Using Green Roofs
Photo: Ryan Merkley
to enhance Biodiversity in the City of Toronto
A Discussion Paper Prepared for Toronto City Planning April 2010
Using Green Roofs to Enhance Biodiversity in the City of Toronto A Discussion Paper Prepared for Toronto City Planning April 2010
Written by: Beth Anne Currie, M.A.Sc & Brad Bass, PhD Acknowledgement of the valuable contributions by Iryna Krukovets, Nicole Puckett and Kristen Hahn
Executive Summary Biodiversity refers to the complex, interconnected community of living organisms in an ecosystem. The importance of preserving and promoting biodiversity has been recognized at the national, provincial and municipal level. Green roofs offer great potential to enhance the biodiversity of urban areas such as Toronto. In recent years, the City of Toronto has instituted a number of policies and initiatives to encourage the implementation of green roofs across the municipality. In 2009, Toronto became the first City in North America to adopt a bylaw to require and govern the construction of green roofs. Green roofs can be classified as either extensive or intensive, depending on the depth of substrate used and the level of maintenance required. It has been recognized that the design of both the substrate and vegetation layers of an installation can be focused to promote habitat creation. Variation in substrate topography and composition, as well as the addition of other materials such as logs and branches can create niche spaces for organisms. Vegetation diversity and structural complexity also contributes to the formation of microhabitats. Research in Europe and America has reported that a diverse abundance of bird and insect species can be supported on green roofs. Local studies have demonstrated similar findings, although the composition of species found on green roofs varies depending on geographic and climatic factors. Emerging biodiversity research from Toronto suggests that green roofs provide similar habitat potential to comparable ground-level urban habitats. This report suggests that green roofs can be used to connect fragmented habitats when installed in aggregation especially if located near fragmented ground-level habitats. Where technically feasible, green roofs should be designed to protect sensitive biological communities and avoid aggressive species. By designing green roof scapes that include important habitat forming and forage species into planting designs Toronto will encourage the proliferation of biodiversity across urban green roofs. In addition to strengthening existing green corridors, green roofs represent an opportunity to create new green space in areas that are otherwise unsuitable for natural restoration. Green roof habitats face several challenges that must be addressed in the creation of a biodiversity strategy for the City. The harsh, dry, and windy conditions present on a roof may support different community assemblages than those present on the ground and may alter the autecology of species that use green roof habitats. Invasive species also pose a threat to green roof communities both in the short and long term. These factors may be exacerbated by climate change and point to the value of ongoing green roof research and observation to further resolve and understand these challenges and opportunities for biodiversity planning on green roofs.
TABLE OF CONTENTS Executive Summary Table of Contents 1.0 Introduction and Background 1.1 Introduction 1.2 Green Roof Policy in the City of Toronto 1.3 What is Biodiversity? 1.4 Biodiversity in a National Context 1.5 Biodiversity in a Provincial Context 1.6 Biodiversity in a Municipal Context 1.7 Biodiversity as a Driver for Green Roofs 2.0 Green Roof Design to Encourage Biodiversity 2.1 What are Green Roofs? 2.2 The Importance of Substrate 2.2.1 Substrate Depth 2.2.2 Substrate Source and Composition 2.2.3 Maturity and Staging 2.3 Vegetation Design 2.3.1 Native versus Non-Native Debate 2.3.2 The Habitat Template Approach 2.4 Structural Diversity and Microhabitats 2.5 Proximity to Existing Urban Landscapes 2.6 Summary of Important Green Roof Design Considerations 3.0 Bird, Insect, and Plant Diversity on Green Roofs 3.1 Evidence from Europe and America 3.2 Birds Response to Green Roofs 3.3 Invertebrate Response to Green Roofs 3.4 Green Roofs Biodiversity Research in Toronto 3.4.1 History of Toronto Green Roofs 3.4.2 Existing Green Roof Biodiversity Research in Toronto 3.4.3 Emerging Green Roof Biodiversity Research in Toronto 3.5 Future Directions for Biodiversity Research 4.0 Opportunities and Challenges for Toronto 4.1 Torontoâ€™s Biodiversity in Context 4.2 Site Scale Opportunities 4.2.1 Biodiversity and Natural Colonization 4.2.2 Sensitive and Rare Plants 4.2.3 Migratory and Breeding Birds 4.2.4 Butterflies 4.3 Landscape Scale Opportunities 4.3.1 Connecting Existing Habitat 4.3.2 Supporting Edge Habitats 4.3.3 Supporting Conservation Source-Sinks 4.3.4 Island biogeography 4.4 A Strategy for Toronto 4.5 Challenges for Green Roofs and Biodiversity 4.5.1 Extreme Conditions 4.5.2 Invasive Species 4.5.3 Climate Change
i ii 1 1 1 2 2 3 3 3 5 5 6 6 7 9 9 10 12 13 13 14 15 15 15 16 18 18 19 20 21 22 22 23 23 23 24 25 26 26 26 27 27 28 31 31 32 32
5.0 Summary and Conclusions 6.0 References List of Tables Table 2.1: Green Roof Comparison Chart Table 4.1: Design Strategies to Enhance Biodiversity and Natural Colonization on Green Roofs in the City of Toronto Table 4.2: Design Strategies to Enhance Biodiversity on Green Roofs in Specific Locations in the City of Toronto List of Figures Figure 2.1 Cross-section through a typical green roof Figure 4.2 Location of buildings with green roof potential in relation to river valleys (forest habitat) and Lake Ontario (shoreline) in the City Toronto. Appendix A â€“ Terrestrial invasive Plant Species for Ontario
34 35 6 28 29 5 30 42
