Leaves’ Pores Help Explain the Longstanding Mystery of Uneven Tree Growth in a Carbon-Enriched World
The basic idea of photosynthesis is familiar to almost everyone: plants take in carbon dioxide, water, and sunlight, and in return they produce oxygen and sugars that fuel growth. With atmospheric carbon dioxide levels steadily rising due to human activity, it has long seemed reasonable to assume that trees and forests would simply grow faster and store more carbon as a result.
But decades of real-world observations have shown that nature rarely follows such a simple rule. Tree growth across the globe has responded to higher carbon dioxide levels in highly uneven ways. Some forests appear to grow faster, some show little change, and others even grow more slowly. This puzzling inconsistency has challenged scientists for years. Now, new research led by scientists at Duke University and Wuhan University offers a detailed explanation, rooted in the tiny pores found on leaves.
The study, published in Nature Climate Change in December 2025, introduces a new model that focuses on how trees balance carbon intake with water loss. By examining this trade-off at the level of leaf pores—known as stomata—the researchers provide a clearer picture of why higher carbon dioxide does not always translate into greater tree growth.
Understanding why higher carbon dioxide doesn’t always boost growth
For many years, a common assumption in climate science was that rising carbon dioxide would act as a kind of fertilizer for plants. This idea, often called the CO₂ fertilization effect, suggested that trees would photosynthesize more, grow faster, and store more carbon as atmospheric carbon dioxide increased.
However, long-term experiments began to challenge this assumption. Two landmark studies played a major role in reshaping scientific thinking. One was conducted at Duke University, where researchers exposed forest plots to elevated carbon dioxide levels over a period of 16 years. The other took place at ETH Zurich, where scientists increased humidity around trees to examine how moisture influenced growth.
These experiments produced rich datasets on tree growth, carbon storage, and physiological behavior. While trees did respond to higher carbon dioxide in controlled conditions, the overall increase in growth and carbon sequestration was far smaller than originally predicted. Clearly, something else was limiting how trees responded.
The missing piece, as the new study shows, lies in how trees manage water.
The role of stomata in tree growth
Trees take in carbon dioxide through microscopic openings on their leaves called stomata. These pores open and close to regulate gas exchange. When stomata open, carbon dioxide enters the leaf, enabling photosynthesis. At the same time, however, water vapor escapes through these same pores in a process called transpiration.
This creates a constant balancing act. Opening stomata more widely allows a tree to absorb more carbon dioxide, but it also increases water loss. In environments that are hotter or drier, this water loss can become dangerous.
The new research shows that as atmospheric carbon dioxide rises, trees often respond by adjusting their stomatal behavior rather than simply absorbing more carbon. In many cases, trees partially close their stomata to conserve water. This reduces water loss, but it also limits how much carbon dioxide enters the leaf. The result is that photosynthesis—and therefore growth—does not increase as much as expected.
This trade-off becomes even more important as trees grow taller. Water must be pulled upward from the roots through the trunk and branches to the leaves. Excessive water loss at the leaf level can disrupt this delicate internal water tension, increasing the risk of hydraulic failure. To avoid this, trees often prioritize survival over faster growth.
An engineering approach to plant biology
One of the most distinctive aspects of this study is its engineering-based perspective. Rather than relying solely on traditional plant physiology theories, the researchers treated stomatal behavior as an optimization problem.
From this viewpoint, a tree operates like a finely tuned system that constantly adjusts to maximize benefits while minimizing risks. The system aims to gain enough carbon for growth while avoiding excessive water loss that could threaten the tree’s survival.
Using detailed measurements from the long-term experiments at Duke and ETH Zurich, the researchers built a model that links stomatal opening, atmospheric carbon dioxide levels, humidity, temperature, and water transport within the tree. The model successfully reproduced the wide range of growth outcomes observed in forests around the world over recent decades.
This approach helps explain why studies of tropical forests, in particular, have produced mixed results. Even though carbon dioxide levels have risen steadily for more than half a century, growth responses differ depending on local climate conditions, water availability, and tree physiology.
Why water-use efficiency doesn’t guarantee more growth
Another key insight from the study concerns water-use efficiency, a measure of how much carbon a plant gains per unit of water lost. Rising carbon dioxide often increases a tree’s intrinsic water-use efficiency, because trees can maintain photosynthesis with smaller stomatal openings.
At first glance, this seems like a clear advantage. But the new research shows that higher water-use efficiency does not automatically lead to higher productivity. If environmental conditions limit how much stomata can open—due to heat, drought, or internal water transport constraints—then gains in efficiency may not translate into faster growth or increased carbon storage.
This finding challenges a common assumption in large-scale climate and carbon cycle models, many of which still rely on simplified relationships between carbon dioxide and plant growth.
Broader implications for climate modeling and forest management
Understanding how trees respond to rising carbon dioxide is crucial for predicting future climate change. Forests play a major role in absorbing carbon dioxide from the atmosphere, acting as a natural buffer against global warming.
If trees absorb less carbon than previously assumed, then future warming could be more severe than some models predict. The new findings suggest that water availability and climate conditions must be fully integrated into models that estimate how much carbon forests can store over time.
The researchers also note that their model can be expanded further. Factors such as soil nutrients, seasonal changes, competition between plants, and interactions with animals all influence tree growth. While the current model operates at the level of individual trees, future work will be needed to scale these insights up to entire forests and regions.
Extra context: how trees manage water under stress
Trees have evolved a range of strategies to cope with water stress. Some species develop deep root systems to access groundwater, while others shed leaves during dry periods to reduce water loss. Leaf size, shape, and surface coatings also influence how quickly water evaporates.
Stomatal control is one of the most immediate and flexible responses. By adjusting the size and opening of stomata, trees can respond within minutes to changes in temperature, humidity, and light. This flexibility helps explain why tree responses to rising carbon dioxide are so varied across ecosystems.
In tropical forests, where temperatures are high and humidity can fluctuate dramatically, stomatal regulation plays an especially important role. The new study’s ability to explain growth variability in these regions is one of its most significant contributions.
A clearer picture of forests in a changing world
Taken together, the findings offer a more realistic and nuanced view of how forests behave in a carbon-enriched atmosphere. Trees are not passive recipients of extra carbon dioxide. Instead, they actively regulate their internal processes to balance growth with survival.
By focusing on the tiny pores in leaves, this research helps resolve a longstanding mystery in climate science. It also underscores the importance of interdisciplinary approaches, combining biology, physics, and engineering, to better understand the natural systems that shape our planet’s future.
Research paper reference:
https://www.nature.com/articles/s41558-025-02504-w
