Pollinator dynamics within intensively managed early seral conifer forests: Bee community diversity is driven by time since harvest in Douglas-fire plantations

Despite widespread concern about the anthropogenic drivers of pollinator declines, little is known about the impacts of land management practices on wild bees outside of agricultural systems, particularly for forests managed intensively for wood production. We assessed bee communities and habitat characteristics (i.e., floral and nesting resources, understory vegetation, and early seral forest in the surrounding landscape) across a chronosequence of 60 intensively managed Douglas-fir (Pseudotsuga menziesii) stands spanning a typical harvest rotation in 2018 and 2019. We found that bee abundance and richness declined rapidly with stand age and both observed and estimated diversity measures were lowest in stands >11 y post-harvest (the mean age of canopy closure). Changes in bee community composition across the stand age gradient were driven by species loss rather than turnover, and did not relate to species sociality, diet breadth, or nesting biology. Bee abundance–but not richness–was positively associated with floral resource density, and neither was associated with floral richness. 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. Our study demonstrates that Douglas-fir plantations develop diverse wild bee communities shortly after harvest, but those communities erode rapidly as forest canopies close. Therefore, stand-scale management activities that prolong the pre-canopy closure period and enhance floral density during this stage of stand regeneration will provide the greatest opportunity to enhance bee diversity in intensively managed conifer forests.