As the days get shorter and temperatures drop in the Northern Hemisphere, the leaves begin to turn. We can enjoy brilliant fall colors while the leaves are still hanging on the trees and later kick through a red, brown and gold carpet while walking.
When the temperature rises again in the spring, the growing season for trees resumes. During the warmer months, trees remove carbon dioxide from the atmosphere and store it in complex molecules, releasing oxygen as a byproduct. In a nutshell, this is the process of photosynthesis. The more photosynthesis, the more carbon is trapped.
We know that carbon dioxide is a major driver of climate change, so the more plants that can get out of the atmosphere, the better. With the warmer climate leading to a longer growing season, some researchers have suggested that more carbon dioxide would be absorbed by trees and other plants than in previous times. But a new study has turned this theory on its head and could have profound implications for how we adapt to climate change.
Reach the limit
The researchers, led by Deborah Zani of the Swiss Federal Institute of Technology, studied the extent to which the timing of color changes in fall leaves was determined by the plant’s growth in the preceding spring and summer.
Temperature and day length have traditionally been accepted as the main determinants of when leaves changed color and fell, leading some scientists to assume that warming temperatures would delay this process until later in the season. Studying deciduous European tree species, including horse chestnut, silver birch and English oak, the authors of the new study recorded how much carbon each tree absorbed per season and how that ultimately affected when the leaves fell.
Using data from the Pan-European Phenology Project, which has tracked some 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 earlier in the year. For every 10% increase in photosynthetic activity during the spring and summer growing season, trees drop their leaves an average of eight days earlier.
Climate-controlled experiments with five-year-old beech and Japanese meadowsweet suggest what could be behind this unexpected result. In these trials, the trees were exposed to full sun, partial shade or shade. The results show that there is a limit to the amount of photosynthesis a tree can perform during a growing season. Think of it like filling a bucket with water. It can be done slowly or quickly, but once the bucket is full there is nowhere for water to go.
This research shows that deciduous trees can only absorb a certain amount of carbon per year and once that limit is reached, no more can be absorbed. At that point the leaves begin to change color. This limit is determined by the availability of nutrients, especially nitrogen, and the physical structure of the plant itself, especially the inner vessels that move water and dissolved nutrients. Nitrogen is an important nutrient that plants need to grow, and it is often the amount of available nitrogen that limits overall growth. This is why farmers and gardeners use nitrogen fertilizers to overcome this limitation.
Earlier fall colors
In a world with increasing levels of carbon in the atmosphereThese new findings imply that warmer weather and longer growing seasons keep temperate deciduous trees from absorbing more carbon dioxide. The study’s predictive model suggests that by 2100, when trees’ growing seasons are expected to be between 22 and 34 days longer, leaves will fall from trees three to six days earlier than they are now.
This has significant implications for climate change modeling. If we accept that the amount of carbon absorbed by deciduous trees in temperature countries such as the UK will remain the same every year regardless of the growing season, carbon dioxide levels will rise faster than previously expected. The only way to change this is to increase trees’ ability to absorb carbon.
Plants not limited by the amount of nitrogen available may be able to grow longer in the warming climate. These are the trees that can get nitrogen from the air, like alder. But these strains still lose their leaves at about the same time as always, thanks to less daylight and colder temperatures.
But on the bright side, 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 long-lasting fall color – and more time for us to kick through the leaves.