Assessing the Introduction and Age of the Acer platanoides (Norway
Maple) invasion within Wilket Creek ravine in Toronto, Ontario.
by
Madison Postma
A capstone project submitted in conformity with the requirements
for the degree of Master of Forest Conservation
Daniels Faculty of Architecture, Landscape and Design
University of Toronto
©Copyright by Madison Postma 2020
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Abstract:
After over a century of disturbance the property that encompasses the Toronto
Botanical Garden and the Wilket Creek ravine in Toronto, Ontario has fallen victim to
the invasive Norway maple (Acer platanoides). The objectives of this study were to
improve the overall knowledge of Norway maple invasions within the Wilket Creek
ravine, to determine when and where Norway maples were introduced in the study area,
and to improve the overall understanding of Norway maple age dynamics within the
property. The results show that Norway maple was introduced into the Wilket Creek
ravine in the 1940s and 50% of the sampled Norway maple within the study were
established between 1980s and 2000s (18 and 40 years old). The results also show that
Norway maple regeneration is present in almost all wooded areas. To control Norway
maple, it is recommended to implement an intensive management plan that includes
mechanical and chemical control methods, strict invasive species policy, and the
development of public education and outreach programs to halt the regeneration and
growth of the invasive tree species.
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Acknowledgements
I would like to thank my internal supervisor, Danijela Puric-Mladenovic for her
undying guidance and support throughout the entire process of this graduate capstone
project. I would also like to thank my external supervisor, Katherine Baird for assistance
in collecting a large portion of the data needed to complete this project, as well as
providing additional VSP monitoring data. To Tony Ung, Dr. Jay Malcolm, and Paul
Piascik for their constant support and expertise throughout the entire core sampling
process. To Adam Tweedle and Krishna Selvakumar who assisted with core sampling.
To Project Learning Tree Canada, the Daniels Faculty, and the University of Toronto
Work Study Program for funding this project. And finally, to the 2020 Master of Forest
Conservation class at the University of Toronto for their support throughout this entire
program.
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Table of Contents
Introduction & Background…………………………………………………………………….8
Objectives………………………………………………………………………………………13
Methods………………………………………………………………………………………...13
Results………………………………………………………………………………………….16
Discussion………………………………………………………………………………………20
Conclusion……………………………………………………………………………………...23
Recommendations…………………………………………………………………………….23
References……………………………………………………………………………………..26
Appendices……………………………………………………………………………………..30
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List of Tables
Table 1: Summary of Norway maple abundance in shrub and ground layer of study
area based on data provided by TBG (Baird, 2020b)…………………………………….17
Table 2: Summary statistics showing minimum, mean, and maximum values from
collected data…………………………………………………………………………………19
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List of Figures
Figure 1: Map of Rupert Edward's property in 1947. The red circle marks the project
study area……………………………………………………………….……………………….9
Figure 2: Map of Rupert Edward's property in 1956. The red circle marks the project
study area. The image shows the increase in development near Edward’s property from
1947 to 1956 …………………………………………………………………………………….9
Figure 3: Map of Study Area. Map Author: Madison Postma…………………………….14
Figure 4: Histogram showing the number of Norway maple in each DBH class based on
data provided by TBG (Baird, 2020a)………………………………………..…….…….….17
Figure 5: Distribution of Norway maples within the study area, based on data provided
by TBG (Baird, 2020a). Map Author: Madison Postma…………...……..……….……….18
Figure 6: Histogram showing the number of sampled Norway Maple within each age
range…………………………………………………………………………………………….19
Figure 7: Linear regression analysis for the tree age and diameter at 1.05m
(y= 3.026112 + 0.019360x R² =0.5569)………………….…………………………………20
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List of Appendices
Appendix 1: Statistical Regression Results (Summary)…………….…………………..…30
Appendix 2: Archival Aerial Photographs of Study Site (City of Toronto, n.d.).……..31-33
Appendix 3: Tools & Materials………………………………………………………………..34
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Introduction & Background:
The History of the Toronto Botanical Garden and Edwards Gardens
The land that occupies the Toronto Botanical Garden and Wilket Creek ravine
was once a part of the 202 ha (500 acres) property purchased by Scottish immigrant
and prosperous wool and lumber producer Alexander Milne in 1827 (Goldenburg,
2020). In 1832 Milne moved his wool and sawmill east to the Don River as the Wilket
Creek (then known as the Milne Creek) was unable to provide a steady supply of water
to power his three-story mill (Toronto and Regional Conservation Authority, 2018).
Although Milne left the property, the land stayed within the Milne family for over 100
years (Toronto Botanical Garden, 2020). In 1944 Rupert Edwards purchased the
overgrown property and with a twelve-person crew cleared the land and completely
transformed it by adding elaborate gardens, ponds, a 9-hole golf course, and one of the
largest rockeries in Canada (Toronto Botanical Garden, 2020; Goldenburg, 2020). The
Wilket ravine slopes were “planted with thousands of bulbs, shrubs, and trees” and
Edward’s “dammed the creek and constructed an electricity-generating waterwheel to
irrigate his gardens” (The Cultural Landscape Foundation, n.d.).
