COS 241-4 - Boosts in photosynthetic capacity linked with post-fire tree survival

Background/Question/Methods

Forests mitigate climate change by storing massive amounts of carbon. Increases in wildfire activity threaten forest carbon storage, but mechanisms controlling tree carbon uptake and survival remain unresolved. Our current understanding is largely informed by laboratory and low-intensity fire experiments, none of which have focused on mature trees. In August 2020, a wildfire burned through a long-term AmeriFlux research site, providing the opportunity for post-fire monitoring in a site containing decades of data. Our objectives were to 1) to quantify carbon uptake by a wide-spread tree species immediately following wildfire that caused variable degrees of damage, 2) determine the extent to which above- and belowground measurements correlate with observed tree damage, and 3) provide empirical support for physiological mechanisms active during initial tree recovery.

Results/Conclusions

Overall, burned trees exhibited higher leaf water potentials than unburned trees (p = 0.01) during summer drought yet maintained similar rates of photosynthesis (p = 0.12). These data indicate continued carbon uptake in burned trees, despite a more conservative response to water stress. Within the burned trees, the maximum rate of carboxylation (V_Cmax) increased with tree damage (p = 0.046), representing higher photosynthetic capacity in new and remaining needles. Furthermore, root density decreased with tree damage (p < 0.01), and soil respiration was lowest in the most severely burned areas (p < 0.05). Our results indicate that boosts in carbon uptake efficiency at the leaf-level help to minimize tree water loss and compensate for whole-tree fire damage (i.e., losses in leaf area and belowground function). V_Cmax is an integrative tree characteristic representing the capability of a tree to take in carbon and is therefore used as a model parameter for predicting ecosystem-level photosynthesis (gross primary productivity). We couple our field-based measurements of V_Cmax on mature trees with a physiological tree model to predict possible GPP outputs for various degrees of tree damage. These findings fundamentally change how Earth system models should represent post-fire carbon dynamics.