As fall approaches, many in Toronto are noticing an early change in leaf colors. This year, the vibrant reds, yellows, and browns are appearing sooner than usual. Experts warn that this could lead to less colorful and patchy foliage this autumn.

Monday marks the official start of fall in Toronto, but the city is still experiencing warm temperatures. The lingering heat from a hot and humid summer is affecting the trees. Susan Dudley, a biology professor at McMaster University, explains that the extreme heat and limited rainfall this summer are causing some trees to brown prematurely. "Some tree leaves might die off before turning red," she said.

Certain tree species in Toronto are particularly vulnerable to early leaf death. Dudley has observed this phenomenon on the McMaster campus in Hamilton. Sean Thomas, a professor at the University of Toronto specializing in tree biology, noted that southern Ontario experienced record-breaking heat and a prolonged dry spell in August. "What you usually see under those conditions is premature senescence of leaves," he said, referring to the browning leaves and dying branches.

The transpiration process in trees is influenced by heat and humidity. If trees lose too much water from their leaves, they may trigger drought responses, even if the soil has adequate moisture. As fall arrives, cooler temperatures and shorter days cause chlorophyll in leaves to break down, revealing underlying yellow and orange hues. When leaves turn red, they produce anthocyanin pigments, which help protect the leaf during the senescence process, allowing the tree to recover nitrogen from the breakdown of chlorophyll. Dudley describes this as a "type of sunscreen" for the leaf's metabolism.

Not all trees in Toronto are affected equally. Deciduous trees like sugar maples and oaks typically display vibrant fall colors. However, heat and drought can lead to browning leaves and less vibrant foliage. Trees planted in urban areas, such as school campuses and streets, often experience more stress than those in shaded forests. Urban trees may suffer from soil compaction due to foot traffic, which limits water infiltration.

Dudley noted that maple trees are more susceptible to stress than oaks, as maples prefer cooler, moist conditions, while oaks are more drought-resistant. Residents can expect to see patchy colors this fall, with some trees changing early while others remain green. Trees under significant stress may even turn brown, indicating dead leaves. "Early change is a sign of stress very often, even if it’s a nice red color," Dudley said.

Looking ahead, Thomas explained that one year of drought is not typically harmful to trees. However, repeated early color changes over multiple seasons could compound stress. "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," he said. While there is no widespread dieback, he emphasized the importance of hoping for better conditions next year.