stands should be preserved, and emphasizing forest management which results in the enhancement of superior-quality deciduous hardwood is not recommended, despite its economic benefits. - In stands with predominant grey alder and olive willow, frequent forest management procedures include coppice-felling and grazing in thinned forests. There are also several pioneer forest communities. These stands are characterised by no or minimum forest management, which is done only by means of deforestation aimed at gaining agricultural land. Forest management in these stands should be carried out with regard to preservation: on small areas and at low intensity. Any measures taken should aim at encouraging the boosting of growing stock, improving stand quality prospects, and controlling the mixture of tree species in a particular stand. Grazing in these forests is possible only by achieving permission from the Slovenian forest Service. - In terms of preserving the grey alder community, caution needs to be employed also when constructing forest roads and temporal gravel depots, and when making other interventions in the forest area. When balancing the suitability of certain of the above-mentioned interventions, the protective role of the forest should always be given priority. - Any measures taken should be directed toward the preservation of their habitat and the conditions in which they thrive. In order to ensure the relevant level of soil water, the human impact on river dynamics should be minimised. Gravel excavation should be limited, as should construction of barriers, watercourse regulation and the construction of artificial riverbanks Pioneer forests on higher-lying river terraces General The pioneer forests on higher-lying river terraces stand type is characterised as riparian forests which have developed on Holocene river terraces in recent succession. Away from the riverbank to elevated areas located above the high-water line, there are habitats that are not reached by river waters, not even during the highest waters, and the tree roots in such habitats are not in touch with high soil water. In terms of morphology, the above-mentioned terraces are high-lying, and their edges are partly wavy and criss-crossed with dikes. These are the 53

68 conditions in which grey alder stands are gradually replaced by stands classified in thecommon hornbeam-white sedge (Carici albae-carpinetum betuli) phytosociological community. In the tree layer of this community, European hornbeam (Carpinus betulus) and European ash (Fraxinus excelsior)are predominant, mixed with small-leaved lime (Tilia cordata),norway spruce(picea abies), wych elm (Ulmus glabra), common maple (Acer campestre), singleseeded hawthorn (Crataegus monogyna)and olive willow (Salix incana). The shrub layer is well developed. The following species are found here: common hazel (Corylus avellana), Cornelian cherry (Cornus mas),dogwood (Cornus sanguinea), common hawthorn (Crataegus monogyna), fly honeysuckle (Lonicera xylosteum), February daphne (Daphne mezereum), guelder rose (Viburnum opulus)and wayfaring tree (Viburnum lantana). The herbaceous layer consists of characteristic white sedge (Carex alba), the distinctive black false hellebore (Veratrum nigrum), and spring snowflake (Leucojum vernum). In addition, the following species can be found here: European wild ginger (Asarum europaeum), wild garlic (Allium ursinum), pink woodruff (Asperula taurina), three-leaved anemone (Anemone trifolia), primrose (Primula vulgaris) and purple cyclamen (Cyclamen purpurescens) Stand description The forests of this stand type are rather homogeneous and are established mainly within the area between Ograjenca and the mouth of the Volarja. Stands in the Ograjenca area are of even size, ranging from pole- to skinnier-sawlog-size of normal to low density. The forest is managed and there is permanent human presence in this economically utilised forest. On sites where the forest stands meet agricultural land, some agricultural auxiliary facilities are under construction and grazing in the forest is common practice. The tree layer growing stock consists of small-leaved lime (40%), European ash (33%), European hornbeam (10%), sessile oak (5%) and spruce (7%). In the shrub layer, common hazel, dogwood, common spindle and common barberry are found. The share of a particular species depends mostly on forest management. In the region downstream from Ograjenca, characterised by saturated soil due to the streams of the neighbouring hillsides, there is an increased share of grey alder and spruce stands (L3 54

69 figures 7.14 and 7.13). In structural terms, these forest stands can be classified in the pole size category. In managed forests, European ash is encouraged. These stands have good quality prospects; when properly managed, valuable assortments of European ash can be grown. Figure 7.13: Pioneer forest stands on higher-lying river terraces downstream from Ograjenca. A large share of European hornbeam (32%) is found in the tree layer of stands located very close to the river course, on sites located below the Church of St. Lawrence and the village of Ladra, on the left bank of the Soča River (L2 Figure 7.14). Here, the forest is characterised by uneven-aged stands, and woodcutting is frequently combined with forest grazing and the construction of agricultural auxiliary facilities. Therefore, forest management in these stands is inconsistent and less intensive. 55

70 Figure 7.14: Extent (highlighted in brown) and areas (L1, L2, L3) of the pioneer forests on higher-lying river terraces stand type Forest functions The scope of the functions of pioneer forest stands on higher-lying river terraces is rather limited. They primarily have a timber industry function. Other functions of these forests are less characteristic of riverine forest communities and more characteristic of other communities thriving on beech vegetation sites. Function of protecting forest lands and stands Pioneer forest stands on higher-lying river terraces are not in connection with river dynamics; therefore, their pedogenesis does not depend on alluvium and detritus brought by floodwaters. Decomposition of plant detritus occurs on vegetated sites and thus contributes to the production of the humus layer. 56

71 Climate function Pioneer forest stands on higher-lying river terraces buffer the influence of the wind in open agricultural areas and increase soil moisture, which is of great importance to the original, gravelly soil of these vegetation sites. Timber industry function Woodcutting in these forest stands is carried out primarily in terms of obtaining wood in stacked cubic metres for fuel. However, due to the pioneer character of these stands, the amount of wood cut is rather modest. The condition and quality of the assortment is a bit better in stands located downstream from Ograjenca (L3 Figure 70). Figure 7.15: Values of forest functions in terms of pioneer forest stands on higher-lying river terraces (value 1: function determines forest management practices; value 2: function exerts an important influence on forest management practices; value 3: function only partly influences forest management practices). 57

72 Management guidelines for pioneer forests on higher-lying river terraces Because of the pronounced timber industry role of pioneer forest stands on higher-lying river terraces, the main measures are those directed towards improving the usage of the particular habitat-production ability. - Management measures are directed towards improved stand structure (of both even and uneven-aged forest), quality prospects and volume of growing stock. - When felling takes place, it is mandatory that special attention is given to preserving the proportion of the given tree species. The encouragement of some species (spruce and deciduous hardwood) and clearance of more valuable tree species for the benefit of economically less interesting species is not acceptable. - A suitable management model is uneven-aged forest management. The diverse ownership structure in the Ograjenca area led to so-called agricultural sorting practices. This silvicultural strategy is based on the felling of assortments of the same size category. Moderate intensity of such practice enhances the accumulation of the growing stock and ensures proper care of the forest. - Special attention should be given to any interventions in the forest area. They need to be regulated and directed properly. Grazing in these forests is possible only by receiving permission from the Slovenian Forest Service Riparian corridor stands General Man pushed the forest far into remote areas of the agricultural landscape so that he could have the land for his own use. The woody riparian vegetation stand type consists of corridors of trees and shrubs which thrive next to water and perform various roles in the agricultural landscape, where they are usually the only belt of woody vegetation. Due to the fact that stands of riparian vegetation are spread-out along watercourses which usually cross agricultural land, they have a significant role as a corridor and consequently their agricultural importance is prominent when compared to communities thriving in forest patches and/or to free-growing woody vegetation in the agricultural landscape. Corridor usage is distinct from the neighbouring dominant land use, but they have a significant connecting role in the area. In terms of their origin, these corridors are remnants of former land use. Stands of woody 58

