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
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.
of species which represent
more than 5% of the total trees in the survey
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
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.
of municipally-owned trees
versus privately owned trees
of native and non-native trees
of trees in each of six diameter classes
- 15.5 cm
- 30.5 cm
- 45.5 cm
- 60.5 cm
- 76.5 cm
of trees by height classes
to 11 metres
than 11 metres 65%
of native and non-native trees
by each diameter class
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
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
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.
benefits of trees go up exponentially as their
size and total leaf area increases.
this conservative valuation concluded that the average tree
in the survey was worth over $2100.
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
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
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 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.
2002: Summary Report (512 KB in PDF format)
gratefully thanks the Toronto Atmospheric Fund for
its generous support of the Trees Count Project.