Ten years later as the urban development started to inch toward the property
(Figure 1 and Figure 2) Edwards decided to sell it. However, he wanted to ensure that it
was preserved as a public park. He sold his private country garden oasis to the City of
Toronto and in 1956 Edwards Gardens was opened to the public (Toronto Botanical
Garden, 2020). The Garden Club of Toronto, which occupied in the original Milne
farmhouse, established the Toronto Civic Garden Centre to provide horticultural support
to the public. In 2006 the Toronto Civic Garden Centre was transformed into the Toronto
Botanical Garden (hereby known as TBG). TBG opened a series of themed public
gardens over four acres, allowing visitors to “enjoy and engage the splendor of nature
while learning practical applications for their own gardens” (Toronto Botanical Garden,
2020). In early 2019 TBG, partnered with the City of Toronto, began phase 1 of the
“Edwards Gardens and Toronto Botanical Garden Master Plan and Management Plan”,
a project involving the expansion of the botanical garden from four to 35 acres (Toronto
Botanical Garden, 2020).
The growth of suburbia around the TBG began the unsolicited expansion of
invasive plants and trees from surrounding urban areas into the Wilket Creek ravine,
including Norway maple (Acer platanoides).
9
The Natural History of the Norway maple
The Norway maple, a large deciduous tree, is native across Europe. Its natural
range is from southern Scandinavia to Northern Italy and further from Eastern Europe to
Asia Minor (Nowak & Rowntree, 1990). In the Balkan Peninsula, Norway maples have
been known to live up to 200 years, but in its more common European ranges the
lifespan varies between 100 and 150 years. Along with its use as a street tree
throughout Europe, Norway maples are also harvested and used for veneer wood as
well as “speciality items such as tool handles, gunstocks and violins” (Nowak &
Rowntree, 1990).
Due to its vigorous growth rate, size and shape, and its ability to withstand
different environments and urban pressures, the Norway maple was introduced to
England in 1683. Soon after, in 1756, John Bartam introduced the species to
Philadelphia, USA (Nowak & Rowntree, 1990). The Norway maple was soon considered
one of the rarest and “finest” maples, and thereby deemed “one of the finest ornamental
trees” in North America (Nowak & Rowntree, 1990). Soon, many high society
Americans, including George Washington, were asking for Norway maple seedlings.
The introduced maple species was considered suitable for streets and avenues by 1833
and from there onward became the most popular urban tree species (Nowak &
Figure 1: Map of Rupert Edward's property in 1947. The red circle marks the project study area.
Figure 2: Map of Rupert Edward's property in 1956. The red circle marks the project study area. The image shows the increase in development near Edward’s property from 1947 to 1956.
10
Rowntree, 1990). By 1861, the demand for Norway maples had crossed the country and
Norway maples began to grow in California tree nurseries (Nowak & Rowntree, 1990).
Norway maple increased in popularity after World War II when the native white
elm (Ulmus americana) population was killed off by Dutch elm disease (Ophiostoma
ulmi) (Nowak & Rowntree, 1990). Norway maple’s ability to grow quickly and provide
ample shade made the species one of the replacements for the dying elm population.
Unfortunately, the invasive properties of Norway maple were not observed or of concern
at the time. Norway maple can self-establish in native forests and can outcompete
native trees, and therefore is considered a harmful and invasive species throughout
urban areas and woodlots in North America (Webb, Pendergast & Dwyer, 2001).
The Ecology & Biology of the Norway Maple
The Norway maple can survive in a variety of temperatures and climates;
however, it thrives where the mean annual temperature is approximately 12°C,
comparative to the annual temperature of 8.6°C in Toronto, Ontario (Munger, 2003:
Nowak & Rowntree, 1990; Climate Data, n.d.). While Norway maples are considered a
resilient species to urban stressors, their growth rate is low when subjected to excessive
heat, cold, evapotranspiration or high soil pH. Norway maple growth is optimal in
environments with a lot of precipitation or in areas with fresh soils (Munger, 2003:
Nowak & Rowntree, 1990). Deep, moist, and fertile soils (i.e. loamy soil) with sufficient
moisture and a pH of 5.5-6.5 is optimal for Norway maple’s high growth rate.
The Norway maple’s average height ranges from 18-22m with average crown
spread of 15m at the age of maturity in a closed canopy (approximately 30-45 years)
(Munger, 2003: Nowak & Rowntree, 1990). The Norway maple develops round clusters
of small, green flowers that are approximately 1cm across, and rely on pollination
insects. One can identify a Norway maple by the size of its leaves and the milky white
sap produced when a leaf stalk is broken. Norway maples leaves are opposite on the
branches and each leaf is palmately lobed, ranging from 8-16cm long and 10-18cm
wide (Munger, 2003).
Norway maple bloom between April and early June, earlier than most native
maples in North America (Munger, 2003). The Norway maple is also known to produce
many seeds that are widely dispersed by wind due to their “winged” shape (Munger,
2003). Its seeds can be carried over 100m from the source tree (Bertin, Manner,
Larrow, Cantwell & Berstene, 2005; Munger, 2003). Norway maple fruit- paired samaras
(or keys) are grown in clusters at the tip of branches and are considerably larger than
North American native maple keys (ex. 65% larger than sugar maple), ranging from 3-
5cm in length (Meiners, 2005; Webb & Kaunzinger, 1993).
Seedlings and young Norway maples are considered highly shade tolerant and
can grow in many different soil types. Besides, rapid regeneration allows the Norway
maple to outcompete other species in the understory and reach canopy openings
11
(Webster, Nelson & Wangen, 2004). Norway maples are also known to hold on to their
leaves longer than most native species in North America.
