Tree leaves may fall earlier in autumn due to climate change, rather than later as previously thought. The finding suggests forests will store significantly less carbon than expected as temperatures rise, and earlier leaf-fall may have knock-on effects on insects and other species.
Constantin Zohner at ETH Zurich in Switzerland and his colleagues looked at autumn leaf-fall data from 1948 to 2015 for six temperate tree species, including common oak (Quercus robur), across nearly 4000 sites in central Europe. They then ran two experiments to see what role CO2 and sunlight play in leaf-fall’s timing. The first compared trees in chambers at close to today’s atmospheric CO2 levels with those at double that amount, while the second tested the impact of shade.
Putting the results together, they modelled what would happen by 2100 if humanity’s carbon emissions stay high. Instead of the established expectation that warmer autumns will bring a longer growing season with leaf-fall occurring about 2 to 3 weeks later than today, Zohner’s team found it would probably happen 3 to 6 days earlier than now. “The key finding is this huge difference to when autumn happens compared to previous models,” says Zohner.
The team’s experiments and the 67-year tree record suggest higher CO2 levels, temperatures or light levels are driving the leaves to be more productive in spring and summer, hastening their demise in autumn. “What we think is happening is plants seem have this internal limit to how productive they can be,” says Zohner. Though the study looked at European trees, he thinks the results will hold true for temperate trees in North America and Asia too.
If proved right, this reversal has big global ramifications beyond when tourists flock to see leaf-fall and for phenologists studying the interactions between trees and animals and other plants. Zohner calculates that the switch from a delay to an advance in leaf-fall amounts to about 1 gigatonne less carbon stored globally each year by temperate forests, roughly a tenth of what humanity emits annually. “It’s a quite huge number,” he says.
Journal reference: Science, DOI: 10.1126/science.abd8911
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