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Autumn forest with beeches in Bavaria
Photo: S. Meyers / blickwinkel / imago images
In our latitudes it is an ancient cycle of nature: In autumn, deciduous trees adjust to the approaching winter, to a certain extent throttle the system and go into a kind of winter standby.
Triggered by the length of the day and the temperature, a process begins that removes nutrients from the leaves.
They color, in the end they just fall off.
Biologists call this annually recurring cycle abscission.
Due to climate change and longer warm periods, the leaf phases on the deciduous trees are lengthening, which researchers have been observing for a long time.
Because the growing seasons have increased due to global warming.
Spring comes earlier, winters are milder.
However, researchers from Switzerland have now found that the leaves do not automatically extend with them - on the contrary, they even die off earlier.
Behind this is a self-regulating mechanism of deciduous trees that limits their growth phase, write researchers led by first author Deborah Zani from ETH Zurich.
Trees that photosynthesize more in spring and summer shed their leaves earlier in autumn, according to the study published in the journal Science.
In a large-scale study, the researchers evaluated environmental observations from 3800 locations in Central Europe between 1948 and 2015.
Horse chestnuts, birches, red beeches, larches, pedunculate oaks and rowan trees were examined.
To this end, the researchers carried out experiments with young trees outdoors and in climatic chambers.
Temperature, daylight and CO2 content were varied and the respective effects on photosynthesis and leaf aging were investigated.
In the years with increased photosynthesis in spring and summer, leaf aging also occurred earlier in autumn for all species examined.
"If photosynthesis continues to rise, the leaves will age three to six days earlier than today over the course of the century - and not later," explains Zani.
According to this, the growing season would only be extended by eight to twelve days by the end of the century instead of the time predicted by models of up to three weeks.
"Up to now, it has hardly been possible to make precise predictions about the tree's growing season."
Photosynthesis, autumn temperature and the length of the day dominated for all factors that have an influence on the processes of senescence, as leaf aging is technically known.
The remaining factors such as CO2 content, summer temperatures, light intensity and precipitation only have an indirect effect on the autumn senescence.
"Precise prognoses about the growing season of trees have so far hardly been possible because the causes of leaf senescence were not sufficiently understood," study director Constantin Zohner from the Crowther Lab at ETH Zurich is quoted in a communication.
He suspects that the phenomenon of the limited carbon sink is behind the observed mechanism.
Among other things, scarce soil nutrients such as nitrogen limit the amount of CO2 that a plant can absorb during the season.
If the maximum amount of CO2 is reached, the leaf aging begins accordingly earlier.
And what does this message mean for climate change?
Due to the faster leaf aging, the trees could store significantly less carbon than previously assumed.
"With rising temperatures, the CO2 uptake will probably increase less strongly than older models predicted," says Zohner.
Trees play a key role in controlling the climate because they can pull large amounts of carbon dioxide from the air.
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