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A large number of trees are slated to be taken down on Mildenhall Rd. in north Toronto. The neighbourhood and its trees are photographed on June 5 2015.Fred Lum/The Globe and Mail

Trees have been fixtures of the Lawrence Park community ever since it was dubbed "the garden suburb" a century ago.

Now, the city is in the planning stages of a road and sewer replacement project to mitigate storm water and basement flooding that would require removal of 349 trees.

"It's going to be sad to lose those trees," said Bruce Grantier, a resident of the area for 18 years.

The ice storm two winters ago badly damaged the city's canopy, and emerald ash borers killed 14,863 city-owned trees in 2014. Lawrence Park residents worry the sewer project will take out too many – especially because similar projects, such as in nearby Hoggs Hollow, affected much fewer trees.

"The canopy is pretty important in the neighbourhood for a variety of reasons, whether it's oxygen-creation, keeping houses cooler, and just the character of Lawrence Park," resident Tim Bradshaw said.

At public consultations last month, many community members objected to the construction's impact on the trees.

There is no doubt the sewer system needs an update. Deteriorating roads have led to serious drainage problems, including sewage backup and flooding in at least 82 basements last year.

Far less clear is the reason so many trees must go. And why – in a neighbourhood that was hit hard by the ice storm – more is not being done to save those still standing.

The city says it is impossible to put in the new sewer system and roads without removing trees.

"We know if we were going to go in and basically reconstruct that road network in accordance with today's standards, forgetting about anything else, trees would be impacted," said Michael D'Andrea, the city's executive director of engineering and construction services.

Mr. D'Andrea said measures will be taken during the design process – such as narrowing roads near larger trees, hand-digging around trunks to protect roots, and pruning instead of removing trees where possible – to limit the impact.

But the decision on how those measures will be implemented, and when, is years down the road, Mr. D'Andrea said. Staff will present the community with a revised recommendation in the fall before a finalized report is submitted to council.

The city wants to increase urban forest coverage to 40 per cent from its current 28 per cent, primarily by planting the type of trees that grow large canopies.

Addressing basement flooding will take priority over saving trees in Lawrence Park, Mr. D'Andrea said. "We're dealing with an existing problem in the area."

And although the city has promised to plant a new tree for every one removed, Hans Schreier of the faculty of land and food systems at the University of British Columbia said it will be years before they can take the place of the ones lost. "It's going to take a long time before you get the same benefits," he said.

Construction projects, disease and weather all reduce the urban canopy. In 2014, the city removed 29,953 trees from its property, about 50 per cent of which because of the emerald ash borer.

About 3,000 street trees were removed after the December, 2013, ice storm.

The city does not track how many trees are removed for projects.

Other cities have taken a different tack: Portland, Ore., converted about 500 streets from the conventional sewer piping system to "greenways" that use trees and other green infrastructure to manage stormwater, reducing the risk of sewer backups in basements.

Urban trees are extremely useful for reducing runoff, Dr. Schreier said.

A study he oversaw in North Vancouver found that conifer species, including Douglas fir and cedar, took in moisture from the ground through their roots and released it through their leaves – up to 50 per cent of all rainfall. Trees also remove water in the soil after a flood, he said.

Extreme rainfall is becoming more common, and that means more strain on the city's drainage system, and trees are less effective at mitigating the water flow.

Dr. Schreier said infrastructure upgrades are a perfect time to green street drainage systems.

"It's very expensive to re-engineer cities, but why not do it starting in the neighbourhoods," he said.

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