Forest bees need compositionally diverse canopies

Abstract

Forests provide critical habitats for pollinating insects, including forest-dependent and habitat generalist species, yet it is unknown how these assemblages are shaped by overstory tree composition. We sampled bees in closed canopy plots in the southeastern United States representing a continuum of forest age and tree composition, from younger conifer-dominated forests to older forests dominated by deciduous broadleaf trees. Species-specific responses of bees to forest composition, and the influence of their traits on responses, were estimated using a joint species distribution model. Additionally, we investigated species richness trends of nesting, sociality, and phenological trait groups. Forest composition greatly influenced bee species occurrence: 48% of species had positive relationships with the diversity of insect-pollinated broadleaf trees and 46% had negative relationships with the proportion of conifer basal area. Bee species with early phenological activity drove these responses and richness patterns supported these trends. Our results indicate that phenology is an important factor determining bee species’ forest dependency and sensitivity to forest composition in this region. We conclude that diverse broadleaf forests are crucial to maintaining bee diversity by providing floral resources that support forest-dependent species even in closed canopy conditions. Conifer forests can also provide valuable habitat to bee pollinators when restored to open canopy conditions. However, because no traits are indicative of conifer forest dependency and bee species respond to understory flora rather than tree attributes, open conifer forests may more strongly favor habitat generalists than forest specialists.

Characterization of the bee community and pollination network in a southeastern U.S. pine savanna

“…For example, the flowers of many broadleaf tree genera (including Acer, Liriodendron, Quercus, etc.) are known to be visited by bees, and bee diversity has been shown to be positively correlated with flowering tree diversity (Traylor et al, 2024). It is likely that riparian forests and other corridors or patches of hardwood trees provide an additional benefit to bees within Coastal Plain landscapes.…”

“…Such resources can include the nectar or pollen produced by broadleaf trees or dead wood used by many species for nesting. There is growing evidence that eastern deciduous forests of the U.S. provide important floral resources to bees in the canopy (Urban-Mead et al, 2021, and recent work from southeastern forests suggests that forest bee diversity increases as the diversity of flowering trees increases (Traylor et al, 2024). By contrast, forest bee diversity within the Piedmont has been shown to decrease as the amount of pine in the surrounding landscape or as a proportion of local basal area increases (Traylor et al, 2024;Ulyshen et al, in press).…”

Local factors influence the wild bee functional community at the urban-forest interface

“…Traylor et al (2022) detected a positive correlation between bee diversity and flowering tree diversity in southeastern U.S. forests, suggesting that bees require compositionally diverse canopies. Further, many native bee species are detected in greater abundances in canopies dominated by insect-pollinated broadleaf species rather than conifer dominated canopies (Traylor et al, 2024). Wind-pollinated tree species such as oaks (Quercus spp.)…”