Carbon uptake by forests will be compromised by climate change and leaf temperature

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CORVALLIS, OR — A new study led by Oregon State University (OSU) suggests that the leaves of tree canopies are not able to cool below the temperature of the surrounding air, which likely means that the ability of trees to avoid temperature increases damage and extracting carbon from the atmosphere will be compromised in a warmer climate. , drier climate.

The results of an international collaboration that included researchers from several universities and government agencies contrast with a prevailing theory in the scientific community that canopy leaves can maintain their temperature within an optimal range for photosynthesis – the process by which green plants make their food from sunlight and carbon dioxide.

Posted in the Proceedings of the National Academy of Sciencesthe research is important for understanding and predicting plant responses to climate change, said lead author Chris Still of the OSU College of Forestry, who notes that numerous studies suggest that many of the world’s forests are approaching their thermal limit carbon absorption.

“A hypothesis known as limited leaf homeothermy holds that through a combination of functional traits and physiological responses, leaves can maintain their daytime temperature near the best temperature for photosynthesis and below that which is detrimental to them. “, said Still. “Specifically, leaves must cool below air temperature to higher temperatures, typically above 25 or 30 degrees Celsius. This theory also implies that the impact of global warming on forests will be partially mitigated by leaf cooling response.

Still and his collaborators have used thermal imaging to observe canopy leaf temperature at many well-instrumented sites in North and Central America – from the Panamanian rainforest to the high-altitude treeline in Colorado – and found that canopy leaves do not consistently cool below. daytime air temperatures or remain within a narrow temperature range as predicted by the theory of limited leaf homeothermy.

The thermal cameras were mounted on towers equipped with systems that measure the “flows” of carbon, water and energy – the exchanges between the forest and the atmosphere – as well as a host of environmental variables.

“Using continuous high-frequency thermal imaging to monitor forest canopies really changes what we can learn about how forests are coping with the stress of rising temperatures,” said Andrew Richardson, a professor at the Northern Arizona University and study co-author. . “Before thermal cameras, if you wanted to measure canopy temperature, you had to tape thermocouples to the leaves with bandaids and wait for the wind to rip them off. But these cameras allow us to measure changes 24 hours a day, seven days a week, over many seasons and years. »

The study showed that canopy leaves warm faster than air, are warmer than air for most of the day, and only cool below air temperature in mid-air. or late afternoon. According to scientists, future global warming will likely lead to even higher leaf temperatures, which would negatively impact the forest carbon cycle and increase the risk of forest mortality.

“Leaf temperature has long been recognized as important to plant function because of its influence on carbon metabolism and water and energy exchange,” Still said. “If canopy photosynthesis decreases with increasing temperature, the ability of forests to act as a carbon sink will be reduced.”

Leaf temperature in different habitats is affected by how leaf size varies with climate and latitude as well as canopy structure, Still says. Large leaves mainly occur in hot and humid climates, and leaf traits like higher reflectance and smaller sizes, which improve the ability to carry heat away and lead to greater cooling, occur mainly in plants growing in hot, dry areas.

In most hot, humid tropical regions, leaf temperatures are already approaching or exceeding thresholds for net positive photosynthesis – the rate of carbon fixation minus the rate of carbon dioxide lost during plant respiration.

“If the leaves are generally warmer than the surrounding air, as our results suggest, trees may approach critical thresholds of heat stress faster than expected,” Richardson said.

“Our results have big implications for understanding how plants acclimate to warming, and they suggest a limited ability of canopy leaves to regulate their temperature,” Still added. “Our data and analyzes suggest that a warming climate will lead to even higher canopy leaf temperatures, which will likely result in reduced carbon assimilation capacity and possibly heat damage.”

Chad Hanson and Hyojung Kwon of the OSU College of Forestry also participated in the study, as did scientists from the University of Colorado, Princeton University, University of Pennsylvania, University of State of Florida, University of California Santa Barbara, University of California, Irvine, NASA, US Geological Survey, Canadian Forces Base Trenton, US Forest Service Pacific Northwest Research Station, Commonwealth Scientific and Industrial Research Organization of Australia and Los Alamos National Laboratory.

– This press release was provided by Oregon State University

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