Green alder communities in the Alps: phytosociological variability and ecological features

Alpine Alnus viridis ( = Alnus alnobetula) communities were studied. A total of 306 phytosociological relevés of Austrian, French, German, Italian and Swiss alpine regions were analysed. Data analysis showed the existence of two associations distinguished on an ecological basis. The first agrees with the classic description of Alnetum viridis found on hygronitrophilous soils in avalanche gullies and moderately steep slopes and is characterized by perennial tall herbs. The second, proposed as new association Rhododendro ferruginei–Alnetum viridis, colonizes mesoacidophilous soils on moderate slopes and disused pastures and is characterized by shrubby invasive species. The association's variability was also examined. Four subunits were identified, distinguished by ecological factors. Comparison tests and constrained ordination of the ecological features (i.e. geomorphology, biodiversity, Ellenberg's indicator values) allowed the discrimination of the main ecological factors that affect the distinction of the coenoses. These were soil reaction, water and soil nitrogen availability, slope, altitude and the influence of grazing. The absence of biogeographical influences and its biodiversity contribution are also discussed.