Year of Award

2025

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Ecology and Evolution

Department or School/College

Division of Biological and Biomedical Sciences

Committee Chair

Anna Sala

Commitee Members

Lila Fishman, James Elser, Philip Higuera, Aaron Ramirez

Keywords

Carbon balance, Drought, Fire ecology, Hydraulic function, Post-fire mortality, Tree physiology

Abstract

Wildfires are inherently spatially heterogenous, with many trees within larger burned areas not immediately killed by fire. These surviving trees often sustain some level of fire injury, yet we lack a mechanistic understanding of how these injuries affect post-fire tree physiological function, thus affecting whether trees go on to recover or die. 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 impacts make trees more vulnerable to post-fire disturbances like drought. Via field experiments utilizing prescribed fires and two widespread western U.S. conifers, I aimed to improve our understanding of the physiological effects of fire on tree function by answering three overarching research questions: (1) How do fire injuries impact tree carbon balance?, (2) What is the relative importance of fire-caused impacts to tree carbon balance and hydraulic function, and how to these impacts interact?, and (3) What is the potential for fire-caused injuries and their physiological impacts to offset benefits from fire-induced reductions in competition on tree water relations and growth under drought? I first show that fire injuries to tree crowns of ponderosa pine (Pinus ponderosa Lawson & C. Lawson) directly relate to fire-induced declines in tree non-structural carbohydrates (NSC) after a spring prescribed fire, and that trees with the strongest NSC declines eventually died. This work suggests that fire-caused tree carbon imbalance is a cause of delayed post-fire tree mortality. Next, I show that fire-caused impacts to NSC are immediate, persistent, correlated with crown injury severity, and strongly related to post-fire mortality in both ponderosa pine and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). In contrast, hydraulic impacts are delayed and not directly attributable to fire-caused injuries, although some burned trees do exhibit signs of increased hydraulic dysfunction and water stress prior to death, suggesting that fire may indirectly affect tree water relations possibly through an interaction with direct fire impacts on NSC. Finally, I show that trees may experience a period of reduced growth and increased vulnerability to drought in the first few growing seasons post-fire as a result of fire injuries. Altogether, my dissertation adds to our fundamental understanding of how fire affects tree physiological function and suggests how this knowledge can improve our ability to predict patterns of delayed post-fire mortality and tree vulnerability to post-fire drought.

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