Forest restoration treatments indirectly diversify pollination networks via floral- and temperature-mediated effects

Authors: DAVIES, CORA - Colorado State University; DAVIS, THOMAS - Colorado State University; Griswold, Terry

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Thinning programs have often been conducted in national forests without looking at the impact on their plant and animal communities. Such thinning has been done on forests in the Front Range region of Colorado. This study examines the effects on bee pollinators of this thinning. It used multiple methods (trapping and netting) to compare diversity and flower visitation by bees between stands that had been thinned 3-10 years ago with non-thinned stands. Three important results were: (1) Native bee abundance, richness, and diversity were all greater, in thinned stands. Bee visits to flowers were more frequent and by more kinds of bees. (2) Bee richness and abundance was greater when trees were farther apart. (3) Four species of flowers were most important for bee communities even though they were not the most abundant flowering plants. The results indicate that native bees and bee connections with flowers respond positively to thinning. So the conclusion is that thinning helps conserve native bees.

Technical Abstract: In North American conifer forests a variety of federally initiated thinning programs are implemented to restore pre-European settlement forest structures, but these changes may impact ecosystem function via impacts on sensitive biotic communities. Across the wildland-urban interface of the Front Range region of Colorado, agencies associated with the ‘Collaborative Forest Landscape Restoration Program’ (CFLRP) have implemented thinning treatments across thousands of hectares of ponderosa pine forest; here we leverage these treatments as an experimental framework to examine thinning effects on a pollinator community. We measured variation in forest structure and sampled bee community assemblages using multiple methods (trapping and netting) to compare bee biodiversity and patterns of floral visitation by bees (bee-flower networks) between mechanically thinned stands that were 3-10 years post-treatment and non-thinned stands. Three key findings emerged: (1) Native bee abundance, richness, and diversity were 120, 53, and 37% greater, respectively, in thinned stands. Nestedness, richness, and abundance of bee-flower interactions were all substantially higher in thinned stands, and there was increased functional redundancy in bee assemblages after thinning. (2) Structural equation modelling indicated that variation in temperature and floral abundance were mediated by canopy openness and were correlated with bee richness and abundance, thereby indirectly driving variation in bee-flower interactions. (3) Four floral species (Achillea millefolium, Cerastium arvense, Erysimum capitatum, and Heterotheca villosa) were identified as key connectors in bee-flower interaction networks, though these were not the most abundant flowering plants. Our analyses indicate that native bee a-diversity and bee-flower interactions positively responded to thinning treatments, and these effects were indirectly driven by canopy removal. We conclude that CFLRP treatments have conservation value for native bee communities. Further monitoring is warranted to evaluate the longevity of these effects.