73 riparian vegetation may cover only the edge of a watercourse or a wider area on the banks of a watercourse, or be present in an area encircled by agricultural land. Corridor width has the greatest influence on the nature of various roles this belt of riparian vegetation has in the landscape and the ways of carrying them out successfully. According to Prosen (1993), the belt of woody riparian vegetation should be at least 5 m wide on watercourses characterised by riverbeds wider than 2.5 m (the Soča River falls into this category), however the ideal width on this belt is around 10 m. A 10 m-wide alder belt can retain the majority of phosphorus and around 50% of the nitrogen from the neighbouring land this is the quantity that would otherwise end up in the water (Papež et al., 1997) Stand description Stands of woody riparian vegetation are spread out along the Soča River throughout the region between Kobarid and Tolmin (O1 Figure 7.17). They are characteristic in regions where cultivated river terraces reach the Soča River via steep slopes and where agricultural land usage is not possible all the way down to the river. Less steep slopes where the terrain gradually approaches the watercourse are more prone to forest interventions, such as holiday structures, grazing and tourism. On some limited parts of the riverbanks, as for example in the vicinity of the Volarje bridge, there is a preserved belt of tree species that is narrower in width than the stand s tree height. Gap width is more than 15 m throughout the stands. The gaps are in various structural stages: from shrubs to coppice, uneven-aged forest and sawlog. In some areas, the maximum tree diameter at breast high exceeds 30 cm. The stand has low quality prospects and is characterised by light to gappy density. Forest management has a significant influence not only on the structure of stands, but also on the proportion of tree species in stands. In the area around Selišče, the share of Norway spruce in stands is larger due to the planting practices employed. Where the belts of woody riparian vegetation are too narrow, lateral illumination gives rise to a dense layer of shrub and tall herb species, which consequently makes natural regeneration of these stands impossible. The characteristic species of this stand s growing stock are as follows: European ash (35%), small-leaved lime (30%), European hornbeam (15%), spruce (5%), sessile oak (4%), beech (3%) and grey alder (3%). There are also other less representative species, such as olive willow, black locust, common walnut and common maple, whose share in the stand is much lower. The dense shrub layer consists of common hazel, common spindle and European hornbeam. 59

74 Figure 7.16: Riparian corridor close to the village of Volarje. Due to deforestation, erosion of agricultural areas occurs. Hedgerows with a similar tree composition and structure to woody riparian vegetation stand types are established in the vicinity of Tolmin, between agricultural areas or next to streams/water channels (O2 Figure 7.17). Management of these areas is somehow more intensive some areas are periodically cleared for firewood. Regeneration takes place via coppice methods. 60

75 Figure 7.17: Extent (highlighted in green) and areas (O1, O2) of the riparian corridor stand type. Figures 7.18, 7.19: Where corridor vegetation is cleared for agricultural land use, erosion of steep riverbanks occurs. LIDAR scan (left) and orthophoto (right) of an eroded riverbank below the village of Idrsko. 61

76 Forest functions The purpose of woody riparian vegetation stands in the landscape is manifold: Function of protecting forest lands and stands Stands of woody riparian vegetation with their root systems prevent direct erosion of the riverbank and decrease the amount of alluvial sediment flowing into the watercourse. The significance of this function is obvious on parts of the riverbank where this type of stand is not established. In the majority of such cases, eroded riverbanks occur as a consequence (figures 7.18, 7.19). Hydrological function When the belt of riverine vegetation is wide enough, it plays a crucial role as a chemical and mechanical filter and thus prevents uneven and saturated deposits from reaching the watercourse. Any clearing of woody riparian vegetation stands increases the amount of surface matter being washed downstream to accumulate elsewhere. This is especially important in areas of intense agricultural activity, because the riparian vegetation absorbs part of the organic material and dissolved minerals flowing from the neighbouring agricultural lands and thus influences the cycle of mineral and organic substances. Clearance of woody riparian vegetation stands contributes to a greater inflow of surface water. This happens due to decreased transpiration and increased surface runoff taking place as a result of the fact that bare-soil areas are less prone to water infiltration. Biodiversity preservation function Woody riparian vegetation stands have a significant influence on the biocenosis of the river for the following reasons (Papež et al., 1997): Their influence on the zoocenosis of watercourses. Most of the organisms living in watercourses protect themselves from light; therefore, woody riparian vegetation stands exercise a favourable influence on organisms living in rivers. 62

77 Their influence on oxygenation. Due to the lower amount of sunlight penetrating the water, the water temperature is lower during the warmer months of the year and thus more oxygen is dissolved in the water; at the same time, the lower rate of photosynthesis produces less oxygen in the terrestrial parts of the area. Organic-substance intake. An important source of organic matter in the watercourse is plant litter from woody riparian vegetation stands. Plant litter is at the bottom of most food chains. Limited growth of aquatic macrophytes. Clearance of woody riparian vegetation stands contributes to a greater inflow of light rays and thus greater primary production in the watercourse. Ability to bind to inorganic salts. The woody riparian vegetation stand type has the ability to bind dissolved organic matter (e.g. fertilisers), which is of great importance in areas of intensive agricultural practices (Tolmin Basin, Volarje Plain). Stands of woody riparian vegetation form bio-corridors whose role is twofold: they enable the migration of many animal species and represent a connection between various biotopes. Birds migrate along river valleys via these corridors as well they find rest and nutrition in biocorridors. In these terms, fruit-tree species within the woody riparian vegetation play a significant role in birds survival during migration. Climate function Stands of woody riparian vegetation regulate temperature extremes in and next to the watercourse. Felling of woody riparian vegetation stands can change the water temperature, which in turn can change the living conditions of organisms in the watercourse. Stands of woody riparian vegetation are often the only belt of trees in the agricultural landscape and therefore play an important role in buffering the wind. As they act as a protective belt against the wind in open areas of agricultural land, they decrease wind erosion and increase soil moisture. Furthermore, they increase turbulence and mix the air, which has a favourable impact on the output of the neighbouring lands. Stands of woody riparian vegetation have an important influence on the physiological exchange of substances for animals and ensure an even supply of oxygen in the water. The specific conditions of the riparian belt, namely its shade, ensure limited growth of macrophytes. 63

