Wildfire, management, and regeneration of quaking aspen (Populus tremuloides) in the Sierra Nevada and Glass Mountain, California, USA

Aspen (Populus tremuloides) is considered a foundation species and adds significant biological diversity to conifer-dominated western forests, yet represents less than 1% of forest cover in the Sierra Nevada. Aspen are also particularly sensitive to climate, and are currently a species of concern due to wide-scale, drought-induced mortality in the Intermountain West, declining vigor of current populations in the Sierra Nevada, and predictions of increased aridity in the next century. Many California land management agencies have identified aspen restoration as a priority and are conducting conifer removal and prescribed fire treatments in an effort to rejuvenate declining aspen populations. This research evaluates the efficacy of these treatments (3 sites of conifer removal, 2 sites of prescribed fire) and compares these treatments with recent wildfires (4 sites, each randomly stratified by fire severity). Experimental questions include: Resistance Strategies: Are prescribed fire and conifer removal restoration treatments effective in stimulating asexual aspen regeneration (sprouting)? How do these simulated disturbances compare to unplanned wildfires? Resilience strategies: How do pre-fire stand composition and fire severity impact post-fire aspen regeneration? Response strategies: Is it likely that aspen will be able to successfully migrate via sexual reproduction (seeds) in an era of changing climate? Generalized linear mixed models show that both conifer removal and prescribed fire can be effective restoration treatments but that unplanned wildfire produces significantly higher ramet (sprout) density than either treatment. Conifer removal was found to be ineffective in one site where heavy aspen over-story mortality was observed 2 years post-treatment. Sun-induced aspen bole damage and spatial dependence between aspen mortality and removed conifers indicates sunscald as the mortality agent, and provides conifer thinning guidelines for future treatments. A significant negative relationship was found between pre-wildfire conifer basal area and post-fire aspen ramet density, indicating that conifer encroachment negatively impacts aspen resilience to fire. Additionally, a significant positive relationship was found between fire severity and post-fire aspen ramet density and growth rates, indicating that increased disturbance severity favors aspen regeneration and persistence in these study sites. Five sites of aspen seedling establishment were identified in recently burned areas. All seedling sites were in areas of high fire severity, high soil moisture, concave micro-topography, and high incident solar radiation. The ability of aspen to migrate in order to accommodate climate change will depend heavily on the occurrence of high severity fire and successful seed dispersal to microsites that will allow successful seedling establishment.