Sustainable forest management increasingly favors mixtures, but effects on many ecosystem characteristics, such as biocontrol by predators and their biodiversity, are still not well understood. This knowledge gap can be particularly problematic when these mixtures include non-native tree species, such as Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), which are planted because of their high growth rates and climatic suitability but might have negative effects on forest biota. Available studies have mostly focused on epigeic communities, even though tree-associated predator communities can be assumed to make an important contribution to biocontrol that differs significantly from conditions on the forest floor. Here, we analyzed the responses of spiders, as generalist arboreal predators, along a tree species richness gradient in a young tree diversity experiment in Thuringia, Germany, utilizing four of the most economically important deciduous and coniferous tree species in Europe: European beech (Fagus sylvatica L.), Norway spruce (Picea abies (L.) H. Karst.), Douglas fir and sessile oak (Quercus petraea Liebl.), planted in stands of 1–4 tree species. We tested for effects of tree species richness, tree identity and composition (tree species proportion and position of individual trees) on arboreal spider species richness, functional diversity, abundance, biomass and community structure at both the tree and plot level. Tree species richness promoted arboreal spider species richness and functional richness at the plot level and community-weighted mean spider biomass at the tree level. Tree identity and composition had a stronger influence than tree species richness on structuring spider communities, with arboreal spiders having higher numbers per tree on conifers, especially on Douglas fir. However, at the plot level spider species richness, abundance and biomass decreased with an increasing proportion of conifers due to lower species turnover among coniferous trees. The observed effects of tree species richness, tree identity and tree species proportions early in the establishment of forest stands might influence the strength of top-down control of herbivores in critical stages of development of forest stands. Tree species selection and the proportion in which these trees are mixed with other species should be determined with care in order to concomitantly promote biodiversity and biocontrol. In this context, an important finding of our analyses is that potentially negative effects of non-native Douglas fir on arboreal arthropods were not more severe than effects of native conifers that are planted outside of their natural habitats.