78 Aesthetic function Stands of riparian vegetation enjoy a high aesthetic value in the area. They act against the monotony of larger areas of agricultural use, have a positive influence on the scenic quality of the landscape, and help in orientation in the landscape. Timber industry function Based on the extent and purpose of woody riparian vegetation stands, their timber industry function is quite limited. It is a bit more pronounced in hedgerows located in the vicinity of Tolmin, where its firewood function comes to the fore. Figure 7.20: Values of forest functions in terms of the riparian corridor stand type (value 1: function determines forest management practices; value 2: function exerts an important influence on forest management practices; value 3: function only partly influences forest management practices) Ecosystem services Riparian corridors have great potential to offer ecosystem services in the domain of regulation, namely protection against soil erosion. Frequently, erosion leading to eroded riverbanks 64

79 occurs in the parts of riverbanks where there is no forest community, which further leads to damage to agricultural land and certain losses to farmers Riparian corridor management guidelines Stands of riparian vegetation have numerous functions in the landscape that we are not conscious of; therefore, they usually don t play an important role when decisions are made on activities which affect the physical environment. One of the reasons why woody riparian vegetation is not valued more highly lies in the fact that in general, people fail to connect woody riparian vegetation and the woody vegetation of the agricultural landscape with forestry and the forester s work. Based on current findings in the area of woody riparian vegetation management, the following guidelines should be followed: - An uneven-aged forest structure seems to be the most suitable structure which provides the various functions offered by woody riparian vegetation stands; therefore, any measures taken should strive toward establishing and maintaining such a structure. - In order to ensure the sustainable performance of all the roles carried out by woody riparian vegetation, the only acceptable measure in terms of forest management is the individual tree selection regeneration method, which emphasises the preservation of the uneven-aged forest structure and the accumulation of growing stock. - On riverbanks prone to erosion, stands of a lower age/size category should be promoted. - Where the belts of woody riparian vegetation stands are too narrow, dense undergrowth makes natural regeneration of these stands impossible; therefore, artificial regeneration is recommended in terms of planting bigger seedlings (this way, nurturing costs are minimised). In the case of natural regeneration, relevant nurturing procedures and measures must be employed until the seedlings outgrow the most aggressive species of undergrowth. Autochthonous tree species are to be planted. For birds, planting fruit-tree species is recommended. - Leaving some sawlog-size trees to naturally decay ensures habitat for cavity-nesting birds. Management guidelines for woody riparian vegetation can be successfully followed only when the status of these stands is legally defined in terms of stimulating the owners to establish, regenerate and maintain corridors of woody riparian vegetation. 65

80 7.1.6 Anthropogenic riverine forest stands General The anthropogenic riverine forest stand type consists of riparian stands where human influence conditions their growth, development and existence. Preserving and managing this woody stand type within the forest and its area is of limited purpose, because they can be classified into a special category within the woody stands in the landscape. With regard to this study, these are stands in the vicinity of the Tolmin aggregate extraction site, characterised as anti-noise, anti-dust belts of trees and some areas in parks aimed at recreational and tourist activities. These stands play an important role in the landscape and therefore deserve special treatment and prudent management to perform their social functions successfully and permanently Stand description Some areas of riparian forest stands, namely those with easy access and closer to settlements, already primarily perform social functions spontaneously, without human physical planning. Frequently, these are areas connected with gravel exploitation (Soča River detritus), which demands temporary roads aimed at gravel pickup that are also used by other people as access to the gravel sites. In the vicinity of larger settlements, these areas are becoming walking zones, and during the summer some larger festivals (like the Metalcamp festival) are taking place there. When the water reaches the temperature suitable for bathing in the summer, the riparian forest stands at the Soča River are crowded by bathers. Sometimes, the increased visitation to these sites initiates the development of on-site seasonal catering and other tourist activities. Some forest stands located between the confluence of the Tolminka and Soča rivers and a World War I German military cemetery (hereinafter: German cemetery) are characterised by intense human influence and are aimed at learning, tourist and recreational activities (A1 Figure 7.22). The 4b GGE Tolmin area has been pronounced a forest stand with a special purpose by the Municipality of Tolmin. This area is partly populated by white willow stands (at the confluence), which are fairly degraded in terms of ecology due to the high human impact (bathing area, event venue, food-and-beverage-serving facilities, walking areas). The 66

81 terrace above the confluence is characterised by overgrown areas of pioneer stands and a forest stand behind the hotel. The latter also consists of non-native species (London plane and horse chestnut) and used to serve as park nowadays it serves as an event venue and a campsite. In the greater area the understory was removed and the forest serves as a park. The area that includes forest stands located along the Soča River to the German cemetery consists of initial willow stands on the river, while on the slopes and/or higher-lying river terraces initial forest stands with predominant European hornbeam are established. The forest stands are mostly occupied during the bathing season and festival-tourism season, but their numerous trails act as a walking area for the nearby town of Tolmin throughout the year. Towards the German cemetery, there is a former military-drill area where Scots pine was planted in the past. Part of this area is designated as the Sovodenj forest learning trail by the Slovenian forest Service. This area is particularly rich in non-native tree species. In the vicinity of the German cemetery, there is common yew (Taxus baccata), a species that was planted in the past and is regenerated via natural seeding with the assistance of birds. This way it is not growing only in this area, but is spreading to the neighbouring forest stands. This is yet another area that is especially crowded during the summer when festival activities are taking place. In vicinity of the Pod Gabrjami campsite and the Labarca bathing area, the corridor forest stands of the riverine belt are also characterised by human impact for tourist purposes (A2 Figure 7.22). In the past, the area under the village of Gaberje was used for military purposes. There was a military-drill area. Some individual trees of Scots pine, planted in the neighbourhood of the current campsite, are from that time. A portion of the stands growing at the campsite is thinned and the shrub and herbaceous layers are removed. The site is used as a bathing area and is a natural meadow flanked by a white willow stand (Figure 7.21). The only function of the willow canopy layer is to provide shade during the summer months. Due to all of the human activities taking place here, natural regeneration is practically impossible; therefore, the next generation will probably have to think about planting trees for shade. 67

82 Figure 7.21: Bathing area on the Soča River below the village of Volče. Figure 7.22: Extent and areas of the anthropogenic riverine forest stand type (A1, 2, 3, 4, 5). 68

