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Which Factors Trigger Leaf Die-Off in Autumn?

Ever wondered what would cause the leaves on trees to change color and fall off? We discuss all the factors to leaf die-off in autumn.

Research done by people at ETH Zurich were able to identify an interesting mechanism in deciduous trees that happens to limit that trees growing season length. This mechanism is trees that photosynthesise more in spring and summer lose their leaves in the early months of autumn.

Leaves of temperate deciduous trees glow in all their yellow and red glory just before falling, signalling that autumn has come. This process, called leaf senescence, allows trees to prepare for the coming winter by suspending their growth and extracting nutrients from the foliage. In the trees’ phenological cycle, leaf senescence marks the end of the productive period during which they absorb CO2 through photosynthesis.

Global warming has resulted in longer vegetation periods in recent years, with spring leaf emergence in European trees happening about two weeks earlier than 100 years ago and autumn senescence about six days later. It is generally expected that senescence will continue to be delayed in a warming climate, increasing the amount of carbon captured by these plants under climate change.

However, researchers at ETH Zurich have now come to the opposite conclusion. In a study published in the journal Science, they have demonstrated a self-regulating mechanism that limits the productive period. Increased photosynthesis in spring and summer leads to earlier senescence, which could result in earlier leaf fall in autumn.

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Strong effect of photosynthesis

Long-term observations of six European deciduous tree species over the last six decades formed the basis of the study. Using this data, Zohner’s team tested the relative influence of various factors on the timing of autumn senescence, including leaf emergence in spring, seasonal photosynthesis, CO2 concentration, temperature and precipitation.

In addition, the researchers also performed a set of experiments with saplings in climate chambers and outdoors. This enabled them to isolate the effects of temperature, daylight and CO2 content that drive the correlation between photosynthesis and leaf senescence.

The long-term observations revealed a strong effect of photosynthesis: in years with increased photosynthesis in spring and summer, leaf senescence began earlier, with each ten percent increase in photosynthetic activity advancing leaf senescence by eight days. The experiments supported these findings.

A new autumn senescence model

Warmer autumns under climate change therefore tend to postpone senescence. This effect, however, is counteracted by increasing photosynthesis in spring and summer through rising CO2 concentrations, warmer summer periods and earlier leaf emergence.

Zani and Zohner developed a new model of autumn phenology that takes all factors into account according to their relevant weight. This model enabled the researchers to predict the timing of autumn senescence over the last six decades with up to 42 percent more accuracy compared to previous models.

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