Prescribed burns, such as this one in the Coconino National Forest, are an effective way to reduce accumulated fuels. New insights into the physiological response of trees to heat stress will help land managers determine the likelihood of tree mortality following a burn.
U.S. Forest Service photo by Brady Smith
When woody tree tissues reach 86 to 140 °F (30 to 60 °C) during a fire, three physiological mechanisms may be triggered that, individually or combined, impair aerobic respiration energy production and usually induce synthesis of ethanol—a bark beetle attractant.
The lethal temperature limit for plant tissues, including trees, is well established at 140 °F (60 °C). However, cells and tissues may die at lower temperatures if exposed long enough, indicating that physiological changes occur below this threshold and create stress. During wild or prescribed fires, woody tissues in tree roots, stems, or crowns are exposed for variable times to temperatures below 140 °F (60 °C) that cause heat stress, but the physiological mechanisms are poorly understood. Heat stress from sublethal temperatures is important because it can kill trees directly, or make them more vulnerable to additional stress agents.
Rick Kelsey and Doug Westlind, scientists with the Pacific Northwest Research Station, formulated a mechanistic model, proposing that woody tree tissues reaching sublethal temperatures of 86 to 140 °F (30 to 60 °C) may experience physiological changes to (a) oxygen supply, (b) membrane function, or (c) enzyme activity that individually or simultaneously impair cellular energy production by oxidative phosphorylation in the mitochondria, often accompanied by induction of ethanol synthesis. These mechanisms were formulated from studies of ethanol concentrations in fire-injured trees and other experiments that focused on stress-induced ethanol synthesis.
This new insight into tree heat stress and its influence on interactions with other stress agents such as bark beetles and pathogens provides valuable information to land managers considering the use of prescribed fire to reduce fuel loads.