Why do Leaves Fall in Autumn?

Researchers at ETH Zurich have found that deciduous plants have an adjustment mechanism that serves to limit their growing season: Trees that photosynthesize more in Spring and Summer, shed their leaves in Autumn. The leaves of deciduous trees in temperate regions shine with yellow and red colors in all their glory just before they fall, announcing that Autumn is approaching. This process, called leaf senescence, allows trees to prepare for winter by suspending their growth and removing nutrients from their leaves. During the phenological cycle of trees, leaf senescence marks the end of the productive period, when carbon dioxide is absorbed by photosynthesis.

Global warming in recent years; European trees delayed leaf formation by about two weeks from 100 years ago, and the Autumn senescence delayed by about six days, leading to a longer vegetation cycle period. It is thought that the generally warmer climate will cause these plants under climate change to capture more carbon, which will continue to delay this senescence. However, researchers at ETH Zurich have come up with a view that advocates the opposite.

In an article published in the scientific journal, these researchers stated that there is a self-regulation mechanism that limits the productive period; They suggested that increased photosynthesis in Spring and Summer caused early senescence leading to earlier leaf fall in Autumn.  Constantin Zohner said, “In the past, it was very difficult to make accurate predictions about the growing season of trees because the factors of leaf senescence were not well understood.” says.

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Until now, scientists thought that the main factors in determining the timing of leaf senescence were the Autumnal fall in day length and air temperature that began after the end of Summer. However, some research has shown that in addition to this, leaf formation in Spring has a great effect on autumn leaf fall. “But because the importance of these mechanisms remained unclear, phenological models could only partially account for such possibilities at best,” says the biologist. Zohner suspected that the link between the Spring and Autumn phenologies could be explained by photosynthetic activity – or rather, the phenomenon of carbon sink limitation.

In this hypothesis, scarce soil nutrients such as nitrogen, among others, limit the amount of carbon dioxide a plant can absorb during the season. In Spring and Summer, the more carbon a tree absorbs, the sooner the leaf senescence begins. This role of photosynthesis has long been known in many plants, including crops, but has never been tested in trees. This was what motivated the ETH Zurich researchers to investigate Autumn phenology with a mix of field observations, laboratory tests and modeling.

Powerful effect of photosynthesis

Long-term research on six different European deciduous tree species over the past six decades has formed the basis of these experiments. Using this information, Zoner’s team tested the relative impact of various factors on the timing of the Autumn season, including Spring leaf formation, seasonal photosynthesis, carbon dioxide concentration, climate, and precipitation.

In addition, the researchers conducted a series of experiments with seedlings in climate chambers and outdoors. This enabled them to also isolate the factors of climate, sunlight and carbon dioxide concentration that affect the relationship between photosynthesis and leaf senescence. These long-term observations revealed a powerful influence on photosynthesis. With increasing photosynthesis in Spring and Summer, leaf senescence starts earlier each year. Every ten percent increase in photosynthetic activity extends leaf senescence eight days further. Experiments also support these discoveries.

A new Autumn seasonal model

“Our analysis suggests that seasonal photosynthesis, the climate in Autumn, and the length of days are key senescence factors,” says Deborah Zani. says. “Atmospheric carbon dioxide concentration, summer climate, light levels and precipitation are also some of the factors affecting the senescence, but indirectly because they affect photosynthesis.” Therefore, the Autumn season, which is warmer with climate change, is being delayed. However, this effect decreases with increased carbon dioxide concentration in Spring and Summer, hotter Summer periods and early leaf formation. Zani and Zohner developed a new Autumn phenology model that takes into account all factors according to their respective weights. This model allowed researchers to predict the fall season timing of the next sixty years up to 42% more consistently than previous models.

Later, the authors used this model to generate a revised estimate of the timing of leaf senescence for the remainder of the century. And the results were unexpected. By this time, the senescent was expected to be as late as two or three weeks by the end of the century. “Our new model argues the opposite: if photosynthesis continues to increase, the leaves will undergo senescence earlier than now, between three and six days.” says Zani. “This means that the growing season will stretch only eight to twelve days by the end of the century, about two to three times less than we previously thought.” adds Zani. He directed this data analysis and modeling as part of his master’s thesis at the Crowther Lab.

Effect on carbon balance

In their work, the researchers used data from the Pan European Phenology Project, evaluating a total of 434,000 phenological observations they made in 3,800 locations in Europe between 1948 and 2015. Six species were representatively studied: horse chestnut with white flowers, common birch, European beech, European hybrid, stalked oak, and rowan. The authors view their research as evidence of limited carbon dioxide absorption capacity in temperate forests. “Seasonal carbon dioxide intake is likely to be lower than other models predicted, due to rising temperatures.” says Zohner. Now, ETH Zurich researchers want to better understand the carbon sink limitation in our planet’s forests.

Source: https://www.bbc.co.uk/bitesize/articles/zbxh47h