Native earthworms and roe deer in temperate forests: Friends in the sun, foes in the shade.

Given increasing deer populations and rising canopy openness through tree mortality in European forests, understanding how these changes affect above-belowground communities and ecosystem functioning is becoming more important. Earthworms comprise the largest animal biomass in European forest soils and play a central role in decomposition, carbon and nutrient cycling, and soil formation. However, their responses to canopy openness and deer browsing have been studied almost exclusively in northern North America, where interactions involve white-tailed deer (Odocoileus virginianus) and mainly non-native and invasive earthworms. Here, we examine how roe deer (Capreolus capreolus) and canopy openness interactively affect native earthworm communities in a temperate European forest across 75 experimental patches: 54 closed-canopy and 21 experimentally created canopy-gap patches. Five years after gap establishment, we sampled earthworms and associated environmental variables from two plots per patch: exclosure (fenced) and control (unfenced) plots. We found that roe deer and canopy gaps interactively influenced earthworm communities. Under closed canopy, roe deer reduced earthworm biomass and density. If roe deer were absent, canopy gaps had no effect on earthworms, but if roe deer were present, canopy gaps increased biomass and density through vegetation-mediated pathways and enhanced soil water content. The magnitude of these effects differed among earthworm ecological groups (anecic species, i.e., earthworms that create deep, permanent burrows and connect surface litter with deeper soil layers, vs. endogeic species, i.e., earthworms that live and feed entirely within the soil) and life stages (juveniles vs. adults). Indirect effects of roe deer were particularly beneficial for juveniles. These results contrast with patterns reported for white-tailed deer and non-native earthworms in northern North American forests, where white-tailed deer presence typically promotes earthworms. Our results on the direct and indirect effects of roe deer on belowground earthworms shed new light on the complex interactions of deer populations and canopy structure on forest soil biodiversity and ecosystem functioning.