What could drought-stricken forests mean for Canada's fall foliage?

Drought-stricken parts of Canada could be in for some underwhelming fall foliage if stressed trees lose out on the energy needed to generate some of the season's most brilliant colours, experts say. 

Biology professor Susan Dudley at McMaster University says trees in dried out parts of the country could see their leaves die off rather than turn red. 

"I can't look at it and make a prediction for a good fall foliage season at all," she said. 

As the days shorten, green chlorophyll in tree leaves starts to break down and reveals the yellow and orange pigments underneath.

Yet Dudley says some trees, such as maple, oak and sumac, synthesize a pigment in the autumn responsible for turning their leaves into the reds and purples associated with the most brilliant foliage. 

If a tree is too stressed, such as from intense drought, Dudley says the leaves may die off before that new pigment can fully develop and give off its most vibrant colour. 

Around 70 per cent of Canada was experiencing abnormally dry or drought conditions as of the end of July, according to the national drought monitor. Climate change, driven by the burning of fossil fuels, is increasing the likelihood and severity of drought conditions, scientists say. 

While it may be too early to know how the fall spectacle will play out, Dudley said it's not looking good but favourable weather could alter the gloomy outlook. 

Leaves get their red or purple colour from anthocyanins, a pigment scientists believe may act as "sunscreen," shielding cells from excess sunlight so they can continue to absorb good nutrients from the leaves even while the photosynthetic machinery is breaking down, Dudley said. 

"These stressed trees are not going to have the energy to do as much protection as they would have," she said.

In southern Ontario, Dudley said what she's seeing so far is brown leaves associated with rapid stress-induced death, not an early start to the fall colours. 

The same goes for Nova Scotia, said Zoe Panchen, associate professor of plant biology at Acadia University. 

"The leaves are just shrivelling up and dying because there's not enough water there for them to thrive longer into the autumn season," she said. 

Those dried out trees have helped fuel intense wildfires across Canada. Panchen said drought can also cut into how much planet-warming carbon dioxide trees absorb. 

The fall foliage could improve if autumn brings sunny days and consistently cool nights, Panchen said. That combination helps trap sugars in tree leaves, which spur those red and purple anthocyanins.

Picture Courtesy: THE CANADIAN PRESS/Christinne Muschi