New research indicates that California redwoods experience reduced photosynthetic capacity and leaf stress when exposed to extreme temperatures.

A study published September 22 in the journal JGR Biogeosciences reveals that California redwoods are struggling to photosynthesize during extreme heat events. Researchers found that as temperatures rise, these trees experience significant physiological stress, which could impact their long-term survival and ability to store carbon from the atmosphere.

Research Methodology and Findings

Between 2022 and 2024, researchers from the University of California, Davis, studied redwood leaves in the Garcia River Forest in Mendocino County. By measuring leaf fluorescence and temperature, the team determined that on hot days, leaves exposed to sunlight were up to 18 degrees Fahrenheit (10 degrees Celsius) hotter than the surrounding air. This temperature disparity suggests the trees close their stomata to prevent water loss, which in turn limits carbon dioxide intake and reduces energy production. Laboratory tests further confirmed that heat directly limits photosynthetic capacity. Even when leaf samples were provided with water and kept with open stomata, photosynthesis decreased as temperatures increased. This indicates that heat imposes a direct limitation on the trees' biological systems rather than solely acting through water-conservation mechanisms.

Implications for Ecosystems

The findings suggest that the iconic California redwoods, which evolved in historically mild climates, are increasingly vulnerable to the region's rising frequency of extreme heat days. Because these forests serve as major aboveground carbon stores, their reduced ability to photosynthesize has broader implications for carbon sequestration. While the study highlights the immediate impact of heat on leaf physiology, researchers note that it remains unclear whether these stress responses are reversible or if the trees are sustaining permanent damage. Future research aims to determine the thresholds at which heat-induced damage becomes irreversible and to understand how these effects manifest at the ecosystem level.