The importance of vertically stratified, multi-year sampling for understanding native bee assemblages in southeastern temperate deciduous forests

Abstract

Bees are highly effective pollinators facing declines. We need a better understanding of our existing native bee assemblages to preserve them. Recent studies have shown that bee assemblages are vertically stratified in forested habitats, but our understanding of how that translates to the southeastern United States is lacking. Most of our knowledge is restricted to how bees utilize the forest understory, potentially biasing monitoring efforts. We sampled and compared the bee assemblages of a temperate deciduous forest (Tuskegee National Forest, Tuskegee, Alabama, USA) in the canopy and understory from April-September 2021 and March-October 2022. Blue vane traps were deployed at four sites across Tuskegee National Forest with four replicates per site. Each replicate had a blue vane in the understory (1 m) and in the upper canopy (20 m average). In total, we captured 1,494 bees representing 37 taxa belonging to 15 genera and 5 families (Andrenidae, Apidae, Colletidae, Halictidae, and Megachilidae). Abundance varied seasonally and by year. There were just over two times as many bees in the canopy as in the understory. The four most commonly captured species (Lasioglossum bruneri, Melissodes bimaculatus, Augochlora pura, and Bombus impatiens) were present in significantly higher abundances in the canopy. We found no significant differences in species richness between the two strata but found that Simpson Diversity was significantly different. Interestingly, in 2021 Simpson Diversity was higher in the canopy, while in 2022 it was higher in the understory. Based on our findings, we suggest multi-year, multi-strata sampling when surveying for bees in forested environments.