2018 ESA Annual Meeting (August 5 -- 10)

COS 77-3 - Herbivory slows forest recovery following bark beetle outbreaks

Wednesday, August 8, 2018: 2:10 PM
333-334, New Orleans Ernest N. Morial Convention Center
Robert A. Andrus, Department of Geography, University of Colorado, Boulder, CO, Thomas T Veblen, Geography, University of Colorado, CO and Sarah J. Hart, School of the Environment, Washington State University, Pullman, WA
Background/Question/Methods

In western North America, recent, widespread tree mortality from bark beetle outbreaks followed by potentially unfavorable environmental conditions (e.g. climate warming) for recovery has prompted questions about forest resilience, particularly where multiple disturbances overlap. Subalpine coniferous forests in the southern Rocky Mountains, dominated by Engelmann spruce (Picea engelmannii) and interspersed with subalpine fir (Abies lasiocarpa) and aspen (Populous tremuloides), exemplify such recent massive mortality. From 2003 to present, outbreaks of spruce beetles (Dendroctonus rufipennis) killed > 90% of Engelmann spruce in many stands in the San Juan Mountains of southern Colorado. Concurrently, subalpine fir decline–caused by the combined effects of western balsam bark beetle (Dryocoetes confusus) and root disease (Armillaria spp.)–led to mortality of subalpine fir across > 15,000 ha of the spruce beetle affected area. In stands where bark beetle outbreaks were no longer active, we collected extensive field data on the pre- and post-beetle outbreak stand structure and composition and assessed the effect of ungulate herbivory on seedlings to determine whether post-disturbance vegetation trajectories will promote a return to pre-disturbance stand conditions (i.e. resilience) or whether the ecosystem is vulnerable to a potential shift in community type.

Results/Conclusions

Across the study area, spruce beetles caused widespread declines in live basal area and stand density, while subalpine fir decline resulted in more heterogeneous patterns of mortality. Pure stands of Engelmann spruce with high-levels of mortality typically have very low densities of live trees and exhibit a strong understory response (e.g. shrub) to the outbreak, suggesting a tendency to convert to a non-forest community. In mixed-species stands, spruce beetle outbreaks led to a shift in tree species dominance from spruce to fir and/or aspen. When stands were also affected by subalpine fir decline, relative dominance was balanced between spruce and fir or shifted to aspen dominance. Greater densities of surviving trees in mixed-species stands suggests that they will likely recover to a forest-dominated state more quickly than pure spruce stands. However, persistent ungulate herbivory will likely slow recovery by reducing the release of seedlings from the seedling bank into the main canopy. Fir and aspen seedlings are more abundant than spruce in the seedling bank, but ungulate preference for these species may counteract this perceived advantage and may strongly regulate the future density and composition of forests.