Why are trees in Ontario changing colour earlier than usual this fall?

Canada’s dramatic summer weather has altered the fall colours this year, researchers say.

Author of the article:

The Canadian Press

Lyndsay Armstrong

Published Oct 09, 2023 • Last updated 13 hours ago • 3 minute read

File photo/ The fall colours are starting to show near the Rideau Canal in Ottawa. Photo by Tony Caldwell /Postmedia

The customary reds, oranges and yellows of the trees, marking the arrival of fall, may have appeared early this year, or not at all.

The dramatic summer weather that brought wildfires in some parts of the country and heavy rain in others is being reflected in fall colours across Canada, researchers say.

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Trees in Ontario have begun displaying the natural pigments earlier than usual.

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Sean Thomas, a professor of forest ecology at the University of Toronto, says there is anecdotal evidence to suggest that smoke from wildfires in northern Ontario this summer may have triggered trees to reveal their colours prematurely.

“Wildfire smoke is a kind of chemical cocktail,” Thomas said.

That cocktail of carbon dioxide, ozone, sulphur dioxide, carbon monoxide and particulate matter “includes trace quantities of chemicals that play a sort of hormonal role with plants,” he said, which can cause trees to drop their leaves early.

“In our case, we got those acute smoke effects earlier in the summer, but it still may be partly what’s accounting for the earlier fall colouration,” Thomas said.

He said there is lots of colour in much of Ontario this season, but added there is reason to believe that ongoing impacts of climate change will see fall colours diminish in the coming years.

“There is good reason to think that climate change will disrupt that normal leaf colouration that we see,” Thomas said.

The combination of warmer temperatures and delayed first frosts will potentially lead to duller fall colours, he said. This effect, he added, will likely be most acute in major cities, which are generally hotter than rural areas because of the urban heat-island effect — when structures such as roads and buildings absorb and re-emit the sun’s heat.

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If climate change delays the longer, colder nights in places like Toronto, “that’s a recipe that is likely to lead to much less fall colouration right in the city, and that will be exacerbated by climate change,” Thomas said.

In Nova Scotia, by contrast, where summer started with forest fires and ended with stretches of overcast and stormy days, dull brown has replaced the vibrant hues usually seen this time of year in much of the province.

Mason MacDonald, an assistant professor of environmental sciences and agriculture at Dalhousie University, said the colours he’s seen so far this autumn don’t compare to the brilliant reds and oranges Nova Scotians are used to.

“What you’re probably seeing is a lot of really dull-looking colours this year. Even the reds are probably more of a dull, darker red, or rust colour,” MacDonald said in a recent interview.

“I’ve heard it from a lot of people this year, especially people from the southern parts of Nova Scotia,” he said.

As the nights get longer in the fall, trees receive less direct sunlight, and chlorophyll, which trees use to absorb sunlight during photosynthesis, begins to break down, displaying the natural pigments within the leaves.

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One of these pigments — anthocyanins — creates the red shade in leaves, and it requires consistent sunlight through late summer to be produced. A drop in anthocyanins isn’t harmful to a tree, MacDonald said, adding that if next summer is sunnier, those vibrant reds will likely be back in full force.

“This year we got more rain than is typical for us, and along with that we had a whole bunch of these dark, grey, cloudy days. That’s what happened. We didn’t get the sun we would normally get. Therefore, they can’t make those colours,” MacDonald said.

In Cape Breton, where the Celtic Colours International Festival is underway, an event spokesperson said in an interview Thursday that trees across the region are beginning to change colour. Dave Mahalik said he’d heard that colours are less vibrant in parts of Nova Scotia this year, “and it made me wonder a bit about where we’re at and how we’d fare,” he said.

“But I’m in Sydney and there’s trees here that are starting to pop, and they’re looking like they usually do,” he said.

Mahalik said he’s hearing from his colleagues that colours are looking bright elsewhere on the island, which bodes well for the festival that’s hosting about 50 concerts in communities across Cape Breton.

“I’m excited as can be and I’m sure the colours will be as vibrant as ever,” he said.

With files from Fakiha Baig in Toronto.