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

2

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.

3

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.

4

Table of Contents

Introduction & Background…………………………………………………………………….8

Objectives………………………………………………………………………………………13

Methods………………………………………………………………………………………...13

Results………………………………………………………………………………………….16

Discussion………………………………………………………………………………………20

Conclusion……………………………………………………………………………………...23

Recommendations…………………………………………………………………………….23

References……………………………………………………………………………………..26

Appendices……………………………………………………………………………………..30

5

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

6

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

7

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

8

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

12

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).

13

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

14

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

Al Miley & Associates (2020). Cost of Removing a Tree in Toronto. Retrieved from https://www.almileytree.com/tree-removal-cost- toronto/#:~:text=Expect%20to%20spend%20anywhere%20between,be%20remo ved%20from%20your%20property. Baird, K. (2020a). Invasive Norway Maple Mapped Locations and DBH Information. [Data file]. Toronto Botanical Garden. Baird, K. (2020b). Vegetation Sampling Protocol (VSP) Forest Inventory Data. [Data file]. Toronto Botanical Garden. Bertin, R.I., Manner, M.E., Larrow, B.F., Cantwell, T.W., & Berstene, E.M. (2005). Norway maple (Acer platanoides) and other non-native trees in urban woodlands of central Massachusetts. Journal of the Torrey Botanical Society. 132(2): 225- 235. CABI (2020). Acer platanoides (Norway Maple). Invasive Species Compendium. Retrieved from https://www.cabi.org/isc/datasheet/2883 Cincotta, C.L., Adams, J.M., & Holzapfel, C. (2008). Testing the enemy release hypothesis: a comparison of foliar insect herbivory of the exotic Norway maple (Acer platanoides L.) and the native sugar maple (A. saccharum L.). Biol Invasions. 11: 379-388. ClimateData.org (n.d.) Toronto Ontario Climate Data. Retrieved from https://en.climate- data.org/north-america/canada/ontario/toronto- 53/#:~:text=Toronto%20Climate%20(Canada)&text=This%20location%20is%20cl assified%20as,is%20785%20mm%20%7C%2030.9%20inch. City of Toronto (n.d.). Aerial Photographs. City of Toronto Archives. Retrieved from https://www.toronto.ca/city-government/accountability-operations-customer- service/access-city-information-or-records/city-of-toronto-archives/whats- online/maps/aerial-photographs/ Conklin, J.R. & Sellmer, J.C. (2009a). Germination and Seed Viability of Norway Maple Cultivars, Hybrids, and Species. Horttechnology. 19(1): 121-126. Conklin, J.R. & Sellmer, J.C. (2009b). Flower and Seed Production of Norway Maple Cultivars. Horttechnology. 19(1): 91-95. Dahir, S.E., & Lorimer, C.G., (1996). Variation in canopy gap formation among developmental stages of northern hardwood stands. Can. J. For. Res. 26. 1875- 1892.

27

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.

28

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.

29

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

(Acer platanoides). Bulletin of the Torrey Botanical Club. 123(3): 197-205.

Yamaguchi, D.K., (1990). A simple method for cross-dating increment cores from living

trees. Can. J. For. Res. 21. 414-416.

30

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