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

<p><span>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 bio</span><span>diversity and notably the modifying role of tree species growing outside their natural range remain poorly understood. </span></p> <p><span>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 (<em>Fagus sylvatica</em>) and economically important conifer species planted outside their natural range (the native <em>Picea abies</em> and non-native <em>Pseudotsuga menziesii</em>). </span></p> <p><span>We found that bird species richness and functional diversity were generally higher in pure and mixed stands of native <em>F. sylvatica</em> than in pure conifer stands, especially in comparison to non-native <em>P. menziesii</em>. These differences were particularly strong at the landscape scale. Pure conifer stands harbored only a reduced set of functionally similar bird species. Structural diversity based on tree microhabitat availability emerged as a key predictor of bird diversity. </span></p> <p><span>Synthesis and applications: Our study suggests that tree species mixtures do not necessarily increase bird diversity compared to pure stands of native trees, but can promote bird diversity relative to pure stands of species planted outside their natural range. Moreover, local mixtures, rather than a mosaic of pure stands, may promote bird diversity also at the landscape scale. By contrast, pure stands of tree species planted outside their natural range can increase the biotic homogenization of forest birds. Promoting structural diversity of microhabitats via tree retention and ensuring that non-native trees are planted in mixtures with native trees may alleviate potential limitations of climate change-oriented management for biodiversity. </span></p>