The role of wildfire and topography in shaping aspen regeneration after the Hayman Fire, CO, USA

Abstract

Unusually rapid and widespread mortality in Quaking aspen (Populus tremuloides) has been documented throughout the 21st century primarily as a result of warm, dry weather conditions. Although aspen are not drought-adapted, aspen are well adapted to wildfire. Increases in wildfire severity and extent as a result of climate change may provide opportunities for aspen regeneration, especially at mid- and 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 which reduce drought stress on vegetation. While aspen mortality is demonstrated to occur at low elevation sites, aspen establishment and persistence is known to occur at high elevation sites due to cooler, wetter conditions. 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 use 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 has occurred, we ask 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 sites in which 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. Ecosystem 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.