Bee diversity decreases rapidly with time since harvest in intensively managed conifer forests

Abstract

Despite widespread concerns about the anthropogenic drivers of global pollinator declines, little information is available about the impacts of land management practices on wild bees outside of agricultural systems, including in forests managed intensively for wood production. We assessed changes in wild bee communities with time since harvest in 60 intensively managed Douglas-fir (Pseudotsuga menziesii) stands across a gradient in stand ages spanning a typical harvest rotation. We measured bee abundance, species richness, and alpha and beta diversity, as well as habitat characteristics (i.e., floral resources, nesting substrates, understory vegetation, and early seral forest in the surrounding landscape) during the spring and summer of 2018 and 2019.We found that bee abundance and species richness declined rapidly with stand age, decreasing by 61% and 48%, respectively, for every 5 years since timber harvest. Asymptotic estimates of Shannon and Simpson diversity were highest in stands 6-10 years post-harvest and lowest after the forest canopy had closed, ~11 years post-harvest. Bee communities in older stands were nested subsets of bee communities found in younger stands, indicating that changes were due to species loss rather than turnover as the stands aged. Bee abundance-but not species richness-was positively associated with floral resource density, and neither metric was associated with floral richness. The amount of early seral forest in the surrounding landscape seemed to enhance bee species richness in older, closed-canopy stands, but otherwise had little effect. Changes in the relative abundance of bee species did not relate to bee functional characteristics such as sociality, diet breadth, or nesting substrate. Our study demonstrates that Douglas-fir plantations develop diverse communities of wild bees shortly after harvest, but those communities erode rapidly over time as forest canopies close. Therefore, stand-scale management activities that prolong the precanopy closure period and enhance floral resources during the initial stage of stand regeneration will provide the greatest opportunity to enhance bee diversity in landscapes dominated by intensively managed conifer forests.K E Y W O R D S bees, early seral forest, forest biodiversity, intensive forest management, pollinator, timber plantation

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

“…Recent studies from a variety of systems (e.g., Breland et al, 2018; Campbell et al, 2018; Coulin et al, 2019; Gelles et al, 2022; Odanaka et al, 2019; Zitomer et al, 2023, present study) collectively converge on the finding that thinning of forest stands increases bee richness. In the present study, nestedness was much higher in thinned stands and deviated from null model predictions; nestedness in nonthinned stands was not different from that predicted by null model distributions, indicating more robust bee–flower networks in areas treated under FR‐CFLRP guidelines.…”

“…Forest thinning via creation of canopy openings and bare ground is a potential strategy for the conservation of forest bee communities across landscapes. A growing body of evidence, including results presented in this study, show that certain disturbances (either anthropomorphic disturbances, such as thinning, or nonmanaged disturbances, such as wildfire) can improve both pollinator habitats and plant–pollinator networks (e.g., Davis et al, 2020; Foote et al, 2020; Gelles et al, 2022; Kaiser‐Bunbury et al, 2017; Rodríguez & Kouki, 2015; Zitomer et al, 2023). These benefits are likely to diminish as canopies regrow and subsequently close (Zitomer et al, 2023); however, the longevity of thinning‐mediated increases in forest bee biodiversity warrant further investigation as effects may vary across bioclimatic regions and forest types.…”

“…A growing body of evidence, including results presented in this study, show that certain disturbances (either anthropomorphic disturbances, such as thinning, or nonmanaged disturbances, such as wildfire) can improve both pollinator habitats and plant–pollinator networks (e.g., Davis et al, 2020; Foote et al, 2020; Gelles et al, 2022; Kaiser‐Bunbury et al, 2017; Rodríguez & Kouki, 2015; Zitomer et al, 2023). These benefits are likely to diminish as canopies regrow and subsequently close (Zitomer et al, 2023); however, the longevity of thinning‐mediated increases in forest bee biodiversity warrant further investigation as effects may vary across bioclimatic regions and forest types. We conclude that forest restoration efforts implemented via the CFLRP program in the Front Range region are consistent with conditions that support bee biodiversity and robust bee–flower interactions, with enhanced network structure (nestedness) and functional redundancy of bee assemblages in post‐thinning landscapes.…”

“…Other authors have suggested that thinning‐mediated changes in abiotic conditions are a primary driver of increased bee diversity in post‐thinning stands (Zitomer et al, 2023) and may be related to visual attractiveness of flowers in stands with low tree density (Coulin et al, 2019). Here, the effects of variation in floral assemblages had a larger effect size on bee diversity than thermal conditions, although both factors were statistically significant.…”

Effective dispersal patterns in prairie plant species across human‐modified landscapes

“…Plectritis congesta is a generalist and is visited by a variety of bee and butterfly species (Young-Matthews, 2012), and dispersal is likely determined by the foraging and habitat preferences of visiting pollinators, some of which can travel tens of kilometres (Goulson, 2010). A recent study by Zitomer et al (2023) found wild bee abundance and diversity in Oregon decreases with stand age of forested areas, and the widespread anthropogenic disruption in our study region's forests may be exposing P. congesta populations to more diverse pollinator communities. In addition, bumble bees, an effective pollinator for P. congesta, frequently use forested areas for forage and nesting (Mola et al, 2021), and proximity to these areas may have increased bee visitation rates, causing regions with natural spaces and mid tree canopy coverage to experience higher gene flow rates.…”