1.0 Introduction and Background 1.1 Introduction The City of Toronto is developing policies and programs to promote the use of green roof technology. These initiatives are founded on research that has demonstrated the City-wide environmental and economic benefits of green roofs. While previous research (Banting et al., 2005) identified â€œhabitat preservationâ€? as one of the environmental benefits of green roofs, it did not examine in detail the substantial and growing body of research concerning how green roofs can contribute to biodiversity in urban areas. This report seeks to review the literature on green roofs and biodiversity and examine opportunities for the City of Toronto to use green roof design templates as well as location and design strategies to help promote local biodiversity over time. This study is divided into four parts: (i) literature review; (ii) case studies; (iii) opportunities based on analysis of the literature and case studies; and (iv) using green roofs to enhance biodiversity in the City of Toronto. Green roofs have the potential to assist the City of Toronto to enhance its biodiversity index at the local level and by extension, to contribute to biodiversity at the national and global levels. 1.2 Green Roof Policy in the City of Toronto In 2004 the City commissioned a study on the potential environmental benefits of widespread implementation of green roofs in the City of Toronto (Banting et al., 2005). The study estimated that roof building roof areas make up 21% of the total land area throughout the City of Toronto. Approximately 8% of that area would be suitable for the installation of green roofs (i.e., flat roofs with an area of at least 350 square metres). The study quantified a number of the benefits of green roofs including: stormwater flow reduction, improvement in air quality, reduction in direct energy use and reduction in urban heat island effect. The study also stated that green roofs could provide habitat for birds and invertebrates. The potential benefits, and risks, of green roofs in cities to migrant birds are further discussed in a report on Migratory Birds in the City of Toronto (Dougan and Associates and Environmental Inc., 2008). Following the implementation study (Banting et al., 2005), the City of Toronto developed a Green Roof Strategy (City of Toronto, 2006) to encourage green roof construction in the City. The strategy includes four main components, as follows: 1. 2. 3. 4.
installation of green roofs on City buildings, a pilot incentive program to encourage green roof construction, use of the development approval process to encourage green roofs, and publicity and education.
As a result, the number of green roofs in the City is increasing (J. Welsh, personal communication, 2008). The 2006/2007 pilot incentive program resulted in about 7000 square metres of new green roof construction. The resulting Eco-roof incentive program, launched in 2009, has approved applications for 8100 sq m of green roofs on 14 projects. Green roofs have been constructed on at least three City buildings and are currently proposed at nine other locations. Well known green roofs in Toronto include the Manulife Centre (with a 25 year old green roof containing mature trees up to 10 m tall), the Mountain Equipment Cooperative building at King Street and Spadina Avenue (maintained since 1998), and a 30,000 square foot garden at York University that was created in 2003 (City of Toronto 2006).
In June 2007, the City of Toronto won the Federation of Canadian Municipalitiesâ€™ FCM-CH2M Hill Sustainable Community Award for its green roof strategy. This award recognizes municipal leadership in sustainable community development and gives national recognition to projects/ that demonstrate environmental excellence and innovation in service delivery. In May 2009, the City of Toronto became the first City in North America to adopt a bylaw to require and govern the construction of green roofs. The bylaw applies to all new building permit applications made after January 31, 2010 for residential, commercial and institutional development and January 31, 2011 for all new industrial development. The new by-law requires green roof coverage of 20-60% on all new development above 2,000 m2 of gross floor area (City of Toronto, 2009). 1.3 What is Biodiversity? Biodiversity is a term that can be used to describe the multi-complex myriad of living things on earth (Wilson, 1999; 2002). Biodiversity refers to the independent and dependant variations within all life forms, from the smallest molecular organizations in soils to the unabated complexities of life forms within entire ecosystems (Wilson, 1999). Biodiversity contributes to a relentless and often invisible ecosystem service that is provided within atmospheric, hydrologic and biogeochemical life cycles where air and water and living and dead elements are cycled and recycled in a continuous circle of life. An area is considered to have a high biodiversity score if it contains many different species of plants and animals and enough of these individuals so that each species can maintain adequate population size to allow for the persistence of species through subsequent generations (Millenium Ecosystem Assessment, 2005). Ecologists have added to this description to suggest that species diversity must be of sufficient richness to maintain ecosystem functions despite habitat loss, disturbance or other processes that operate at multiple spatial scales (Hannah et al. 2005; Hansell and Bass, 1998; Bass, 1996). In an urban area, biodiversity is a concept that involves not only the quantification of available habitat areas, but also the description of species diversity within these spaces. An ecosystem's capacity to support a particular level of biodiversity is dependent on several factors. Climatic phenomena such as the amount of incident solar radiation, seasonal variations and amounts of precipitation are all variable depending on the location of the ecosystem. These variations will affect the resource pool from which biological organisms draw their energy and nutrients. Within any given ecosystem, microclimatic variation exists due to the structural complexity of its biotic (natural materials from living organisms) and abiotic (non-living chemical and physical) components. Different sub-areas within an ecosystem will experience different levels of shading, wind exposure, runoff infiltration, and many other factors. These factors lead to the creation of niche spaces wherein certain organisms can be established and proliferate as they are able to optimally exploit a particular portfolio of available resources. 