Quirks & Quarks

Climate change might make autumn leaves appear — and disappear — earlier

Trees will become more productive earlier in the season in a warming climate and could drop their leaves sooner as a result

Early spring and summer productivity could mean leaves are exhausted by autumn

Expect to see fall colours like this much earlier, according to a new study (Ben Nelms/CBC)

Something unexpected could soon happen to temperate trees like oak, birch, maple and ash in the forests of Europe, and likely around the world. For a long time now, scientists have expected that climate change will result in temperate trees dropping their leaves much later than normal.

That is because a warming climate brings a longer growing season for trees. Cold temperatures, one of the stressors that starts the process of leaves falling, would happen later. 

Constantin Zohner measures autumn senescence (Submitted by Constantin Zohner)

Expect fall colours earlier

The opposite is more likely to be the case according to a study by Constantin Zohner. His work suggests trees will soon start to lose their leaves slightly earlier in autumn in response to climate change. That's because the increase of carbon in the atmosphere and a warming climate is resulting in trees becoming more productive much earlier in the season according to his modelling.

Climate change will result in leaves like these falling sooner, not later as was previously thought (Constantin Zohner)

Zohner's work suggests that because trees in temperate latitudes are adapted to a growing season of a certain length, starting that season early means that trees will essentially have exhausted their seasonal growth capacity before autumn temperatures drop. They will then begin their genetic program of leaf senescence, or death, which brings fall colours and eventual dropping of leaves. 

This means trees will stop the process of photosynthesis earlier as well. The tree, including its roots will stop using or even storing the carbon that the leaves have captured at a point much earlier in the growing season than usual. As a result the anticipated carbon sequestration benefits of a longer growing season may not be as great as scientists had anticipated.

Written and produced by Mark Crawley