Town of Oakville: Our Solution to Our Pollution Recognizing Trees as Green Infrastructure

January-February 2007

DURING AN INTERVIEW that was conducted with Dr. Kim Coder at last year’s London conference, he was asked what our industry should be doing to address our declining urban forests. Two of his recommendations included that we should build public awareness about the plight of our urban forest with terms that people can relate to and that we should promote trees for their utility value. The following article is the third in our series focusing on municipal urban forestry and details the Town of Oakville’s recent progressive report Our Solution to Our Pollution by John McNeil, Manager of Forestry and Cemetery Services. As work on this report began long before the 2006 conference, the Town of Oakville along with John McNeil and his staff should be applauded for taking on this initiative. This is a document that I am sure Dr. Coder would endorse, and it can serve as a model for other towns and municipalities.

In 2004, in an effort to improve air quality in Oakville, Oakville’s Town Council committed to establish an Urban Forestry Management Plan. Required background data for this plan was information about the entire urban forest. To capture this information, forestry staff recommended using the Urban Forest Effects Model (UFORE). As part of the Town’s 2005 capital project program, a UFORE project was approved and the results are being incorporated into the Urban Forest Strategic Management Plan – a major 2006 capital project.

The Urban Forest Effects Model (UFORE), designed by the United States Forest Service, has been used to quantify urban forest structure, function and values in numerous communities throughout the world. Randomly generated plots stratified by land use type combined with local pollution and meteorological data quantified the ecological benefits provided by trees and shrubs growing throughout Oakville in 2005.

Project Methodology
In 2005, Oakville Town Council approved $144,500 for the UFORE project and the Tree Canada Foundation contributed an additional $33,000 as a grant under Green Streets Canada. Using the UFORE protocol for plot selection, measurement and quality control to ensure reliable results, a local database with 372 permanent sample plots were established. The plots will be re-measured every four years and a State of the Urban Forest “report card” will be prepared.

Approximately 500 residents and businesses participated in the Town of Oakville’s UFORE project, a partnership involving the Town of Oakville, the U.S. Forest Service, the Faculty of Forestry, University of Toronto, and the Tree Canada Foundation. Emphasis is placed on the urban forest canopy cover and how to achieve an optimum target in order to optimize air quality benefits.

Major Findings
Oakville’s urban forest was responsible for filtering 172 tonnes of criteria pollutants in 2005; the criteria pollutant that was most reduced was ground level ozone (85 tonnes). Ground level ozone when combined with particulate matter forms smog, which, as we all know, is a deadly risk to human health. This finding was the inspiration for the title Our Solution to Our Pollution because Oakville’s urban forest plays a role in reducing the risks to the health of Oakville’s residents by reducing the amount of smog formed from the local emissions of criteria pollutants.

While significantly reducing local air pollution, Oakville’s urban forest does not filter a significant volume of the trans-boundary pollutants coming into Halton Region’s airshed. However, even small amounts of air quality improvement are significant. The Ontario Medical Association reported in 2005 that Halton Region had 190 premature deaths, 540 hospital admissions and 2,010 emergency room visits due to smog; this resulted in $18 million in health care costs.

Air Pollution Control – The Tree Factor
1. Structure. Oakville has 1.9 million trees – most are privately owned (57%). The top three species by leaf area are sugar maple (Acer saccharum), Norway maple (Acer plantanoides) and silver maple (Acer saccharinum). Municipal trees will be subject to an inventory/asset management project beginning in 2007. The average urban forest canopy cover is 29.1% and varies considerably by land use type. Average urban forest canopy cover is almost twice as high in the “Low Density Residential” land use type than in the “Residential Medium + Residential High Density” land use type plus it has a higher percentage of ‘plantable space.’

2. Functions. Oakville’s urban forest filtered all (102%) of the local industrial and commercial emissions of particulate matter (PM10 ) and 7% of PM2.5 and over two times (243%) the amount of sulpher dioxide plus other criteria pollutants. A total of 22,000 tonnes of carbon dioxide were filtered and 6,000 tonnes of carbon were sequestered. Carbon storage and sequestration are most affected by: a) Size of Tree. Large-stature trees are the most efficient at storing and sequestering carbon and; b) Land Use Type. ‘Woodlots’ are the most efficient air filters while ‘Public Use’ sections, which contain schools, are the least efficient.

3. Values. The value of the annual ecological services provided by trees within the Town of Oakville is $2.1 million. In addition, trees save local industry $1.1 million each year by avoiding the expenditure on mechanical methods to remove the 172 tonnes of criteria pollutants emitted at source. Trees save Oakville residents $812,000 annually in reduced energy bills. The annual revenue potential through trading the Town’s qualifying carbon credits was $5,191 on the Chicago Climate Exchange on June 21, 2006.

Implications for Forest Management
There are several important management and planning tools resulting from this project and these will be incorporated into a current major project: The Urban Forest Strategic Management Plan. For example, action item #14 in Our Solution to Our Pollution will result in an Official Plan amendment to recognize the municipal urban forest as a component of the municipality’s ‘infrastructure.’

The UFORE Grow-out Module simulated a 100-year canopy cover projection. Using an assumption of a 2% tree mortality rate and no future tree planting resulted in a projection to 40% urban forest canopy cover by 2046. Additional simulations will be modeled which reflect the impact of other town departments.

The UFORE Tree Locator Module indicates the best locations to plant trees in order to improve air pollution filtration. Forestry staff will use this as a guide to identify priority areas to plant trees in combination with the “Best Species for Air Quality Improvement” table developed for Oakville. The top three species for air quality improvement in Oakville are the tulip tree (Liriodendron tulipifera), American basswood (Tilia americana) and the Japanese zelkova (Zelkova serrata).

Identifying the ‘best locations’ and supplying the ‘best species’ relies on good quality tree habitat for success. This is often far from the case for the majority of urban sites (see The Town of Aurora’s Take on Tree Planting in the November issue). Instead, urban trees are too often planted in a nutrient poor, compacted medium that is too small in volume to sustain large-stature trees, i.e. sites are known as ‘tree coffins.’ Compounding this challenge is the fundamental impact that the planner and the engineer have on tree habitat as recognized in the ‘ formula’: (street) Tree Habitat = Zoning* + Engineering** where *Zoning is density plus front yard setback and **Engineering is the municipal road cross section plus plan view of the building lot entry.

A paradigm shift in urban design is necessary to bridge the gap and bring together those professional disciplines responsible for impacting tree habitat on a single plan that accurately renders the conditions for the green and grey infrastructure. Merging these professions and adopting sound urban forestry practices such as structural soil combined with sophisticated management tools such as i-Tree must become standard practice.

Major Conclusions
(1) Oakville’s urban forest canopy filters the majority of the criteria air pollutants generated by local commercial and industrial sources;
(2) Municipal trees can contribute to a corporate greenhouse gas emission reduction program;
(3) Trees play a role in contributing to the health of the people of Oakville. Trees can reduce the number of premature deaths, hospital admissions and emergency room visits due to air pollution;
(4) A paradigm shift towards recognizing the urban forest as ‘green infrastructure’ is required in order to create and maintain the type of tree habitat necessary to grow and support a healthy urban forest canopy and recognize that expenditures in the urban forest are an investment in a healthier environment. Adopting an asset management based approach towards green infrastructure should become a statutory requirement at senior government levels.

Copies of the staff document Oakville’s Urban Forest: Our Solution to Our Pollution are available at

— John McNeil, Town of Oakville (Introduction by Ontario Arborist Editor John Wilson)

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