Interactive effects of tree diversity, tree identity and management on generalist predator taxonomic and functional diversity

Stand diversification can increase structural and resource diversity, benefitting multitrophic diversity and ecosystem functioning. However, for higher trophic level organisms such as spiders, ambiguous tree diversity effects have been reported. Instead, tree species composition seems more relevant, especially in species-poor forests, where individual tree species contribute strongly to habitat complexity. Spiders might benefit from management interventions directly targeting stand structure, but such interventions have rarely been studied together with the effects of tree diversity and identity. In a temperate tree diversity experiment comprising four economically and ecologically relevant species (European beech, sessile oak, Norway spruce, and Douglas fir) we compared effects of tree species richness, tree identity, recent management (thinning with deadwood retention), and their interactions on epigeal spider abundance, taxonomic and functional diversity. In our study, tree diversity had no direct effects on spider diversity. However, some tree identity effects were only apparent in monocultures. Tree species identity was a key driver: Sessile oak promoted spider abundance, while Norway spruce promoted diversity, likely due to differences in stand structure. By comparing our study to findings from sites of varying ages, we conclude that tree identity effects change with stand structural development. Thus, predators can benefit from mixed-species stands even when direct tree diversity effects are absent, as mixtures can temper temporary negative tree identity effects. Moreover, moderate management during early stand development can enhance structural diversity, thereby promoting biodiversity and top-down control potential. • No direct tree species richness effect on epigeal spiders. • Tree diversity modulates management and tree identity effects on epigeal spiders. • Management benefits epigeal spiders, especially in structurally poor monocultures. • Tree identity effects on epigeal spiders can change during forest development. • Stand diversification may temper some temporary negative tree identity effects.