Norway maple cultivars
The Norway maple has over 100 cultivars. Many of the Norway maple cultivars
were bred in Germany, France, and Belgium and imported to North America (Nowak &
Rowntree, 1990). Norway maple cultivars are just as resilient as the typical species and
are often used for their distinctive colours and ability to create large areas of shade
(Roussy, Kevan, Dale, & Thomas, 2008). Cultivars of Norway maple differ in some
phenotype and/or functional characteristics which often determine their suitability for
urban areas (Conklin & Sellmer, 2009a). For example, Acer platanoides “Crimson King”
is by far the most popular due to its deep marron coloured foliage (Roussy, Kevan,
Dale, & Thomas, 2008), while Acer platanoides “Columnare” develops narrow, columnar
tree canopy.
For instance, a study was conducted by J. Conklin and J. Sellmer (2009b) to see
the differences in flower and seed production throughout six Norway maple cultivars.
The study found that the cultivars Acer platanoides “Columnare” and Acer platanoides
“Emerald Queen” produced more seeds than Norway maple cultivars “Crimson King”,
“Globosum”, “Faasen’s Black”, and “Rubrum” (Conklin & Sellmer, 2009b). The study
suggests that the cultivars that produce the largest number of seeds would be more
problematic in non-native landscapes, whereas those with lower seed yield were
considered less invasive alternatives (Conklin & Sellmer, 2009b).
The Role of Norway maple in Urban Forests: Issues & Impacts
Municipalities and landowners are drawn to Norway maple for many reasons; its
resilience to pests and urban stresses (i.e. pollution, road salt, and compact soil), its
ability to provide dense shade, its rapid growth rates, and its overall aesthetics
(Lapointe & Brisson, 2015; Roussy, Kevan, Dale, & Thomas, 2008). For these reasons,
Norway maples and Norway maple cultivars are still commonly sold in tree nurseries
and evidently continue to expand beyond sidewalks and private gardens into native
forests and woodlots (Bertin, et al. 2005; Lapointe & Brisson, 2015; Kloeppel & Abrams,
1995).
However, Norway maple is not a perfect urban tree species, as it develops
structural problems such as girdling roots and contorted branching. Girdling roots that
emerge toward the surface of the soil and cut into the tree’s trunk. This is an issue
because it restricts movement of water and nutrients and puts pressure on the wood
and trunk, which eventually leads to the tree’s death (Fraedrich, n.d.; Munger, 2003). It
is not uncommon to observe girdling roots on older and larger Norway maples due to
compact soils (Munger, 2003) and/or due to tree nursery practices (trees kept in
containers for too long). Other issues that can often be observed on larger and older
Norway maples are that branches start to contort and become esthetically displeasing
to many landowners (Munger, 2003; Roussy, Kevan, Dale, & Thomas, 2008). Contorted
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branching and girdled roots often mean larger Norway maples are more susceptible to
storm and ice damage, resulting in fallen trees, maintenance costs, and property
damage (Roussy, Kevan, Dale & Thomas, 2008).
Despite its evident invasive characteristics, it was not until the late 1970s that the
Norway maple was observed as a potential invader of North American woodlands.
Despite this, it was not until the 1990s that intensive research on the Norway maple and
its adverse impacts on urban woodlots and forests would be conducted (Bertin, et al.
2005).
The largest issue caused by Norway maples is its seed dispersal from trees
planted across urban areas into urban woodlots and forests, due to their high shade
tolerance and quick growth rate. Due to their high shade tolerance and quick growth
rate, Norway maple seedlings and saplings quickly outcompete native tree species in
woodlots. For example, compared to the native sugar maple (Acer saccharum), the
Norway maple is known to hold it’s leaves for approximately 12 days longer, and utilizes
light, water, and nutrients more efficiently (Kloeppel & Abrams, 1995; Bertin, et al.,
2005). It has been also shown that native tree seedlings often struggle with
regeneration in the presence of Norway maple due to the invasive species’ desne
shade, surface roots and lower rates of predation (Cincotta, Adams, & Holzapfel, 2008;
Meiners, 2005; Martin, 1999). The deep shade and intense seed dispersal of Norway
maples make it difficult for native trees and flora to recover once Norway maple
dominates a forest stand (Bertin, et al. 2005; Martin, 1999).
A study conducted by S.L. Galbraith-Kent & S.N. Handel (2008) looked at sapling
growth of Norway maple and native saplings under various canopy tree species. The
study found that native tree species grew significantly less seedlings under a canopy of
Norway maple than under a canopy of native trees (Galbraith-Kent & Handel, 2008). It
is recommended that Norway maple be eradicated from urban woodlots and native
forests before it reaches the canopy, as the invasive maple was found to be detrimental
to the health and growth of native seedlings and other flora (Wyckoff & Webb, 1996;
Galbraith-Kent & Handel, 2008).
However, a study done by Lapointe & Brisson (2011) comparing the growth and
survival of tar spot disease on Norway maples show some reassuring information. The
study was conducted in Mount Royal, an urban forest in Montreal, Quebec, and
compared the growth of Norway maple and sugar maple seedlings and trees before and
after the tar spot disease outbreak. The study showed that before the outbreak, the
Norway maple had higher growth rates than to the sugar maple (Lapointe & Brisson,
2011). However, once the disease was introduced, the roles were reserved. With the
introduction of the disease, Norway maple seedling and tree growth saw a sharp
decline, as well as an increase in the mortality rate (Lapointe & Brisson, 2011). The
study concluded that the tar spot disease might be used to reduce the invasive Norway
maple in natural areas without negatively affecting native maples or other tree species
(Lapointe & Brisson, 2011).
