Trees struggle to ‘breathe’ as climate warms, researchers say

Trees have difficulty sequestering heat-trapping carbon dioxide (CO).₂) in hotter, drier climates, meaning they may no longer serve as a solution to offsetting humanity’s carbon footprint as the planet continues to warm, according to a new study by researchers at Penn State.

“We found that trees in hotter, drier climates cough instead of breathe,” 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 send CO₂ directly into the atmosphere, much more so than trees in cooler, wetter conditions. »

Through the process of photosynthesis, trees eliminate CO₂ of the atmosphere to produce new growth. However, under stressful conditions, trees release CO₂ return to the atmosphere, a process called photorespiration. By analyzing a global dataset of tree tissue, the research team demonstrated that the rate of photorespiration is up to twice higher in warmer climates, particularly when water is limited.

They found that the threshold for this response in subtropical climates begins to be crossed when average daytime temperatures rise above about 68 degrees Fahrenheit and worsens as temperatures rise further.

The findings complicate a widely held belief about the role of plants in extracting or using carbon from the atmosphere, offering new insights into how plants might adapt to climate change. Importantly, the researchers noted that as the climate warms, their results demonstrate that plants may be less able to absorb CO.₂ of the atmosphere and assimilate the carbon necessary to cool the planet.

“We have thrown this essential cycle out of balance,” Lloyd said. “Plants and climate are inextricably linked. The greatest absorption of CO₂ of our atmosphere come from photosynthetic organisms. It’s a big knob on the composition of the atmosphere, meaning small changes have a big impact. »

Plants currently absorb around 25% of CO₂ emitted by human activities each year, according to the U.S. Department of Energy, but that percentage is likely to decline in the future as the climate warms, Lloyd said, especially if water is more scarce.

“When we think about the future of climate, we predict that CO₂ will increase, which in theory is good for plants because these are the molecules that they breathe,” Lloyd said. “But we have shown that there will be a trade-off that some dominant models do not take into account. The world will get warmer, which means plants will be less able to absorb this CO2₂“.

In the study, the researchers found that variation in the abundance of certain isotopes in part of the wood called methoxyl groups serves as a tracer of photorespiration in trees. You can think of isotopes as varieties of atoms, Lloyd explained. Just like you can 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 around 30 tree specimens from various climates and conditions around the world to observe trends in photorespiration. The specimens come from archives at the University of California, Berkeley, which contains hundreds of wood samples collected in the 1930s and 1940s.

“The database was originally used to train foresters to identify trees from different places around the world, so we reused it to essentially reconstruct these forests to see how well they absorbed CO.₂” Lloyd said.

Until now, photorespiration rates could only be measured in real time using live plants or well-preserved dead specimens that retained structural carbohydrates, meaning it was almost impossible to study the rate at which plants absorbed carbon on a large scale or in the past. Lloyd explained.

Now that the team has validated a way to observe the rate of photorespiration using wood, he said the method could offer researchers a tool to predict how well trees might “breathe” when future and how they behaved in past climates.

The amount of carbon dioxide in the atmosphere is increasing rapidly; it’s already larger than at any time in 3.6 million years, according to the National Oceanic and Atmospheric Administration. But this period is relatively recent in geological time, Lloyd explained.

The team will now work to uncover photorespiration rates in the ancient past, tens of millions of years ago, using fossilized wood. The methods will allow researchers to explicitly test existing hypotheses regarding the changing influence of plant photorespiration on climate over geologic time.

“I’m a geologist, working in the past,” Lloyd said. “So if we’re interested in these big questions about how this cycle worked when the climate was very different from today, we can’t use living plants. We might have to go back millions of years. go back years to better understand what our future could look like.”

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.