Species diversity of forest floor biota in non-native Douglas-fir stands is similar to that of native stands

Cultivation of non-native tree species is a promising option to adapt managed forests to climate change. However, consequences of non-native tree species on flora, fauna, and microorganisms that occur in forest ecosystems (forest associates) need to be considered when managing forests. We lack a solid understanding of how cultivating non-native tree species in pure stands and in mixture with native tree species impacts abundance and species diversity of forest associates. We compared abundance, alpha-, beta- and gamma-diversity of eight forest-floor-associated taxa that are relevant for ecosystem functioning (including fungi, plants, arthropods, and small mammals) between different forest ecosystems. We addressed pure stands of non-native coniferous Douglas-fir (Pseudotsuga menziesii) and two native species, broad-leaved European beech (Fagus sylvatica) and coniferous Norway spruce (Picea abies), as well as mixed stands of European beech with each conifer in two regions with differing site conditions in temperate Northwest Germany. Diversity indices revealed differences among species groups. Gamma-diversity and alpha-diversity of forest associates in Douglas-fir and spruce stands were usually higher than in beech stands. Estimates of species diversity of mixed stands are intermediate between estimates for the respective pure stands. Differences in the diversity between the two study regions were highly taxon specific with no clear support for a general trend toward a higher or lower species diversity of forest associates depending on site quality. Abundance values show a pattern similar to our diversity estimates, but with a higher statistical uncertainty. Non-native Douglas-fir stands provided habitats to support associated species communities of equally high or higher diversity than stands of native beech or spruce. Mixed stands of non-native and native tree species may be a management option to achieve different goals, that is, to provide habitats for species that are strictly associated with beech and to increase resilience to climate change. However, the overall diversity of forest-floor-associated biota is not improved by cultivating Douglas-fir or spruce in mixture with beech.