1.4 Biodiversity in a National Context Early in the 1990s, the world community acknowledged the threat posed by degradation of ecosystems and loss of species and genetic diversity by successfully negotiating the United Nations Convention on Biological Diversity (CBD). With the support of the provinces and territories, Canada became the first industrialized country to ratify the Convention, which entered into force on December 29, 1993. Canada's Response to the Biodiversity Convention has
provided opportunities for us to re-examine our relationship with nature, create new global partnerships, harmonize national activities and develop new economic opportunities. The objectives of the Convention are (CBD, 2008): • • •
the conservation of biodiversity; the sustainable use of biological resources; and fair and equitable sharing of benefits arising from the use of genetic resources.
Federal, provincial and territorial governments, in cooperation with members of the public and other stakeholders, are pursuing strategic directions set out in the Canadian response to preserving biodiversity across Canada. 1.5 Biodiversity in a Provincial Context Within Canada, Ontario is a biologically diverse province. Most of the northern part of the province remains untouched by anthropogenic change, while ecosystems in the southern regions are under enormous development pressures. The southern regions however, provide habitats for nearly 40% of Canada’s endangered species. In order that questions about Ontario’s species identity, genetic variation, ecological roles and ecosystem processes – especially with climate change – be answered, a novel, multi-disciplinary approach will be required. The diverse elements of such an approach are now fostered at the Biodiversity Institute of Ontario (BIO) located on the University of Guelph campus (BIO, 2008). Ontario, along with all provinces and Territories, is working on an Ontario Biodiversity Strategy (OBS) that seeks to protect biodiversity and ensure the sustainable use of its biological assets. One means to achieve this objective was the creation of an Ontario Biodiversity Council. The Council has members from 22 provincial organizations and an independent chair. Under the OBS, the Council is to lead, coordinate and report on the implementation of Ontario's Biodiversity Strategy. The Ontario Biodiversity Council has released an Interim Report on Ontario’s Biodiversity. This report contains information on important milestones towards reporting on the state of Ontario’s biodiversity in 2010 and provides an overview of biodiversity in Ontario (Ontario Ministry of Natural Resources, 2008). 1.6 Biodiversity in a Municipal Context The City of Toronto Official Plan recognizes the importance of biodiversity as part of a healthy environment. Official Plan Policy 3.4.1 indicates that public and private city-building activities will support biodiversity (City of Toronto, 2007a). A “Biodiversity Strategy for the City of Toronto” is currently being developed and is expected to be available in 2010 to coincide with Canada’s commitment to the Convention on Biodiversity (K. Snow, personal communication, 2009). The recent publication “Birds of Toronto; a Guide to their Remarkable World” is the first in the City of Toronto’s planned Biodiversity Series. The Series will include guides that will help increase understanding of biodiversity in the City, re-connect people with the natural world and inform people how they can help reduce biodiversity loss (City of Toronto, 2007b). 1.7 Biodiversity as a Driver for Green Roofs In North America, enhancing biodiversity has not traditionally been viewed as a key driver for green roof policy development and proliferation. Green roofs are an emerging industry in North America and evidence on the role and value of green roofs for biodiversity (see chapter 2) may strengthen their position as cities consider ways in which the built environment can contribute to
ecosystem degradation, habitat loss, migratory bird stopover habitat and species-at-risk (Millenium Ecosystem Assessment, 2005). The European view of green roofs has been more ecologically focused as the potential for this technology to provide habitat for a multitude of species has been studied and recognized in available literature.(Gedge, 2003; Gedge and Kadas, 2004; Brenneisen, 2008). The UK Biodiversity Action Plan supports the use of green roofs as important linkages between habitat fragments to facilitate dispersal and additional habitat for rare and protected species (Design for London, 2008). In Basel, Switzerland, research focused on the potential to support species biodiversity has led to amendments in their building and construction law. As part of Baselâ€™s overall biodiversity strategy, green roofs are now mandatory on new buildings with flat roofs. Design criteria supported by a team of specialists recommend the use of varied and local plants and soil substrates, combined with varied soil depths that are predictive of re-creating existing ecological habitats. Studies of green roofs in Zurich, Switzerland, have shown that the use of natural soils can encourage biodiversity through their suitability for locally and regionally endangered species (Brenneisen, 2008). It has also been suggested that the incorporation of topographic variation into green roof design may further enhance a green roofâ€™s capacity to mimic natural habitats (Dunnett and Kingsbury, 2008). Further research is needed in order to better understand the relationships between green roof design, installation location, and biodiversity.
