Outbreaks of native bark beetles are key natural disturbances that shape the structure and function of conifer forests across the northern hemisphere. Bark beetle outbreaks structure forests directly via mortality of overstory trees and also indirectly by releasing the growth of understory trees as light, water, and nutrient availability all increase for survivors. This compensatory response of trees that survive beetle outbreaks can preserve ecosystem function at the stand scale. Population aggregate or average growth releases of surviving trees are often used to detect the occurrence and effects of past outbreaks. Within a stand, however, processes governing the magnitude of individual tree growth releases following beetle outbreak are less well understood. Here we use a spatially explicit long-term monitoring dataset of a lodgepole pine (Pinus contorta var. latifolia) forest (>9,000 individually mapped trees in three 2-hectare plots) impacted by a severe mountain pine beetle (Dendroctonus ponderosae) outbreak to explore interactions among fine scale drivers of post-outbreak growth releases (quantified as difference between pre- and post-outbreak mean annual growth increment). Using a Bayesian spatial modeling approach, we evaluated how tree-scale and tree neighborhood-scale characteristics interact to mediate compensatory responses following mountain pine beetle outbreak in the Southern Rocky Mountains.
Results/Conclusions
Overall, growth releases of surviving non-host trees [Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa)] were greater in magnitude than growth releases of surviving host trees (lodgepole pine) following the outbreak. Tree neighborhood characteristics (pre-outbreak live tree density and beetle-caused mortality within a 10 meter radius of the focal tree) mediated the magnitude of growth release, and the direction and magnitude of these mediating effects varied with tree size. Tree-scale growth release decreased with pre-outbreak neighborhood live tree density and increased with proportion of neighborhood density killed by mountain pine beetle. These effects were detected for small to medium diameter surviving trees (~10 to 25 cm diameter) but were not detected for large surviving trees (>30 cm diameter). Within-stand topographic variability was not an important predictor of growth release. By explicitly considering how within-stand spatial pattern mediates individual tree scale compensatory responses following bark beetle outbreak, our findings bridge an important gap in understanding how spatial heterogeneity affects resilience to disturbance.