Forest vegetation of Trešnjik near Banja Luka
Due to wide range of different ecological conditions, variety of human influence and historical development of flora and vegetation, forest vegetation of Bosnia and Herzegovina is very heterogeneous, and, at the same time, very poorly investigated in terms of their composition, ecology, distribution and syntaxonomy (Redžić, 2007; Beus & Vojniković, 2012). This can be applied also to the Pre-Pannonian region of BiH which is potentially occupied mainly by zonal sessile oak - com mon hornbeam forests, submontane beech forests and azonal dry, more or less acidophilous sessile oak forests (Stefanović et al., 1983). Although area of Trešnjik hill is very close to Banjaluka city center, its vegetation was uninvestigated until present. As this area is one of the potential Natura 2000 sites in BiH (Milanović & Golob, 2015), its habitats and wild species are to be preserved in a favorable conservation state. To facilitate this, one of the main objectives of this paper was to investigate forest habitat types in Trešnjik area. The other, not less important objective was to continue with the study of BiH vegetation, in order to contribute in solving the above mentioned problems. The research area Trešnjik is located in the NW part of Starčevica masiff (Figure 1), just a few kilometers soth from the Banjaluka city center (Figure 2). The area is orographicaly and geologicaly heterogeneous with the predominance of the flysch bedrock. The climate is temperate with average annual temperatures of 11.0°C, and annual precipitation of 1024 mm. All 55 relevés collected in the field were made using the standard Central European phytosociological method (Braun-Blanquet, 1964). After analysis of the collected relevés, 48 relevés were synthetized and presented in two phytosociological tables. Classification was done according to floristic and ecological simmilarirties between relevés, without the use of numerical classification methods. Total of seven floristically and ecologically well defined types were distinguished, and classified in two groups:
• Group of beech habitat types (Table 1)
1. Habitat type Athyrium filix-femina-Fagus sylvatica (pure beech forests)
2. Habitat type Fagus sylvatica-Tilia tomentosa (forests of beech and silver lime)
3. Habitat type Quercus petraea-Tilia tomentosa (degradation of the former type with the domination of silver lime)
• Group of sessile oak habitat types (Table 2)
1. Habitat type Quercus petraea-Carpinus betulus (zonal forest of sessile oak and common hornbeam)
2. Habitat type Vaccinium myrtillus-Quercus petraea (acidophilous sessile oak forests)
3. Habitat type Carpinus orientalis-Quercus petraea (thermophilous sessile oak forests)
4. Habitat type Fraxinus ornus-Carpinus orientalis (oriental hornbeam – mana ash scrub)
Habitat types dominated by beech are ecologicaly and physiognomicaly quite similar, they are mainly found on the northern exposures with very steep slopes, but on deeper and nutrients richer soils. Sessile oak group on the other hand occurs on the milder slopes, but ecologicaly it is represented with three types: mesophilous one, found on deeper soils on predominantly carbonate bedrock, acidophilous type on acidic soils on flysch, and thermophilous type on shallow, rocky soils on dolomites or limestone. In this type there is also secondary succession stage of oriental hornbeam-mana ash scrub. Both groups are characterized by presence of relict species: Ruscus hypoglossum, Ruscus aculeatus (absent only from the most mesophilous beech type) and Ilex aquifolium. There is also great frequency of Rubus hirtus which abundance is in direct proportion with the level of stand degradation. It can be concluded that area of Trešnjik has relatively high diversity of forest vegetation (seven types on relatively small area). Also, results pointed out several problems in the phytosociology science in BiH that need to be adressed in the future: floristic composition, ecology, distribution, syntaxonomy and nomenclature of the main part of forest communities.
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