Authors: Le Souchu, Elodie; Parmain, Guilhem;Bankhead-Dronnet, Stéphanie;Brand, Matthias;Damoiseau, Sébastien;Sallé, Christian;Bouget, Christophe;Sallé, Aurélien;

Climate change is increasing the frequency and spatial extent of forest declines and diebacks in Europe. This leads to major shifts in trophic resources and microhabitats, especially in the canopy layer, which is enriched in deadwood resources, but depleted in foliage. We assessed the effects of oak decline on the community of canopy-dwelling insects. To do this, we sampled insects in the canopy of 21 oak stands with contrasting levels of decline, located in three forests, using green multi-funnel and flight-interception traps. We collected more than 129,000 individuals belonging to 1,359 species and 16 larval trophic guilds. Oak decline significantly reshaped the community composition but did not affect the overall species richness and abundance. Decline had contrasted effects depending on the larval trophic guilds considered. Surprisingly, no effect was detected concerning xylophagous and saproxylophagous guilds, but decline negatively affected gall-inducers. However, decline promoted several guilds depending on ground-related resources like rhizophagous and pollinivorous/nectarivorous insects. This suggests that the impact of canopy opening on ground microclimates and resources may be a major driver of compositional change in canopy-dwelling communities during declines. These results also highlight that declining stands shelter peculiar communities and can be an asset for biological conservation purposes.