Wildfire Catalyzed Shift from Conifer to Aspen Dominance in Montane Zone, Colorado
Climate change impacts on forest systems will likely be concentrated through influences on climate-sensitive ecological disturbances, such as wildfire. Over the last two decades, a shift towards megafires is evident and projected warming this century will continue to increase the frequency of these events. Forest recovery following megafires is challenging due to increased seed dispersal distances, harsh post-fire environment, and weather conditions. Species such as Quaking aspen (Populus tremuloides) with long seed dispersal distances and the ability to establish or resprout in harsh sites should be favored to expand their range in the context of wildfire trends. Aspen are well adapted to wildfire, however they are not drought-adapted. Unusually rapid and widespread mortality in aspen has been documented throughout its range during the 21st century, primarily as a result of warm, dry weather conditions. Increases in wildfire severity and extent caused by climate change may provide opportunities for aspen regeneration, especially at cooler, high- elevation sites. Aspen's sensitivity to drought suggests that regeneration following fire might be constrained to cooler and wetter topographic locations on the landscape that reduce drought stress on vegetation. Aspen establishment and persistence are known to occur at high elevation sites due to cooler, wetter conditions, while aspen mortality is demonstrated to occur at low elevation sites. Low- and high-elevation aspen persistence is well-understood; however, patterns of aspen regeneration and persistence at mid-elevation sites is still relatively unexplored. We studied the 2002 Hayman fire (Colorado, USA) to explore whether high-severity wildfire has provided opportunities for aspen regeneration at mid-elevations in which aspen was not observed before the fire. If regeneration had occurred, we asked if regeneration is contingent on topographic conditions. Cool, wet microclimates created by fine-scale topography at mid-elevations may allow for increased aspen regeneration, however this is unexplored in the Hayman landscape. Our findings demonstrate that the Hayman fire provided opportunities for aspen regeneration at mid-elevation where aspen were not observed before the fire and that the density of regeneration is contingent on topography. Specifically, aspen regeneration is most dense at mid-elevations on steep slopes. Forest management may focus on threats to aspen health and vigor (i.e., ungulate herbivory) on steep slopes at mid-elevations rather than at low-elevation sites for efficiency.
Porter, A., T. Assal, N. T. Hobbs, and J. Sibold. 2022. Wildfire Catalyzed Shift from Conifer to Aspen Dominance in Montane Zone, Colorado. SSRN.