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Understanding Species Invasions
When managing forests that have been altered by invasive tree species, it is
important to know when, where, and how the invasive was introduced. Fortunately, with
enough data predictions of invasive species age and introduction within natural areas
can be made. Understanding the timelines and history of introduction is also useful in
knowing when native trees and flora began to be negatively impacted by the invasive
species’ presence (Martin, 1996; Webster, Nelson, & Wangen, 2004). Knowing the age
of an invasion allows for a more in depth understanding of the species and how it may
have been introduced into the natural area; creating effective management strategies
(Martin, 1996; Webster, Nelson, & Wangen, 2004). Webster, Nelson and Wangen
(2004) collected tree cores from the largest trees on Mackinac Island. Tree ring analysis
from the cores was used to understand and recreate the original recruitment canopy
over 80 years prior to the study (Webster, Nelson, & Wangen, 2004). This analysis,
along with a gap capture method and radial growth patterns were used to determine
how long after the first Norway maple was introduced, the invasive species began to
take over the forest (Webster, Nelson & Wangen, 2004).
Objectives
The overarching objective for this project is to better understand the history of Norway
maple invasion within the Wilket Creek Ravine, Toronto, Ontario as well as its
distribution and abundance within the study area. My specific objectives for this project
are as follows:
1. To improve the overall knowledge of Norway maple invasions within the Wilket
Creek ravine in Toronto using abundance data from the ground, shrub, sub-
canopy, and canopy layers of the forest.
2. To determine the age of the invasion, and investigate when, where, and how
Norway maples were introduced in Wilket Creek ravine.
3. To improve our understanding of the post-invasion dynamics of Norway maples
within Wilket Creek ravine.
Methods
Study Area
The study area, which encompasses the TBG, Edwards Gardens, and the Wilket
Creek ravine portion in Toronto, Ontario (see Figure 1) is 15.5 hectares. The forested
portion covers 4.1ha (or 25.9%) and the Wilket Creek ravine floodplain covers 2.2ha (or
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14.1%) (Toronto Botanical Garden, 2018). The area generally has clay loam soil type,
with some sandier soil near the creek (Toronto Botanical Garden, 2018). The natural
areas within the study area range from hardwood to mixed wood stands dominated by
native species such as sugar maple (Acer saccharum), white cedar (Thuja occidentalis),
black cherry (Prunus serotina), American beech (Fagus grandifolia), eastern hemlock
(Tsuga canadensis), and red oak (Quercus rubra) (Toronto Botanical Garden, 2018).
Wilket Creek ravine is a tributary of the Don River watershed, and a portion of the study
area encompassing Wilket Creek Forest is designated an Environmentally Significant
Area (North-South Environmental Inc, 2002).
Beyond the site’s natural area are “ornamental gardens, cultural plantations,
manicured lawns, and urban cover” (Toronto Botanical Garden, 2018). There are also
numerous paved and woodchip paths throughout the property.
Data Collection
Norway maples within the TBG property and Wilket Creek ravine were mapped in
the summer of 2020 by Katherine Baird, ecologist at TBG (Baird, 2020a). The TBG
dataset which included locations of all Norway maples above 5cm in diameter at breast
height (DBH), was used to select 44 trees for this study and for which to collect more
detailed data (Baird, 2020a). Sample size of 44 trees was chosen based on the size of
Figure 3: Map of Study Area. Map Author: Madison Postma
15
the study site and methodology of the Webster, Nelson, & Wangen (2005) study. Ten
trees from each DBH class (10-19.9 cm, 20.29.9 cm, 30-39.9 cm, 40-49.9 cm, ≥50 cm)
were randomly sampled, with exception of 40-49.9 cm and ≥50 cm as there were less
than 10 trees (8 trees between 40-49.9cm and 3 trees ≥50 cm) within those diameter
classes in the study area (Baird, 2020a; Webster, Nelson, & Wangen, 2005). Three
trees with a diameter at breast height (DBH) of 5-9.9cm were also sampled to retrieve a
representation of younger sub-canopy trees.
Four measurements were taken at each of the 44 sampled trees: DBH, diameter
at tree core sample height (1.05m), total tree height, canopy base height, canopy
closure (%), and percent canopy dieback. DBH was measured at 1.30m from the base
of each tree. Both tree height and canopy base heights were measured using a
rangefinder and clinometer (Webster, Nelson, & Wangen, 2005). Canopy closure was
assessed by looking up into the canopy 1m from the base of the tree and determining
the percentage of canopy present (Dahir & Lorimer, 1996). Collection of canopy dieback
data was based the Neighbourwoods© tree inventory protocol (Kenney & Puric-
Mladenovic, 1995). Dieback was determined based on a scale of 0 to 3, 0 indicating no
sign of dieback and 3 indicating severe dieback over 75% of tree canopy (Kenney &
Puric-Mladenovic, 1995; Lapointe & Brisson, 2011). All measurements, including any
additional observations, were recorded in an ArcGIS Collector application that was
downloaded onto field tablets.
Two cores were taken from each tree (one on each side of the trunk) 20cm or
greater in diameter to ensure all rings were captured. Trees with a DBH under 20cm
had one core sampled as the increment borer was able to go through the tree and
therefore retrieve both sides of the pith (Grissino-Mayer, 2003). Cores were collected
using an increment borer 1.05m from the base of the tree and the diameter was
measured. Although a study conducted by Webster, Nelson, & Wangen (2005) collected
cores 30cm from the base of the tree, it was decided for this study to collect at 1.05m to
ergonomically collect a whole core sample (Webster, Nelson, & Wangen, 2005;
Grissino-Mayer, 2003). Each extracted core was placed into a labelled plastic straw.
