COS 238-4 - Does industrial timber harvest act as a fire-analogue for wild bee populations in early seral forests?

Abstract: Understanding the degree to which intensive forestry supports biodiversity can inform whether practices promoting complex habitat structure and composition are necessary to improve conservation outcomes of timber production. The early seral stage of forest succession immediately following stand-replacing disturbance can support insect pollinator populations, especially in productive landscapes dominated by closed-canopy forests. Clearcut timber harvest is an anthropogenic disturbance that generates early seral forest conditions, and therefore may be important for supporting forest pollinator biodiversity where other disturbance agents have been suppressed, such as wildfire. However, timber harvest and post-harvest management practices may alter the characteristics of regenerating stands relative to early seral areas generated by natural disturbances. In this study, we sought to answer the question: to what degree do intensively managed regenerating forests support the wild bee assemblages of stands regenerating after stand-replacing wildfire?

We conducted a large-scale mensurative experiment in southwest Oregon, comparing bee communities across a chronosequence of early stand development (2-20 years) following stand-replacing wildfire, post-fire salvage logging, and intensively managed clearcut harvest. We sampled bee communities using blue vane traps in late spring (late May – early June) and mid-summer (late July – early August) and evaluated community composition, diversity, and abundance. We captured nearly 9,800 bees in regenerating forests, representing 140 species. For the most recently disturbed stands (2-5 years), average bee abundance was 69% higher in intensively managed stands than fire-generated early seral forests in spring samples, and 119% higher in summer samples. However, bee abundance declined more rapidly with stand age in intensively managed stands. Rarefied estimates of bee species richness and diversity showed no clear differences between early seral stands generated by wildfire or intensively managed clearcuts, but salvage-logged stands had, on average, 46% lower species richness and 43% lower diversity than similar-aged stands that were not salvage-logged. Community composition varied among disturbance agents, and life history characteristics were important predictors of this variability. Specifically, solitary species and species that nest aboveground were more strongly associated with fire-generated early seral forests than intensively managed or salvage-logged stands. Despite high bee abundance and diversity in some intensive harvest-generated early seral, differences in species composition suggest that they do not fully support the bee assemblages associated with fire-generated early seral in southwest Oregon. Therefore, practices that focus on retention or regeneration of aboveground nesting substrates may improve conservation outcomes for forest pollinators in managed landscapes.