83 In the area located below the village of Gaberje, upstream from the Gaberje campsite, there is a corridor forest established on overgrown agricultural land where some former agricultural facilities (sheds and hay barns) have been converted into holiday homes (A3 Figure 7.22). Forest intervention in this area is dictated by the needs of holiday-home owners; therefore, due to human impact, these stands are losing their ecological function. The fragmented remnants of these forest stands are still acting as a park area and are retaining their protective function. In the area of former Italian military garages, downstream from the Volče bridge, there are alder stands with low quality prospects along with initial willow stands. (A4 Figure 7.22). These stands are under intense human influence because of the gravel site at the Volče bridge, which is used as bathing area. Away from the watercourse, in the area of former temporary gravel depots, a temporary food-and-beverage facility was established, which together with the bathers creates great pressure on the environment during the summer months. The Municipality of Tolmin has developed a plan for this area to become a tourist-recreational area. Directing summer bathers to individual, limited areas would somehow unburden more preserved, dense stands of riparian forest in the vicinity of the town of Tolmin. There are some small patches of pole/sawlog-size white willow stands established on a river terrace in the area of the Tolmin aggregate extraction site (A5 Figure 7.22). It is greatly affected by human impact (gravel excavation aggregate extraction site). Gravel exploitation causes the riparian forest stands to be in the successional shrub stage, while in former gravel depots fragmented forest stands prevail, with European ash and small-leaved lime tending to be predominant. Here and there, some remnants of white willow and grey alder stands with low quality prospects and gappy crown closure can be found. Forest interventions are unplanned and depend completely on the construction industry. Unfortunately, this practice is not in use only for clearing and thinning, but also for construction of temporary roads, which results in the existing stands being filled with excavated gravel as well as in the abandonment of temporary gravel depots, which are then overgrown (figures 7.23, 7.24). 69

84 Figures 7.23, 7.24: Relief characteristics of natural white willow stands (upper part), agricultural land meadows (right), and the anthropogenic Tolmin aggregate extraction site, which influences the succession and growth of riparian forest stands Forest functions Function of protecting forest lands and stands Riverine forest stands with their root system prevent erosion of the riverbank and decrease the amount of alluvial sediment flowing into the watercourse. However, the anthropogenic forest stands in the vicinity of the bathing area are less overgrown and their understory has been removed; therefore, they cannot perform this role in an optimal fashion. Hygiene/health-protection function The forest stands in the vicinity of the Tolmin aggregate extraction site perform an emphasised hygiene/health-protection function. These stands represent the only tree belt close to a pollution source and thus decrease the negative effects of pollution emissions and protect the nearby settlements against the noise caused by the aggregate extraction site. Therefore, they exert a favourable influence on the nearby settlements and agricultural areas. 70

85 Recreational function Throughout the year, recreational activities (walking, jogging) are carried out via numerous trails leading through these stands. Tourist function These riverine stands are heavily visited during the summer months. Remnants of autochthonous forest stands serve as park trees in the vicinity of the catering and tourist establishments. Furthermore, in areas where bathing tourism takes place (around the larger gravel sites), they offer shade against the summer heat. Informative function An informative/learning function is performed by forest stands at the confluence of the Tolminka and Soča rivers, where there is a centre for outdoor school activities, and by the Sovodenj forest stands, where a forest learning trail is established Cultural-heritage-protection function An emphasised cultural-heritage-protection function is noted in the forest stands and alley of yew trees in the vicinity of the German cemetery. 71

86 Figure 7.25: Values of forest functions in terms of the anthropogenic riverine forest stand type (value 1: function determines forest management practices; value 2: function exerts an important influence on forest management practices; value 3: function only partly influences forest management practices) Ecosystem services The anthropogenic forest stand type has the potential to carry out the cultural ecosystem services of recreation and tourism. Currently, the price for such ecosystem services is included in the price of the services offered by catering providers and providers of other tourist services on anthropogenic-forest-stand sites (e.g. the value of shade in the bathing area is paid for via the price for the campsite). However, these financial means are not invested into the preservation and nurturing of these stands. The stands in the vicinity of the Tolmin aggregate extraction site have a regulation potential, namely to perform the function of noise regulation. 72

87 Anthropogenic riverine forest stand management guidelines The anthropogenic forest stands on this part of the river play a special role in the riparian forest stand type, because their existence is important for other forest stands on the Soča. Directing visitors to individual, limited forest areas can to a large extent unburden the somehow more preserved, dense stands of riparian forest elsewhere. - The management guidelines for anthropogenic forest stands are similar to the guidelines for urban and suburban park management. - Forest management is directed towards the final goal of ensuring healthy, stable and unevenaged forest growth on small areas with a predominant share of older structural stages. This is a kind of forest that can successfully perform its social functions. - Forest interventions should be performed not only in terms of stand preservation, but also in terms of increasing stand stability (sanitation felling, tree surgery, maintaining a high stand density, planting autochthonous trees of a larger size). - Forest management of anthropogenic forest stands needs to take into account their extent in the area. These forest stands should not be established and then left to disappear or their sites changed in an uncontrolled fashion, otherwise they will lose their initial purpose and become nothing more than degraded areas of riparian forest. It is of great importance that these stands are limited in the area and are managed accordingly. - It is wise to take these areas out of common management and manage them via municipal ordinance (e.g. as a suburban forest or a forest of particular importance). 73

88 Stand type comparison Classification of stand types into groups via hierarchical clustering (Figure 7.26) shows the greatest similarity between the initial willow stand and white willow stand types, followed by the grey alder stand type. All three stand types have an emphasised ecological function of protecting forest lands and stands, hydrological function and biodiversity-preservation function, as well as a social function of natural-value protection. The similarity of the three types is also obvious in the tree structure deciduous softwood (olive and white willow and grey alder) prevails in the tree layer of the growing stock. The three syn-dynamically related stand types cover ha of the forest area. Initial willow stands White willow stands Grey alder stands Pioneer forest stands Riparian corridor stands Anthropogenic forest stands Figure 7.26: Dendrogram of riverine forest stand type functions in the analysed area. The second cluster consists of pioneer forest on higher-lying river terraces and the riparian corridor stand type (Figure 7.26). These two stands are not characterised by a variety of emphasised functions. The only emphasised functions are climate function and the function of protecting forest lands and stands. However, the similarity of the two types is more obvious in the tree structure, where a great share of small-leaved lime, European ash, European hornbeam, and spruce is in the tree layer of the growing stock. The majority of these stands thrive on rather dry vegetation sites, and their roots have no contact with soil water. This cluster covers ha of the forests included in this study The third cluster consists of the anthropogenic forest stand type The main characteristic of this stand type that makes it distinct from the other two clusters is its extremely emphasised social function, namely its recreational, tourist, informative, hygiene/health-protection and cultural-heritage-protection functions. This cluster covers ha of the forests included in this study 74

