Data from: Bee diversity decreases rapidly with time since harvest in intensively managed conifer forests

Despite widespread concern about the anthropogenic drivers of global
pollinator declines, little information is available about the impacts of
land management practices on wild bees outside of agricultural systems,
including forests managed intensively for wood production. We assessed
changes in wild bee communities with time since harvest in 60 intensively
managed Douglas-fir (Pseudotsuga menziesii) stands across a gradient in
stand ages spanning a typical harvest rotation. We measured bee abundance,
species richness, and alpha and beta diversity, as well as habitat
characteristics (i.e., floral resources, nesting substrates, understory
vegetation, and early seral forest in the surrounding landscape) during
the spring and summer in 2018 and 2019. We found that bee abundance and
richness declined rapidly with stand age, decreasing by 61% and 48%,
respectively, for every five years since timber harvest. Asymptotic
estimates of Shannon and Simpson diversity were highest in stands 6-10 y
post-harvest and lowest after the forest canopy had closed, approximately
11 y post-harvest. Bee communities in older stands were nested subsets of
bee communities found in younger stands, indicating changes were due to
species loss, rather than turnover, as stands aged. Bee abundance – but
not richness – was positively associated with floral resource density, and
neither metric was associated with floral richness. The amount of early
seral forest in the surrounding landscape seemed to enhance bee species
richness in older, closed-canopy stands, but otherwise had little effect.
Changes in the relative abundance of bee species did not relate to bee
functional characteristics such as sociality, diet breadth, or nesting
substrate. Our study demonstrates that Douglas-fir plantations develop
diverse communities of wild bees shortly after harvest, but those
communities erode rapidly over time as forest canopies close. Therefore,
stand-scale management activities that prolong the pre-canopy closure
period and enhance floral density during the initial stage of stand
regeneration will provide the greatest opportunity to enhance bee
diversity in landscapes dominated by intensively managed conifer forests.