Insect pollinators, especially bees, are an essential component ecosystem function. Native bees provide key ecosystem services and shape the structure and composition of plant communities. However, recent research suggests a large-scale decline in bee populations, compelling the need for further research of the drivers and mechanisms influencing this decline. Within ponderosa pine ecosystems, fire suppression policies in the late 19th and early 20th century have led to the growth of dense stands with closed canopies and low understory production-this forest structure is widely considered undesirable for a variety of reasons. One approach to restoring the historic structure of these forests is to re-introduce fire disturbances to the landscape. Although the effects of managed or ‘prescribed’ fire on vegetation structure and composition are well-studied, relatively few studies have investigated whether prescribed fires have cascading effects on ecological communities important to ecosystem function, including native bees.

To address this knowledge gap, blue vane traps were used to sample native bee community assemblages across the growing season in ponderosa pine-forest sites in northern Colorado to evaluate the effects of prescribed fire restoration treatments, and time since treatment (1-yr post-fire, 3-yrs post-fire. non-treated controls), on bee populations. We quantified bee abundance, richness, and diversity as well as foraging resources (floral abundance and richness) and nesting habitat (coarse woody debris). From this, 5 key findings emerged: (1) Overall γ-diversity consisted of 5 families (Andrenidae, Apidae, Colletidae, Halictidae, and Megachilidae), 25 genera, and at least 58 species. Predominant genera include bumblebees (Bombus sp.), mason bees (Osmia spp.), and digger bees (Anthophora spp.) which accounted for 61.4%, 13.9%, and 8% of collected specimens, respectively. (2) Pooled bee abundances varied across the season, with highest captures occurring early in the growing season; bee species richness and α-diversity varied across treatment type and were highest within 1-year post-fire stands. (3) Unique bee community compositions were associated with different treatment types but also varied across the season. (4) Treatment type and seasonality were associated with differences in bee nesting habit. (5) Floral resource abundances and richness were associated with increased bee abundances, richness, and diversity, though stand basal area was negatively correlated with bee abundance and species richness.

Results here provide evidence that fire-disturbed forest stands generally promote bee site occupancy, but this effect is likely to peak shortly after fires and then decline. In addition, distinct bee assemblages were found in stands that were in varying states of time-since-fire, indicating that a mosaic of treatments likely support the greatest bee biodiversity at a landscape-scale. Further, findings here elucidate habitat structural components, specifically stand basal area and floral resource richness, that can be targeted by land managers to facilitate bee site occupancy. With this, we conclude the use of prescribed fire as a forest restoration method likely promote pollinator abundance and diversity in semi-arid ponderosa pine forests of the southwestern United States.