2.0 Green Roof Design to Encourage Biodiversity 2.1 What are Green Roofs? Green roofs consist of a living layer of vegetation established over a layer of substrate or growing media that extends over a specific perimeter of flat or sloped roof surface. Under the substrate layer, there are stormwater drainage materials, fleece and/or landscaping cloth to prevent erosion and a root protective barrier that prevents the penetration of both water and roots into the waterproofing membrane (Figure 2.1). Green roofs are typically classified as being either 'extensive' or 'intensive' (Dunnett and Kingsbury, 2008) (Table 2.1).
Figure 2.1: Cross-section through a typical green roof (Moran et al., 2003). Intensive green roofs are constructed with deeper growing media generally greater than 25 cm (10 in.) and can include water features, concrete walking pathways, pergolas and other amenities (Currie, 2005). Because of their increased soil depths, intensive roofs can support a great variety of vegetation such as trees and shrubs. Intensive roofs are more expensive to construct and usually require considerable maintenance and irrigation. This type of roof is generally constructed for installations where structural load restrictions are negligible or can be incorporated into the initial building design. Extensive green roofs tend to be thinner with typically 5 - 15 cm (2 - 6 in.) of substrate (Currie, 2005). Typically, extensive green roofs are composed of a smaller number plant species. Drought-resistant, hardy perennials such as stonecrops (Sedum spp.) are commonly used in extensive green roof designs. Extensive green roof systems can be installed on new or existing buildings including heritage buildings as they are lightweight, relatively inexpensive and may require less irrigation and maintenance after initial plant communities are set up (Dunnett and Kingsbury, 2008). Extensive roofs have received the most attention in the literature because they have been preferred across most installations, primarily due to cost factors and weight restrictions. European studies on deeper substrates suggest that intensive green roofs may be more successful in supporting biodiversity, however, shallow extensive green roofs with varied substrate depths have also been shown to be ecologically productive. Roofs with media depths and vegetation characteristics that fall in between the ranges stated for intensive and extensive roofs may be referred to as being 'semi-intensive' (Dunnett and Kingsbury, 2008). Studies of green roofs and biodiversity in both Europe and North America suggest that substrate depth and
composition, topography, vegetative composition, green roof age and local landscape context are variables that can be incorporated into green roof design and location in order to target opportunities for biodiversity (Somerville and Counts, 2007). Table 2.1: Green Roof Comparison Chart (Adapted from Currie, 2005) Roofing Element
Extensive Green Roof
(Semi) Intensive Green Roof
Cost Vegetation Layer and Plant Selection
$18 – 25 per square foot 2-6 inches planting media minimal to no irrigation stressful conditions for plants requires low, drought-resistant species light weight structural strengthening usually not required suitable to cover large surface areas shallow, well-drained substrates and hot dry conditions generally only suitable for drought tolerant species such as mosses, sedums and herbaceous plants can support meadow, grassland or prairie habitat can be designed to simulate forest understory, riverbanks, ravines and/or wetlands can provide habitat for common species of invertebrates can be attractive to some species of birds (although not been well-researched)
$40– 50 + per square foot 6–25 + inches planting media more likely to require irrigation favourable for many varieties of plants and shrubs, and trees
heavier in weight requires structural engineering used over smaller surface areas or in landscaped containers
Deeper substrates can support
greater variety of habitats than extensive green roofs Can be designed to simulate forest understory, riverbanks and wetlands. Suspected to promote habitat for native plant species and may provide habitat for the conservation of some heritage species and their seeds. Provides habitat for greater variety of beetles, spiders, wasps, bees and other invertebrates, including butterflies May benefit migratory birds as can simulate habitat for birds to forage, breed and rest.
