Tree mixtures increase bird taxonomic and functional diversity over pure stands of tree species planted outside their natural range—but not over pure native stands

Recent biodiversity loss has emphasized the necessity to critically evaluate the consequences of human alterations of forest ecosystems. Stand diversification via tree species mixtures and the use of non-native tree species are two such alterations currently gaining importance as climate change adaptations. However, the effects of local versus regional tree mixing on associated biodiversity and notably the modifying role of tree species growing outside their natural range remain poorly understood.
We assessed how monocultures and mixtures of native and introduced tree species influence the taxonomic and functional diversity of northwest German bird communities at stand and landscape scales. We focused on the dominant natural tree species (Fagus sylvatica) and economically important conifer species planted outside their natural range (the native Picea abies and non-native Pseudotsuga menziesii).
We found that bird species richness and functional diversity were generally higher in pure ...