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Trees are struggling to sequester carbon in warmer, drier climates – meaning they may no longer serve as carbon sinks, according to a new study.
Planting trees has emerged as one of the few universally accepted approaches to tackling climate change. Forests are natural carbon sinks, sequestering colossal amounts of atmospheric carbon dioxide.
However, researchers and environmental groups have raised concerns about the efficacy of tree-planting programmes in practice, with complications around biodiversity, land use, threat of forest fires and premature felling. A 2023 King’s College London study warned that tree-planting was a risky and unreliable solution to climate change.
New research from Penn State University has found that trees are increasingly struggling to sequester carbon as the planet heats up.
“We found that trees in warmer, drier climates are essentially coughing instead of breathing,” said Professor Max Lloyd, lead author of the study. “They are sending CO2 right back into the atmosphere far more than trees in cooler, wetter conditions.”
Trees remove CO2 from the atmosphere through photosynthesis, allowing them to produce new growth. However, under these “stressful” conditions, they release CO2 back to the atmosphere in a process called photorespiration.
Until now, photorespiration could only be measured in real time using living plants or well-preserved dead specimens, making it almost impossible to study the rate at which plants draw down CO2 at scale. However, the researchers discovered that variation in the abundance of certain isotopes served as a tracer of photorespiration in trees. They studied the presence of the isotope in wood samples from around 30 specimens of trees from a variety of climates and conditions to observe photorespiration trends.
They found that the rate of photorespiration is up to two times higher in warmer climates – especially when water is limited.
The threshold for this response in subtropical climates begins to be crossed when the average daytime temperatures exceed 20°C (68°F), and worsens as temperatures rise further.
The findings complicate assumptions about plants and their role in helping to draw carbon from the atmosphere. As the climate warns, the researchers said, plants could be less able to draw CO2 out of the atmosphere. “We have knocked this essential cycle off-balance,” said Lloyd. “Plants and climate are inextricably linked. The biggest draw down of CO2 from our atmosphere is photosynthesising organisms ... so that means small changes have a large impact.”
Plants currently absorb around 25% of the CO2 emitted by human activity each year, although this fraction is likely to decrease as the climate warms, Lloyd said, and especially if water becomes scarcer.
