Numerical classification and syntaxonomical revision of the Belgrade Forest vegetation (Istanbul, Türkiye)

The Belgrade Forest, located in the European part of Istanbul, represents an ecological and biogeographical transition zone where nemoral and Mediterranean elements coexist. Although comprehensive phytosociological studies have been conducted in the study area, differences in data structure, sampling intensity, and nomenclatural problems have prevented the establishment of a unified vegetation classification. In the present study, a total of 685 relevés derived from previous surveys were compiled and digitized in TURBOVEG, standardized, and analysed using numerical classification and ordination methods. The vegetation data were classified by TWINSPAN, and diagnostic species were identified based on the phi coefficient and constancy ratio. Detrended correspondence analysis (DCA) was applied to determine major compositional gradients, and Ellenberg-type indicator values were used to interpret ecological differentiation. The analysis resulted in four forest associations and one pseudomaquis community, corresponding to Alnus glutinosa-Carpinus betulus riparian forest, Acer campestre-Carpinus betulus forests on humid soils, meso-thermophilous mixed beech-oak-hornbeam forests, and pseudomaquis vegetation. Among these associations, Equiseto telmateiae-Alnetum glutinosae ass. nov. and Hyperico calycini-Ericetum arboreae ass. nov. are here described as new. The principal ecological gradient reflected a transition from humid to relatively dry site conditions, consistent with slope aspect (site exposure to solar radiation) and soil depth variations. The resulting syntaxonomic scheme provides a harmonized framework for the classification and monitoring of forest vegetation in the Belgrade Forest and offers a baseline for assessing future ecological change and restoration priorities.