Forests could prove less effective in combating the climate crisis than previously thought. This is suggested by new research published by Columbia University, according to which many trees continue to absorb carbon dioxide through photosynthesis even when they have already stopped growing. A detail that might seem marginal, but which actually radically changes the way we estimate the capacity of forests to store carbon in the long term.
For years, scientists have believed that increasing the concentration of CO₂ in the atmosphere could promote photosynthesis and, consequently, accelerate the growth of trees. More growth means more wood and therefore more carbon trapped for decades or even centuries within trunks, branches and roots. However, the new study shows that the link between photosynthesis and growth is much less direct than previously thought.
The team led by the ecoclimatologist Mukund Palat Rao from the Columbia Climate School analyzed oak trees distributed across 137 sites across the eastern United States and California. To do this, it combined satellite images capable of detecting photosynthetic activity, continuous measurements of CO₂ in the canopy and sensors applied to the trunks to monitor tree growth in real time.
The results revealed a surprising disconnect between carbon uptake and wood growth. In oaks in the eastern United States, for example, growth is concentrated between May and July, while photosynthesis continues into October. In practice, approximately 36% of the carbon assimilated annually is captured when the tree has already stopped increasing its woody biomass. In California, the phenomenon is similar: about 26% of annual carbon absorption occurs after growth stops.
Where does the absorbed carbon go?
The crucial question is precisely this. If it is not transformed into new wood, the carbon captured through photosynthesis can be used for the production of leaves, roots and fruits, temporarily stored in the form of starches or used in the metabolic processes necessary for the survival of the plant. Some may even be released into the soil to feed the microbial communities living around the roots.
The problem is that these forms of storage are generally much more temporary than the carbon contained in wood. While a trunk can retain carbon for decades or centuries, leaves and other plant tissue quickly decompose again, returning some of the accumulated CO₂ to the atmosphere.
According to the authors, the phenomenon is closely linked to climatic conditions. As temperatures rise and water becomes scarce, trees lose the internal pressure needed to expand their tissues and produce new wood. Growth stops almost immediately, while photosynthesis can continue for weeks or months, albeit at a reduced rate.
A particularly worrying aspect concerns the fact that this separation between photosynthesis and growth was more marked in years characterized by strong oscillations between very humid periods and very dry periods. Precisely those extreme conditions which, according to climate forecasts, will become increasingly frequent in the coming decades.
What changes for the climate?
The implications are far from negligible. Many climate models used today assume that an increase in photosynthesis automatically results in greater tree growth and therefore a higher capacity of forests to sequester carbon.
This study suggests instead that a significant part of the absorbed CO₂ may not end up in the wood and therefore not contribute to long-term storage. If the phenomenon is also confirmed in other forest species and other ecosystems, it will be necessary to review the role of forests as natural allies in the mitigation of climate change downwards.
Forests remain critical to the health of the planet, but research reminds us that they cannot be considered an infinite or sufficient solution on their own. Protecting forests is essential, but drastically reducing fossil fuel emissions remains the most effective strategy to limit global warming.
