Bees in the trees: Diverse spring fauna in temperate forest edge canopies

Abstract

Temperate hardwood deciduous forest is the dominant landcover in the Northeastern US, yet its canopy is usually ignored as pollinator habitat due to the abundance of wind-pollinated trees. We describe the vertical stratification of spring bee communities in this habitat and explore associations with bee traits, canopy cover, and coarse woody debris. For three years, we sampled second-growth woodlots and apple orchard-adjacent forest sites from late March to early June every 7–10 days with paired sets of tri-colored pan traps in the canopy (20–25 m above ground) and understory (<1m). Roughly one fifth of the known New York state bee fauna were caught at each height, and 90 of 417 species overall, with many species shared across the strata. We found equal species richness, higher diversity, and a much higher proportion of female bees in the canopy compared to the understory. Female solitary, social, soil- and wood-nesting bees were all abundant in the canopy while soil-nesting and solitary bees of both sexes dominated the understory. Canopy cover increased with leaf-out, and was negatively associated with understory but not canopy bee abundance. Site-level volume of coarse woody debris (CWD) did not predict bee abundance, while approximated landscape-scale availability of woody debris was positively correlated with understory wood-nesting and solitary-bee abundance. This work expands our understanding of habitats where bees are likely foraging and reveals vertically stratified behavior. We emphasize deciduous forests as an important habitat for wild bee conservation and recommend further research into the behavior and diets of bees occupying the canopy, speculating that females forage for anemophilous tree pollen. Forest management plans that conserve above-ground deadwood may provide nest sites for wood-nesting bees.