Oak (Quercus robur) in autumn
Anne Gilbert / Alamy
Due to climate change, tree leaves can fall earlier in autumn than later, as previously assumed. The finding suggests that forests store significantly less carbon than expected as temperatures rise, and earlier leaf fall can affect insects and other species.
Constantin Zohner from ETH Zurich in Switzerland and his colleagues examined autumn leaf data from 1948 to 2015 for six temperate tree species, including oak (Quercus robur), at almost 4,000 locations in Central Europe. They then performed two experiments to determine the role CO2 and sunlight play in the timing of leaf fall. The first compared trees in chambers that were close to today’s atmospheric CO2 levels with those that were twice that, while the second tested the effects of shade.
Taken together, they modeled what would happen by 2100 if human carbon emissions remained high. Instead of the established expectation that warmer autumn months would bring a longer growing season, with leaf fall around 2 to 3 weeks later than today, the Zohner team determined that this would likely happen 3 to 6 days earlier than now. “The most important result is this big difference to the autumn compared to earlier models,” says Zohner.
The team’s experiments and the 67-year-old tree record suggest that higher CO2 levels, temperatures or light levels make leaves more productive in spring and summer and accelerate their decline in autumn. “We believe that plants have this internal limit for their productivity,” says Zohner. Although the study looked at European trees, he believes the results will apply to temperate trees in North America and Asia as well.
If this turns out to be correct, this reversal will have major global implications for both tourists flocking to watch leaf fall and phenologists studying the interactions between trees, animals, and other plants. Zohner calculates that switching from a delay to an increase in leaf fall is about 1 gigaton less carbon stored in temperate forests around the world each year, which is roughly a tenth of what mankind emits annually. “It’s a pretty big number,” he says.
Journal reference: Science, DOI: 10.1126 / science.abd8911
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