Predictors of bark beetle activity and scale-dependent spatial heterogeneity change during the course of an outbreak in a subalpine forest

Abstract

Climate conditions and forest structure interact to determine the extent and severity of bark beetle outbreaks, yet the relative importance of each may vary though the course of an outbreak. In 2008, we conducted field surveys and reconstructed forest conditions at multiple stages within a recent mountain pine beetle (MPB) outbreak in Rocky Mountain National Park, Colorado. At each stage in the outbreak, we examined changes in (1) lodgepole pine mortality and surviving stand structure, (2) the influence of topographic versus stand structure variables on mortality rates, and (3) stand complexity and landscape heterogeneity. Lodgepole pine mortality reduced basal area by 71 %, but only 47 % of stems were killed. Relative to pre-outbreak stands, surviving stands had lower mean dbh (11.0 vs. 17.4 cm), lower basal area (8.5 vs. 29.3 m2 ha−1), lower density (915 vs. 1,393 stems ha−1), and higher proportions of non-host species (23.1 vs. 10.6 % m2 ha−1). Factors predicting mortality rates changed through the course of the outbreak. Tree mortality during the early stage of the outbreak was associated with warm, dry sites and abundant large trees. During the middle and late stages, mortality was associated with stand structure alone. Stand complexity increased, as defined by stand-scale variability in density, basal area, and the proportion of susceptible trees. Landscape heterogeneity decreased according to semi-variograms of tree diameter and basal area. Increased stand complexity may inhibit future MPB population development, but decreased landscape heterogeneity may facilitate outbreak spread across the landscape if a future outbreak were to irrupt.