🌳 These Large Trees Absorb CO₂ Without Growing, and That's Not Good News

Current climate models often assume that increased CO₂ stimulates photosynthesis, leading to enhanced tree growth, a phenomenon known as the carbon fertilization effect. Yet, researchers of the study, published in Science Advances, show that in oaks, carbon assimilation and wood production can be disconnected. This decoupling occurs especially when environmental conditions are unfavorable for growth, such as during drought.

To reach these results, the scientists combined several types of data. Satellites measured photosynthetic activity at 137 sites in the eastern United States and California. Instruments installed near tree canopies recorded CO₂ exchanges hour by hour, while sensors attached to trunks tracked minute variations in their diameter. Growth rings and historical temperatures completed this picture.


These measurements showed that in the eastern United States, oaks stop growing between May and July, but their leaves remain active until October. About 36% of their annual carbon absorption occurs after growth stops. In California, the pattern is similar: growth stops in August, but 26% of carbon is captured afterward. Thus, leaves can continue functioning while the tissues responsible for wood expansion are already at rest.

This lag has a biological explanation. Tree growth depends on internal water pressure that allows cells to expand. Under hot and dry conditions, this pressure drops and growth stops quickly. Photosynthesis, however, can continue at a reduced rate. The study reveals that the disconnection is strongest during years marked by abrupt alternations between wet and dry periods, a phenomenon expected to intensify with climate change.

So, where does this carbon absorbed after growth go? Part of it is stored as starch to restart growth the following year. Other portions are used to produce leaves, roots, or to keep cells alive during winter. But this carbon does not end up in durable wood: it returns to the atmosphere more quickly. Researchers are now exploring whether this phenomenon occurs in other species and in other ecosystems, in order to refine models predicting the future role of forests in the carbon cycle.