89 7.2 Dynamics of the growth of riverine forest tree species dendrological analysis Growth in height, diameter and volume of six tree species growing in riverine forest stands in the study area is described in this chapter by means of growth and increment curves Growth in height Growth in height depends on the tree species, its habitat and the silvicultural treatment received throughout the tree s lifespan. Figure7.27: Growth in height of tree species in riverine forest stands. Within riverine stands, the most rapid growth in height can be observed in the following species: white willow (Salix alba), European ash (Fraxinus excelsior) and grey alder (Alnus Incana) (Figure 7.27). White willow and grey alder species show continuous growth in height without culmination; they are still growing at the age of 45. Due to the fact that some individual white willow and grey alder trees of this age are in the process of decay and are drying up, it can be speculated that culmination will not be achieved at all. The small- 75

90 leavedlime (Tilia cordata) and European ash growth curves are typical sigmoid curves. Between the age of 35 and 50, the growth rate declines in these two species. The growth rate of sessile oak (Quercus petraea) and European hornbeam (Carpinus betulus) is constant throughout their lifespan (55 and 85 years, respectively) (Figure 7.28). Table 7.4: Parameter values of regression curves for growth in height of tree species. CHAPMAN-RICHARDS CURVE PARAMETERS TR EE S PEC IES a b c CORRELATIO N CO EFFICIENT White willow European hornbeam Sessile oak Small-leaved lime Grey alder European ash r 2 Figure 7.28: Current height increment of tree species in riverine forest stands. In general, the more productive the habitat, the larger the height increment and the earlier the culmination, followed by more rapid decline (Kotar, 2005). The current annual height increment shows early culmination for the majority of tree species, which is no surprise as 76

91 this is general practice in even-age stands in which regeneration hasn t occured with shelterwood cutting or selective logging (Figure 7.28). Kotar (2005) states that the approximate time of culmination for black alder and European ash is between one and 15 years of age. Grey alder and white willow species show large height increments in their very early years (i.e. willow up to 1.14m/year), then, the increments rapidly decline to the breaking point at the age of five, when a slowdown occurs. When compared to other species at the age of 45, these two species still show the greatest annual height increment (white willow: 0.27 m/year; grey alder: 0.19 m/year). Kotar (2005) states that alder, willow and European ash species may show a height increment of one meter or more at the time of culmination. European hornbeam has a similar height increment curve to grey alder; however, its rapid increment in the early years, breaking point of decline at five years, and later slowdown in decline is somehow shallower (initial height increment of 37 cm/year decreases to 19 cm/year at the age of 50). In small-leaved lime and European ash, the current height increment increases during their youth and reaches culmination at the age of five, with an increment of 75 cm/year (European ash) and 54 cm/year (small-leaved lime). In both species, the annual current growth increment is very low at the age of and reaches the zero point asymptotically. Sessile oak shows the following dynamic: a somewhat low current height increment in its early years (12 cm/year), which increases at the age of 40 and asymptotically reaches the value of 20 cm/year. This dynamic is characteristic for trees growing under the cover of other trees or those unable to adapt to the conditions of their habitat Diametric growth Diametric or secondary growth occurs in the vascular cambium and is seen as the annual loading of wood layers around the trunk. The diametric increment depends on the tree species and even more on the habitat and its conditions as well as on the environmental conditions. 77

92 Figure 7.29: Diametric growth of tree species in riverine forest stands. A tree grows in diameter until its physical death, although the tree rings at older ages are very narrow (Kotar, 2005). Among the studied tree species, the largest diametric growth is shown by white willow and grey alder; however, European hornbeam shows the most even diametric growth throughout the lifespan (Figure 7.29). The diametric growth of sessile oak is somehow slow during its early years. To achieve the same diameter (i.e. 15 cm), it needs 20 more years than European ash and 60 more than white willow. Table 7.5: Parameter values of regression curves for diametric growth of tree species. CHAPMAN-RICHARDS CURVE PARAMETERS TR EE S PEC IES a b c CORRELATIO N CO EFFICIENT White willow European hornbeam Sessile oak Small-leaved lime Grey alder European ash r 2 78

93 Figure 7.30: Current diametric increment of tree species in riverine forest stands. In even-aged stands, the diametric increment culminates in the early years and then slowly declines. This is especially characteristic for light-demanding species (Kotar, 2005). The current diametric increment of all tree species, with exemption of sessile oak, culminates in their youth, then decreases rapidly and reaches the breaking point at the age of five (Figure 7.30). After that, the current diametric increment decline is slower. When comparing the studied species, it is obvious that white willow has the largest diametric increment throughout its lifespan at the time of culmination it reaches 1.4 cm/year. At the age of 45, when individual white willow trees are drying up and thus dying, the diametric increment of the stand is still 0.6 cm/year. The current diametric increment of European hornbeam shows the lowest values in the group; however, its diametric increment is more constant during its lifespan: from 0.4 cm/year in its early years to 0.25 cm/year at the age of 50. The diametric increment of sessile oak is low during its youth and then slowly increases during its lifespan. When compared to small-leaved lime, European ash and European hornbeam, its diametric increment is larger at the age of

94 7.2.3 Volumetric growth The volume of a tree and its increment depend on the diameter at breast height and total tree height. Therefore, the volumetric increment includes characteristics of both height and diametric increments. Figure 7.31: Volumetric growth of tree species in riverine forest stands. The volumetric growth of the studied tree species is rather slow during their youth and increases over time (Figure 7.31). In the group of studied tree species, the largest volumetric growth is achieved by white willow. Volumetric growth of European hornbeam is low, but even. At the age of 45, white willow achieves 2.5 times the volume of grey alder and 18.5 times the volume of European hornbeam. 80

95 Table 7.6: Parameter values of regression curves for volumetric growth of tree species. EXPONENTIAL CURVE PARAMETERS TR EE S PEC IES a b CORRELATIO N CO EFFICIENT White willow European hornbeam Sessile oak E Small-leaved lime Grey alder European ash r 2 Figure 7.32: Current volumetric growth increment of tree species in riverine forest stands. In the group of studied species, culmination of the current volumetric increment takes place later than culmination of the current height and diametric increments. In general, lightdemanding tree species reach culmination of the current volumetric increment at a younger age than shade-tolerant tree species; however, Kotar (2005) is of the opinion that fast-growing trees never reach culmination of the mean volumetric increment, because stand regeneration overtakes it or the trees die of old age. The tree species on the Soča River do not reach culmination of the current volumetric increment (Figure 7.32). The largest current volumetric 81

