Responses of the hyper‐diverse community of canopy‐dwelling Hymenoptera to oak decline

Forest decline and dieback are growing phenomena worldwide, resulting in severe, large‐scale degradation of the canopy. This can profoundly alter the provision of trophic resources and microhabitats for canopy‐dwelling arthropods. In 2019, we assessed the effect of oak decline on the community of canopy‐dwelling Hymenoptera. We selected 21 oak stands and 42 plots, located in three forests in France, presenting contrasting levels of decline. Insects were sampled at the canopy level with green multi‐funnel and flight‐interception traps. We collected a particularly diverse community of 19,289 insect individuals belonging to 918 taxa, 10 larval trophic guilds and 7 nesting guilds. Oak decline had no effect on the abundance or richness of the overall community but significantly reshaped the community assemblages. Decline had contrasting effects depending on the taxa and guilds considered. Specialist parasitoids were more abundant at intermediate levels of decline severity while generalists were negatively affected. Taxa depending on ground‐related resources and microhabitats were promoted. Saproxylic taxa were more abundant while xylophagous insects were negatively impacted. Reduced leaf area index promoted several guilds and the diversity of the overall community. While an increasing tree mortality rate enhanced the abundance and diversity of deadwood resources, it had negative impacts on several Hymenoptera guilds. Our results suggest that micro‐environmental changes at the ground level due to canopy decline have major cascading effects on the communities of canopy‐dwelling Hymenoptera. Our study highlights the relevance of studying Hymenoptera communities to investigate the outcomes of disturbances on forest biodiversity. Hymenoptera dwelling in oak canopies are hyper‐diverse. Forest decline reshapes the community of canopy‐dwelling Hymenoptera, and trophic and nesting guilds have contrasting responses to decline. Species relying on ground‐related resources (floral responses and soil microhabitats) are promoted by decline‐induced canopy openness.