Not Dead Yet: Physiological Resilience of Pinus ponderosa to Wildfire and Prolonged Drought

Wildfire causes immediate and delayed tree mortality over days to years. Reductions in photosynthesis and changes in ecosystem respiration reduce net ecosystem carbon uptake immediately, but the magnitude and trajectory of recovery is dependent on stochastic climate conditions and the severity of fire effects. Here we present above- and belowground physiological data from mature conifer trees immediately following a mixed-severity wildfire. While much progress has been made regarding the impact of fire on conifers, few studies have observed the immediate physiological responses of mature conifers to fire, and none have done so at varying fire severities. In August 2020, amidst a prolonged drought, a wildfire burned through our study site in Metolius OR. The fire grew to higher-than-normal intensities due to an accumulation of carbon-rich understory that had developed in the absence of necessary low-intensity burns over the last several decades. Due to the localized wind patterns, fire effects were highly heterogeneous, providing a unique opportunity to observe the immediate physiological responses of trees burned at varying severities. Our results indicate that by the following growing season, trees that experienced 50% canopy consumption were exhibiting sap flow rates similar to unburned control trees. Furthermore, leaf water potential and gas exchange in burned trees were similar to that of unburned trees, indicating resumed physiological activity despite considerable damage from the fire. Soil respiration decreased across all burn severities immediately following the fire but increased during the following growing season. Soil respiration was lowest in the most severely burned areas but was highest in the moderately burned areas. These data provide useful insights about the immediate response of mature conifers to mixed-severity wildfire and the immense capacity for resilience in even severely burned trees.