Trees may store less planet-heating carbon than hoped, study suggests
Trees may store less planet heating – Research published this week challenges the long-held belief that forests act as a reliable long-term solution for capturing carbon dioxide from the atmosphere. A study conducted across 137 locations in the United States reveals that the process of photosynthesis, which is central to tree growth, does not always translate into increased wood accumulation. This finding has significant implications for how scientists model the planet’s ability to mitigate climate change through natural carbon sinks.
Photosynthesis and Wood Growth: A Decoupling
Scientists discovered that tree growth halts several months before photosynthesis ceases in a given season. This means that a substantial portion of the carbon absorbed by forests is not being permanently stored in wood, as previously assumed. Instead, much of this carbon may be temporarily retained in other forms, such as leaves or internal tree processes, before being released back into the atmosphere.
The study highlights that forests, while vital to combating climate change, are not as effective at locking away carbon as models had estimated. These models often assume a direct correlation between photosynthesis rates and wood growth, but the researchers found that this connection is weaker than previously thought. For example, in the eastern United States, they observed that 36% of annual carbon uptake occurs after growth has stopped in late summer. In California, the figure was slightly lower, at 26%.
“Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” said Mukund Palat Rao, a carbon cycle scientist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study. “Just because there is more photosynthesis might not necessarily mean more tree growth in the future.”
Rao and his team also noted that wood growth is constrained by environmental conditions such as aridity and temperature. Their analysis of four specific sites showed that growth occurs primarily during periods of low heat and moisture stress. As global temperatures rise, these optimal conditions are becoming less frequent, which could reduce the capacity of forests to act as long-term carbon reservoirs.
“The moment you have dry and hot conditions, growth activity stops pretty instantly, while photosynthesis seems to continue at a slightly decreased rate,” Rao explained.
The Role of Forests in Climate Mitigation
Forests play a crucial role in slowing climate breakdown by absorbing carbon dioxide from the atmosphere. When trees convert CO₂ into wood, the carbon remains sequestered for decades or even centuries, far longer than carbon stored in other forms. This long-term storage is what makes forests a key component of climate strategies, alongside efforts to reduce emissions through renewable energy and industrial reforms.
However, the new findings suggest that the efficiency of this process may be underestimated. If the carbon absorbed by trees is not effectively directed into wood, its potential to combat rising atmospheric CO₂ levels diminishes. This could mean that forests, while still valuable, may not be as powerful a tool for carbon removal as current climate models indicate.
Implications for Carbon Uptake Models
The researchers argue that existing climate models may overestimate the future carbon sequestration potential of forests. These models typically rely on photosynthesis rates to estimate how much carbon is being stored in wood, without accounting for the decoupling between the two processes. As a result, projections about forest carbon sinks could be inflated, leading to misplaced confidence in their ability to offset emissions.
“Earth system models that assume consistently tight coupling between photosynthesis and growth may therefore overestimate future forest carbon sequestration under rising atmospheric moisture demand,” the team wrote in their analysis. This suggests that even as carbon dioxide levels continue to climb, forests might not be able to absorb and retain as much as expected, requiring more precise strategies for carbon management.
Broader Climate Context and Technological Needs
While land-based actions like reforestation remain the primary method for removing carbon dioxide from the atmosphere, they may not be sufficient to meet global climate goals. A recent report emphasized that humanity must accelerate the deployment of carbon removal technologies, such as direct air capture systems and bioenergy with carbon capture and storage (BECCS), to counteract the growing emissions from fossil fuels.
Land-based efforts currently account for the vast majority of carbon dioxide removal, with machines and chemical processes contributing just 0.1% of the 2.2 billion tonnes of CO₂ taken out of the atmosphere annually. This imbalance raises concerns about whether the planet’s natural carbon sinks can keep pace with the increasing demand for carbon storage. The study’s findings add to this debate, suggesting that forests may not be as reliable a solution as once believed.
The researchers are now investigating whether this decoupling of photosynthesis and wood growth applies to other tree species and regions beyond the US. If confirmed, it could lead to a reassessment of global carbon storage strategies. For instance, areas experiencing frequent heatwaves or prolonged droughts may see reduced forest capacity to act as long-term carbon reservoirs, even as atmospheric CO₂ levels continue to rise.
These insights underscore the need for more accurate representations of forest dynamics in climate models. By factoring in the nuanced relationship between photosynthesis and wood growth, scientists can better predict how forests will respond to future environmental changes. This, in turn, could inform more targeted conservation and management practices to maximize their carbon sequestration potential.
As the world grapples with the urgency of climate action, the study serves as a reminder that natural solutions must be understood in their complexity. While trees remain a cornerstone of climate mitigation, their effectiveness depends on a delicate balance of environmental factors. This research not only refines our understanding of forest carbon dynamics but also highlights the importance of integrating advanced technologies with natural systems to achieve sustainable carbon reduction targets.
