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

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 (Penstemon virens, Cerastium arvense, Erysimum capitatum, and
Geranium caespitosum) were identified as key connectors in bee-flower
interaction networks, though these were not necessarily the most abundant
flowering plants. Our analyses indicate that native bee α-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.