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According to new research, global warming appears to be causing leaves to fall from trees earlier, confusing the idea that warmer temperatures are delaying the onset of fall.
The discovery is significant because trees draw huge amounts of carbon dioxide from the air and therefore play a key role in climate management.
Rising temperatures also mean that spring is coming sooner, and overall the growing season for trees in temperate areas of the planet is lengthening. However, the early fall means that far less carbon can be stored in trees than previously thought, which is less of a drag on global warming.
Scientists are still figuring out the likely magnitude of the effect, but it could add up to 1 billion tonnes of CO2 per year, which would be more than Germany’s annual emissions.
The new research is based on a large dataset of European tree sightings, experiments that varied light and CO2 levels, and mathematical models. She showed that in addition to temperature and day length, the amount of carbon a tree has taken up over a season is a key factor in determining when it no longer needs its leaves and throws them away. Scientists compare the effect to a person who becomes full after a heavy meal and is unable to eat more food.
Other recent research has shown that fast growing trees have a shorter lifespan, and the climate crisis and massive tree felling have shortened their lifespan overall.
“For decades we assumed that the growing seasons were increasing and the autumn leaves were later,” said Professor Thomas Crowther from ETH Zurich in Switzerland, who was part of the team at study. “However, this research suggests that as tree productivity increases, leaves fall sooner.”
Earlier models that did not include the amount of carbon a tree absorbs during a season indicated that fall could be two to three weeks later by the turn of the century on current trends in emissions. But the scientists’ new model says fall could actually arrive up to six days earlier. “So the increases [in carbon storage] will be nowhere near as performing as we expected, ”said Crowther.
Christine Rollinson, an environmentalist at the Morton Arboretum in Illinois, US, who was not on the study team, said earlier models were known to be simplifications, but were the best available.
“The big challenge is that fall has always been a bit tricky,” she says. “Depending on where you are and what species you are looking at, there is some evidence that leaf fall occurs earlier and some that it occurs later. But understanding how well a tree grows during the season really helps explain this tree-to-tree variation.
“What’s particularly amazing about the study is how it provides such different evidence to come to the same conclusion,” she said. But she was cautious about the impact of the first autumns on global carbon storage: “We really do not yet know how this will cascade.”
Rollinson said reducing emissions from fossil fuel combustion and deforestation remained essential to tackle the climate emergency: “We cannot put all the blame on [growing] trees.”
The research, published in the journal Science, used more than 430,000 observations of leaf fall from trees at 3,800 sites in central Europe from 1948 to 2015, as well as experiments and modeling.
Photosynthesis in leaves converts CO2 of the atmosphere into carbon compounds that the tree uses to live and grow. If the tree can no longer use carbon, it stops holding its leaves and they fall.
It is not known what specific factor, or combination of factors, triggers this growth arrest, but it may be the availability of nitrogen. About 94% of deciduous trees cannot supply their own nitrogen, so the researchers believe their findings will likely apply to most trees in temperate zones around the world.
Estimates of how much early fall leaf fall will reduce CO2 captured by trees, compared to previous models, is in progress. But it can be on the order of 1 billion tonnes per year, according to scientists, based on previous research.
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