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Hot, dry spell expected to put damper on fall colours


Caroline Floyd
Meteorologist

Monday, September 25, 2017, 7:31 PM - The blast of summer-like weather that's marked the first few days of fall across southern Ontario could drain some of the colour out of the season's changing leaves.

Danielle Way, an assistant professor at Western University, told CBC News that the unseasonable heat may be leading trees to continue producing chlorophyll past the point they usually slow down for the oncoming cold season, and that could mean less of the brilliant red southern Ontarians are used to in their fall forests.

Record-breaking high temperatures were the story of the weekend across all of southern Ontario, with some records from the late 1800s falling as high pressure and sunny skies dominated the first official weekend of the fall season.

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Way told CBC News the red pigment trees like sugar maples produce is in response to colder days. "Early in the morning on those cold fall days," Way said, "you need the [red] pigment to help protect photosynthesis because the leaf can't cope with the [bright] light."

While pigments like yellow and orange are always present in leaves, the dark green of chlorophyll masks them during the summer. Once the weather turns colder, chlorophyll productions slows down, the green fades, and the other colours become visible.


Red pigments are a bit different, however. Professor Dana A. Dudle of DePauw University, who researches red pigment in plants, explained the difference to EarthSky:

The red color is actively made in leaves by bright light and cold. The crisp, cold nights in the fall combine with bright, sunny days to spur production of red in leaves – especially in sugar maple and red maple trees. Burgundy leaves often result from a combination of red pigment and chlorophyll. Autumn seasons with a lot of sunny days and cold nights will have the brightest colors.

Adding to the stress on the region's foliage is the dry spell that's accompanied the heat. Toronto is currently working toward its longest stretch since 2009.

Given the stretch of record-breaking and near-record heat, along with drought-like conditions, across much of southern Ontario, Dr. Way says to expect more yellows and browns this year than reds.

"You're probably going to see much more drab foliage and that's not exactly what we get in our cars to look at on the weekend," Way told CBC.

Sources: CBC News | EarthSky

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