The contribution of canopy samples to assessments of forestry effects on native bees

Abstract

Forest management is often practiced to enhance conditions for wildlife, including native bees. Evaluations of the effects of forest management on bees have shown that abundance and diversity are higher in newly created earlysuccessional conditions. To date, studies have restricted sampling to the forest understory; however, recent research finds that bee abundance is as high or higher in forest canopies than in understories, suggesting that previous observations of substantially greater bee abundance and diversity in recently managed areas could be an artifact of incomplete sampling of the vertical gradient within forests. To examine the potential implications of sampling biases associated with the failure of previous studies to include canopy samples in comparisons of managed and unmanaged forests, we sampled bees within a recently harvested forest as well as the understory and canopy of adjacent unmanaged forest. Bee abundance and diversity were an order of magnitude higher in managed areas compared to the unmanaged forest, even when understory and canopy samples were combined. These results suggest that not sampling the canopy is inconsequential with respect to the broadly reported conclusion that managed areas support more abundant and diverse bee communities than surrounding forest cover.

Abundant, distinct, and seasonally dynamic bee community in the canopy-aerosphere interface above a temperate forest

“…Our study is the first to demonstrate that bees occupy the aerosphere immediately above the forest canopy; furthermore, the community above the canopy was compositionally distinct with similar abundances compared to lower strata (understory, midstory, canopy). These findings expand our understanding of forest bee communities and build on earlier research that revealed differences between understory and canopy bees (Cunningham-Minnick & Crist, 2020;Milam et al, 2022;Ulyshen et al, 2010;Urban-Mead et al, 2021). However, when attempting to characterize the forest bee community, the importance of sampling the canopy-aerosphere interface hinges upon the question of whether these bees should be considered as part of the forest community, or if they are transients moving among resources.…”

“…Therefore, studies that quantify potential nesting substrates for wood-nesting bees within the canopy, including those that nest in 'soft' wood, are clearly needed to resolve these discrepancies (Harmon-Threatt, 2020). Milam et al (2022) found that the inclusion of canopy sampling in addition to understory sampling did not influence their ability to characterize the forest bee community. Our study supports their conclusion when only considering bees below the maximum height of the canopy (i.e., understory, midstory, and canopy strata), but further demonstrates that the bee community above the canopy is distinct from lower strata.…”

“…Studies examining bee communities within temperate forests have largely restricted sampling to the understory (Milam et al, 2022) with the presumption that most bees remain in this lower stratum. However, recent evidence indicates that bees are vertically distributed within temperate forests (e.g., Ulyshen et al, 2010;Urban-Mead et al, 2021), suggesting a potentially large knowledge gap in the ecological role of these important forest pollinators.…”

“…Thus, our understanding of pollinator ecology within forests will remain incomplete until the distribution of forest bee communities along the entire vertical gradient of vegetation structure is documented. Moreover, if the current understanding of bee abundance and diversity patterns in forests are inaccurate, forest management recommendations for bee conservation may be biased or potentially misguided (Milam et al, 2022;Urban-Mead et al, 2021), further highlighting the importance of understanding the distribution of bee communities along the full vertical gradient of temperate forests, including the canopy-aerosphere interface.…”

“…Bees are expected to be spatially and temporally distributed throughout temperate forests in response to local resource availability. Studies have demonstrated that forest bee communities are diverse and vertically stratified on sun-exposed edges (e.g., Cunningham-Minnick & Crist, 2020), within forests near edges (e.g., Urban-Mead et al, 2021), and within the forest interior (e.g., Campbell et al, 2018;Milam et al 2022;Ulyshen et al, 2010) when floral resources of the forest are available, as well as when they are not. Inferences and observations further suggest that bees will both forage on floral resources and nest at different vertical strata within forests (Cunningham-Minnick & Crist, 2020;MacIvor et al 2014;Russo & Danforth, 2017;Smith et al, 2019;Sobek et al, 2009;Urban-Mead et al, 2021;Wood et al 2018).…”