Fly Me to the Canopy: Diptera Communities in Oak Forest Crowns As Bioindicators of Stand Decline

Diptera are among the most diverse and ecologically important insect orders in forest ecosystems and hold strong potential as bioindicators of ecosystem transformations under anthropogenic and environmental pressures. We examined the effects of oak decline, a process that alters canopy structure and resource availability, on canopy-dwelling Diptera across 30 oak stands in six French forests. Diptera were sampled using green multi-funnel traps placed in the lower canopy. Decline severity was derived from Sentinel-2 imagery at multiple spatial scales. In total, more than 27,000 specimens were collected, representing 60 families and approximately 500 taxa. Overall diversity increased with decline severity, particularly for saproxylic and floricolous guilds. This pattern may result from changes in microclimatic conditions in more open, declining stands that promote Diptera activity, as well as from increased availability of deadwood and floral resources. Families such as Dolichopodidae, Empididae, Hybotidae and Anthomyiidae responded positively to decline severity, whereas Mycetophilidae and other mushroom-associated taxa declined, possibly due to reduced fungal productivity and drier microhabitats. Stand characteristics, including height, density and oak basal area, generally had negative effects on richness and abundance, potentially through their influence on microclimate or resource distribution in denser stands. Co-occurrence analyses indicated that species associations were predominantly random, followed by positive associations, and this pattern did not vary with decline severity. These results suggest that Diptera may serve as sensitive indicators of stand condition and structural changes associated with climate-driven canopy decline.