PS 17-244 - Let it bee? Responses of flowering plant and native bee communities to passive restoration in Pacific Northwest grasslands

Abstract: More than 99% of North American grasslands have been converted to agricultural and other land uses since European settlement. However, many lands that were formerly farmed have been abandoned, especially in areas where climate and soil conditions make crop production impractical. As agricultural land abandonment has increased exponentially across North America since the late 1800s, there is heightened interest in restoring these grasslands to more native states for the purpose of enhancing biodiversity and ecosystem services. Approaches for restoring native grasslands can include passive restoration (i.e., removing the disturbance from the system and waiting for native species to re-colonize) vs active restoration (i.e., removing the disturbance and actively planting native species). Passive restoration is less expensive and time-consuming, but it is unclear whether passive approaches are effective in restoring native biodiversity, including plant and bee species, especially in arid and semi-arid systems. To address this question, we examined passively restored grasslands located within relatively intact interior Pacific Northwest bunchgrass prairie. Our specific objective was to compare the abundance, richness, diversity, and community composition of blooming plant and bee communities in intact native bunchgrass prairie to passively restored grasslands. Our study was conducted at The Nature Conservancy’s Zumwalt Prairie Preserve in northeastern Oregon. We counted blooming stems and sampled bees with 15 pan traps at eight sites in each of the grassland types three times during the growing season over two years. We counted 35,568 blooming stems of 94 forb species and collected 7,980 individuals of 130 bee species. Native prairie had higher abundance, richness, and diversity of blooming plants than passively restored sites, and blooming plant community composition differed significantly between the two types of sites. However, the two types of sites did not differ consistently in any of the bee variables (abundance, species richness, diversity, community composition). This work adds to growing evidence that passive restoration of formerly cultivated grasslands in the inland Pacific Northwest may not be sufficient to recover flowering forb communities in a timeframe relevant to most conservation goals. The lack of bee response to decreased floral resources in passively restored sites may indicate that large tracts of intact prairie adjacent to formerly cultivated grasslands may compensate for decreased floral resources associated with those sites.