Historic Quaking Aspen Fire Regimes in Utah (USA) Forests

Quaking aspen (Populus tremuloides) is the most widely distributed tree species in North America. In the Intermountain West, aspen occurs across a broad range in elevation and is found in relatively pure stands and in association with various conifer species. Reproduction is primarily asexual (root suckering) resulting in longlived, spatially-stable clones that benefit from periodic disturbance. Our objective was to characterize historic aspen fire regimes in Utah, a state with abundant aspen across a broad range of forest types. We extracted fire-scar and tree-recruitment data for 122 plots (aspen present) from 11 sites in which plot grids were designed to span a broad range in elevation and vegetation type (299 total plots). Aspen plot elevation ranged from 2340 to 3204 m for southern sites and from 2255 to 3172 m for northern sites. Based upon pre-EuroAmerican (1860) reconstructions, the proportions of study plots that were aspen- (≥ 50 percent stem density) versus conifer-dominated were approximately equal across all sites. Plot evidence of non-lethal, surface fires in the form of annually-dated fire scars (on conifers) was sufficient to estimate mean fire intervals for 19 plots. Estimates ranged from 7 to 109 years with a mean of 32.4 years (1650-1900). We inferred fire severity using criteria that took into account the presence (or absence) of fire scars and recruitment cohorts and tree mortality patterns. Plots were classified as 10 percent low-, 25 percent mixed- and 20 percent high-severity, with 45 percent unclassified. Results suggest that historically, aspen in Utah persisted in both aspen- and conifer-dominated stands, the composition and dynamics of which varied in response to temporal and spatial variation in fire regime parameters