Tree-related microhabitats harbor distinct micro-invertebrate communities and support complex food webs.

Microscopic soil invertebrates are known to play an important role in forest ecosystems through their interactions with the rhizosphere and belowground food webs. However, little is known about the abundance, diversity, distribution, and ecological roles of micro-invertebrates above the forest floor, particularly within tree-related microhabitats (TreMs). In this study, we sampled 18 distinct types of TreMs in the UNESCO World Heritage old-growth beech forest of La Massane, located in the southeastern Pyrenees. We extracted and counted various groups of micro-invertebrates (nematodes, tardigrades, rotifers, and mites) associated with these TreMs, and identified nematodes to the species level. Additionally, we measured the stable isotopic signatures of carbon and nitrogen in various resources and invertebrate groups to reveal the structure of TreM-associated food webs in unprecedented detail. TreMs emerged as hotspots of both abundance (averaging 195 individuals per gram of dry TreM substrate) and diversity (98 nematode morphospecies across 20 families). We found significant differences among TreM types in terms of community composition and food web structure, revealing distinct species assemblages and trophic pathways. These differences could be linked to factors such as the stage of wood decomposition, occupation by larger animals (e.g., insects, birds), and the position of TreMs within trees. Our findings suggest that micro-invertebrates serve as valuable ecological indicators, adding a new layer of biodiversity and functional understanding to TreM typology. This, in turn, supports more comprehensive strategies for nature conservation and forest management.