Abstract: Wildfire and drought are expected to become increasingly frequent and severe with global environmental change, particularly in the American West. As interactions between drought and fire become more common across western forests, it is imperative that we understand how these may affect trees differently than either fire or drought alone. Previous research in ponderosa pine (e.g., Partelli-Feltrin et al. 2022) suggests that fire scars can damage water-transporting xylem tissues and reduce trees’ resistance to drought stress. Based on this, we predicted that fire would increase vulnerability to drought in other western tree species as well. To test this hypothesis, we directly measured drought physiological traits (xylem vulnerability curves, leaf area: sapwood area, active xylem area) in individual trees in burned and unburned stands in three forest types (ponderosa pine, quaking aspen, and Engelmann spruce-subalpine fir) in the San Juan National Forest in Colorado, USA. Recent large fire years (2002, 2018) as well as drought years (2018, 2020) have devastated forests in this region, making it an ideal study area to test our hypothesis. We found that fire can impact vulnerability to drought, but that the directionality and magnitude of this impact varies by species, with one species (subalpine fir) actually showing increased resistance to drought after wildfire. These results suggest that fire may significantly impact forest physiological health and responses to drought, and that these impacts will likely vary significantly by forest type, as the climate warms further.
