Fire effects on tree physiology, growth, and drought vulnerability: Non-structural carbohydrate, hydraulic function, water potential, and tree growth data

The mechanistic links between fire-caused injuries and post-fire tree function are poorly understood. Current hypotheses differentiate effects of fire on tree carbon balance and hydraulic function, yet critical uncertainties remain about the relative importance of each, how they interact, and whether these physiological impacts have consequences for post-fire growth and drought vulnerability. We utilized two prescribed burn sites and paired unburned control sites with Pseudotsuga menziesii and Pinus ponderosa to examine i) the relative evidence for fire-caused changes in hydraulic function (vulnerability to cavitation, hydraulic conductance) and carbon dynamics (non-structural carbohydrates; NSC), and how such impacts relate to fire injuries, ii) which impacts most likely lead to post-fire mortality, and iii) how these impacts affect post-fire radial growth and drought vulnerability (water potential, total water deficit). We found that fire-caused impacts to NSC were immediate, persistent, correlated with crown injury severity, and strongly related to post-fire mortality. In contrast, hydraulic impacts were delayed and not directly attributable to fire-caused injuries, although some burned trees did exhibit signs of increased hydraulic dysfunction and water stress prior to death, possibly due to an interaction with direct fire impacts on NSC. Fire-caused injuries also showed a strong negative relationship with post-fire growth, and tree-level total water deficit was generally higher in burned trees, particularly burned trees that experienced a post-fire drought treatment. This suggests a period of higher drought vulnerability and low growth post-fire, likely until recovery from fire injuries occurs. These findings offer a more nuanced understanding of fire’s effect on post-fire tree function and mortality and are important in the context of increased fire activity in forests globally.