Forest composition change after a mountain pine beetle outbreak, Rocky Mountain National Park, CO, USA

• The recent MPB outbreak has created more diverse forest stands.
• These stands have higher proportions of Engelmann spruce and subalpine fir.
• Disturbances may not increase the rate of vegetation adaptation to climate change.

Recent severe and extensive mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have created novel conditions in Southern Rocky Mountain lodgepole pine forests which historically had disturbance regimes dominated by extensive, stand-replacing fires. The goal of this study is to investigate patterns of and potential mechanisms in post-outbreak forest change in order to better understand the ecological legacy of the recent outbreak in the context of its implications for resilience to future disturbances and adaptation to climate change. To this end, we collected field data on forest structure and species composition in 2012 in lodgepole pine dominant forests in Rocky Mountain National Park. We then used a combination of modeling and statistical methods to identify possible mechanisms in post-outbreak forest conditions and evaluate the effect of the MPB outbreak on forest heterogeneity. We found that the outbreak initiated a shift in forest structure from single-cohort lodgepole pine stands to stands with greater diversity in age classes and species composition. This increase in landscape asynchrony may increase resiliency to future disturbances. However, this heterogeneity is a result of more spruce and fir on the landscape, species which are less adapted to projected future climate conditions. Our results indicate that disturbances do not necessarily increase the rate at which vegetation adapts to a changing climate, and that it is essential to consider disturbance type and available seed sources when predicting future forest conditions.