As the days shorten and temperatures drop in the northern hemisphere, leaves begin to turn. We can enjoy glorious autumnal colours while the leaves are still on the trees and, later, kicking through a red, brown and gold carpet when out walking.

 

随着北半球白昼缩短、气温下降，叶子开始变黄。当叶子还在树上时，我们能观赏到秋日的绚丽色彩，在外出散步时，还能踩着红色、棕色和金色的“地毯”。

 

When temperatures rise again in spring, the growing season for trees resumes. Throughout the warmer months, trees take carbon dioxide from the atmosphere and store it in complex molecules¹, releasing oxygen as a byproduct. This, in a nutshell, is the process of photosynthesis². The more photosynthesis, the more carbon is locked away.

 

We know that carbon dioxide is a major driver of climate change, so the more that can be taken out of the atmosphere by plants, 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 effects on how we adapt to climate change.

 

The researchers, led by Deborah Zani at the Swiss Federal Institute of Technology, studied the degree to which the timing³ of colour changes in autumn tree leaves was determined⁴ by the growth of the plant in the preceding spring and summer.

 

Temperature and day length were traditionally accepted as the main determinants of when leaves changed colour and fell, leading some scientists to assume that warming temperatures would delay this process until later in the season. Studying deciduous⁵ European tree species, including horse chestnut⁶, silver birch and English oak, the authors of the new study recorded how much carbon each tree absorbed per season and how that ultimately affected⁷ when the leaves fell.

 

Using data from the Pan European Phenology Project, which has tracked some trees for as long as 65 years, the researchers found in their long-term observational study that as the rate of photosynthesis increased, leaves changed colour and fell earlier in the year. For every 10% increase in photosynthetic⁸ activity over the spring and summer growing season, trees shed their leaves, on average, eight days earlier.

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