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Lightening lights up the night sky in an east end neighbourhood on Aug. 24, 2011 during a severe thunderstorm warning and a tornado watch in Toronto, Ontario, Canada.Deborah Baic/The Globe and Mail

With the onslaught of lightning, rain and wind storms recently, Toronto residents have witnessed the trees in their neighbourhoods bending and sometimes breaking under the assault of the elements.

There are reasons for this.

The reasons for this go back 80 years ago, when the city went on a tree-planting blitz as part of Depression-era make-work projects. Norway maples were the "it" tree then, and now make up 6.5 per cent of the urban tree population.

Unfortunately city planners in the 1930s weren't thinking long-term, and the relatively short-lived and now-aging trees are falling victim to the increasingly common severe-weather conditions across the city, especially in the north-end.

After Wednesday night's storm, the city had reports of more than 30 fallen branches or trees in North Toronto. There were 10 such incidents in other areas.

The maples don't live much beyond 80 years, which means many of the trees in the city are due to die in the next decade or two.

"North Toronto is also just where a lot of older trees are," said Mark Procunier, manager of the city's tree protection and planing review.

A few years ago, Philip van Wassenaer, an arborist with Urban Forest Innovations Inc., was in a neighbourhood north of Eglinton Avenue and east of Bayview Avenue when a freak 10-minute storm occurred. "It was scary; trees were falling all over the place," he said. He had to find a parking lot to wait out it out.

Mr. Procunier said trees being uprooted and falling over is not a common occurrence. What usually happens is branches fall, or the trees break off at the trunk. But what makes the situation in Toronto particularly problematic is how densely populated the city is.

"When a branch falls in North Toronto, it often hits something," he said.

The issue isn't just about a particular species – it's about a particular species that has reached a particular age in a particular part of the city. Heavily developed neighbourhoods with pavement, buildings, concrete and compact, over-trodden soil, are not ideal for trees – especially older ones. They suffer from drought and are left weakened.

Norway maples also have an underdeveloped, girdled root system that grows in a circular pattern, and makes it harder for them to thrive in drought-like conditions.

Another factor is how the city has maintained these trees over the years , Mr. van Wassenaer said, including whether they've been properly pruned.

The Norway maples, once considered a miracle tree that would replace the native elms that were being wiped out by Dutch Elm Disease, is a species that doesn't recover well from injury or being pruned. Falling over or losing branches is the result of long-term weakness in the trunk and limbs. "Trees that haven't been properly maintained tend to fall first in those storms," Mr. van Wassenaer said.

The trees develop pruning wounds, as city workers remove lower branches to create a higher canopy. It leaves Toronto full of tall trees with weak trunks near the ground, which can make them susceptible in severe storms. Mr. van Wassenaer warned that this will become more common, as climate change affects weather patterns.

"The real question is how is the city going to handle this," said Mr. Procunier. Right now, he says, the city is monitoring the areas with older trees and working with communities to address the challenges of public safety versus the preservation of mature trees.

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