New insight to how exposure to sub-lethal temperatures affects trees
The lethal temperature limit for plant tissues, including trees, is well established at 140 degrees Fahrenheity (60 degrees Celsius). However, cells and tissues may die at lower temperatures if exposed long enough, indicating 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 degrees Fahrenheit and cause heat stress, but the physiological mechanisms are poorly understood. Heat stress from sub-lethal temperatures is important because it can kill trees directly, or make them more vulnerable to additional stress agents. Scientists with the Forest Service’s Pacific Northwest Research Station formulated a mechanistic model, proposing that woody tree tissues reaching sub-lethal temperatures of 86 to 140 degrees Fahrenheit (30 to 60 degrees Celsius) may experience physiological changes to oxygen supply, membrane function, or 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 focused on stress-induced ethanol synthesis. This new insight on 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.