In North American conifer forests, thinning operations are broadly implemented as a means of fire hazard mitigation, ecological restoration, and timber harvest. Effects of thinning on forest bee communities are poorly understood but could be important for conservation of biodiversity and ecosystem services. Here, we test the hypothesis that thinned forest stands have greater diversity of native bee species than non-treated forests. To address this, native bee assemblages were collected across the growing season and compared between ponderosa pine stands treated by mechanical thinning and non-treated stands. Associations between native bee communities and forest conditions were analyzed. Forest structure, floral resources, nesting habitat, and bee assemblages differed between treated and non-treated stands. Forest basal area at non-treated sites was on average 3.5 times greater than treated sites, and canopy openness was greater at treated sites. Fuel loads were similar between treated and non-treated sites. Floral resources were >2.5 times more abundant at treated sites; floral abundance was highest in June and decreased throughout the summer. Native bees were two times more abundant in treated stands. Our results suggest that (1) forest thinning has significant impacts on both floral resources and bee nesting habitats within 2-8 years post-treatment; (2) bee assemblages likely respond to this variation, and this difference is especially apparent later in the growing season. We conclude that forest thinning for ecological restoration in ponderosa pine habitats is likely to improve resources utilized by native bees and are associated with increased bee abundances in the wildland-urban interface.
