Wildfire Affects Forest Responses to Drought in Southwestern Colorado, USA

As the climate warms, both wildfire and drought are expected to become increasingly frequent and severe across much of 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. Fire is already known to interact with drought to affect forest health in myriad, often seemingly contradictory, ways. With respect to tree health, these interactions may be negative (e.g., heat plume embolism or heat-induced xylem tissue damage that further reduce trees' ability to transport water in drought conditions), or positive (e.g., leaf area reduction or reduced competition post-fire that reduce water losses in forests). Additionally, some physiological adaptations that allow plants to withstand drought may also be advantageous during fire events, and vice versa. Topography can also alter stand-level responses to fire and drought due to differences in moisture availability and solar radiation. We combined landscape-level topographic analysis and remote sensing of vegetation health with stand-level surveys of mortality and tree drought physiological traits to elucidate potential effects of fire on forest drought recovery in southwestern Colorado, USA. We found a positive correlation between low-moderate severity fire and reduced declines in late summer EVI in spruce-fir forests that burned prior to the 2020 drought. We also found that subalpine fir (Abies lasiocarpa) trees that recently recovered from fire were more resistant to drought than unburned trees, but that Engelmann spruce (Picea engelmannii) did not experience significant changes in drought resistance after fire.