Stand‐replacing fires reduce susceptibility of lodgepole pine to mountain pine beetle outbreaks in Colorado

Aim  As climate change is increasing the frequency, severity and extent of wildfire and bark beetle outbreaks, it is important to understand how these disturbances interact to affect ecological patterns and processes, including susceptibility to subsequent disturbances. Stand-replacing fires and outbreaks of mountain pine beetle (MPB), Dendroctonus ponderosae, are both important disturbances in the lodgepole pine, Pinus contorta, forests of the Rocky Mountains. In the current study we investigated how time since the last stand-replacing fire affects the susceptibility of the stand to MPB outbreaks in these forests. We hypothesized that at a stand-scale, young post-fire stands (< c. 100–150 years old) are less susceptible to past and current MPB outbreaks than are older stands. Location  Colorado, USA. Methods  We used dendroecological methods to reconstruct stand-origin dates and the history of outbreaks in 23 lodgepole pine stands. Results  The relatively narrow range of establishment dates among the oldest trees in most sampled stands suggested that these stands originated after stand-replacing or partially stand-replacing fires over the past three centuries. Stands were affected by MPB outbreaks in the 1940s/1950s, 1980s and 2000s/2010s. Susceptibility to outbreaks generally increased with stand age (i.e. time since the last stand-replacing fire). However, this reduced susceptibility of younger post-fire stands was most pronounced for the 1940s/1950s outbreak, less so for the 1980s outbreak, and did not hold true for the 2000s/2010s outbreak. Main conclusions  Younger stands may not have been less susceptible to the most recent outbreak because: (1) after stands reach a threshold age of > 100–150 years, stand age does not affect susceptibility to outbreaks, or (2) the high intensity of the most recent outbreak reduces the importance of pre-disturbance conditions for susceptibility to disturbance. If the warm and dry conditions that contribute to MPB outbreaks concurrently increase the frequency and/or extent of severe fires, they may thereby mitigate the otherwise increased landscape-scale susceptibility to outbreaks. Potential increases in severe fires driven by warm and dry climatic trends may lead to a negative feedback by making lodgepole pine stands less susceptible to future MPB outbreaks.