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Global warming appears to cause leaves to drop before trees, confusing the idea that warmer temperatures delay the onset of autumn, according to new research.
The discovery is important because trees absorb huge amounts of carbon dioxide from the air and thus play a key role in climate management.
Rising temperatures also mean that spring is coming earlier and, overall, the growing season for trees in the planet’s temperate zones is lengthening. However, early autumns mean that significantly less carbon can be stored in trees than previously thought, providing less of a brake on global warming.
Scientists are still evaluating the magnitude of the effect, but it could amount to 1 billion tonnes of CO2 per year, which would be more than Germany’s annual emissions.
The new research is based on a huge dataset of observations of European trees, experiments that varied light and CO2 levels, and mathematical models. It showed that, in addition to temperature and day length, the amount of carbon a tree has absorbed in a season is a key factor in determining when it no longer needs its leaves and loses them. Scientists compare the effect to a person getting full after a heavy meal and unable to eat more food.
Other recent research has shown that fast-growing trees have a shorter lifespan and that the climate crisis and mass felling of trees have overall reduced their lifespan.
“For decades we have assumed that growing seasons are increasing and fall foliage is coming later,” said prof. Thomas Crowther of ETH Zurich in Switzerland, one of the members of the study group. “However, this research suggests that when the productivity of trees increases, the leaves fall off earlier.”
Previous models that did not include the amount of carbon a tree absorbs during a season indicated that autumn could be two or three weeks later by the end of the century on current emissions trends. But the scientists’ new model indicates that autumn could actually come up to six days earlier. “So the increases [in carbon storage] it won’t be anywhere near as big as what we expected, “Crowther said.
Christine Rollinson, an ecologist at Morton Arboretum in Illinois, USA, who was not part of the study group, said previous models were known to be simplifications but were the best available.
“The big challenge is that autumn has always been a bit of a mess,” he said. “Depending on where you are and the species you are observing, there is some evidence that leaf fall is happening earlier and some that it happens later. But understanding how well a tree grows during the season really helps explain this tree-to-tree variation.
“What’s particularly surprising about the study is how it provides such different lines of evidence to reach the same conclusion,” he said. But she was cautious about the impact of early autumns on overall carbon storage: “It’s not yet clear how this will turn out.”
Rollinson said it is still crucial to reduce emissions from fossil fuel burning and deforestation to tackle the climate emergency: “We cannot entrust all responsibility to [growing] trees. “
The research, published in the journal Science, used more than 430,000 observations of leaf fall from trees at 3,800 sites across Central Europe from 1948 to 2015, as well as experiments and models.
Photosynthesis in leaves converts CO2 from 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 off.
It is not known what specific factor, or combination of factors, triggers this stunting, but it could be the availability of nitrogen. About 94 percent of deciduous trees cannot provide their own nitrogen, so the researchers believe their findings will apply to most trees in temperate zones around the world.
Estimates of how early autumn leaf fall will reduce CO2 caught from trees, compared to previous models, is underway. But it could be on the order of 1 billion tons per year, the scientists said, based on previous research.
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