Abstract 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.

中文翻译：

---

树种丰富度、树种特性和非本地树种比例影响树栖蜘蛛的多样性、丰度和生物量

摘要 可持续森林管理越来越有利于混合，但对许多生态系统特征的影响，如捕食者的生物控制及其生物多样性，仍不清楚。当这些混合物包括非本地树种时，这种知识差距尤其成问题，例如花旗松 (Pseudotsuga menziesii (Mirb.) Franco)，这些树种因其高增长率和气候适宜性而被种植，但可能对森林产生负面影响生物群。现有的研究主要集中在表生群落上，尽管可以假设与树木相关的捕食者群落对生物防治做出了重要贡献，这与森林地面的条件有很大不同。在这里，我们分析了蜘蛛的反应，作为一般的树栖捕食者，在德国图林根州的一项幼树多样性试验中，沿树种丰富度梯度，利用欧洲四种经济上最重要的落叶和针叶树种：欧洲山毛榉 (Fagus sylvatica L.)、挪威云杉 (Picea abies (L.) H. Karst.)、花旗松和无梗橡树 (Quercus petraea Liebl.)，种植在 1-4 种树种的林分上。我们测试了树种丰富度、树种特性和组成（树种比例和个体树木的位置）对树栖蜘蛛物种丰富度、功能多样性、丰度、生物量和群落结构的影响。树种丰富度在样地层面促进了树栖蜘蛛物种丰富度和功能丰富度，在树木层面促进了群落加权平均蜘蛛生物量。树木特性和组成对构建蜘蛛群落的影响大于树种丰富度，在针叶树上，尤其是在花旗松上，树栖蜘蛛在每棵树上的数量更高。然而，在样地层面，由于针叶树之间物种周转率较低，蜘蛛物种丰富度、丰度和生物量随着针叶树比例的增加而下降。在林分建立早期观察到的树种丰富度、树种特征和树种比例的影响可能会影响林分发展关键阶段对食草动物的自上而下控制的强度。应谨慎确定树种选择以及这些树与其他树种混合的比例，以同时促进生物多样性和生物防治。在这种情况下，