As carbon dioxide levels in the atmosphere continue to rise, a long-standing assumption has guided climate thinking: more CO₂ should help forests grow faster, absorb more carbon, and slow global warming. However, real-world forest data collected over decades has repeatedly challenged this idea. Tree growth and long-term carbon storage have shown inconsistent responses, ranging from slight increases to no change and even declines in some regions.
A new study led by researchers from Duke University and Wuhan University suggests the answer lies not in carbon alone, but in water. By examining how plant leaves manage the trade-off between absorbing carbon dioxide and conserving water, scientists are gaining a clearer picture of why forests do not always respond predictably to rising CO₂.
Plants absorb carbon dioxide through tiny pores on their leaves known as stomata. These openings allow CO₂ to enter for photosynthesis, but at the same time, they release water vapor into the air. In theory, higher carbon dioxide levels mean plants can take in enough carbon while keeping these pores partially closed, improving water efficiency and boosting growth.
In reality, warming temperatures and drier air complicate this balance. As heat increases and humidity drops, water loss through open stomata accelerates. To avoid damaging their internal water transport systems, trees often respond by closing these pores, which limits both water loss and carbon intake. This protective response can reduce or cancel out any growth advantage from higher CO₂ levels.
To better understand this process, the research team developed a model that treats stomatal behavior as a daily optimization problem. Trees, in effect, “decide” how much to open their leaf pores by weighing the benefits of carbon gain against the risks of excessive water loss. This engineering-inspired approach successfully reproduced decades of puzzling field observations that traditional models could not explain.
The model was tested using data from two rare long-term experiments. At Duke University, a forest plot was exposed to elevated carbon dioxide for 16 years. At ETH Zurich, researchers increased local humidity to observe how trees responded. In both cases, trees stored far less additional carbon than earlier models had predicted. The key factor was not carbon availability, but atmospheric moisture.
When the air was moist, trees could afford to keep their stomata open longer, allowing higher carbon uptake. In hot, dry conditions, however, stomata closed more frequently to protect the trees’ hydraulic systems, limiting growth despite higher CO₂ levels. These findings explain why forest responses differ so widely across regions and climates.
Applying this framework to tropical forest studies spanning the past 50 years helped resolve long-standing contradictions. In areas where warming and dry air intensified, tree growth gains from higher carbon dioxide were muted or reversed. In regions where moisture buffered heat stress, forests were more likely to show increased growth.
The researchers stress that carbon dioxide fertilization is not a myth, but its impact is conditional. Forest growth depends on the balance between carbon supply and water demand, mediated by microscopic leaf structures and the plant’s internal plumbing. Other factors such as soil nutrients, species composition, pests, and forest age also play critical roles.
The findings carry important implications for climate policy and modeling. Relying on forests to automatically absorb increasing amounts of carbon may be unrealistic in a warming, drying world. Protecting forest water balance by conserving soil moisture, reducing heat stress, and limiting additional environmental pressures could be just as important as reducing emissions.
Ultimately, the study highlights a simple but powerful insight: more carbon in the air does not automatically translate into more carbon stored in forests. Between those two lies a complex biological system shaped by survival, not speed. If forests are to remain effective allies against climate change, keeping them hydrated may matter as much as the carbon they breathe in.