Divergent Responses of Soil Predatory Macroarthropods to Elevation Gradients in the Picea crassifolia Forests of Qilian Mountains

Picea crassifolia forests are a key plant resource protected within the Qilian Mountain National Park and also constitute a vital component of the regional ecological barrier. While extensive research has examined variation in vegetation and soil conditions along elevational gradients in forests, an understanding of how soil fauna diversity changes with elevation remains limited. Predatory soil macrofauna play a critical role in the soil fauna food webs of alpine ecosystems, and shifts in their diversity along elevational gradients are important for ecosystem stability and functioning. In this study, we assessed changes in predatory arthropod communities along an elevational gradient in a P. crassifolia forest in the Qilian Mountains. We found that community composition varied distinctly with elevation, with average dissimilarity increasing progressively at higher altitudes. Predatory arthropod abundance followed a unimodal pattern along the elevational gradients, whereas diversity indices showed an opposing trend. The abundance of Lithobiidae and Thomisidae increased notably at the upper tree line, in contrast to declines observed in Linyphiidae, Staphylinidae, and two carabid beetle species (Carabus sewertzowi and Poecilus singularis). Predatory arthropod assemblage varied significantly with elevation, particularly at the tree line, primarily driven by herbaceous coverage, tree density, and total nitrogen, which together explained 50.9% of the variation. In short, the tree lines of P. crassifolia forests were key regions for predatory arthropods, with the upper tree line exerting a greater influence than the lower tree line.