Modeling individual lodgepole pine mortality from mountain pine beetle outbreak in a spatially explicit framework

Abstract

Outbreaks of native bark beetles (Curculionidae: Scolytinae) are key natural disturbances that shape the structure and function of conifer forests across the northern hemisphere. While drivers of bark beetle outbreaks have been studied extensively at spatial scales ranging from stands to continents, within-stand processes driving individual tree mortality in an outbreak are less well understood. Here we use a spatially explicit long-term monitoring dataset of lodgepole pine (Pinus contorta var. latifolia) forest impacted by a severe mountain pine beetle (Dendroctonus ponderosae) outbreak to explore interactions among fine-scale drivers of beetle-caused tree mortality. Using a hierarchical Bayesian spatial modeling approach, we evaluated whether and how within-stand neighborhood structure and topographic setting interact with tree size to mediate tree level susceptibility to mountain pine beetle outbreak in the Southern Rocky Mountains (USA). We found evidence that both tree growth rate preceding the outbreak and stand structure around the host tree mediated the effect of tree size. However, we did not find evidence that topographic position within a stand mediated the effect of tree size. Mortality probability increased with pre-outbreak growth rate for small to medium sized trees (~10-25 centimeters diameter), but that same effect could not be detected for larger trees. Conversely, mortality probability increased with greater neighborhood density across tree sizes, with the most pronounced effects for medium to large sized trees (~15-30 centimeters diameter). Within-stand topographic variability was not an important predictor of mortality probability; among stands, however, the driest stand conditions experienced the greatest overall mortality. By explicitly considering how natural within-stand heterogeneity mediates individual tree level susceptibility to mountain pine beetle outbreak, our findings bridge an important gap in understanding multi-scale drivers of disturbance dynamics. Identifying factors influencing individual tree mortality in these systems informs our understanding of both the structural development of forest stands and reciprocal feedbacks between stand structure and outbreak dynamics.