Longer periods of drought and warmer global temperatures has favored the occurrence of wildfires in forest ecosystems. With predictions of increase in severity and extent of wildfires due to climate change, understanding physiological mechanisms that lead to post-fire tree mortality is urgently needed to improve model post-fire carbon processes and inform management decisions. However, the physiological mechanisms that lead to tree death after a forest fire is not well understood. In this study, two separate experiments were conducted with well-watered Pinus ponderosa saplings exposed to fire. Xylem hydraulic conductivity was assessed 1-day and 21-months after fire exposure to better understand the short- and long- term fire effects on Pinus ponderosa saplings hydraulic system. In the short-term experiment, saplings were divided in two treatments: unburned and burned with a lethal fire intensity of 1.4 MJ m-2. In the long-term experiment, plants were placed in three treatments: unburned and burned with 0.7 and 1.4 MJ m-2. Native percentage loss of conductivity (nPLC), vulnerability to cavitation, and deformation of xylem tracheid were assessed.
Results/Conclusions
nPLC was not affected in either experiment. Plants evaluated 1-day post fire did not show any evidence of being more vulnerable to xylem cavitation. However, we found that plants were more vulnerable to cavitation 21-months post-fire. In neither experiment we did observe deformation of the xylem of plants exposed to the fire. We conclude that hydraulic failure was not the main cause of post-fire tree mortality and suggest that other physiological mechanisms such as depletion of carbohydrates could lead to tree mortality. We also did not find any evidence of conduit deformation that has been proposed as the consequence of hydraulic failure and vulnerability to cavitation in post-fire trees. However, saplings exposed to the fire are more vulnerable to cavitation after 21-months, thus we suggest that the irregularity of the newly grown xylem cells closer to the wound caused by the heat may contribute to plants that survive fires being more vulnerable to xylem embolism.