Recurring wildfires provoke type conversion in dry western forests

Abstract

Recent wildfires across western North America have burned with uncharacteristically high severity, representing a substantial departure from natural fire regimes. In mixed-conifer and pine–oak ecosystems of the southern Cascade Range, widespread shifts in stand structure and composition have led to a diversity of post-wildfire vegetation responses. When recent wildfire “footprints” reburn in subsequent fires, their recovery pathways are complex. In order to understand the effects of overlapping mixed-severity fires, we quantified changes in overstory and midstory structure and species composition for time periods prior to and following two large overlapping wildfires in the southern Cascades: the 2000 Storrie and 2012 Chips Fires. Plots were stratified into 16 severity combinations (unburned, low, moderate, and high in the Storrie Fire combined with the same four categories in the Chips Fire: e.g., moderate Storrie/high Chips) across the 9000-ha overlapping burned area. Following the two fires, tree quadratic mean diameter and stand density declined for most species, but changes were species-specific. Compared with preburn values, importance values for fire-sensitive white fir (Abies concolor) were reduced by 66%, while resprouting fire-resilient California black oak (Quercus kelloggii) importance values increased by 37% in severity combinations that included at least one high-severity fire. Greatest shifts were documented in sites that burned twice at high severity, where resulting vegetation was dominated by oak sprout clumps and resprouting and fire-stimulated montane chaparral species, while unburned and low-severity strata retained a substantial component of Douglas-fir (Pseudotsuga menziesii) and white fir. Results suggest that repeated moderate- and high-severity fires can result in ecosystem state shifting toward fire-resilient oak-shrub communities in this fire-prone landscape. Managers seeking greater landscape resilience can implement treatments such as thinning and prescribed burning, while taking advantage of fire-created patches such as these in areas where the likelihood of a hotter and drier future makes the reestablishment of continuous forest cover unrealistic.