Mid-Seral Thinned Stands in Pacific Northwest Coastal Forests Show Enhanced Floral and Bee Diversity without Increased Parasite Prevalence in Wild Bees

Abstract

Management practices, such as forest thinning, may support bee communities by increasing the abundance and diversity of floral resources in forest ecosystems. However, these enhancements could also concentrate foraging bees, potentially facilitating disease spread. In this study, we surveyed understory plant and bee communities and the prevalence of bee parasites across forest stands of different ages and management histories in the coastal Pacific Northwest, including early seral stands, two size classes of mid-seral thinned stands, and mature unthinned stands. We found that both categories of thinned stands supported levels of flowering plant diversity and abundance and overall bee abundance comparable to those found in early seral stands, although bee diversity was highest in early seral stands. In contrast, mature stands exhibited substantially less abundant and diverse plant-pollinator communities than early seral stands. Though we expected that thinned stands would primarily affect flowering plants and bees through reduced canopy cover, within thinned stands increased canopy openness was only associated with a modest increase in floral abundance. We did not observe a strong influence of canopy openness on floral diversity or bee diversity and abundance. The prevalence of the common parasite genus Crithidia spp. was not strongly related to bee or floral community characteristics but was negatively related to bumble bee foraging distance capacity. These findings suggest that thinned forests have enhanced forage opportunities for bees but do not exacerbate the spread of disease—offering a promising management strategy for sustaining healthy pollinator populations in restored forest ecosystems.

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