Trees are struggling to "breathe" and store CO2 as the climate warms

Recent research has unearthed alarming findings about trees’ diminishing ability to “breathe” and combat climate change by storing CO2.

Contrary to previous beliefs, trees in warmer and drier climates struggle to absorb carbon dioxide (CO2), posing a serious threat to our efforts in mitigating global warming.

Alarming reality: Trees are “coughing” CO2

Max Lloyd, Assistant Research Professor of Geosciences at Penn State and the study’s lead author, elucidates this worrying trend.

“Trees in warmer, drier climates are essentially coughing instead of breathing. They are sending CO2 right back into the atmosphere far more than trees in cooler, wetter conditions,” he explains.

Trees typically remove CO2 from the air through photosynthesis, using it to grow. However, stressful conditions like high temperatures and limited water supply trigger a reverse process called photorespiration, where trees release carbon dioxide back into the atmosphere.

The Penn State team, through a comprehensive analysis of global tree tissue data, discovered that photorespiration rates are up to twice as high in warmer climates, particularly when water scarcity is a factor.

This reaction starts to occur when average daytime temperatures exceed about 68 degrees Fahrenheit and intensifies with rising temperatures.

Rethinking plants’ role in carbon sequestration

This revelation challenges the widespread assumption about plants’ role in carbon sequestration.

“We have knocked this essential cycle off balance. Plants and climate are inextricably linked. The biggest draw down of CO2 from our atmosphere is photosynthesizing organisms,” Lloyd points out.

“It’s a big knob on the composition of the atmosphere, so that means small changes have a large impact.”

Currently, plants and trees absorb roughly 25% of the CO2 emitted by human activities annually, according to the U.S. Department of Energy.

However, Lloyd warns that this percentage is likely to decrease as the climate warms, particularly if water becomes scarcer.

Lloyd highlights a significant tradeoff, explaining, “When we think about climate futures, we predict that CO2 will go up, which in theory is good for plants because those are the molecules they breathe in.”

He continues, “But we’ve shown there will be a tradeoff that some prevailing models don’t account for. The world will be getting warmer, which means plants and trees will be less able to draw down that CO2.”

New method for studying photorespiration

The study also introduces a novel method for tracking photorespiration in trees.

The team discovered that variations in certain isotopes within wood, specifically in methoxyl groups, act as indicators of photorespiration rates.

You can think of isotopes as varieties of atoms. 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.

To observe photorespiration trends, the team actively examined the methoxyl isotope “flavors” in wood samples from approximately thirty tree specimens.

Predicting the future, understanding the past

These specimens, representing a diverse range of climates and conditions from across the globe, originated from an extensive archive at the University of California, Berkeley.

This archive houses hundreds of wood samples, meticulously collected during the 1930s and 40s, providing a rich resource for the study.

“The database was originally used to train foresters how to identify trees from different places around the world, so we repurposed it to essentially reconstruct these forests to see how well they were taking in CO2,” Lloyd said.

This breakthrough in methodology allows for a more comprehensive understanding of how trees have responded to past climates and how they might react in the future.

Unearthing ancient climate secrets

Previously, assessing photorespiration rates was limited to living plants or well-preserved specimens.

Now, this new approach using wood enables researchers to predict future tree behavior and to delve into the historical data.

Looking ahead, the team plans to extend their research to fossilized wood, aiming to uncover photorespiration rates from tens of millions of years ago.

This will help test hypotheses about the historical interplay between plant photorespiration and climate.

As Lloyd, a geologist by trade, puts it, “We may have to go back millions of years to better understand what our future might look like.”

Trees, CO2, and climate change

In summary, Penn State’s disturbing study, led by Max Lloyd, fundamentally alters our understanding of the role trees play in CO2 sequestration and mitigating climate change.

Lloyd’s research reveals that trees in warmer, drier climates contribute less to carbon sequestration, challenging previous assumptions about their role as natural carbon sinks.

This shift in perspective, underpinned by innovative methods to analyze photorespiration rates in trees, emphasizes the urgency of reassessing our strategies in combating global warming.

As the climate continues to change, this study underscores the need for a more nuanced approach to understanding the complex relationship between plants and the atmosphere, guiding future efforts towards more effective climate action.

The full study was published in the Proceedings of the National Academy of Sciences.

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