As carbon dioxide levels continue to rise in the atmosphere, scientists have long believed that forests would grow faster, absorb more carbon, and help slow climate change. The idea seemed logical: plants use carbon dioxide, water, and sunlight to produce sugars and grow. More carbon in the air should mean more growth. But real-world forests are not responding the way simple theories once predicted.
Long-term measurements from forests across the world show a mixed and often confusing picture. In some places, tree growth has increased slightly. In others, it has stayed flat or even declined. This inconsistency has left scientists questioning why higher carbon dioxide does not consistently translate into stronger forest growth and higher carbon storage.
A new study by researchers from Duke University and Wuhan University suggests the answer lies not just in carbon, but in water. The researchers argue that trees constantly balance a trade-off between absorbing carbon dioxide and conserving water, especially under hotter and drier conditions driven by climate change.
Trees take in carbon dioxide through tiny pores on their leaves called stomata. These pores open to let carbon in, but at the same time, they allow water to escape. When carbon dioxide levels are high, trees can partially close these pores and still absorb enough carbon, reducing water loss. This improves what scientists call water-use efficiency. However, this does not automatically lead to faster growth.
As temperatures rise and air becomes drier, water loss through open pores increases sharply. To avoid damage, trees respond by closing their stomata more often. This protects their internal water transport system but also limits the amount of carbon dioxide they can absorb. In effect, trees prioritise survival over rapid growth.
The research team developed a model that treats this daily decision-making process as an optimisation problem. Trees, in simple terms, try to maximise carbon gain without risking dangerous water loss. The model was tested using detailed data from two rare long-term experiments.
At a forest site in North Carolina, trees were exposed to elevated carbon dioxide levels for 16 years. At another site in Switzerland, scientists increased air humidity to study how moisture affects tree behaviour. In both cases, trees did not store as much extra carbon as earlier models had predicted.
The results showed that higher carbon dioxide alone does not guarantee stronger growth. When air is moist, trees can keep their leaf pores open longer with less risk, allowing more carbon intake. But when air is hot and dry, trees restrict carbon intake to protect their water systems, cancelling out much of the potential growth benefit from higher carbon dioxide.
When the researchers applied this framework to decades of tropical forest data, many previously puzzling results began to make sense. Forests growing in warmer and drier conditions showed little to no growth boost, despite rising carbon dioxide. In wetter regions, growth gains were more likely, though still limited.
The study highlights that forest responses to climate change are highly conditional. Nutrients, soil moisture, heat stress, tree height, species composition, and local climate all influence how trees respond to rising carbon levels. Carbon fertilisation is not a universal solution and cannot be relied upon as a guaranteed climate buffer.
Scientists say the findings carry important lessons for climate policy and carbon modelling. Relying on forests to automatically absorb more carbon as emissions rise may lead to overconfidence. Protecting forests from heat stress, drought, and land degradation may be just as important as reducing emissions themselves.
The research reinforces a simple but critical message: more carbon in the air does not automatically mean more carbon stored in forests. Between the atmosphere and the wood lies a fragile system of water balance and biological limits. If forests are to help slow climate change, they must first be kept healthy, hydrated, and resilient in a warming world.