Variability of alder-dominated forest vegetation along a latitudinal gradient

Plant species establishment and survival along environmental gradients have commonly been attributed to different environmental niches, life-history and adaptive traits [1]. Effective phenotype adaptation of alder species to extreme soil conditions allows them to form canopy-closed forest vegetation on alluvial and marshland localities. Alder forests represent azonal plant communities developed independently of the variables determining vegetation zonality. The hydrological regime was most often identified as the underlying source of variation in their floristic spectrum. Seasonal dynamic of groundwater table and flood regime are important ecological phenomenon creating microhabitats with various environmental qualities [2–5]. The diverse ecological requirements and distribution patterns of black alder (Alnus glutinosa) and grey alder (Alnus incana) [6,7], considered as native species to Central Europe, result in various syntaxonomical positions of their stands. Alder-dominated forests on permanently waterlogged soils are traditionally assigned to the Euro-Siberian alder carr forests of the Alnion glutinosae alliance (Alnetea glutinosae class), whereas the more mesophilous streamside forests are recorded within the European broad-leaved floodplain forests of the Alnion incanae (= Alno-Ulmion) alliance (Querco-Fagetea class). These ecologically and floristically distinctive vegetation types are scattered relatively evenly all over the Central European landscape [3,8–12]. The presented classification concept belongs to the widely accepted and frequently used in vegetation surveys, namely in Poland [13], Ukraine [14], Hungary [15], Austria [16], the Czech Republic [4], Germany [17] and also in Slovakia [18]. On the other hand, there is a considerable inconsistency in internal differentiation of these two alliances resulting probably from diverse level of variability in their species composition. While the higher number of associations has been distinguished in the Alnion incanae forests (e.g. [4,19]), the opposite pattern has been reported from the Alnion glutinosae communities [11,16]. The Slovak phytosociological data set contains especially relevés from local vegetation studies performed in lower mountains Abstract