COS 88-3 - Effects of site characteristics, native ungulates and forest thinning on bee and blooming plant communities

Abstract: Insects are among the most diverse creatures on Earth and are vital to a wide range of ecosystem functions such as pollination. Within most North American rangelands and forests, native bees are by far the most important and speciose group of pollinators. Native bees and blooming plant communities can be influenced by a wide range of abiotic factors (e.g., solar radiation), biotic interactions (e.g., ungulate herbivory), and human-mediated processes (e.g., forest thinning). However, the ways in which communities respond to these processes is not well understood. Our objectives were to determine how 1) habitat characteristics, 2) fuels-reduction thinning, and 3) ungulate herbivory influences bee and plant communities. To address these objectives, we sampled 24 sites in eastern Oregon forests in May and June, from 2020 to 2022. We used a two-by-two factorial design with forest type (warm dry or cool moist) and elk density (high or low) as factors. To evaluate the effects of forest thinning, we used a before-after (two years pre-thin, one year post-thin), control-impact (thinned or un-thinned) design with half of our sites thinned in 2021 after sampling and half our sites left un-thinned. We collected bees in vane traps and pan traps, counted blooming stems along transects and measured site characteristics including canopy density, solar radiation, ground cover, slope, aspect, and elevation. Since elk density can vary tremendously, even within a treatment group, we used GPS collar data to estimate elk density immediately within and around our sites. Over the three years of sampling, we identified and counted 30,511 blooming stems and collected 6,015 native bees. We found that bee and blooming plant communities were strongly, significantly, shaped by sample month within years. We found that site characteristics such as canopy density and variability of canopy density influenced blooming stem abundance. While we did not find that categorical elk density significantly affected bee or bloom communities, we found that site specific elk density was positively correlated with blooming plant diversity in the following year, indicating that elk herbivory plays a role in moderating plant diversity over time, with a one-year lag effect observed in our study system. Finally, there was some indication, though non-significant, that communities differed between thinned and control sites following experimental thinning but did not differ before. Our results indicate that human management actions (e.g., forest thinning, ungulate population control) can influence bee and plant communities and may interact with abiotic site characteristics.