2.2 The Importance of Substrate To date, substrate quality and quantity has been the focus of much research on green roof biodiversity. Variations in substrate including depth, source, composition and age exert an important influence on green roof biodiversity. . 2.2.1 Substrate Depth European studies have demonstrated that substrate depth has a direct influence on species diversity and abundance and that variation in substrate depth across a green roof can increase biodiversity. Dr. Stephan Brenneisen, has studied green roofs in Basel and Zurich, Switzerland
for over twenty years (Brenneisen, 2008). His observations suggest that there are a number of green roof design considerations that may facilitate urban biodiversity. He reports that substrate depth either constrains or facilitates species richness and abundance and hence biodiversity on a green roof (Brenneisen, 2006). Thin, shallow substrates commonly used in the creation of extensive green roofs reduce the ability of a roof to support biodiversity by intensifying the already extreme ecological conditions of roof environments. These roof environments are typically subject to intense temperature and moisture changes and tolerant pioneer species have found this design to be a suitable form of habitat (Grant, 2006). However, some reviewers have shown that even these shallow, monoculture green roofs can support a measure of diversity (Dunnett et al., 2008; Hahn, 2009). Brenneisen (2006) in Switzerland and Kadas (2003) in UK, have both demonstrated that by varying the depths of the medium used across a green roof it is possible to create a series of different microclimates, and subsequently microhabitats, within the same green roof zone. Their observations show that thin substrate layers on roofs support sparse vegetation to develop, whereas small hills or mounds of thick substrate support taller, more dense vegetation. The sections of these test roofs where soil layers were kept thicker were able to retain more moisture and were not as likely to dry out as rapidly compared to the shallow sections of the study roofs. The deeper sections were able to support increased abundance and diversity of vegetation, creating greater structural diversity and supporting more invertebrate species. Varying the depth of substrate along with creating unevenness in the green roof terrain can also reduce heat loss in the winter, as this varied terrain serves to reduce the wind speed and chill across roof surface areas. Use of stones, limbs of trees and variances in plant structure and height are design elements that can be used to enhance biodiversity on green roofs. Other studies in North American support European findings whereby deeper planting media favours quick plant growth, long term survivability and roof coverage. For example, the Edgewater Condominium in Minneapolis was designed with varied depths of growing medium to enhance wildlife habitat value of the green roof. On this roof, even the thinnest substrate studied (2.5 cm) supported a number of varied species (Durhman et al., 2007). This finding was confirmed by Brenneisen (2006) who reports that thin substrate segments on Swiss roofs characterized by sparse vegetation, dryer and warmer temperatures that North America, were found to attract specialist species. Both Brenneisen and Kadas (2003) found that drought tolerant invertebrates were well adapted to these roof conditions and able to successfully establish healthy populations. Early work by Jones (2002) in London, UK, found that extreme, dry conditions on extensive green roofs supported a number of uncommon invertebrates (ants, beetles, and other species). Jones noted however, that overall diversity was low on a study green roof and concluded that this low, overall diversity was a result of uniform substrate depth and poor diversity in vegetation types. The empirical relationship between substrate depth and species diversity from the Swiss studies suggests a significant relationship between these two variables. Creating variations in substrate depth as a tool to target increased biodiversity in Basel created habitat for 79 beetle and 40 spider species (Brenneisen, 2003; 2004). Of these, 13 of the beetle species and 7 of the spider species were classified as rare or endangered (Brenneisen, 2003; 2004). The deeper, wetter areas were found to be a key factor in attracting beetles. The density of beetle populations were found to significantly increase with the roofâ€™s ability to retain water in both studies from the UK and Switzerland respectively (Kadas, 2003; Brennesien, 2006).
2.2.2 Substrate Source and Composition The origin and composition of the planting media are important design criteria when creating roofs to specifically enhance biodiversity. The use of natural, local soils and substrates has been found to assist biodiversity and benefit regional and endangered species because local species are already adapted to that particular soil environment (Brenneisen, 2006; Gedge, 2003). It has been suggested that substrate composition and depth is a strong indicator of future successes in biodiversity for green roofs and advocates for the use of a mixture of local soil substrates and local and native seed for self-sustaining green roofs (D. Gedge, personal communication, 2008). A recently issued study of urban biodiversity conducted by the Sheffield Biodiversity in Urban Gardens (BUGS) project also supports claims that invertebrate diversity is influenced significantly by the composition and distribution of substrate and less reliant on the composition of plants (BUGS, 2007). Soils from any particular local reflect cumulative combinations of climatic, geological and biological degradation, intrinsic factors and organic constituents. Locally sourced substrates can expedite the colonization by microorganisms, particularly a range of mychorrhizal fungi and plant propagates that reflect how vegetation has adapted to that particular soil (Dunnett, 2006). Colonization of animal and plant species may be enhanced with substrates that are extracted from an existing and functional ecosystem to make way for a new building project or other construction zone that may be proximal to a green roof. Coffman (2007) postulated in his biodiversity study on the Michigan Ford Motor green roof, that colonization of the sedum mats was probably influenced by the fact that the mats were grown beside the new building and its target green roof in a 10 acre field over three (or more) months. Findings from Brenneisen’s (2003; 2004) green roof research helped shape