Season, habitat characteristics, and forest thinning shape bee and blooming plant communities in an inland PNW forest

Forested landscapes of the United States are increasingly recognized as important habitats for native bees; however, not all forested regions have been well-studied. One understudied area is the inland Pacific Northwest (PNW), United States. Our study objectives were to: 1) describe bee and blooming plant communities in an inland PNW forest, and 2) evaluate how fuels-reduction thinning affects those communities. To address our study goals, we conducted a before-after, control-impact (BACI) experiment with 12 paired treatment (thinning) and control sites; we sampled in spring and summer for two years before and two years after treatment. We evaluated the effects of spatiotemporal variation and habitat characteristics on bee and blooming plant communities in the first two years and effects of thinning, using data from all four years, with a BACI analysis. We collected 10,197 bees from 156 species or morphospecies and counted 19,392 blooms on 101 flowering species across the study years. We found that season and forest type (cool-moist versus warm-dry) significantly influenced community composition of bee and bloom communities. Several habitat variables (e.g., heat load index, woody debris cover) were positively correlated with bee abundance. Bee abundance and diversity increased in response to thinning, but the effect was partially dependent on forest type. By contrast, bloom communities were not significantly affected by thinning. Our results show that inland PNW forests harbor species-rich bee communities and that fuels-reduction thinning, designed to ameliorate wildfire risk, may also improve bee habitat in these forests.