Martini, M.; Dadda, M.; Fornoff, F.; Feldhaar, H.; Luo, A.; Rehling, F.; Spitz, J. E.; Staab, M.; Thorn, S.; Zhu, C.-D.; Klein, A.-M.

1. Saproxylic community assembly is structured both by deadwood and forest habitat gradients, as well as biotic interactions such as competition, predation, and parasitism. However, covariation between abiotic and biotic conditions in natural systems have limited our ability to disentangle these mechanisms. Furthermore, focus towards beetles and fungi in temperate or boreal forests has led to important taxonomic and geographic knowledge gaps. 2. Here, we tested how experimentally-manipulated tree diversity, deadwood position (lying vs. standing), and biotic interactions with a dominant antagonist (ant exclusion) structure the community assembly of deadwood-cavity-nesting bees, wasps, and their parasitoids in a subtropical forest. 3. Our findings reveal that lying deadwood supports a nested subset of the communities occurring in standing deadwood, with less diversity and abundance of hosts and parasitoids. We found that increased moisture, rather than ant activity, was the primary mechanism filtering Hymenoptera communities, as deadwood in contact with the forest floor retained twice as much moisture as standing substrate. Moreover, moisture gradients within each substrate type further reduced host abundance, likely due to reduced brood cell production and survival. In contrast, forest habitat (tree species richness, canopy cover, and coarse woody debris) had comparatively minor roles in shaping cavity-nesting community assembly. 4. Our results provide a mechanism for the positive association between cavity-nesting Hymenoptera and standing deadwood in forests. Because standing deadwood is typically scarce in many managed forests, these findings support the retention and enhancement of such substrates to promote these ecologically-important insects.