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Why Trees Aren't Turning Red In Ontario And Quebec This Fall

Unseasonably warm weather is confusing the trees.

10/23/2017 19:32 EDT | Updated 10/23/2017 19:33 EDT
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Light shines behind two green leaves on a tree in Toronto.

Unseasonably warm weather in parts of Canada may deprive some areas of one of their trademark natural attractions — rich fall colours.

A forestry expert says the vivid red leaves that draw crowds of tourists to areas of Ontario and parts of Quebec are triggered by bright sunshine combined with cold temperatures.

Sean Thomas, a professor of forest ecology at the University of Toronto, says trees start breaking down the chlorophyll in their leaves in the fall in order to draw out nutrients such as nitrogen and store them over the winter.

He says chlorophyll is what gives leaves their green colour, so as it is broken down, other pigments such yellow and orange are revealed.

Thomas says that process can produce other chemicals that damage plant tissues if exposed to UV radiation.

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Maple trees with orange leaves are seen in Toronto in 2015.

To protect leaves from UV rays, Thomas says red pigments — a trademark of sugar maple trees, among others — emerge as a type of "leaf sunscreen."

"The need for that protection is even more acute under cold temperatures," he said. "So those things together, high light plus cold temperature, also have evolved to be the trigger for synthesis of anthocynanins (red or purple pigments)."

This year, however, the weather conditions have disrupted the typical process.

"It's been unseasonably warm and so the trees haven't received the signal that they usually would have," Thomas said. "And it's been a bit cloudy too."

Some trees were set off by a bout of cold weather in early September before the mercury rose again, but those that weren't are more likely to just turn yellow as they prepare for winter, he said.

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If the leaves start to die before the tree can extract nutrients from them, in case of a sudden frost for example, then they simply turn brown, he said.

A drop in temperature now probably wouldn't do much to bring out the dramatic reds, Thomas said.

"There's kind of a time window when that can happen and that's probably passed now," he said.

The lack of bright fall colours, however, doesn't indicate poor tree health, Thomas noted.

"The fact that the trees aren't synthesizing the red pigments, that's not necessarily a bad thing for them," he said. "It's a bad thing for tourism in the autumn, but it's not necessarily a signal that the trees are unhealthy."

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