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Why are the fall leaves so slow to change this year?

In many parts of Canada, we're still seeing green. A plant biologist explains what's up

In Oct. 2015, Toronto's High Park was an explosion of fall colours. Not so much this year. Here's why.

Cole Burston/Torstar News Service

In Oct. 2015, Toronto's High Park was an explosion of fall colours. Not so much this year. Here's why.

Do boring fall bike rides have you feeling green with envy over parts of Canada where brilliant colours have begun? Every autumn, shortening days set off a series of genetic changes in trees. A build-up of cells at the base of leaves cuts off the flow of nutrients: That’s part of it. But many other factors determine when and how the leaves change. York University plant biologist Dawn Bazely explains.

Andres Plana/Metro

DAYTIME SUN
Bright sunlight causes chlorophyll to decompose, Bazely explained. But sunlight also triggers the production of red anthocyanins, which help protect the leaves from sun damage. So a sunny autumn causes more red, and a cloudy one more yellow.

DAY LENGTH
The decrease in the photoperiod (the length of the day) tells leaves when it’s time to change. Short days signal bright green, chlorophyll-containing structures called chloroplasts to turn into gerontoplasts —  geriatric chloroplasts that don’t do photosynthesis anymore.   

NIGHTTIME TEMPERATURE
“Chlorophyll requires sunlight and warm temperatures,” Bazely said. “During summer chlorophyll is constantly breaking down and being remade, but cold causes chlorophyll production to stop. If there’s carotene in there, the leaf will turn bright yellow.” If nights are warm, as in Ontario, leaves stay green longer. Bazely said our warm nights is probably the most important factor causing the disappointing colours this year.

DROUGHT
Trees tend to get by OK even in severely dry weather, thanks to deep roots, Bazely said. But under extreme drought stress, they might drop their leaves early.    

Andres Plana/Metro

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