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.