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And, if you’re an autumn enthusiast, it could also mean a disappointing backdrop for your photos. 

Monday marks the first official day of fall in º£½ÇÉçÇø¹ÙÍøbut the city has yet to feel cooler temperatures, as warm weather from an unbearably hot and humid summer ±ô¾±²Ô²µ±ð°ù²õ.Ìý

The extreme heat and fewer days of rain this summer are contributing to the early browning of some trees, especially those more susceptible to the effects of dry weather, explained McMaster University biology professor Susan Dudley. Instead, she said some tree leaves might die off before turning red. 

As fall arrives, here’s what you should expect to see when going on a stroll throughout the city.

Why are º£½ÇÉçÇø¹ÙÍøtrees changing colour quicker than usual?

Certain species of trees in the city are facing premature leaf death, said Dudley, who has also noticed the early colour change in trees on the McMaster campus in Hamilton. 

This is because º£½ÇÉçÇø¹ÙÍøand much of southern Ontario saw record-breaking heat this summer and a long rainless period in August, according to Sean Thomas, a University of º£½ÇÉçÇø¹ÙÍøprofessor whose expertise is in tree biology.

“What you usually see under those conditions is premature senescence of leaves,” Thomas said, adding that signs of stress are the browning of the leaves and branches dying. 

The transpiration process from tree leaves is related to both heat and humidity so if a lot of water is being lost from the leaves, even if there’s enough water in the soil, drought responses in the trees are induced, he said.

When cooler temperatures and shorter days arrive, a leaf’s green chlorophyll is broken down, revealing the underlying yellow and orange colours.

When a tree’s leaves change to red, they produce anthocyanin pigments that have the function to protect the leaf during the senescence process, so that the nitrogen can be recovered in the tree from the breakdown of chlorophyll. Dudley calls this protection a “type of sunscreen” for the metabolism within the leaf after the chlorophyll is gone.

But this process can be triggered early under drought conditions, Thomas added. 

Does this affect all trees in Toronto? 

Many deciduous trees change colour in the fall including the sugar maple and oak trees that produce the vibrant red, orange and yellow leaves seen in Toronto. 

Although heat and drought conditions can lead to brown leaves and duller fall foliage, some trees can be at more of a disadvantage than others based on location and access to water. 

Trees planted on school campuses, streets or parks are generally more stressed than those in forests where they’re clustered and receive more shade. Trees in city parks and those planted further apart don’t experience as much of the shade that might help protect them from the sun during intense heat waves. They may also suffer more soil compaction from people walking near them, which hinders water infiltration. 

Dudley said maple trees may be more at harm than trees like oaks, as maple trees usually thrive in cooler, moist conditions while oak trees are more adaptable to drought conditions. 

What could the fall foliage look like? 

Due to the early autumnal colouration, residents will notice patchiness in a group of trees where there will be some changing colour early while others won’t be. This all depends on the trees’ access to water during growing season.

Some trees that are “highly stressed” over the heat and lack of water could even see its leaves turn from green to brown, indicating “dead leaves.” As result, the fall colours of the trees will be less vibrant and have a duller appearance. 

“Early change is a sign of stress very often, even if it’s a nice red colour. When they change in September, it’s like an indication that the tree might be stressed,” Dudley said.

Could drought-stressed trees revive next fall? 

One year of drought conditions is not a problem, explained Thomas, but if Toronto’s trees experience early leaf colour change for multiple fall seasons, then this can really add up to the stress the trees experience. 

“I’ve certainly seen some dead trees around, so they might have been on a site where they’ve had other problems or they have disease impacts. So there’s certainly this kind of drought conditions can be a real problem for individual trees. There’s not widespread dieback. But, we need to hope that we won’t see another drought here next year,” he said. 

With files from The Canadian Press