Bark beetle outbreaks are major natural disturbances in northern hemisphere conifer forests, causing extensive tree mortality and altering forest structure, composition, and function. Recent widespread and severe outbreaks have raised concern about forest resilience following outbreaks, and potential interactions with other disturbances such as fire. Thinning (i.e., density reduction) treatments may promote resilience to outbreaks and reduce post-outbreak fire hazard by altering stand structure and composition, but may also present a tradeoff with woody carbon storage. Effects of post-outbreak harvest on future forest resilience and interaction with fire are well studied, but knowledge gaps remain concerning effects of pre-outbreak thinning on forest response to beetle outbreaks, and duration of potential effects relative to the typical outbreak disturbance interval (~40-80 years). We measured stand structure and fuel profiles in lodgepole pine (Pinus contorta var. latifolia) forests thinned in 1940 that later experienced a severe mountain pine beetle (Dendroctonus ponderosae, MPB) outbreak from 2004-2008 to ask: (1) how do historical stand-thinning treatments influence components of resilience to outbreaks (resistance to MPB and post-outbreak successional trajectories), and (2) what are the associated tradeoffs with post-outbreak fire hazard and carbon storage? Thinning treatments spanned five intensity levels ranging from uncut (i.e., control) to clearcut.
Results/Conclusions
Effects of thinning on components of resilience to MPB outbreak differed across spatial scales. At the tree scale, thinning had no effect on resistance (probability of individual tree survival); tree size drove survival, regardless of stand-context. At the stand scale, thinning had a positive effect on resistance (proportion of surviving stem density and basal area), but only for the heaviest thinning treatments. Pre-outbreak differences in live stand structure among treatments were indistinguishable post-outbreak, but underlying drivers that erased these differences varied: thinned plots were dominated by density-dependent mortality of small size classes, whereas control plots were dominated by MPB-induced mortality of larger trees. However, thinning effectively “reset” post-outbreak successional trajectories in understory composition (lower proportions of later-seral spruce-fir saplings compared to control plots). Thinning altered post-outbreak surface fuel profiles, but only for coarse fuels. Compared to the control units, thinned plots had lower sound coarse fuels. All treatments had similar total post-outbreak basal area (preliminary proxy for biomass carbon), but thinned plots had greater live and correspondingly less dead basal area compared to the control plots. Our findings have important implications for consideration of tradeoffs in forest management decisions in the face of climate-mediated changes in disturbances.