The labelling included the tree i.d., core identifier (either A or B) and date (Webster,
Nelson, & Wangen, 2005; Grissino-Mayer, 2003). A total of 75 cores for 44 trees were
taken.
Data Analysis
All tree cores were mounted onto precut wooden blocks using wood glue and
labeled again. After the core samples dried for 38 hours, they were sanded ⅓ down
using a palm sander (Webb & Kaunzinger, 1993). For each core total core length was
measured (in cm) as well as the length from the tree center to the end of the core. Once
scanned onto the computer CooReader software was used to measure and digitize the
annual rings of each core (Webster, Nelson, & Wangen, 2005; Webb & Kaunzinger,
1993; Yamaguchi, 1990).
16
The age of each tree was determined via the number of rings that were present
on each core. For trees with two core samples, the average age between samples A
and B was used. For the few Norway maple cores that were unable to reach the center
of the tree the age was estimated using an equation by Norton, Palmer, & Ogden
(1987):
age = (r-p)/(d/n) + N
“Age was estimated by dividing the length of the missing geometric radius by an
estimate of the mean ring width of the innermost 20 rings and adding the number
of rings counted to the core. R is the geometric radius, p the partial core length, d
the length of the last n (20) rings, and N the total number of rings present in the
partial core. All measurements were made in mm” (Norton, Palmer, Ogden,
1987).
Using the statistical software R, Shapiro-Wilks tests were used to test normality
of tree age and DBH at 1.05m data. Once the tree age data was transformed, a linear
regression statistical test was performed to show the relationship between the two data
sets. The GIS software ArcGIS was used to map the Norway maple diameter and age
distribution within the study site. Maps created in ArcGIS and graphs created in
Microsoft Excel were also used to visualize Norway maple abundance within the shrub,
sub-canopy, and canopy layer of 38 of VSP plots (Baird, 2020b). The abundance data
was collected July to October 2020 from 38 VSP plots (37 400m² plots and one 100m²
located in a narrow forest patch) and provided by Katherine Baird, ecologist at the
Toronto Botanical Gardens (Baird, 2020b).
Results
Abundance Data
The Norway maple data from 38 VSP fixed area plots (Baird, 2020b) (37 plots
400m² and one 100m²) was collected and summarized to determine abundance in the
shrub and ground layers of the study area (Table 1). Within the shrub layer (0.5-3m in
height), Norway maple had an average absolute cover of 0.4% with a summed cover
across all plots of 15%. Within the ground layer (
17
Shrub Layer (0.5-2m)
Ground Cover (
18
All 172 mature Norway maple within the study site were mapped along with their
DBH to show species and size distribution (Figure 5). Norway maple density per hectare
is 34.4 trees with a basal area of 1.31m² per hectare.
Summary statistics were run on the collected data from each of the sampled
Norway maple to determine the minimum, maximum, and average for each of the field
measurements (Table 2). The average tree height for the sampled Norway maple is
16.98m with a minimum of 7.3m and a maximum of 23.6m. The average height to base
of canopy for the sampled trees was 4.9m with a minimum of 1.8m and a maximum of
9.2m. The average canopy width for the sampled trees was 9.2m with a minimum of
4.9m and a maximum of 18.8m. The average canopy closure percentage for the
sampled trees was 79.1% with the minimum percentage of 45% and the maximum
percentage of 95%. The average diameter at breast height for the sampled trees was
28.9cm with a minimum of 6.9cm and a maximum of 57cm. The average diameter at
1.05m for the sampled trees was 29.7cm with a minimum of 7.4cm and maximum of
54.3cm. The average tree age for the sampled Norway maple was 38.7 years with a
minimum of 18 years and maximum of 77 years.
Figure 5: Distribution of Norway maples within the study area, based on data provided by TBG (Baird, 2020a).
Map Author: Madison Postma
19
Minimum Mean Maximum
Tree Height (m) 7.3 16.98 (SE±0.55)
23.6
Height to Base of Canopy (m)
1.8 4.9 (SE±0.26)
9.2
Canopy Width (m) 4.9 9.2 (SE±0.54)
18.8
Canopy Closure (%) 45 79.1 (SE±1.97)
95
Diameter at breast height (cm)
6.9 28.9 (SE±1.99)
57
Diameter at 1.05m (cm)
7.4 29.7 (SE±1.99)
54.3
Tree Age (years) 18 38.7 (SE±1.99)
77
Amongst the 44 sampled trees, 14 Norway maples were between the ages of 39
and 48 years old, making it the most common sampled age, and 12 trees were between
the ages of 29 and 38 years old, the second most common sampled age (Figure 6). Ten
trees were between the ages of 18 and 28 years old and five trees were between the
age of 49 to 58 years old (Figure 6). The least common age ranges amongst the
sampled Norway maples were 59 to 68 years old with only one tree and 69 to 78 years
old with only two trees (Figure 6).
Figure 6: Histogram showing the number of sampled Norway Maple within each age range.
Table 2: Summary statistics showing minimum, mean, and maximum values from collected data.
