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Researchers take stock of how Canada's urban forests protect our health

Montreal's urban forest has a big impact on human health, and yet researchers are just starting to count the trees, and the benefits.

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Alain Paquette is passionate about his research on urban forests — trees that grow in cities — and their potential to improve human health and protect us from the ravages of climate change.

This week, Paquette, a biology professor at the Université du Québec à Montréal, was awarded a $540,000 grant from the Natural Sciences and Engineering Research Council of Canada for a project that will involve researchers at several universities in various disciplines examining the resilience and benefits of Canada’s urban forests in the context of climate change.

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“We want to gain a better understanding of urban forests because they provide important ecosystem services — such as the reduction of heat, better air quality — which favours environmental and human health,” said Paquette, who is working with colleagues at UQAM, Université de Montréal, Carleton University and the University of British Columbia.

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The project has three goals: to better understand how trees grow in the urban milieu, the way climate change affects them, and how that affects the health of people in cities.

“We know almost nothing about our urban trees,” Paquette said. “We think they grow well or not according to this or that. But I have had my students pore through the literature — there is almost nothing that tells us about the rate of growth or the mortality rates of trees in an urban setting. I was shocked.”

Most cities do inventories of the number and types of trees that the municipality is responsible for, in city parks or on city streets. But public trees account for only about half the trees growing in cities, according to estimates. There is no firm data on the types and number of trees private citizens or institutions have planted.

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Meanwhile, thanks to decades of investments by forestry companies and governments, scientists know quite a lot about trees in the boreal forest — their rates of growth, and the environmental or climatic conditions that help them thrive or fail.

“We know so much about these faraway boreal forests, but almost nothing about the trees with which we share our daily living environment,” Paquette said.

And knowing about trees in our local environment is proving more important than ever. Not only do trees sequester carbon, which helps in the struggle against global warming, they provide a number of more immediate benefits to humans.

“There are more and more studies today showing a direct link between the canopy — the quantity of trees — and the physical and mental health of human beings, and also human development. Studies (show that) things like satisfaction, happiness, academic success, etc., are linked to the presence and quantity of trees.”

One of Paquette’s current projects is an inventory of all the trees planted on a plot of land surrounding UQAM’s sciences complex on President Kennedy Ave., bordered by Milton, Clark, Aylmer and Ste-Catherine Sts.

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Students are knocking on doors and asking to count and measure an estimated 2,200 trees in the area. They will identify the species, and measure and record their diameter and height using lidar, a surveying method that measures distance by lighting up the treetops with a laser light and then measuring the reflected light with a sensor.

Next summer, Paquette aims to have his students do the same at a plot around the university’s main campus on Ste-Catherine and St-Denis Sts. The idea is to update the inventory regularly, to gain a portrait of how the urban forest grows and changes. Eventually, Paquette hopes all of Montreal’s universities will take this type of inventory of the trees around their campuses.

Paquette’s main concern is the lack of diversity in the urban forest. Most cities tend to plant too many of the same kinds of trees. In Montreal, three species dominate: Norway maple, silver maple and red ash make up about 50 per cent of municipally planted trees.

“The ash tree by itself represents between 20 and 25 per cent of our trees. So when we lose all of them, and that is what is happening right now, it has a direct impact on the canopy and on our health,” said Paquette, referring to the emerald ash borer beetle infestation that is ravaging ash trees across North America.

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Montreal’s ash trees, ironically, were mostly planted to replace American elm trees that were decimated several decades ago by Dutch elm disease.

Cities also make assumptions that may be incorrect about factors that stress or kill urban trees. Paquette’s recent work suggests air pollution and bad growing conditions are not necessarily as deadly to trees as bark injuries caused by snow removal machinery, cyclists who lock their bikes to young trees, and weed eaters.

“The living part in a tree is barely a couple of millimetres just underneath the bark, so it is really easy to seriously injure or kill a tree by injuring it at the bark level.”

Another major problem for the urban forest is what Paquette calls “masculinization.” Cities and residents have been planting mainly male trees, partly because of the extra work and cost of cleaning up seeds and fruits produced by female trees. The trouble with this is that the flowers from male trees produce more pollen, which increases allergic reactions.

mlalonde@postmedia.com

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