Predators significantly affect ecosystem functions, but our understanding of to what extent findings can be transferred from experiments and low-diversity systems to highly diverse, natural ecosystems is limited. With a particular threat of biodiversity loss at higher trophic levels, however, knowledge of spatial and temporal patterns in predator assemblages and their interrelations with lower trophic levels is essential for assessing effects of trophic interactions and advancing biodiversity conservation in these ecosystems. We analyzed spatial and temporal variability of spider assemblages in tree species-rich subtropical forests in China, across 27 study plots varying in woody plant diversity and stand age. Despite effects of woody plant richness on spider assemblage structure, neither habitat specificity nor temporal variability of spider richness and abundance were influenced. Rather, variability increased with forest age, probably related to successional changes in spider assemblages. Our results indicate that woody plant richness and theory predicting increasing predator diversity with increasing plant diversity do not necessarily play a major role for spatial and temporal dynamics of predator assemblages in such plant species-rich forests. Diversity effects on biotic or abiotic habitat conditions might be less pronounced across our gradient from medium to high plant diversity than in previously studied less diverse systems, and bottom-up effects might level out at high plant diversity. Instead, our study highlights the importance of overall (diversity-independent) environmental heterogeneity in shaping spider assemblages and, as indicated by a high species turnover between plots, as a crucial factor for biodiversity conservation at a regional scale in these subtropical forests.