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The beginning of autumn may begin a week earlier in the future due to climate change causing trees to capture more carbon and drop leaves early in the year.
For decades, scientists expected temperate trees to gradually lose their leaves later in the year, making autumn later as average temperatures around the world rise.
Early observations seem to show this has been happening over the past few decades, driving a longer growing season that could slow the rate of climate change.
However, a new large-scale study of European trees by the Swiss Federal Institute of Technology found that this trend has started to reverse and the leaves drop earlier.
Instead, the new discovery means that autumn begins up to three weeks later, it will begin between three and six days earlier in the century.
Early observations seem to show this has been happening over the past few decades, driving a longer growing season that could hold back the rate of climate change.
The presence of leaves on deciduous trees marks the change of seasons and the length of time the trees store carbon from the air.
The alarming winters cause spring leaves to emerge earlier and is a widespread example of the impact of climate change, according to the team behind the study.
The moment when the leaves fall is more difficult to detect. There may be limits to the amount of greenhouse gases a tree can use or store in a single year, explained Dr. Zani.
If all carbon needs are met, leaves may fall sooner rather than later in the fall.
Changes in the growing season of trees greatly affect the global carbon balance, Zani explained, but future patterns are difficult to predict.
This is due to the fact that the environmental factors of foliar aging are not well understood by scientists.
Autumn shedding in temperate regions like the UK is an adaptation to stressors, such as cold weather, and a common hypothesis is that if you warm the air this would allow leaves to persist longer and fix more atmospheric carbon.
Dr Zani and colleagues used long-term observations from dominant Central European tree species from 1948 to 2015 and experiments designed to modify carbon uptake to assess related impacts.
The study showed an increase in the growing season in spring and summer due to more CO2, light and higher temperatures will lead to early leaf drop, not later.
This is likely because the roots and wood stop using or storing the carbon captured by the leaves in one spot, making the leaves expensive to maintain.
The researchers used the data to build a model to improve the fall forecast in a normal climate scenario – a scenario in which no efforts are made to slow the rate of climate change by preventing the global mean temperature from rising.
The model predicts the possibility that autumn dates fall earlier in the rest of the century rather than later, as previously assumed.
However, a new large-scale study of European trees by the Swiss Federal Institute of Technology found that this trend has started to reverse and the leaves are falling earlier.
Dr. Zani said: “Changes in the length of the temperate tree growing season greatly affect biotic interactions and the global carbon balance.
“However, the trajectories of the future growing season remain highly uncertain because the environmental factors of autumn leaf deterioration are poorly understood.”
Consideration of spring and summer productivity increases due to increased carbon uptake improved forecast accuracy by up to 42%.
“These findings demonstrate the critical role of well limitation in governing the end of seasonal activity and reveal important constraints on the length of the future growing season and the carbon uptake of trees,” said Zani.
The findings “substantially lower our expectations of the extent to which longer growing seasons will increase seasonal carbon uptake in forests,” he added.
The presence of leaves on deciduous trees marks the change of seasons and the length of time the trees store carbon from the air.
The researchers pointed out that the universality of this model in other forest types remains unknown – they can only say with certainty that it applies to temperate regions.
They note that an important next avenue of research is the implementation of such growing season length constraints on a wider range of systems.
Dr Christine Rollinson, a tree scientist at The Morton Arboretum in Illinois who was not involved in the study, said it shows that the forest is not a bottomless carbon sink.
“Therefore, while trees and forests remain a solution to mitigate the impacts of climate change, they cannot be the only means of response.
“A diversified portfolio of actions that includes emission reduction and tree conservation and planting is essential to mitigate anthropogenic carbon emissions and climate change.”
The results were published in the journal Science.
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