Conversion of pure spruce to mixed spruce beech stands: Effects on alpha and beta diversity of multiple taxonomic groups

Abstract

The conversion from pure coniferous stands to mixed forests is a major goal of forest management strategies in the light of climate change. However, the effects of forest conversion on the associated species diversity and the factors driving diversity changes remain poorly understood. To investigate these effects, we established 54 plots in a managed forest in southern Germany, where large areas have been converted from pure spruce to mixed spruce-beech stands since the middle of the 20th century. Our sampling plots consisted of three different age classes and increasing proportions of beech, measured by the number of stems. We sampled fungi, plants, invertebrates, and birds and combined classical methods (identification by specialists) and metabarcoding for species identification. Furthermore, important forest structure and site parameters were recorded to test their influence on species alpha and beta diversity. Alpha diversity of embryophytes, and therein tracheophytes, increased with increasing availability of light, whereas fungi decreased. With increasing humus thickness alpha diversity of insects, especially dipterans and hymenopterans increased, whereas embryophytes, tracheophytes and fungi decreased. Increasing numbers of habitat trees increased the alpha diversity overall, especially in tracheophytes and hymenopterans. The proportion of beech determined the alpha diversity of one out of 12 taxa and influenced the beta diversity of 10 from 12 taxa. However, factors influencing the diversity are taxa dependent. Our study indicates that the proportion of beech had less impact on alpha diversity but impacts beta diversity. Changes in forest structures and environmental conditions occurring during forest conversion are as important as the introduction of beech for alpha and beta diversity.