Disturbance promotes native bee biodiversity in a southwestern ponderosa pine forest
Date
2021
Authors
Gelles, Ryleigh Victoria, author
Davis, Seth, advisor
Stevens-Rumann, Camille, committee member
Ode, Paul, committee member
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Abstract
Native bees are critical components of ecosystems where they provide an estimated 85% of pollination services. In recent decades, reports of global decline in bee populations have drawn concern from conservationists, compelling the need for further research on the drivers and mechanisms influencing the diminishment of native bee populations. In ponderosa pine ecosystems, land management tactics of the late 19th and early 20th century, particularly fire suppression policies, promoted dense stand structures with closed canopies, a suppressed understory, and increased surface fuel loadings. Forest restoration practices including thinning of stands and re-introduction of fire are utilized as a technique to restore historical ecosystem structures, and restoration goals in southwestern ponderosa pine forests may align with conservationists' goals of creating desirable habitat for bees by promoting resource patch connectivity, growth of understory floral species, and landscape heterogeneity. However, despite the widespread implementation of ecological restoration tactics in western forests, the effects of restoration and disturbance in general on native bee communities are not understood but could have important consequences for ecosystem function. To address this knowledge gap, the objectives of this thesis are: (1) describe the response of native bee communities to ecological disturbance, including wildfire, managed fire, and forest density reduction treatments, (2) identify structural components of ponderosa pine forests associated with site occupancy by native bees, and (3) inventory and describe the native bee fauna present in a common forest type of the Colorado Front Range. In two separate studies, native bee communities were sampled within lower-montane ponderosa pine forest systems along the Colorado Front Range over a 2-year period. Sites were representative of various wildfire severities (high and low) and forest management treatments (prescribed fire and mechanical thinning). I quantified bee α- and β-diversity and compared diversity metrics to variation in forest structure, foraging resources (floral abundance and richness), and nesting habitat (woody material). In total, 2,177 bee specimens were collected. Overall γ-diversity consisted of 5 families (Andrenidae, Apidae, Colletidae, Halictidae, and Megachilidae), 25 genera, and at least 57 species. Four main findings emerged: (1) bee species richness and diversity varied across disturbance types and were highest within 1-year post-fire and high-severity wildfire stands, (2) unique bee community compositions were associated with different disturbance types but also varied across the growing season, (3) treatment type (non-treated, thinned, or burned) was associated with differences in bee functional variation, especially nesting behavior, and (4) floral resource abundances and richness were associated with increased bee abundance, richness, and α-diversity, though stand basal area was negatively correlated with bee abundance and species richness. These collective findings have implications for forest management and indicate structural elements of ecosystems that can be managed for enhancing bee biodiversity. The data presented in this thesis provide evidence that fire-disturbed forest stands generally promote bee site occupancy compared to non-burned control stands, but this effect is likely to peak shortly after fires and then decline. In addition, distinct bee assemblages were found in stands that experienced different disturbances (e.g., thinned vs. burned vs. non-disturbed), indicating that a mosaic of disturbance histories is likely to support the greatest bee biodiversity at a landscape-scale. Further, findings here elucidate habitat structural components, specifically stand basal area and floral resource richness, that can be targeted by land managers to facilitate site occupancy by bees. Accordingly, I conclude that forest restoration practices including thinning and prescribed fire use, as well as natural fire disturbances, likely promote pollinator abundance and diversity (and ostensibly pollination services) in semi-arid ponderosa pine forests of the southwestern United States.
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Subject
forest management
ponderosa pine
native bees
disturbance