Warmer and drier climate conditions have exposed forest ecosystems to higher risk of drought and wildfire occurrence. Thus, understanding post-fire and drought tree recovery is vital to better predictions of forest resilience after these events. In this study, we investigated the recovery of well-watered and drought-stressed Pinus ponderosa saplings treated with three different fire radiative energies (FRE) per unit area (0, 0.7 and 1.4 MJ m-2). Sapling water status treatments were determined based on their pre-fire predawn water potential. Well-watered saplings water potential ranged from -0.27 to -0.56 MPa and drought-stressed from -1.4 to -2.4 MPa. To evaluate post-fire seedling recovery, 10 saplings, for each treatment, were kept in the greenhouse for 201 days. Photosynthesis and water potential were measured the day of the burn, and 1, 13, and 27 days post-fire. Other signs of recovery such as development of new buds and leaves were also observed.
Results/Conclusions
FRE had different effects on tree sapling survival depending on their water status. All saplings burned with 0.7 and 1.4 MJ m-2 showed decreased photosynthesis 1 day post-fire, with values approaching zero and remaining near zero at 27 days post-burn. Well-watered and drought-stressed saplings burned with 1.4 MJ m-2 did not show any physiological signs of recovery and had 100% mortality in the end of the experiment. Saplings burned with 0.7 MJ m-2 showed a different recovery pattern than those burned with 1.4 MJ m-2. At 35 days post-fire, 90% of well-watered and 30 % of drought-stressed saplings burned with 0.7 MJ m-2 showed some sign of recovery (such as developing new buds). However, by the end of the experiment, all drought-stressed saplings had 100% of mortality versus 30 % of mortality of well-watered saplings. Our study suggests that sapling tree survival has a direct relationship with FRE and plant water status. For instance, saplings under water stress are more likely to die than well-watered saplings. However this relationship is only true until a certain FRE threshold. More studies with different tree species at different FRE and water status are needed to better understand the interaction between plant water status and FRE. Consequently this will result in better predictions of post-fire tree mortality across different forest ecosystems.