96 increment, which increases rapidly with age, is achieved by white willow (at the age of 45 its volumetric increment is m 3 /year). The current annual volumetric increments of other species increase more slowly with age and are thus significantly lower at the same age of 45: grey alder achieves m 3 /year, European ash m 3 /year, and small-leaved lime m 3 /year. European hornbeam shows a constantly low volumetric increment throughout its lifespan. The increment of sessile oak is very low during the first 30 years ( m 3 /year), but then it increases. At the age of 45, the current annual volumetric increment of white willow is 45 times larger than the increment of sessile oak and 35 times larger than that of European hornbeam. 82

97 7.3 The development of riverine forest land in the study area In the past, man removed the forest or pushed it far into remote areas of the agricultural landscape so that he could have the land for his own use. Consequently, the area of riverine forest in the study area was much smaller in the past (Figure 7.33). Figure 7.33: Riverine forest area through time along the Soča River between the town of Kobarid and the confluence with the Tolminka River. As shown on the map from 1763 (Figure 7.35), the study area encompassed only ha of forest land. The Carta geografica del littorale map from 1830 (Figure7.36), a version from the Franciscan cadastre, somehow gives a different picture for that time, but before jumping to conclusions one has to know that the classification used on this map was different, namely the majority of infertile lands were classified in the forest category, which gives the wrong (oversized) impression of the forest land at that time. Ignoring the disputable data from this map, it is obvious that the size of the riverine forest area remained practically the same for almost 200 years, namely under 50 ha up to 1931 (Figure 7.37). However, after the end of World War II, the riverine forest area rapidly increases and exceeds 200 ha in 2011 (Figure 7.40). Data analysis shows that the locations of riverine forest stands varied greatly up to 1931; therefore, it is impossible to draw a picture of how the forest areas developed due to change of 83

98 land use via old maps. Partly, it is because of the old maps inaccurate mapping and undefined categories of forest and non-forest land. After 1931, the practice of reverse land use (changing forest lands back into agricultural) decreases and the trend of enlargement of forest areas due to overgrown agricultural land can be followed. However, field examinations show that defining stand age via mapping is impossible. The actual age and structural stage of stands in nature is usually much lower than expected from mapping, which clearly shows that the area of riverine forest is subject to great and intensive impacts. Figure 7.34: Development of relative forest share in the study area and in the larger area of the municipalities of Tolmin and Kobarid. When comparing the development of relative forest share in the study area on the one hand and in the larger area of the municipalities of Tolmin and Kobarid on the other (Figure 7.34), one can see that the enlargement of riverine forest land in the study area fits into the broader context of overgrown agricultural lands in Slovenia after World War II. Contrary to the European trend of floodplain forest extinction, the riparian forests along the Soča River show an increase in land following the process of overgrowing agricultural land, their area has become more than 25% larger over the last eight decades. The biggest current threat to riverine forests lies not in land reduction, but rather in the changed habitat conditions (lowering of soil water, gravel exploitation, etc.) forming the basis of their existence. 84

99 Figure 7.35: Riverine forest area as of 1763 (digitalised from the Slovenian military map of , 1995). Total area of riverine forest: ha. Figure 7.36: Riverine forest area as of 1830 (digitalised from the Carta geografica del littorale, 1830). Total area of riverine forest: ha. 85

100 Figure 7.37: Riverine forest area as of 1931 (digitalised from the Catastro forestale Provincia di Gorizia, fascicolo 31, 1931). Total area of riverine forest: ha. Figure 7.38: Riverine forest area as of 1958 (digitalised from aerial photo records from 1958). Total area of riverine forest: ha. 86

101 Figure 7.39: Riverine forest area as of 1995 (digitalised from orthophoto records from 1995). Total area of riverine forest: ha. Figure 7.40: Riverine forest area as of 2011 (digitalised from orthophoto records from 2011). Total area of riverine forest: ha. 87

102 7.4 Land use in the study area When analysing the study area (528.9 ha in total) in terms of land use (figures 7.41, Anex B), it is obvious that forests make up the largest share (43.3 %). There is a good chance that the forest area will remain the same in the future as it is currently; there are relatively small successional areas (1.7 % of the land) and the existing forests are already on the border of areas of intensive farming. Because the riverine terraces of the Soča River are characterised by flat land suitable for farming, the share of agricultural use here is large (25.8% of all land) these are mostly intensively managed, fertilised meadows. The main culprit of the large share of gravel sites and developed lands is gravel exploitation and its related activities (temporary roads, depots and aggregate extraction sites). Figure 7.41: Land use in the study area. The proportion of water areas and gravel sites changes seasonally, depending on the Soča River s water level. During times of heavy flooding, the river can change its course and as a result new gravel sites are established. Usually the wetlands on the Soča River are formed on 88

103 the edges of the lowest-lying river terraces, where the floodwaters accumulate, and sometimes their formation is connected to streams flowing from the slopes of the higher-lying terraces; however, the wetland adjacent to the village of Kamno is established on a cut-off meander. Although the area of ecosystems along the Soča River is rather small, they are very important in terms of preserving the habitats in the area. Upon analysing the land use, one can draw the conclusion that in order to ensure developmentally oriented, sustainable use of the land in the riverine belt, inter-sectoral physical planning (i.e. planning that comprises the various interests of public entities) is crucial. 8. DISCUSSION AND CONCLUSION The Soča River is one of the rare Slovenian rivers that flows into the Adriatic basin. The river basin stretches from the mountainous lands of the Julian Alps to the lowlands of Val Padana in its delta near the Italian town of Grado. On its way from its source to the seaside, the river runs through various, distinct landscapes, and the more distant from its source the more easygoing its flow, but with increased human impacts and pollution. Despite the fact that the Soča River is one of the best preserved Alpine rivers in its upper course, namely from its source to the confluence with the Tolminka River, there have been few studies of its riverine forests, which are an integral part of the river. There is a large amount of data related to the topic, but it was mostly obtained for individual cases and specific fields, and therefore it was never assembled to be analysed, compared or studied integrally. In addition, the interpretation of this data is difficult due to some mostly unplanned and frequently unexpected impacts and changes made in the area as a consequence of the many actors playing a particular role in it: groups (sectors) of various size and influence. The methods selected for analysing the data used in this study are proven appropriate, as they enable (e.g. similarities of stand types) one to grasp the meaning of the situation, which was later confirmed in nature (on-site description of stands). Yet another proof is the comparison with previous analyses of the study area (i.e. vegetation map Appendix D). It may be postulated that there are many appropriate methods in use when making forest management plans, and therefore their utilisation is deemed correct on the micro level of riverine forests of the study area. However, the content tackled by this 89

