As the daylight shortens and temperatures drop in the Northern Hemisphere, leaves begin to turn yellow.

While the leaves are still on the trees, we can enjoy the vibrant autumn colors, and when taking a walk, we tread upon a blanket woven with red, brown, and golden hues.

As spring arrives and temperatures rise, trees rejuvenate. During the warm months, trees absorb carbon dioxide from the atmosphere and store it in complex molecules, releasing oxygen as a byproduct. In essence, this process is known as photosynthesis. The more photosynthesis occurs, the more carbon is sequestered.

Carbon dioxide is a primary driver of climate change, making plants a crucial force in stabilizing the climate. With climate change leading to longer growing seasons for plants, some researchers believed that trees and other plants would absorb more carbon dioxide. However, recent studies have challenged this theory, potentially having profound implications for how humanity adapts to climate change.

A team led by Deborah Zani from the Swiss Federal Institute of Technology investigated the timing of autumn leaf color change. Traditionally, temperature and daylight duration have been the main factors determining when leaves change color and fall, leading some scientists to believe that global warming would delay this process.

The authors of the new study examined deciduous tree species in Europe, including European beech, birch, and English oak. They recorded the amount of carbon each tree absorbed each season and its impact on the carbon sequestration process when the leaves eventually fell.

Using data from the Pan-European Phenology Project spanning 65 years, the researchers found that as the rate of photosynthesis increased, leaves changed color and fell earlier in the year. For every 10% increase in photosynthetic activity during the spring and summer growing seasons, trees, on average, shed their leaves 8 days earlier.

The study suggests that deciduous trees can only absorb a certain amount of carbon each year, and once that limit is reached, they cannot absorb more. At that point, leaves begin to change color. This limitation is determined by the availability of nutrients, especially nitrogen, and the physical structure of the plant itself, including internal vessels for water transport and nutrient dissolution.

Nitrogen is a critical nutrient for plant growth, and its availability often limits growth, leading farmers and gardeners to use nitrogen fertilizers to overcome this constraint.

In summary, these limitations mean that carbon absorption during the growing season is a self-regulating mechanism for both trees and herbaceous plants. There is a limit to the carbon fixed by plants.

Predictive models indicate that by 2100, the tree growing season is expected to extend by 22 to 34 days, with leaves dropping 3 to 6 days earlier than they do now. This has significant implications for climate change modeling. If these conclusions are accepted, the rate of increase in carbon dioxide levels will be faster than previously expected. The only way to change this situation is to increase the ability of trees to absorb carbon.