A new study suggests that trees may not be able to store as much planet-heating carbon as previously anticipated, challenging current assumptions in climate models. Researchers found that the process of photosynthesis, where trees absorb carbon dioxide, does not always result in the growth of wood, which is crucial for long-term carbon sequestration.
Scientists from the Lamont-Doherty Earth Observatory at Columbia University, led by carbon cycle scientist Mukund Palat Rao, examined 137 sites across the United States. Their findings indicated that trees often ceased growing months before photosynthesis stopped for the year. This decoupling of photosynthesis and wood growth is significant because forests' ability to combat climate change largely depends on how much carbon dioxide they convert into wood, trapping the molecule out of the atmosphere for decades or even centuries. Other uses of absorbed carbon, such as for foliage or internal processes, typically have a much shorter lifespan.
Climate scientists generally expect land-based carbon sinks, like forests, to remain stable or grow throughout the 21st century as fossil fuel emissions increase atmospheric carbon dioxide. However, many models estimate this uptake based on photosynthesis levels rather than actual wood growth. Mr Rao highlighted this discrepancy, stating, “Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case. Just because there is more photosynthesis might not necessarily mean more tree growth in the future.”
The study revealed specific regional differences; in the eastern US, approximately 36% of annual carbon uptake occurred after tree growth had stopped in late summer. For sites in California, this figure was around 26%. More detailed measurements at four selected sites further showed that wood growth was restricted to periods of low aridity and temperature. These conditions are becoming increasingly rare as global temperature rises lead to more frequent heatwaves and droughts, potentially limiting trees' capacity for long-term carbon storage even if photosynthesis continues at a reduced rate.
These peer-reviewed findings, published by researchers at Columbia University, add a critical nuance to our understanding of forest carbon dynamics. They suggest that Earth system models that assume a consistently tight coupling between photosynthesis and growth may overestimate future forest carbon sequestration, especially under increasing atmospheric moisture demand. The research team is now investigating whether this observed decoupling extends to other tree species and regions globally, which could have broader implications for global climate change mitigation strategies.
The findings come shortly after a separate report emphasised the urgent need for humanity to remove carbon from the atmosphere at an accelerated pace, beyond the rate at which solar panels have been deployed. Land-based initiatives, including tree planting, currently account for the vast majority of human efforts to remove carbon dioxide, far outweighing contributions from emerging technologies and chemical processes.