Environmental thresholds for plant species richness of black alder (Alnus glutinosa) forests in Central Europe

The diversity of vascular plants in temperate floodplain forests varies between biogeographical regions of Europe. Our study aims to identify the key environmental drivers of plant species richness in forests dominated by black alder (Alnus glutinosa) in Central Europe with four regions: Pannonian lowland, Matricum as the southern part of the Western Carpathians, High Western Carpathians and Polish Plain. We analysed plant species richness and quantified 15 environmental characteristics (soil, climatic and landscape characteristics) in 140 vegetation plots (35 per region). We used model-based regression trees to test the influence of predictors on the richness of both native and alien species. The regression tree analysis identified eight significant variables controlling species richness in three regions and all bioregions together but found no significant predictor in Matricum. The analysis of the joint dataset indicates that native plant richness was controlled by the effects of catchment slope, soil reaction and precipitation of the warmest quarter. In contrast, the richness of alien species was influenced by the precipitation of the warmest quarter, soil phosphorous and temperature. The species richness of native plants in the High Western Carpathians was driven by soil reaction and the presence of artificial surfaces around the plots, while the richness trend in the Pannonain lowland was determined by annual temperature. Alien richness was affected by the proportion of agricultural areas in the High Western Carpathians, by the stream power index in the Polish Plain and by soil reaction in the Pannonian lowland. The explanatory power of the tree models ranged from 22 to 36%. Our results suggest that the predictability of the richness patterns is contingent upon the specific regions, which differ in the length of environmental gradients.