Losses outlast gains in Northern Hemisphere tree growth under climate change

Forests are the lungs of the Earth, absorbing carbon dioxide and storing it in wood, a process that helps regulate the planet's temperature. But trees are not static machines; they are living things that grow faster or slower depending on the weather around them. When the air is warm and moist, many trees grow well. When it is too hot or too dry, they struggle. Scientists have long known that climate change is altering these conditions, but the big question has been whether the world's forests will ultimately grow more or less as the century unfolds. The answer has been frustratingly mixed. In some cold places, warmer weather seems to help trees grow faster, while in hot, dry places, the same warming seems to choke them. Because these local effects pull in different directions, it has been difficult to see the overall trend for the entire Northern Hemisphere.

A new study by researchers at the Hebrew University of Jerusalem and the Swiss Federal Institute for Forest, Snow and Landscape Research cuts through this confusion by looking at the big picture across thousands of forests. They gathered a massive collection of tree-ring records from 4,110 different locations across the Northern Hemisphere, representing more than 12 million individual measurements of tree growth stretching back to 1902. Instead of assuming that trees react to climate in a simple, straight-line way, the researchers used a computer model to learn the complex, curved relationships between climate and growth directly from the data. They then fed this model with future climate projections to see how forests would respond as the world continues to warm. The result is a clear, if sobering, direction: across the Northern Hemisphere, tree growth is projected to decline overall. More importantly, the study reveals a stark asymmetry in how these changes play out over time. Once a forest starts to suffer losses due to climate stress, those losses tend to stick. Gains, however, are fleeting.

The researchers found that while some areas might see temporary boosts in growth, these improvements rarely last. In contrast, when growth begins to decline, it almost never bounces back. By the end of the century, under a moderate climate scenario, 90 percent of the locations that experienced a significant drop in growth were still suffering from that decline. Meanwhile, only about 56 percent of the locations that saw a temporary increase in growth managed to hold onto those gains. This pattern held true in every well-sampled region, from the Mediterranean to northern Europe, and the gap between permanent losses and fading gains grew even wider under a high-emission scenario. The study suggests that forests are not simply swinging back and forth between good years and bad years; they are being pulled steadily downward.

The driving force behind this decline is a combination of rising temperatures and the increasing thirst of the atmosphere. As the air gets warmer, it holds more moisture, creating a higher "vapor pressure deficit," which is a measure of how much water the air is trying to pull from the soil and the trees. This rising demand for water often outweighs any benefits from increased rainfall. In many regions, the negative impact of this drying effect is so strong that it cancels out any positive effects from wetter weather. The researchers broke down the causes of growth changes and found that in the vast majority of grid cells across the network, the combined effect of warming and atmospheric water demand was negative. Even in places where rainfall increased, the heat and dryness of the air often suppressed growth more than the extra water could help.

The study also explains why the same climate changes can have such different results in different places. It comes down to where a specific forest starts on the curve of its growth potential. In some cold regions, a little bit of warming might push a tree into a better growth zone, but if the warming continues, the tree eventually crosses a peak and starts to decline as the heat becomes too intense and the air too dry. In other regions, trees are already near their limit, so any additional warming immediately pushes them into decline. The researchers visualized these relationships as a landscape of hills and valleys. A forest's future depends on the direction and distance it is pushed across this landscape by climate change. For many trees, the path leads over the peak and down the other side, where growth slows. For others, the path leads straight into a valley of decline.

Crucially, the study highlights that these losses are not just temporary setbacks. The analysis of how forests transitioned from their baseline growth to future states showed that once a forest enters a state of significant decline, it almost never returns to its previous state. In contrast, forests that experienced a period of accelerated growth often saw those gains fade away as the climate continued to shift. This suggests that the temporary benefits of a warming climate are fragile, while the damages caused by heat and drought are durable. The researchers noted that their model focused on the radial growth of trees, which is a key indicator of how much wood a tree adds each year, but it did not simulate other factors like carbon dioxide fertilization or insect outbreaks. Despite these limitations, the data-driven approach, which learned directly from millions of tree-ring observations, provides a robust view of the likely trajectory.

The findings challenge the idea that forests might adapt or recover easily from climate stress. Instead, the evidence points to a future where the negative impacts of a warming world accumulate. The study concludes that the growth of trees in the Northern Hemisphere is being drawn steadily downward, with losses that outlast gains. This asymmetry means that even if some regions see short-term improvements, the overall trend is one of reduction. As the century progresses, the balance shifts further, with the negative effects of warming and atmospheric demand becoming the dominant force across most of the network. The research offers a clear warning: the forests that have helped regulate our climate for millennia are becoming increasingly vulnerable, and the damage they suffer is likely to be permanent.