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Recent research led by Penn State indicates that trees in hotter, drier environments struggle to absorb carbon dioxide, compromising their ability to counteract climate change. The study highlights an increase in photorespiration, a process in which stressed trees release CO2, under these conditions, challenging the effectiveness of trees as natural carbon sinks in a warming world. Credit: SciTechDaily.com
Trees are struggling to sequester heat-trapping carbon dioxide (CO2) in hotter, drier climates, meaning they may no longer serve as a solution to offset humanity’s carbon footprint as the Planet continues to warm, according to a new study led by Penn State. researchers.
“We found that trees in warmer, drier climates are essentially coughing instead of breathing,” said Max Lloyd, research assistant professor of geosciences at Penn State and lead author of the study recently published in Proceedings of the National Academy of Sciences. “They are returning CO2 to the atmosphere much more than trees in colder, wetter conditions.”
Through the process of photosynthesis, trees remove CO2 from the atmosphere to produce new growth. However, under stressful conditions, trees release CO2 into the atmosphere, a process called photorespiration. Using an analysis of a global data set of tree tissue, the research team showed that the rate of photorespiration is up to two times higher in warmer climates, especially when water is limited. They found that the threshold for this response in subtropical climates begins to be crossed when average daytime temperatures exceed about 68 degrees. Fahrenheit and worsen as temperatures rise further.
The complicated role of plants in climate adaptation
The results complicate a widely held belief about the role of plants in helping to extract or use carbon from the atmosphere, providing new insights into how plants might adapt to climate change. Importantly, the researchers noted that as the climate warms, their findings show that plants may be less able to extract CO2 from the atmosphere and assimilate the carbon needed to help the planet cool.
“We have unbalanced this essential cycle,” Lloyd said. “Plants and climate are inextricably linked. The greatest reduction of CO2 in our atmosphere is produced by photosynthesizing organisms. “It is a big factor in the composition of the atmosphere, meaning that small changes have a big impact.”
Currently, plants absorb about 25% of the CO2 emitted by human activities each year, according to the US Department of Energy, but this percentage is likely to decrease in the future as the climate warms, Lloyd explained, especially if water is scarcer.
“When we think about the climate future, we predict that CO2 will increase, which in theory is good for plants because those are the molecules they respire,” Lloyd said. “But we have shown that there will be a balance that some prevailing models do not take into account. “The world will warm, which means plants will be less able to absorb that CO2.”
Using an analysis of a global data set of tree tissue, a team led by Penn State researchers showed that the rate of photorespiration in trees is up to two times higher in warmer climates, especially when water is limited. They found that the threshold for this response in subtropical climates, such as this part of the Appalachian Mountain Range and Valley region, begins to be crossed when average daytime temperatures exceed about 68 degrees Fahrenheit and worsens as temperatures rise further. further. Credit: Warren Reed/Penn State
In the study, the researchers found that variation in the abundance of certain isotopes of a part of wood called methoxyl groups serves as a marker of photorespiration in trees. You can think of isotopes as varieties of atoms, Lloyd explained. Just as you might have vanilla and chocolate versions of ice cream, atoms can have different isotopes with their own unique “flavors” due to variations in their mass. The team studied levels of the isotope methoxyl “flavor” in wood samples from about thirty tree specimens from a variety of climates and conditions around the world to observe trends in photorespiration. The specimens come from an archive of the University of California, Berkeleycontaining hundreds of wood samples collected in the 1930s and 40s.
“The database was originally used to train foresters on how to identify trees from different places around the world, so we reused it to essentially reconstruct these forests and see how well they were absorbing CO2,” Lloyd said.
Until now, photorespiration rates could only be measured in real time using living plants or well-preserved dead specimens that retained structural carbohydrates, which meant it was almost impossible to study the rate at which plants take up carbon at scale or in the past. Lloyd explained.
Look at the past to understand the future
Now that the team has validated a way to observe photorespiration rate using wood, he said the method could offer researchers a tool to predict how well trees could “breathe” in the future and how they fared in past climates.
The amount of carbon dioxide in the atmosphere is increasing rapidly; is already larger than at any other time in the last 3.6 million years, according to the National Oceanic and Atmospheric Administration. But that period is relatively recent in geological time, Lloyd explained.
The team will now work to unearth photorespiration rates in the ancient past, up to tens of millions of years ago, using fossilized wood. The methods will allow researchers to explicitly test existing hypotheses about the changing influence of plant photorespiration on climate over geological time.
“I’m a geologist, I work in the past,” Lloyd said. “So if we are interested in these big questions about how this cycle worked when the climate was very different from today, we can’t use live plants. “We may have to go back millions of years to better understand what our future will be like.”
Reference: “Isotopic clumping in wood as an indicator of photorespiration in trees” by Max K. Lloyd, Rebekah A. Stein, Daniel E. Ibarra, Richard S. Barclay, Scott L. Wing, David W. Stahle, Todd E. Dawson and Daniel A. Stolper, November 6, 2023, proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2306736120
Other authors of the paper are Rebekah A. Stein, Daniel A. Stolper, Daniel E. Ibarra, and Todd E. Dawson of the University of California, Berkeley; Richard S. Barclay and Scott L. Wing of the Smithsonian National Museum of Natural History and David W. Stahle of the University of Arkansas.
The work was funded in part by the Agouron Institute, the Heising-Simons Foundation and the US National Science Foundation.