Climate change is causing autumn leaves to change color sooner – here’s why

As the days get shorter and temperatures drop in the Northern Hemisphere, the leaves begin to turn. We can enjoy glorious fall colors while the leaves are still on the trees, and later kick a red, brown and gold carpet when walking.

When temperatures rise again in spring, the tree growing season resumes. During the warmer months, trees take carbon dioxide from the atmosphere and store it in complex molecules, releasing oxygen as a byproduct. This, in a nutshell, is the photosynthesis process. The higher the photosynthesis, the more carbon is blocked.

We know that carbon dioxide is a major driver of climate change, so the more it can be removed from the atmosphere by plants, the better. With warmer weather leading to a longer growing season, some researchers have suggested that more carbon dioxide would be absorbed by trees and other plants than previously. But a new study has overturned this theory and could have profound effects on how we adapt to climate change.

Reaching the limit

The researchers, led by Deborah Zani at the Swiss Federal Institute of Technology, studied the degree to which the timing of color changes in autumn tree leaves was determined by the plant’s growth in the previous spring and summer.

Temperature and day length were traditionally accepted as the main determinants of when leaves changed color and fell, leading some scientists to assume that warming would delay this process until the end of the season. By studying European deciduous tree species, including horse chestnut, silver birch and English oak, the authors of the new study recorded the amount of carbon absorbed by each tree per season and how that ultimately affected leaf fall.

Using data from the Pan European Phenology Project, which has been monitoring trees for 65 years, the researchers found in their long-term observational study that as the rate of photosynthesis increased, the leaves changed color and fell off. beginning of the year. For every 10% increase in photosynthetic activity during the spring and summer growing season, trees shed their leaves, on average, eight days earlier.

Climate-controlled experiments on five-year-old European beeches and Japanese meadowsweet trees suggest what may be behind this unexpected result. In these trials, the trees were exposed to full sun, half shade or full shade. The results show that there is a limit to the amount of photosynthesis a tree can perform during a growing season. Think about how to fill a bucket of water. It can be done slowly or quickly, but once the bucket is full, there is nowhere for more water to go.

This research shows that deciduous trees can only absorb a certain amount of carbon each year and once that limit is reached, it can no longer be absorbed. At that point the leaves begin to change color. This limit is set by the availability of nutrients, in particular nitrogen, and by the physical structure of the plant itself, in particular the internal vessels that move the water and the dissolved nutrients. Nitrogen is a key nutrient that plants need to grow, and it is often the amount of available nitrogen that limits total growth. This is why farmers and gardeners use nitrogen fertilizers to overcome this limitation.

Together, these constraints mean that carbon uptake during the growing season is a self-regulating mechanism in trees and herbaceous plants. It is only possible to absorb so much carbon.

The first autumn colors

In a world with increasing levels of carbon in the atmosphere, these new findings imply that a warmer climate and longer growing seasons will not allow temperate deciduous trees to absorb more carbon dioxide. The study’s predictive model suggests that by 2100, when tree growing seasons are projected to be between 22 and 34 days longer, leaves will drop from trees three to six days earlier than they do now.

This has significant implications for modeling climate change. If we accept that the amount of carbon absorbed by deciduous trees in high-temperature countries like the UK will remain the same every year regardless of the growing season, then carbon dioxide levels will rise faster than previously predicted. The only way to change this will be to increase the ability of trees to absorb carbon.

Plants that are not limited by the amount of nitrogen available may be able to grow longer in hot weather. These are the trees that can take nitrogen from the air, such as alder. But these species will lose their leaves at around the same time as ever, thanks to less daylight and colder temperatures.

But on the upside, with the prospect of some trees losing their leaves sooner and others losing them the moment they do now, there may be the prospect of prolonged fall colors – and more time for us to kick in the leaves.