Basel’s (municipal) building and construction codes. Existing land-use regulations attempt to minimize damage to the natural environment and support the sustainable use of locally accessible soils. Swiss Federal conservation and cultural legislation supports the protection of endangered plant and animal species. Accordingly, the canton of Basel now mandates a biodiversity strategy for green roofs that points to the use of local area soil substrates that are blended for extensive green roof construction. In Basel, green roofs must be constructed on all new buildings with flat roofs (Nature and Landscape Conservation Act § 9; Building and Planning Act § 72). Moreover, on roofs with more than 500 square meters, the green roof substrate must be blended with local area soils and designed and installed at varying depths. Gilbert (1990) and Harvey (2001) noted that brownfield (post-industrial landscapes) made up of dry gravel sites at ground level throughout the UK were particularly valuable as habitat for flora and invertebrates. Similarly, Brenneisen’s early observations (before studying green roofs) focused on exploring rare invertebrates in dry gravel and alluvial river beds that were being impacted by erosion and urbanization. He observed that interesting 'bugs' had shifted from these alluvial beds toward abandoned brownfield sites that existed beside the main river in Basel. One conclusion from these studies was that brownfield substrates could become a template for green roof substrate design in order to support these displaced species. (Brenneisen, 2008). Green roofs modelled after brownfields1 are called brown roofs and have been found to harbour as much invertebrate diversity as green roofs and brownfield habitat at grade level over time. 1
In the U.K., "brownfield" is unused industrial land that can be built on; it is not necessarily contaminated
According to Kadas (2006), brownfield sites provide some of the most species-diverse habitats in the U.K., as they have escaped development and become wildlife refuges or habitat 'islands'. Once these brownfield sites become slated for redevelopment, their flora and fauna could be lost. In 2004, Kadas targeted observations of key species important to the U.K. Biodiversity Action Plan including Araneae (spiders), Coleoptera (beetles) and aculeate Hymenoptera (wasps, ants, bees) on three green (sedum) roofs, two brown roofs2 and four brownfield ground sites. Kadas reported that at least 10% of the species collected at these study sites were designated as nationally rare (UK) or scarce in accordance with Natural England criteria. This study identified 72 different invertebrate species, representing 12% of the total United Kingdom spider fauna and 30% of the total London spider fauna (Kadas, 2006). The data indicated that green and brown roofs can be important habitat zones for both invertebrate and ecological conservation. Some researchers in the UK have used brownfield substrates in their brown roof design out of a need to protect an endangered bird species such as the black redstart (Poenicurus ochruos). This bird species had adapted to abandoned brownfield landscapes that were increasingly vulnerable to housing development (Gedge and Kadas, 2005; Grant, 2006).Conventional rooftops were fortified with substrates typical of brownfield sites and observations over fifteen years have found that these brown roofs support significant populations of the black redstart (Grant, 2006). Recreating local habitat on roofs has involved placing soils from regional riparian areas and other aggregates made up of recycled concrete from brownfield sites, in order to support successful habitats for bird and invertebrate species in Basel and London (Gedge and Kadas, 2005; Brenneisen, 2006). 2.2.3 Maturity and Staging The age or maturity of a green roof and its plant community is another important factor in understanding biodiversity on green roofs (Jones, 2002; Grant et al., 2003; Brenneisen, 2006). A German study of ten vegetated roofs in two different age classes revealed that roof substrates compost over time and follows a similar pattern of degradation as those processes at ground level in disturbed habitats (Schrader and Boning, 2006). Their study revealed that over time, extensive green roof substrates are characterized by reductions in bulk density, increases in organic matter, and increases in species abundance and richness (Schrader and Boning, 2006). The oldest roof in their study exhibited the most unique soil properties of all the test roofs, and reported the highest levels of biodiversity. Very few studies have looked at the influence of age on species abundance and diversity above the substrate level. In a study in Sheffield, UK, bee abundance and diversity were correlated with age of a green roof. This might have something to do with the establishment of wild flowers, which provide greater habitat and foraging opportunities for the bees (Gong, 2007). 2.3 Vegetation Design The vegetation layer of green roofs also plays a significant role in fostering biodiversity. A technical report that supports Londonâ€™s (UK) Biodiversity Action Plan (BAP) recognizes that green roofs can be designed to mimic almost any habitat that is desired including targeted plant species (English Nature, 2003). Habitats that necessitate the use of trees or shrubs may be 2
Brown roofs consist of substrate made from recycled aggregate that has been left to colonize naturally or, in some cases seeded with local wildflower seeds
better suited to intensive roofs (see below), while extensive roofs may be best suited to grasslands and herb communities (Design for London, 2008). The most common extensive green roofs are planted with sedums and can provide habitat for common species, whereas uncommon species, are more likely to colonize on green roofs that mimic a specific habitat at grade level (Design for London, 2008). Both intensive and extensive green roof types can become potential hosts for many common and uncommon varieties of moss in London (Design for London, 2008). Green roofs can support a multitude of different plant species, depending on the depth and composition of the substrate. Green roof plant choices can be a complicated matter and an emerging body of literature has examined various plant design considerations. 