104 study is not encompassed in the plans of the local forest management units. The reason lies in the fact that forest management plans deal with forests in sections which are too broad to enable one to focus on such content, and riverine forests are not classified in a section per se. Based on the above, it is obvious that on the level of a particular section, the riverine forests within it represent a tiny proportion of the entire forest area; therefore, their characteristics are neither noticed nor emphasised. We can ask ourselves, also on the suitability of the approach for describing the riverine forest stands using the classical dendrometric stand parameters. Because it is a particular type of forest stands it is also important to use different approaches, which must be harmonized with the characteristics of the riverine forests and research objectives. For example, due to the high level of heterogeneity and low timber- industry potential of the riverine forests (see the timberindustry function of stand types), knowing the growing stock of shrubs of olive willow, corridor forests and anthropogenic forests is not essential for our research. Instead, we can concentrate on other aspects of riverine forest stands (i.e. forest functions, ecosystem services, stand management guidelines) that are more important. Describing these forests with widely used parameters as for example the parameters used to describe the beech forests of the high karst resulted in currently evident devaluation of these areas. Therefore, the goal of this study was to describe these forests in a somehow different way, where classic forest data serve only for easier understanding of development processes, succession, land changes, forest function and ecosystem services. The research in this thesis encompasses part of the river basin between the towns of Kobarid and Tolmin. It was found that the riverine forests in the study area are not a homogeneous habitat. Rather, they consist of numerous phytosociological communities with a syn-dynamic connection (Chapter 4.2.6). In addition, the study shows that both the distance from the river channel and the height above it play an important role with regard to vegetation as they increase, the chances for flooding and contact with soil water decrease, and both are vital for the existence of riverine forests. Consequently, the further away from the riverbank, the more different the tree structure of the forest: the share of olive and white willow as well as grey and black alder decreases on account of the greater share of other tree species: European ash, small-leaved limeand European hornbeam. In addition, human impact increases as one moves away from the riverbank. The majority of riverine forests in the study area is younger than 50 90

105 years old and were established either by the overgrowing of agricultural land or from degraded forests. Accordingly, the forest structure is characterised mainly by uneven-aged stands or younger structural stages (pole-size, shrub, sapling-size), which are frequently returned to their earlier stages due to human impacts (due to gravel exploitation, initial willow stands remain in the shrub stage). The majority of these forests have low quality prospects. Because of the human impact, some riverine forest stands are limited to corridor forms thriving next to the river and some are so substantially altered that they are more similar in form to park areas or windbreaks. Based on their vegetation and stand characteristics, the riverine forests under study were classified into six stand types. A comparison of the functions and characteristics of particular forest types revealed similarities between the stand types of initial willow, white willow and grey alder. Forests in this cluster are under the influence of soil water and high floodwaters. Their most pronounced function is ecological. In their growing stock, there is a large share of deciduous softwood. Forests in this cluster cover 62% of all riverine forests in the study area. Silvicultural measures are rare in these forests and are less important for the existence of the stands than environmental features like a high level of soil water and minimised human impact on their habitat and the river course. An important characteristic of the second cluster, which consists of pioneer forest on higher-lying river terraces and the riparian corridor stand type is a lack of contact with soil water. In the tree layer of the growing stock in this cluster, species appearing on drier riverine-forest sites prevail (European ash, small-leaved lime, European hornbeam, sessile oak). These forest stands have a greater timber industry potential; therefore, modest measures and limited human impact (especially that which leads to a change in land use like forest grazing and clearing) are recommended. The stands in this cluster cover 30% of the analysed riverine forest area. The third cluster of riverine forests consists of anthropogenic riverine forest stands where the human impact is so significant that they mainly perform a social function. The stand type of this cluster represents 8% of the riverine forests in the study area, and it is suggested that they should be managed as parkland and not as forest areas. Directing visitors to these areas would unburden other, more preserved stand types from the pressures created by human activities (tourism, recreation, construction work). 91

106 Growth and increment analyses of the most common tree species in the riverine forests in the study area show that mainly olive and white willow, followed by grey alder, have substantial current height (both willow types up to 1.14 m/year), diametric (willow up to 1.4 cm/year) and volumetric increments. However, their rapid growth makes these subjects prone to rapid ageing. This is probably the main reason why some white willow trees in the white willow stands located in the vicinity of the town of Tolmin are drying up already at the age of 45. The region of northern Primorska was inhabited early in history. Accordingly, there was great pressure imposed on riverine forests already in the past. The analysis of old maps revealed that up to the beginning of the 20 th century, the riverine forest area within the scope of analysis was under 50 ha. Within the process of the overgrowing of agricultural land in Slovenia, a process that is currently still underway, the aforementioned riverine forest area increased to ha. It is speculated that in the future, this area will probably not increase substantially, because the lowest-lying terraces of the Soča River that have become overgrown with riverine forests are also among the most fertile and therefore most intensively cultivated lands within the broader area of the Upper Soča Valley. It is supposed that the results of this study may be helpful to all land managers, namely to determine the activities affecting the environment and land-use planning (e.g. municipal planning documents). Additionally, professional forestry groups may more easily plan the management of these stands by utilising this study, and the public at large might find it a useful aid in dividing riverine forests into sections when planning their numerous uses. Particular activities (construction industry, tourism, urban interventions, farming) may be orchestrated only if they are performed in accordance with the guidelines and principles of nature protection and forestry professionals, and if they are strictly carried out in a limited fashion in the most suitable areas, the locations of which are precisely defined. Further analysis of the riverine forests of the Soča River in the area between the town of Kobarid and the confluence with the Tolminka River could be carried out with the aim of determining and evaluating the ecosystem services of riverine forests. Means collected by the community as a subsidy for offering ecosystem services in these areas could present a significant source for the management, care and development of this area. In addition, this study may trigger research into what the influence on riverine forests and their succession is of lowering the water level of the Soča River due to human impacts. Mϋller 92

107 (1995) connects lowering the water level of Alpine rivers mainly with increased river flow due to human impacts (regulation of the river course). According to him (Mϋller, 1995), the lowering of the water level has a great influence on the succession of riverine forests. Connecting the rich archive of data on the Soča River s flow rate and water level at the Kobarid measuring station I (Površinske, 2014) with the precise relief model (LIDAR) would bear data on the extent of flooding in terms of area and the influence of floodwater and soil water on riverine forests. This thesis shows that the study area is not an unspoiled wilderness. On contrary, just as in the past, it is a crossroads of numerous and frequently conflicting interests. In the area of riverine forests, numerous professionals and various interests, along with the forest owners themselves, are trying to fulfil their ambitions. In fact, all interventions in riverine forests are multidisciplinary. Their influences are not limited to a particular area rather they are reflected throughout the system (e.g. gravel excavation impacts natural succession and lowers the river channel, but building temporary roads for gravel transportation creates access roads to gravel sites for bathers). On the one hand, this is an extremely sensitive area ecologically, protected by numerous regulations, while on the other it represents one of the most intensively cultivated areas in the Northern Primorska region. Managing riverine forests as an area classified in Natura 2000 based on the existing forest management plans seems not to be effective, even with some supplements. The riverine forest area is too small, in terms of normal forestry practices, to get special attention. As a result, the forest management plans are not refined enough to deal separately with this issue. In addition, the structure of forest management planning seems to be too rigid to allow a multidisciplinary approach in dealing with the various issues and interests of the numerous groups that play a role in the area. It is noted that a strict natural-protection view of riverine forests is diminished due to the fact that it simply ignores various public interests. Because it does not recognise the plurality of interests in the area of these riverine forests, it comes into conflict with the actual situation and sound wishes of the community. Ignoring the community s wishes on the side of lawmakers and taking the many functions offered by these forests free of charge and for granted on the side of various groups results in uncontrollable development leading to the current 93

