Trees Count

A survey of trees conducted by Friends of the Don East (FODE) in east Toronto neighbourhoods has turned up some interesting and disturbing information about the condition of our urban forest.

The "Trees Count" study examined private and public trees on several streets in Cabbagetown, Riverdale, Leaside and East York.

It was conducted in the summer of 2002, with the financial support of the Toronto Atmospheric Fund, to test the usefulness of a tree inventory process called Neighbourwoods© which is specifically designed to allow volunteers to determine the health of trees in residential neighbourhoods and help communities develop strategic plans to protect and enhance their urban forests.

The teams of volunteers examined nearly 400 trees during four weekends in August and September. Two or three streets from each of the four neighbour-hoods were included in the study, and nearly 30 pieces of information were collected for each tree.

Homeowners allowed the teams access to privately-owned trees in most of the backyards in the targeted areas. Trees on city property along the street were also examined, but they formed only 36% of the total. The study found that nearly two-thirds of the trees in the neighbourhoods are on private property. This confirms experience in other cities and shows that homeowners have a very large part to play in the overall health of the urban forest.

Norway Maples made up 34% of all the trees in the sample. A close relative, the Silver Maple, was the second most common species, accounting for a little over 6%. Siberian Elms ranked close behind in third place. Fourth place was occupied by White Mulberry and fifth by a third maple species, the Manitoba Maple.

Contribution of species which represent
more than 5% of the total trees in the survey

Norway Maple
      Siberian Elm
      Silver Maple


Although 45 different tree species were found in the survey, 70% of the trees were from only eleven species, suggesting that biodiversity is quite poor in Toronto's urban forests. The fact that close to half the trees (46%) were maples is disturbing for several reasons.

In the first place, both Manitoba Maple and Norway Maple are non-native. The Norway Maple is a popular choice partly because it comes in a variety of leaf colours including the dark red Crimson King cultivar. It is also better able than many species at withstanding air pollution, compacted soils and other poor growing conditions that face trees planted along city streets.

Proportion of municipally-owned trees
versus privately owned trees

Private
trees
64%

City
trees
36%



Proportion of native and non-native trees

Non-native
70%

Native
30%

 

Number of trees in each of six diameter classes
Diameter Classes
1 0 - 15.5 cm
2 15.6 - 30.5 cm
3 30.6 - 45.5 cm
4 45.6 - 60.5 cm
5 60.6 - 76.5 cm
6 > 76.6 cm


Proportion of trees by height classes

4.5 to 11 metres
20%

Less than
4.5 metres
15 %

Greater than 11 metres — 65%

 

Number of native and non-native trees
by each diameter class

However, Norway Maples are also highly invasive and they have become a very serious problem in stream valleys where they are squeezing out native tree species. They shade out saplings as well as woodland wildflowers, sedges and other groundcover normally found in native forests, and this makes the soils more susceptible to erosion. This highly prolific species has already taken over large areas of Toronto's ravines.

Secondly, such overwhelming dependence on a single family of trees makes Toronto's urban forest particularly susceptible to being decimated by diseases or environmental problems affecting maples. The city had bitter experience of this in the last century when Dutch Elm disease wiped out virtually all of Toronto's stately elm trees, leaving many streets completely barren.

Thirdly, the survey found that maples are an even more dominant part of east Toronto's forest when their size is taken into account as well as their numbers. The Silver and Norway maples formed the overwhelming majority of the largest trees, and together they comprised over 80% of the total leaf area.

The leaves of trees intercept and remove air pollutants, provide air conditioning by evaporating water, and provide protection from sun and wind.

While all trees make important contributions to air quality and local climate, the greatest benefits come from the biggest trees because they have the largest leaf area. Investing nearly all these critical functions in one or two species is obviously risky.

Fourthly, large trees are also the oldest and represent the trees that most likely will have to be removed in the near future. Toronto neighbourhoods that today are graced by large numbers of magnificent large maples may quickly find themselves with drastically fewer trees if action is not taken in time to renew their urban forest. For example, nearly one quarter of all the trees examined already show structural defects that make them potentially hazardous.

The Trees Count survey was conducted with exactly these problems in mind. While the inventory looked at less than 400 trees, the results strongly suggest that east Toronto communities need more information about their trees in order to appropriately plan for the future of their urban forests. FODE hopes to continue the tree inventory work this summer and to concentrate on one or two large neighbourhoods in order to develop a complete picture of the status of their urban forests.

Besides their obvious environmental benefits, trees form a substantial part of the value of a house. Approximately 15% of the house and lot price may be related to its trees, and some real estate appraisers think this number is closer to 30%, especially for large mature trees.

Much of this value is aesthetic and could not be determined from the data collected. However, the analysis did allow calculation of the replacement value of each tree based on its size, species, condition and location using the approach of the Council of Tree and Landscape Appraisers.


The benefits of trees go up exponentially as their size and total leaf area increases.

Even this conservative valuation concluded that the average tree in the survey was worth over $2100.

White Oaks averaged more than $5000 each, with Black Walnuts and Red Oaks close behind.

In addition to inventorying trees, the volunteers who conducted the survey also identifed 129 locations where there was room to plant additional trees.

We must preserve suitable locations to plant new trees. There are increasing pressures to pave over or build over lands that could support trees and other vegetation. When front lawns are replaced with parking pads, the opportunity to plant trees in that location is eliminated.

This preservation of plantable spots is the third "P" in an effective urban forest program. The other two are protecting existing trees and planting new ones.

Most tree programs only emphasize the last "P", but it can't be accomplished effectively without determined efforts to provide and preserve plantable locations.


And while we are waiting for new trees to grow, we need to make every effort to protect the trees we already have. One large tree provides climatic, energy and environmental benefits equal to hundreds of new saplings.

This is proven by the average leaf area of the trees in the study - a stunning 4300 square metres.

The study also identified 83 potential heritage trees and found four regionally rare trees - two very large White Oaks, and two Honey Locusts.

The key finding, however, is that the urban canopy in the study areas is dominated by Norway Maples, a non-native invasive species with negative impacts on natural areas, particularly Toronto's ravines.

In addition, the maturity of many trees, while contributing to neighbour-hood aesthetic and real estate values, and relief during heat waves, means that many parts of Toronto will soon face potentially significant tree loss.

FODE believes that these key conclusions suggest both a huge near-term requirement to plant replacement trees and a corresponding opportunity to reduce the extensive presence of Norway maples and improve the diversity of the existing canopy with more native species.

The Full Report

The Trees Count inventory was conducted by volunteers. They utilized the Neighbourwoods© program developed at the University of Toronto by Andy Kenney and Danijela Puric-Mladenovic, who subsequently crunched the numbers and produced a detailed 108-page summary report. The graphs accompanying this article are taken from that report. See full report.
The appendix provides detailed data on each of the trees examined in the study.

Summary Report

Trees Count 2002: Summary Report (512 KB in PDF format)

FODE gratefully thanks the Toronto Atmospheric Fund for its generous support of the Trees Count Project.
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FODE is a membership-based non-profit organization working to protect and enhance the Don River and to encourage the establishment of healthy and sustainable communities within the central and eastern portions of the Don watershed, Toronto, Ontario. © 2004