Climate change is predicted to result in increased in wildfire season length and wildfire intensity. Understanding how trees and seedlings respond physiologically to fire events is essential for predicting their recovery and mortality, as well as for better managing prescribed forest fires. Despite the scarce knowledge about tree physiological response to fire, hydraulic failure has been suggested to be one of the main mechanisms of post-fire tree mortality. We evaluated the effects of whether fire radiative energy (FRE) per unit area at levels known to cause substantial mortality in pine saplings (0.7 and 1.0 MJ m-2) resulted in impacts on xylem conductivity and photosynthesis of Pinus ponderosasaplings under two levels of water status (well-watered and water-stressed). Burns were conducted in a controlled combustion laboratory. We measured xylem conductivity in burned versus unburned sapling stems. Photosynthesis was measured pre-fire and 1, 7 and 14 days post-fire. For each treatment 5 saplings were kept after fire treatments to evaluate their recovery.
Fire radiative energy did not have significant effects on xylem conductivity, at either watering treatment. All burned saplings had decreased photosynthesis 1 day post-fire, with values approaching zero and photosynthesis remained near zero at 14 days post-burn. Sapling mortality varied among treatments. About 3 months post-fire, water-stressed seedlings had 80% mortality for both fire treatments. Well-watered saplings burned at 1 MJ m-2 had 100% mortality, while well-watered saplings burned at 0.7 MJ m-2 had 40% mortality. Our results suggest that hydraulic failure is not a primary main mechanism of post-fire saplings mortality. However, fire has a significant effect on photosynthesis, indicating that foliar damage can force sapling to rely on their carbon reserves to recover photosynthetic machinery after a fire event. Much additional work is needed to understand the primary cause(s) of post-fire tree mortality.