20
A regression analysis of the tree age and diameter at 1.05m for the sampled
Norway maple shows that there is a positive, and statistically significant, correlation
between the two indicators, with an R value of 0.5569 (p
21
to recreate canopy recruitment of Norway maple in the dominant crown class and
“investigate gap capture rates” for Norway maple and native species on a forested
island in Lake Huron (Webster, Nelson, & Wangen, 2005). The second study, which
was conducted by Webb & Kaunzinger (1993) used increment cores of native oak,
beech, maple trees and Norway maple to assess biological invasions of invasive
species within Drew University Forest Preserve in New Jersey, USA. In addition to the
increment cores, Webb & Kaunzinger counted and identified surrounding woody
saplings to examine the target tree species’ reproductive patterns.
As the research questions of the two Norway maple stand dynamic studies are
similar to those raised in this study it is appropriate that the data collection and analysis
methods used in this project would, to a degree, mimic those executed within those
projects. Those studies had a considerable influence on determining which types of
measurements should be conducted on each sample tree and how many trees to
sample per diameter class. However, unlike the studies conducted by Webster, Nelson,
& Wangen and Webb & Kaunzinger, intensive native tree data was not collected as the
focus of this study is on Norway maple age and overall distribution throughout the
Toronto Botanical Gardens and the surrounding property. Overall, with the guidance of
the three Norway maple stand dynamic studies, our project successfully met its goals.
Abundance
The abundance data show that Norway maple seedlings are present in almost all
areas of the forest. Although the ground cover data showed that the seedlings only
cover an average 0.12% of a 400m² plot, it is significant to note that they were present
in 31 of the 38 plots (82%). The results also show that Norway maple were present in
the shrub layer of 23 of the 38 plots (60%), with an average cover of 0.4%. These
results are important in determining regeneration within the study area because they
show just how far Norway maple can spread away from mature trees.
These results support the findings of a similar study conducted by Martin (1999)
in which the study compared the understory growth and regeneration patterns of
Norway maple and sugar maple. Although this project did not investigate and compare
other tree species regeneration data, it is likely that similar regeneration patterns and
understory consequences are occurring or will occur with the continued regeneration
and expansion of Norway maple within the study area (Martin, 1999; Wyckoff &
Webb,1996).
Tree Age & Establishment of Invasion
The results show that Norway maple density per hectare is 34.4 trees with a
basal area of 1.31m² per hectare. The distribution of Norway maple DBH across the
study site (Figure 5) shows several high-density pockets, especially in the south-west
corner of the property where the largest Norway maples are located. However, we also
see Norway maple with a diameter of 5cm to 10cm seem to be distributed all over the
area, with dense pockets on the eastern and southern portion of the property. This
22
observation fits well with our shrub layer and ground cover data as we can see that
Norway maple seedlings and saplings are not necessarily always located near the
mature trees growing within the subcanopy and canopy of the forest.
The results from our core samples showed that the oldest Norway maple within
our study site was about 77 years old, meaning that the start of the invasion began in
the 1940’s. This correlates well with the history of sub-urban development and Norway
maple introduction and use within North America. As soldiers came home from the
Great War many wanted to settle down outside of major cities, creating an increased
expansion of suburban housing development outside of Toronto (Smith, 2012). Norway
maple was favoured as an ornamental tree and was commonly planted in the yards of
private landowners in the 1940’s and 1950’s (Nowak & Rowntree, 1990). Although there
are no original garden plans, it can be speculated that Rupert Edwards also would have
planted Norway maple within his gardens and especially along his golf course because
of the large, shaded area that their canopies provide (Toronto Botanical Garden, 2020).
While there is no definite pattern of Norway maple distribution on the property, based on
historical aerial images (City of Toronto,n.d.) it is assumed that intentional planting by
Rupert Edwards and Norway maple seeds from encroaching subdivisions began the
invasion into the study area (Appendix 2-aerial photographs) (City of Toronto, n.d.).
The results of this study determined that most of our sampled trees were
approximately 40 years old (Figure 6) and thus established in the 1980s. The linear
regression showed a statistical significance between height and diameter at 1.05m
which allows us to roughly infer the other Norway maple ages in the study area (Figure
7). As the majority of Norway maple within the study site have a diameter of 5cm-25cm
it can be inferred that their age range is approximately 18 to 30 years old, showing that
the Norway maple self-establishment was intensive in the 1980s and 1990s (Figure 4).
Although diameter is not necessarily the best predictor of age (Gibbs, 1963), based on
the results of this study and the other studies which use similar methods, it is believed
that we can still predict the age of the Norway maple invasion within the study area
(Webb & Kaunzinger, 1993; Webster, Nelson & Wangen, 2005; Martin, 1999).
There are multiple explanations for the boom of Norway maple within the study
site in the 1980s and 1990s. By the 1980s the Norway maples planted to replace elm
trees effected by the Dutch elm disease were at full maturity. Therefore, there was an
abundance of Norway maple seeds escaping from backyard gardens and streets into
natural areas (Nowak & Rowntree, 1990). The increase in suburbia surrounding the
study area and public access to Wilket Creek would have also played a large role in the
increased Norway maple invasion. This is also in addition to the original “invaders”
(those planted in the 1940s; 77 years old) reaching full maturity and regenerating within
the surrounding natural areas (Nowak & Rowntree, 1990; Webb & Kaunzinger, 1993).
The findings of this project support the conclusions of similar studies such as
those conducted by Martin (1999), Wyckoff & Webb (1996), and Webb & Kaunzinger
(1993). In a study conducted by Webb & Kaunzinger (1993) on the invasion of Norway
23
maple within the Drew University Forest Preserve in New Jersey they conclude that two
factors that influence the likelihood that an introduced species will become invasive:
characteristics of a site and the life history of the species (Webb & Kaunzinger, 1993).
