Multiple controls of understorey plant richness in mountain forests of the Bavarian Alps

The management of biodiversity in forests requires a basis derived from community ecology. In a transect representative of montane and subalpine forests at the northern fringe of the Alps, I studied the relative importance of the tree layer diversity and age, understorey cover and species pool size for species richness of vascular understorey communities. Regional species pools were estimated for seven community types represented in a large phytosociological database. Local species pool size was modelled for each community type on the basis of regional pools and grid cell data from floristic mapping. Tree-layer diversity was the only study variable unrelated to species richness. Understorey richness increased with understorey cover, species pool and stand age. Combined, these three predictors explained 46 % of the observed variation in richness in a regression model. When the species pool variable was partioned into its components, synecological information was a more powerful predictor of richness than variation in the local flora, which was likely due to the strength of site effects related to soil pH and to the relatively small geographic extent of the study. Species richness in mountain forests can be forecasted by maps of community types. Within the bounds of natural site-dependent diversity, extensive management practices allowing for moderate to high cover of understorey and the development of old growth stands favour the occurrence of forests rich in vascular plant species.