108 situation in these stands. The core problem in managing the riverine forests of the Soča River in the study area seems to lie in there being a somehow too robust planning regimen. In order to manage riverine forests successfully, a management plan whose content is limited only to this area is needed. Furthermore, within the area under study, zonation and consequently diverse land use are of vital importance. A multidisciplinary management plan should include zonation upon which each sector can take initiative. Co-ordintaion and direction should remain under the umbrella of a manager. This study proves that managing this environment on the level of the landowners along with widespread legislation has not been effective. To expropriate from the landowners would be an illusion; therefore, a rational approach could be the granting of subsidies for ecosystem services regardless of ownership. The restrictions imposed by regulations without rewards will not contribute to the successful management of riverine forests. The current miserable situation proves it. The implementation and creation of a management plan seems to be too much for the local communities to handle; therefore, a chance to create a refined management plan within the Natura 2000 area (riverine forest of the Upper Soča Valley) or the establishment of an Upper Soča Valley regional landscape park (the establishment of such a park is an idea that has appeared many times) seem feasible. This way, the successful management of the land adjacent to the Soča River would contribute to more favourable conditions also for the river itself. During the last 200 years, the amount of riverine forest in the study area has never been larger than it is at the moment, despite the fact that current public pressure on these forests is large and varied. The riverine forest situation on the Soča River in the area between the town of Kobarid and the confluence with the Tolminka River may improve if a management plan that takes into account and balances the many wishes of various communities and professionals is adopted and implemented. With proper management, all of the potential of these stands for offering ecosystem services in the landscape can be used, and their undisputable significance will be retained. 94

109 9. SOURCES Čušin B., Šilc U Vegetation development on gravel sites of the Soča river between the towns of Bovec and Tolmin. Sauteria, 14: Dakskobler I., Šilc U., Čušin B Riverline forests in the upper Soča valley (the Julian Alps, western Slovenia). Hacquetia, 3/2: Dakskobler I Fitocenološka in floristična analiza obrečnih gozdov v Posočju (zahodna Slovenija). Razprave IV. Razreda SAZU, XLVIII-2: Dakskobler I Razvoj vegetacije na prodiščih reke Idrijce v zahodni Sloveniji. Folia biologica et geologica, 51/2: Dakskobler I., Kutnar L., Šilc U Poplavni, močvirni in obrečni gozdovi v Sloveniji. Ljubljana, Silva Slovenica: 127 str. Danev G Vrednotenje ekosistemskih storitev. Končno poročilo CRP V DS-1, Ljubljana, Gozdarski inštitut Slovenije: 28 str. Ećimović T., Jan J., Vrhovšek D Slovenija včeraj-danes-jutri, gospodarjenje z vodo, izziv za generacijo Slovencev v tretjem tisočletju. Ljubljana, SEG in IZVOR: 147 str. Evidenca dejanske rabe kmetijskih in gozdnih zemljišč (RABA) Ministrstvo za kmetijstvo, gozdarstvo in prehrano ( ). ( ) Jarc P., Kladnik D., Rojšek D Posočje A-Ž enciklopedični priročnik za popotnika. Ljubljana, ZRC: 244 str. Klimo E., Hager H The floodplain forests in Europe: current situation and perspectives. Leiden, Koninklijke Brill NV: 249 str. Klimo E., Hager H., Matić S., Anić I., Kulhavy. J Floodplain forests of the temperate zone of Europe. Kostelec nad Černymi lesy, Lesnicka prace, s.r.o.: 623 str. Kollman J., Vieli M., Eduards P.J., Tockner K., Ward J.V Interactions between vegetation developement and island formation in the Alpine river Tagliamento. Applied Vegetation Science, 2: Kotar M Zgradba, rast in donos gozda na ekoloških in fizioloških osnovah. Ljubljana, zveza gozdarskih društev Slovenije, zavod za gozdove Slovenije: 500 str. Muller N River dynamics and floodplain vegetation and their alterations due to human impact. Arch. Hydrobiol, Suppl. 101: Odlok o gozdnogospodarskih in lovsko upravljavskih načrtih območij ( ). Ur. l. RS st. 87/12 95

110 Opozorilna karta poplav (Redka območja poplavljanja) MOP Agencija RS za okolje ( ). Papež J., Perušek M., Kos I Biotska raznolikost gozdnate krajine. Ljubljana, Zveza gozdarskih društev - Gozdarska založba: 161 str. Paušič A Rastlinske vrste in njihove morfološke posebnosti kazalci nekdanjega gospodarjenja z krajino. Proteus, 74/7: Pirnat J Obvodna drevnina kot del krajinske infrastrukture. V: Gozd in voda. XVI. Gozdarski študijski dnevi, Poljčane Oktober Anko B. (ur.). Ljubljana, Biotehniška fakulteta, Oddelek za gozdarstvo: Površinske vode dnevne vrednosti MOP Agencija RS za okolje ( ) ( ) Pravilnik o gozdnogospodarskem načrtu gozdnogospodarske enote Kobarid ( ). Ur. l. RS st. 101/09 Pravilnik o gozdnogospodarskem načrtu gozdnogospodarske enote Tolmin ( ). Ur. l. RS st. 5/12 Pravilnik o načrtih za gospodarjenje z gozdovi in upravljanje z divjadjo. Ur. l. RS st. 91/10 Rojšek D Naravne znamenitosti Posočja. Ljubljana, Državna založba Slovenije: 206 str. Slovenija na vojaškem zemljevidu Ljubljana, znanstvenoraziskovalni center Slovenske akademije znanosti in umetnosti, Arhiv Republike Slovenije. Zakon o gozdovih. Ur. l. RS st. 30/93, 56/99, 67/02, 110/02, 115/06, 110/07, 106/10, 63/13, 101/13r, 17/14, 24/15 in 9/16 96

111 APPENDIX Appendix A Map of stand types 97