Abstract
Landscape-scale bark beetle outbreaks alter forest structure with direct and indirect effects on plants and animals in forest ecosystems. Using alpine spruce forest and a native bee community as a study system, we tested how tree mortality from bark beetles impacts bee foraging habitats and populations. Bees were collected across the growing season (early-, middle-, and late-season) for two years using passive trapping methods, and collections were used to analyze patterns in species abundances and diversity. Three important findings emerged: (1) forest stands that were post-outbreak had 62% higher floral density and 68% more floral species during peak bloom, respectively, than non-affected stands; (2) bee captures were highest early-season (June) and were not strongly affected by bark beetle outbreak; however, mean number of bee species and Shannon–Weiner diversity were significantly higher in post-outbreak stands and this effect was pronounced early in the growing season. Corresponding analysis of β-diversity indicated higher accumulation of bee biodiversity in post-outbreak stands and a turnover in the ratio of Bombus: Osmia; (3) bee captures were linked to variation in foraging habitat, but number of bee species and diversity were more strongly predicted by forest structure. Our results provide evidence of increased alpine bee biodiversity in post-outbreak stands and increased availability of floral resources. We conclude that large-scale disturbance from bark beetle outbreaks may drive shifts in pollinator community composition through cascading effects on floral resources, mediated via mortality of overstory trees.
Highlights
Bark beetle outbreaks are a central factor driving large-scale changes in structure, function, and composition of forest landscapes in western North America and in temperate conifer forests g lobally[1,2,3]
Our results suggest that shifts in forest structural characteristics from bark beetle outbreaks in high-elevation spruce-fir forests have impacts on both bee foraging habitats (Fig. 4) and bee species assemblages, and we report evidence that significant overstory tree mortality is likely to result in an overall increase in bee captures and changes in both Shannon-Weiner diversity and β-diversity in post-outbreak stands (Figs. 5, 6, 7, 8)
Mortality of forest basal area from spruce beetle was positively correlated with floral abundance and number of floral species, and understory forb responses to canopy gaps created by overstory tree mortality in post-outbreak stands is likely a mechanism by which bee communities respond to bark beetle outbreaks (Figs. 1, 3, 4)
Summary
Bark beetle outbreaks are a central factor driving large-scale changes in structure, function, and composition of forest landscapes in western North America and in temperate conifer forests g lobally[1,2,3]. In the southern Rocky Mountain region extensive forest mortality has occurred during the past decade in high-elevation alpine Engelmann spruce (Picea engelmannii Parry ex Engelm.) stands due to a widespread outbreak of North American spruce beetle (Dendroctonus rufipennis Kirby), an aggressive phloem-feeding bark beetle species that colonizes live host trees and is a primary agent of tree death when population densities are high. A recent regional study indicates that alpine spruce forest harbors a relatively unique community of native bees with many high-elevation specialists that can maintain viable populations even in extreme h abitats[9]. We test the hypothesis that spruce beetle outbreak-driven variation in forest structure affects floral resources and native bee assemblages in a southern Rocky Mountain landscape (Fig. 1). Our studies link mortality of a dominant forest tree species from a widespread biotic agent of ecosystem disturbance to alpine bee biodiversity, with consequences for the interpretation of key forest disturbance processes and the conservation of ecosystem services
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