Biotic disturbance facilitates range shift at the trailing but not the leading edge of lodgepole pine's altitudinal distribution

Aim Climate change will alter the geographic distribution of many species, but in forest communities, long-lived trees often persist after the climate is no longer suitable for recruitment. Disturbance events may accelerate distribution shifts in forested ecosystems by removing persistent trees and creating new recruitment opportunities for species better adapted to the current climate. Here, we investigate how biological disturbances such as insect outbreaks may interact with climate change to initiate tree species range shifts. Location Southern Rocky Mountains, Colorado, USA. Methods We compared tree and seedling data from long-term monitoring transects established in 1992–1995 and re-sampled in 2012 following a widespread mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak. Transects are located along elevational gradients at the altitudinal range margins of lodgepole pine (Pinus contorta). Results We found that recruitment had declined through time at the trailing edge of lodgepole pine's distribution (lower elevation range margin), where there are now many fewer seedlings compared to the leading edge (upper elevation range margin). At the trailing edge, mountain pine beetle increased mortality of mature lodgepole pines relative to background levels. This mortality was associated with an increase in recruitment of Douglas-fir (Pseudotsuga menziesii), a lower elevation tree species. At the leading edge of lodgepole pine's distribution, overstorey mortality was associated with only a modest increase in the density of lodgepole pine seedlings (mean increase of 362 seedlings·ha−1), and instead favoured more shade-tolerant competitors (mean increase of 2182 seedlings·ha−1). Conclusion While there is some evidence that climate change is beginning to alter the altitudinal range of lodgepole pine, mountain pine beetle disturbance will likely facilitate this process only at the trailing (lower elevation) range margin. At the leading (upper elevation) range margin, range expansion may be limited by competition with trees and seedlings of species not susceptible to mountain pine beetle. A physical disturbance such as fire that can eliminate competing species may be required for the upward expansion of lodgepole pine's leading range margin. For tree species such as lodgepole pine that are affected by periodic insect outbreaks, ranges may shrink in the near term if this disturbance–climate interaction continues to drive range retractions that outpace range expansions.