2.3.1 Native versus Non-Native Debate Native plant communities are defined as a group of plants whose interactions with other organisms in the community have evolved independently of human intervention (Dunevitz Texler and Lane 2007). These native communities are distributed according to climate, landform, and soil patterns and shaped by natural disturbances, primarily due to extreme weather â€“ less so by human intervention and design. Native plant communities have evolved over many centuries and are adapted to the local environmental conditions. They provide important sources of food and shelter to birds, butterflies, and other animals. Their unique profile and vulnerabilities on green roofs have not been well studied or tested on green roofs (S. Benvie, personal communication, 2008). It is thought that diverse native plantings on green roofs can potentially be used to attempt to replicate local native plant communities and their ecological benefits. Research has demonstrated that the use of local vegetation in planting designs allows colonization from native species to occur more quickly, as they are already adapted to such vegetation (BUGS, 2007). As well, local species such as hoverflies and solitary bees have been observed to show high native plant fidelity (BUGS, 2007). Other preliminary studies have found similar patterns. For example, in Alberta, a semi-intensive (deeper than 6 inches) green roof was established with a native mixed prairie community (Clark and MacArthur, 2007). When compared to an extensive green roof made up of non-native short grasses and sedum, the semi-intensive roof was reported to have achieved more biomass in spiders and species variability and a greater overall biodiversity (Clark and MacArthur, 2007). Establishing a protected (conserved) native plant community on a green roof can be a difficult task. It has been suggested that rare plants and invasive species may encroach and threaten native species on green roofs which could increase the amount of maintenance required for such planting designs (S. Benvie, personal communication, 2008). However, some green roof designers have contended that green roofs comprised of native plantings may be more successful as once established they require less fertilizer, water, and maintenance compared with non-natives (T. McGlade, personal communication, 2008). The autecology of a species refers to the manner in which it relates to its environment through its entire life cycle. Green roofs represent a novel habitat for native plant species although a plant's autecology may change in response to this new environment. Factors such as a plantâ€™s regeneration tactics, preferred and critical structural needs, habitat formation, and establishment and generation time may all be affected on a green roof. Moreover, considerations such as the environmental tolerances and preferences required by each species, the market (nursery stock) availability of a species, the threat of invasive species to this particular plant selection and whether any of the target species on the green roof become invasive to other proximate communities are compounding variables for consideration in this debate.
Native plant species may not necessarily be local in origin, depending on market availability. . Dunnett (2006) has raised this issue in his discussions of the need to use local plants as a source of seeds rather than seeds from other areas. The concern is whether native plants from non-local sources are genetically fit to survive during the entire year of their establishment on a green roof. Most urban sites, and particularly green roofs are a drastic transformation from their original natural habitat, thus an assumption that original genotype of native plants will be better suited than non-local plants may not be credible. A species that is not typically invasive may act invasive under certain conditions where it experiences altered hydrology, altered nutrient levels, or other unforeseen environmental pressures. Examples of native species that can dominate in some situations are Canada goldenrod (Solidago canadensis) and Manitoba maple (Acer negundo) (Morton Arboretum, 2009). Evaluating whether or not a species will become invasive in a particular restoration project is an important consideration for each planting3. Dunnett (2006) echoes this warning that many native species are in fact highly invasive, and can dominate the landscape and reduce overall diversity on a green roof. On the other hand, many exotic species can be greatly adaptable to particular environment especially if it is similar to their native habitat. Exotic species are plants or animals that have been introduced into a certain local communities. Some species are introduced with a specific intent in mind, while others are introduced by accident. While some exotic species are unable to compete with native species, certain exotics are able to out-compete native species and may become invasive and take over. For example, the extensive green roof on 1 Pace Plaza in lower Manhattan was planted with approximately 3,200 m2 of sedums and other succulents as well as native meadowland and prairie species that are known to attract birds (Levandowsky, 2006). During the hot, dry summer of 2005, the growing medium became totally desiccated and many native plants died. The survivors included Sedum spp. and other succulents (exotics that have adapted well), as well as a number of typical weeds, including spurge (Euphorbia maculate), nutsedge (Cyperus esculentus), speedwell (Veronica longifolia) and 2 moss species (Levandowsky, 2006). Dunnett’s (2006) review of biodiversity supports the use of both non-native substrates and nonnative exotic species on green roofs and purports that the use of native plants may be a less significant variable on the amount of invertebrate activity compared with other factors such as the age and geographic location of a green roof. Dunnett (2006) also suggests that while native species do have many benefits in terms of supporting biodiversity, that substrate qualities, spatial and vertical structure of vegetation, (regardless of geographical origin), and the overall diversity of content on a green roof is equally important. Dunnett posits: “that more thought should be given to the creation of ideal conditions for the establishment and long-term persistence of naturalistic vegetations, the content of which may vary according to context – in effect it is about ecology and biodiversity as process rather than lists of names. We should view ecological communities, and the possibilities for their creation as continua, rather than absolute and fixed points. In an urban green roof context, these continua should take as much account of aesthetics and visual criteria as they do of scientific ones” (2006, p.11).
