Restoring tree species mixtures mitigates the adverse effects of pine monoculture and drought on forest carabids

Habitat degradation and climate change are main drivers of insect species loss worldwide, raising concern about natural forest replacement by tree monocultures in a context of more frequent disturbances like drought. Carabid beetles are emblematic species in ecology because they are often used as indicators of biodiversity and they have important functional roles, particularly predation. We used a tree diversity experiment with half of the plots irrigated and the other half under summer water stress to test the combined effects of tree species mixing and drought on carabid species and functional diversity. We observed a qualitative effect of drought on carabid communities, with species turnover mainly due to predator species loss in the drier (non-irrigated) plots. We also found that species richness and activity density were lowest in pure pine plots and highest in mixture of pine and birch at low tree density. The likely underlying mechanism is the better provision of food and shelter resources in mixed forests. The association of pine with birch species could compensate for the loss of carabid beetles observed between pine monocultures benefiting from the water regime of the last century and those subject to current droughts. This suggests that diversifying plantation forests is a promising way to increase their resilience to the adverse effects of climate change.