The Disconnect Between Photosynthesis and Wood Growth
Forests serve as a critical defense against climate change, primarily by converting atmospheric carbon dioxide into wood. This process effectively traps carbon for decades or even centuries. However, a recent study involving 137 sites across the United States suggests that the capacity of forests to act as long-term carbon sinks may be lower than previously estimated. Researchers discovered that trees frequently cease their growth months before the biological process of photosynthesis concludes for the year.
Current climate models often rely on photosynthesis rates to predict how much carbon a forest will sequester. According to Mukund Palat Rao, a carbon cycle scientist at the Lamont-Doherty Earth Observatory at Columbia University and the study's lead author, these models operate on the assumption that photosynthesis directly translates to growth. The findings indicate that this assumption is flawed, as the two processes are not as tightly coupled as scientists previously believed.
The Science Behind Carbon Sequestration
The study highlights a significant discrepancy in how carbon is utilized by trees. In the eastern United States, researchers observed that approximately 36% of annual carbon uptake occurred after tree growth had already stopped in late summer. In California, that figure stood at 26%. When trees continue to photosynthesize without producing wood, the carbon is diverted to transient uses, such as foliage or internal processes, which do not store the molecule for the long term.
Detailed measurements at four specific sites revealed that wood growth is highly sensitive to environmental conditions, restricted primarily to periods of low aridity and moderate temperatures. As global temperatures rise, the frequency of heatwaves and droughts is increasing, creating conditions that cause growth to halt almost instantly. While photosynthesis may continue at a reduced rate during these stressful periods, the vital conversion into woodâthe mechanism that keeps carbon out of the atmosphereâis effectively paused.
Public Impact and Climate Strategy
The implications of these findings are significant for global climate policy. Currently, land-based efforts, such as tree planting, account for the vast majority of human-led carbon removal initiatives. In contrast, mechanical and chemical carbon removal technologies currently handle only 0.1% of the 2.2 billion tonnes of CO2 removed globally each year. If forests are less efficient at long-term storage than models suggest, the reliance on these natural sinks may need to be re-evaluated.
The researchers noted that Earth system models, which assume a consistent link between photosynthesis and growth, likely overestimate future forest carbon sequestration. This is particularly concerning as atmospheric moisture demand rises alongside global temperatures. If a larger portion of absorbed carbon is directed toward short-term biological functions rather than wood production, the overall effectiveness of forests as a climate mitigation tool could diminish.
What Researchers Say Next
The research team is expanding its scope to determine if the decoupling of photosynthesis and wood growth is a widespread phenomenon. Future studies will investigate whether these patterns hold true across different tree species and in diverse geographical regions outside of the United States. By refining these models, scientists hope to gain a more accurate understanding of how forests will perform as carbon sinks throughout the 21st century, especially as environmental stressors become more common.
Source: The Guardian.
