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When assessing the value of a tree, it helps to take a step back.

On a blistering day in Etobicoke this week, that one step - into the cool, shady respite of a green ash outside John G. Althouse Middle School - was particularly effective at demonstrating one of the unheralded favours trees do for us all the time.

It also afforded a pair of University of Toronto forestry students a chance to record its other attributes, and thus put the tree into the context of the estimated 27,000 that dot the yards of the city's 542 public schools.

"If we look at this front yard, you could see a bunch of trees and not really think about what they're doing," said Angus McAuslan, a 23-year-old master's student who is spending the summer, along with undergrad Heather Morrison, 19, documenting trees for the Toronto District School Board.

Indeed, this particular handful isn't doing much beyond decorating the school's front yard, which was the intent when they were planted in previous decades. These days, however, trees are taking their place in the sun as far more important than that, as concerns about climate change, energy use and children's health mount.

Counting, measuring and assessing the condition of its trees will give the school board the information it needs to plan improvements to its urban forest, which yield wide-ranging results - from cooler, quieter schools to lower energy and maintenance costs and better learning opportunities for its 250,000 students.

"We've got lots of asphalt," said Bruce Day, grounds supervisor for the board. "We're trying to change that."

If the average school-owned tree costs $2,000 to replace, the board is sitting on a $54-million asset, said Mr. Day, who hopes that figure will convince "the powers that be" of the need to better protect and enhance it.

When they arrived at John G. Althouse, in a 1960s-era suburb in the Kipling-Eglinton area, Mr. McAuslan and Ms. Morrison found a relative abundance of trees, with some obvious deficiencies that could have been avoided with more thoughtful planning.

Along the school's front wall, a cypress tree stands lopsided; it was planted far too close to the building. And the green ash, out toward the sidewalk, was probably not the best choice of species, given its drooping limbs that tend to fall off due to storms and heavy snowfall.

"If branches are breaking off, they're constantly having to prune the trees," Ms. Morrison said, and that means higher maintenance costs - not to mention the potential hazard of falling limbs - for the school board.

Behind the school, a row of Norway maples marks the back of the lot, where students rarely go. "We need to be doing more tree-planting where children actively play," Mr. Day said, not only to prevent overexposure to the sun, but to encourage learning. "This is a place where kids spend a great deal of their youth; I think we need to pay more attention to the positioning of trees in the schoolyard."

On the bright side (the north side, to be precise), the John G. Althouse property is home to a stand of towering trees, mostly oak and hickory, on land donated to the board in years past.

"You can imagine the educational opportunities in here," Mr. McAuslan said, referring not only to the trees, but to the animals and insects their presence supports. The species of trees within the stand are diverse, and "the more diversity you have, the healthier the system will be," he said, echoing a principle oft applied to the people within school buildings.

This is the fourth consecutive summer that the board has employed a pair of U of T students, at $14 an hour, to catalogue its trees, school by school. By the end of August, 225 schools - just over half of the board's properties - will have been ticked off the list, Mr. Day said. It will likely take another four or five summers to finish the job.

The board undertook the task after Mr. Day met Andy Kenney, a senior lecturer in the U of T forestry program, at a conference several years ago. In the late 1990s, Dr. Kenney and a colleague, Danijela Puric-Mladenovic, developed a program called Neighbourwoods, which equips untrained volunteers to collect meaningful data about trees, including those on private property, which can then be used to develop plans to preserve and enhance the urban forest.

Similar inventories have since been conducted in neighbourhoods around the city and across the country.

With growing environmental interest, the timing couldn't be better. The city's trees, already under stress from compacted soil, road salt, pollution, loss of water infiltration from pavement and damage from collisions with lawn mowers, are now struggling through a dry summer, "and I think that's a concern," Dr. Kenney said.

Perhaps the biggest threat to the urban forest, which currently shades about one-fifth of Toronto, is the threat of removal from development. While public bodies such as city departments and utilities have become far more sensitive and vigilant about conservation, "there's still huge pressure from development," Dr. Kenney said.

Many cities, including Toronto, have ambitious goals to increase the canopy cover of their trees, but if they want to succeed, "the first thing you have to do is start protecting the trees that are already there," he said, "and I think we've still got a long, long way to go in that."

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