Why trees deciduous in the fall

Researchers at the Swiss Federal Institute of Technology (ETH Zurich) have discovered a self-regulating mechanism in autumn deciduous trees in Europe shortens the growth time of trees.

Those are plants that photosynthesize more in the spring and in summer will shed their leaves early in the fall.

The leaves of temperate deciduous trees turn bright yellow and red before falling, signaling the arrival of autumn. This is the aging process of leaves, helping plants prepare for winter by stopping their growth and taking nutrients from the foliage. During the plant’s posterior cycle, aging leaves mark the end of the growth phase where the plant absorbs CO2 through photosynthesis.

Global warming has resulted in longer growth stages in recent years, with the emergence of leaves in the spring two weeks earlier than 100 years ago and leaves aging six days later. Leaf aging will continue even more slowly if the climate continues to be warm, increasing the amount of carbon the plant absorbs under changing climate conditions.

However, scientists at ETH Zurich recently concluded the opposite. In a study just published in the journal Science, they explained the self-regulation mechanism shortens the growth period of plants. Increased photosynthesis in the spring and summer leads to earlier aging, causing the autumn leaves to fall off earlier.

A decrease in carbon pool is a determinant of leaf aging

In the past, it was difficult to accurately predict the growth season of a tree because the factors that lead to aging of the leaves were not fully understood. Up to now, scientists believe that when the summer is over, the autumn weather makes the temperature lower, the day shortens. These are the main factors in the plant’s leaf vitality. Some other studies have also shown that leaf growth during the spring has an effect on leaf loss in autumn.

“But because of how important these mechanisms are, we still do not fully understand, the post-physics models can only apply these effects somewhat” – Dr. Constantin Zohner, lead research of ETH Zurich said.

The relationship between autumn and spring post-histology can be explained by photosynthesis, or more accurately, carbon sink depletion. In this hypothesis, when the soil is scarce for nutrients such as nitrogen, the amount of CO2 that the plants absorb during autumn is also reduced. The more carbon a tree absorbs during spring and autumn, the earlier the season of withered leaves begins.

This role of photosynthesis in controlling leaf aging has long been known, for example in seasonal crops, but has never been tested in other plants. . That is the reason why scientists at ETH Zurich have studied the post-physics determinants by combining field observations with experiments and laboratory models.

Strong effect of photosynthesis

Through long-term observations of six species of deciduous trees in Europe over the past six decades, scientists have had the basis to conduct this study. From the data collected, Dr. Zohner’s team tested the relative effects of different factors on the time the leaves fade in the fall, including the following factors: strong growth of leaves in spring, seasonal photosynthesis, CO2 concentration, temperature, and precipitation.

In addition, the researchers also conducted a kind of experiment with outdoor saplings and in experimental cages to be able to distinguish how the effects of temperature, daylight and CO2 concentrations affect. correlation between photosynthesis and leaf aging.

Long-term observation also shows a strong effect of photosynthesis: during the years when the leaves are more photosynthesis in spring and summer, the earlier leaves age at a rate of 10% increase in photosynthetic activity leading to older leaves. 8 days early. Experiments have further confirmed these findings.

The new model is more accurate

Research shows that seasonal photosynthesis, autumn temperature and day length are the main factors that influence leaf aging. Other factors, such as CO2 concentrations in the air, summer temperatures, light intensity and precipitation, also have an influence, but only indirectly through their influence on photosynthesis.

Warm fall due to climate change slows down leaf aging. However, this effect is blocked by increased photosynthesis in spring and summer through increased CO2 concentrations, warmer summers and earlier leaf development.

The researchers have built a new fall post-physics model that takes all of these factors into account at the impact level of each. The new model helps them predict when the leaves age in fall with a 42% increase in accuracy over the old models. Previous projections suggested that leaf aging would appear two to three weeks later by the end of this century, but the new model has the opposite result, meaning that if photosynthesis continues to increase, the leaves will age 3 to 6 days earlier than now. That means the growing season will extend 8 to 12 days by the end of this century.

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