Vegetation–environment relationships in the alderwood communities of Caspian lowlands, N. Iran (toward an ecological classification)

Abstract

Hyrcanian (Caspian) lowland forests (northern Iran) include alderwood communities, dominated by Alnus glutinosa ssp. barbata. A data set of these alderwoods, including floristic relevés and environmental variables (groundwater level, soil physical and chemical properties from two depths) was analyzed in order to describe the relationships between floristic composition and environmental variables. Classification of relevés using two-way indicator species analysis (TWINSPAN) and a clustering technique displayed five clear vegetation groups of A. glutinosa ssp. barbata, each with specific indicator species. Principal component analysis (PCA) displayed two major gradients in environmental variables, namely a gradient of acidity-related variables and one of groundwater level-related variables. The five vegetation groups were confirmed by detrended correspondence analysis (DCA) and can be interpreted with these two major environmental gradients and also life form data which were passively projected on the diagram. Hydrophytes and helophytes were mostly found in swampy or wet groups where they influenced the first axis, while other life forms were mostly concentrated in the drier groups on the second axis. The results of both direct canonical correspondence analysis (CCA) and indirect (DCA) analyses of vegetation–environmental data were almost the same. The main environmental variables controlling the separation of these vegetation groups on the first two axes are groundwater level and acidity. There is a little difference between environmental variables analysis by PCA and vegetation–environment analysis by DCA and CCA mainly on different effects of CaCO 3 on two first axes. Comparisons between habitat ecology of European alderwoods (stands of A. glutinosa ssp. glutinosa) and the measured environmental variables in the Hyrcanian alderwoods indicate some similar trends of variation of pH and C/N over these habitats in both areas. Three major types of A. glutinosa ssp. barbata habitats in Hyrcanian lowlands are distinguished mainly based on groundwater regime and geomorphology. These major types are compared with similar alderwoods in Europe.