Similar to the site in this study, the Drew University site is a small (18 ha) natural area
with a disturbance history (Webb & Kaunzinger, 1993). The study also states that
Norway maple, can invade natural areas within proximity to urban landscapes (Webb &
Kaunzinger, 1993; Nowak & Rowntree, 1990; Lapointe & Brisson, 2011). Like the Drew
University study, the Norway maple invasion within the Wilket Creek results from not
recognizing and not managing invasive species within natural areas within the first few
decades of their introduction (Webb & Kaunzinger, 1993). This study shows that over a
period of 70 years, Norway maple has spread throughout the entire study area and has
an ideal reverse J curve indicating successful regeneration. If not managed effectively,
invasive Norway maple will continue to proliferate and expand further into Wilket Creek
ravine (Dong, 2015). This poses harm to the ravine’s ecological integrity, which is also
identified as an Ecologically Significant Area (North-South Environmental Inc, 2002) and
creates an even more costly predicament for future generations (Webb & Kaunzinger,
1993).
Conclusion
The study’s objectives were to improve our understanding of the Norway maple
invasion within the Wilket Creek ravine and determine when, where, and how this
invasive species was introduced. From the collected data and results of the analysis this
study has determined that Norway maple was introduced into the study area in the
1940s due to the increase in housing development around the ravine and the extensive
gardens and recreational areas developed on the property by Rupert Edwards (City of
Toronto, n.d.; Toronto Botanical Garden, 2020). Analysis of the 172 mapped mature
Norway maple within the study area show that the invasive species has a density per
hectare of 34.4 trees with a basal area of 1.31m² per hectare. Norway maple
regeneration is indeed abundant in almost all areas of the property and if not managed.
it will continue to harm the ravine’s ecological integrity.
Recommendations
Selecting the appropriate treatment is often difficult when managing forests that
have an abundance of invasive woody species, such as the Norway maple. With the
knowledge obtained from this study we have developed three recommendations to
address the Norway maple invasion and associated impacts within Wilket Creek study
area and prevent its introduction in other natural areas.
The first recommendation would be to use mechanical and chemical methods to
remove and control the Norway maple within the site. Mechanical control of Norway
24
maples would involve removing saplings and seedlings from the forest (including all root
systems) and cutting mature Norway maple trees close to the ground, or girdling
(removing bark and phloem layer from 10cm around the trunk) (Webb, Pendergast, &
Dwyer, 2001; CABI, 2020). Wood from the removed trees would be distributed and left
as snags or downed woody debris throughout the forest where appropriate (Webb,
Pendergast, & Dwyer, 2001; CABI, 2020).
The use of chemical control is also recommended for Norway maple
management and removal. Chemical control would involve applying herbicide to stumps
or girdled trees, or to the base of saplings that are
25
live in harmony with nature” (Toronto Botanical Garden, 2020). Since invasive exotic
species are harmful to the natural environment, invasive species education should be
an important component of this mission. It is believed that public education through the
botanical garden would greatly impact on the public view of the Norway maple and its
cultivars. Partnering with the City of Toronto and implementing educational signage
along the Edwards Gardens and TBG trails would be a great way to highlight why
Norway maples are so problematic (Roussy, Kevan, Dale, & Thomas, 2008). Norway
maple focused outreach events hosted by the TBG would also be extremely beneficial.
Some of these events could include native vs. non-native education workshops and
seminars geared explicitly towards backyard trees and how they influence nearby
natural areas, or even hands-on events that have volunteers remove Norway maple
seedlings and saplings throughout the study site.
26
References
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Dong, A. (2015). Ecological Integrity in the Park Drive Ravine: 1977 to 2015. (Master of Forest Conservation Capstone Project). Retrieved from https://torontoravinesdotorg.files.wordpress.com/2016/04/anqi-dong_ecological -integrity-in-the-park-drive-ravine_1977-to-2015.pdf Fraedrich, B.R. (n.d.) Research Laboratory Technical Report: Girdling Roots. Bartlett Tree Experts. 1-2. Galbraith-Kent, S. L. & Handel, S.N. (2008). Invasive Acer platanoides inhibits native sapling growth in forest understorey communities. Journal of Ecology. 96: 293- 302. Gibbs, C.B. (1963). Tree diameter a poor indicator of age in West Virginia hardwoods. Research Note NE-11. Upper Darby, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 1-4. Retrieved from https://www.fs.usda.gov/treesearch/pubs/11607 Goldenburg, S. (2020). North York’s Edwards Gardens Named after Millionaire who Cut City Good Deal. North York Historical Society. Retrieved from https://nyhs.ca/history/north-yorks-edwards-gardens-named-after-millionaire- who-cut-city-good-deal/ Grissino-Mayer, H.D., (2003). A Manual and Tutorial for the Proper Use of an Increment Borer. Tree-Ring Research. 59(2). 63-79. Invasive Plant Control Inc. (2016). Invasive Plant Control Inc. GSA Price List/Services Offered: On the Ground Management. Retrieved from https://www.gsaadvantage.gov/ref_text/GS21F0146X/0VO5IH.3REIH8_GS-21F 0146X_IPCTERMS.PDF Kenney, A. & Puric-Mlednovic, D. (1995). Neighbourwoods© Kloeppel, B.D., & Abrams, M.D. (1995). Ecophysiological attributes of the native Acer saccharum and the exotic Acer platanoides in urban oak forests in Pennsylvania, USA. Tree Physiology. 15: 739-746. Lapointe, M. & Brisson, J., (2011). Tar spot disease on Norway maple in North America: Quantifying the impacts of a reunion between an invasive tree species and its adventive natural enemy in an urban forest. Ecoscience. 18(1): 63-69. Martin, P.H. (1999). Norway maple (Acer platanoides) invasion of a natural forest stand: understory consequence and regeneration pattern. Biological Invasions.1: 215- 222.
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Meiners, S.J. (2005). Seed and Seedling Ecology of Acer saccharum and Acer platanoides: A Contrast Between Native and Exotic Congeners. Northeastern Naturalist. 12(1): 23-32. Munger, G. (2003). Acer platanoides. In: Fire Effects Information System. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available Online: http://www.fs.fed.us/database/feis/. North-South Environmental Inc. (2002). Environmentally Significant Areas (ESAs) in the City of Toronto. (Technical Report). Retrieved from https://www.researchgate.net/publication/316256779_Environmentally_Significan t_Areas_ESAs_in_the_city_of_Toronto Norton, D.A., Palmer, J.G. & Ogden, J. (1987) Dendroecological studies in New Zealand 1. An evaluation of tree age estimates based on increment cores. New Zealand Journal of Botany. 25:3, 373-383. Nowak, D. and Rowntree, R. (1990). History and Range of Norway maple. Journal of Arboriculture. 16(11): 291-296. Roussy, A., Kevan, P., Dale, A., & Thomas, V.G. (2008). Norway Maples- Friend of Foe: A Wolf in Sheep’s Clothing. Ontario Arborist. 35-37. Smith, N. (2012). Comrades and Citizens: Great War Veterans in Toronto, 1915-1919 (Doctoral dissertation). The Cultural Landscape Foundation (n.d.). Toronto, On Canada: Edwards Gardens. Landscape Information. Retrieved from https://tclf.org/landscapes/edwards -gardens Toronto Botanical Garden (2018). 5.0 Management Plan. Edward Gardens/Toronto Botanical Garden Master Plan and Management Plan. 124-171. Toronto Botanical Garden (2020). Overview: History. Retrieved from https://torontobotanicalgarden.ca/about/overview-history/ Toronto and Region Conservation Authority (2018). Walk the Don: Wilket Creek. Retrieved from http://www.trca.on.ca/dotAsset/93715.pdf Webb, S.L. & Kaunzinger, C.K. (1993). Biological Invasion of the Drew University (New Jersey) Forest Preserve by Norway Maple (Acer platanoides L.). Bulletin of the Torrey Botanical Club. 120(3): 343-349.
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Webb, S.L., Pendergast, T.H., & Dwyer, M.E. (2001). Response of Naive and Exotic Maple Seedling Banks to Removal of the Exotic, Invasive Norway Maple (Acer platanoides). The Journal of the Torrey Botanical Society. 128(2): 141-149. Webster, C.R., Nelson, K., & Wangen, S.R., (2004). Stand dynamics of an insular population of invasive trees, Acer platanoides. Forest Ecology and Management. 208: 85-99.
Wyckoff, P.H. & Webb, S.L. (1996). Understory Influence of the Invasive Norway Maple
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Appendix 1: Statistical Regression Results (Summary)
y= 3.026112 + 0.019360x
R² =0.5569
31
Appendix 2: Archival Aerial Photographs of Study Site (City of Toronto, n.d.)
Map of Study Site in 1938
(http://jpeg2000.eloquent-systems.com/toronto.html?image=ser97/s0097_fl0009_id0008.jp2)
32
Map of neighbourhood just west of study site in 1938
(http://jpeg2000.eloquent-systems.com/toronto.html?image=ser97/s0097_fl0009_id0007.jp2)
33
Map of Study Site in 1971
http://jpeg2000.eloquent-
systems.com/toronto.html?image=ser12/s001
2_fl1971_it0117.jp2
Map of Study Site in 1961
http://jpeg2000.eloquent-
systems.com/toronto.html?image=ser12/s00
12_fl1961_it0145.jp2
Map of Study Site in 1981
http://jpeg2000.eloquent-
systems.com/toronto.html?image=ser12/s
0012_fl1981_it0031.jp2
http://jpeg2000.eloquent-systems.com/toronto.html?image=ser12/s0012_fl1971_it0117.jp2http://jpeg2000.eloquent-systems.com/toronto.html?image=ser12/s0012_fl1971_it0117.jp2http://jpeg2000.eloquent-systems.com/toronto.html?image=ser12/s0012_fl1971_it0117.jp2http://jpeg2000.eloquent-systems.com/toronto.html?image=ser12/s0012_fl1961_it0145.jp2http://jpeg2000.eloquent-systems.com/toronto.html?image=ser12/s0012_fl1961_it0145.jp2http://jpeg2000.eloquent-systems.com/toronto.html?image=ser12/s0012_fl1961_it0145.jp2
34
Appendix 3: Tools & Materials
The equipment and resources used to conduct this study were provided by the
University of Toronto Daniels Faculty of Architecture, Landscape, and Design.
Data Collection Equipment: Data Analysis Equipment: DBH tape (x2) Sandpaper (80, 250, and 400 grit) Rangefinder (x2) Palm sander Clinometer (x2) Wooden mount blocks Increment borer (x2) Wood glue Plastic straws (x80) CooRecorder Software Sharpie marker (x2) R statistical software Collection bag Microsoft Excel Masking tape (x2) Tablet (x2)
Geographic Information System (GIS) mapping software
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