3 Researchers believe there are several factors that contribute to a plant becoming invasive including: fast growth rate; seeds that germinate quickly in high percentage; prolific seed production, which begins within the first few years of the plant’s life; easy seed dispersal by animals, water, and wind; ability to reproduce by seed as well as vegetatively, e.g. through suckering; longer flowering and fruiting periods; and adaptability to a wide range of soil and growing conditions
Dunevitz, Texler and Lane (2007) cite a number of reasons not to plant rare or native plant species including that many of these rare species have been reduced to a small number of fragile populations that are located in sequestered global communities that could be damaged or impacted by the introduction of varied genes from similar plants from different geographic areas (Havens, 1999). Another reason is that since many rare plants have very specific and complex habitat requirements, it is unlikely that planting or transplanting them to green roofs will be successful as a long term strategy. Green roofs in Switzerland and the UK rely to varying extents, on natural colonization to seed new green roofs. Some researchers caution against a reliance on natural colonization to vegetate green roofs and as natural colonization may result in sparse vegetation cover over the first few years of growth coupled with the fact that the roofs may come to be dominated by a few aggressive, primary succession species (Grant, 2006). Lundholm (2006) reflects that to date, research on green roofs has largely proceeded from the engineering considerations for better stormwater runoff quality and quantity and related building energy benefits. Lundholm (2006) suggests that the use of entire communities of plants on green roofs requires more understanding of habitat characteristics and the relationships between plant community structure, climate and ecosystem functions. He predicts that these concerns will shift research on building-surface vegetation to the forefront. Opinions regarding the use of native species in green roof planting designs thus remain mixed and further research in this area is needed in order to resolve this debate. Instead of focusing on the characteristics of specific species, another approach to green roof vegetation design considers species assemblages from a community perspective. 2.3.2 The Habitat Template Approach Prairie grassland habitat creation is one of the few examples of a specific landscape that has been successfully re-created on North American green roofs, often created with native plants. Tall grass prairies are important habitats for several species of migratory birds (D. Gedge, personal communication, 2008), and these habitats on green roofs could play a similarly important role in the City of Toronto. Extensive green roofs, which are low in nutrients, free draining and 'hot' generically can mimic many of the characteristics of dry meadow grassland (Dunnett and Kingsbury, 2008). Native plants found in this habitat are characterized by thickened cuticles, hirsute stems and leaves, highly reflective surfaces, fine or narrow leaves, sticky surfaces that can hold onto water, leathery rough leaf textures that reduce the speed of wind traveling over leaves, and many water storage cells. The attempt to use local prairie habitat as a design template has led to trial and use of three native analogues in Minneapolis, MN (MacDonagh et al., 2007). In Minnesota, green roof designers are learning lessons from the native substrate that supports bedrock bluff prairie, a subtype of dry prairie, which occurs on bluffs along the Mississippi and its tributaries in south east Minnesota, as well as occasionally along the St. Croix River. Prairie ecosystems are found to support more than 25 species of prairie grasses and forbs that grow 12 cm to 76 cm tall and over thin soils. More research on prairie grass species that adapt well to green roof conditions are recommended, however some successes on exiting green roofs can be observed. Minnesota also boasts the Phillips Eco-Enterprise Center (PEEC), a state-of-the-art green building and business centre that opened in 1999. It has a monitored, extensive green roof with 18 native and 11 European plant species. Traditional European green roof plants were planted
in swale-like depressions with 2â€? of growing medium. Bedrock bluff plants native to Minnesota were planted in 2-6â€? of growing medium. After 15 months, the percent cover of Grey Goldenrod (Solidago nemoralis) exceeded that of all other species while Little Bluestem (Schizachyrium scoparium) had the highest percent cover among the native grasses. Subsequently, the Lundâ€™s Market green roof in downtown Minneapolis was planted with sedums and chives as well as Little Bluestem (Andropogon scoparius), Spiderwort (Tradescantia occidentalis) and Prairie Onion (Allium stellatum). Similarly, the green roof on the Edgewater Condominiums, located adjacent to the Minneapolis Chain of Lakes region, was designed to visually express the connection between the green roof and adjacent Lake Calhoun. The roof was planted with a traditional white sedum in 7.5 cm of substrate with a diversity of other sedums and short native bedrock bluff prairie plants in 10-13 cm of growing medium. These species may adapt well to green roof conditions, however, ongoing monitoring will be required (Green Roof Projects, 2009) A green roof ecologist and plant grower/installer in southwestern Ontario and has been collecting seed from grade-level, naturalized tall grass prairie systems and experimenting with direct hand distribution and nursery grown applications on green roofs over the past few years (M. Natvik, personal communication, 2008). Results of this experimentation would suggest that the use of little blue stem, black-eyed Susan, wild bergamot and other naturalized species seeds help conserve existing prairie system species, enhance biodiversity and improve overall system performance where drought and winds dominate. 2.4 Structural Diversity and Microhabitats Structural complexity arises as a result of the interaction between the substrate and the plant species present in a given area. On green roofs, other factors such as the presence of rooftop equipment and the shading influence of adjacent structures (parapets, HVACs) also contribute to the habitat's structural complexity. Microhabitat creation is an important determinant of biodiversity within the plant and soil communities on a green roof. While often limited by roof size and load bearing capacity, green roofs can be designed purposefully to provide a range of structural complexity within plant communities. Large roofs or roofs with high load bearing capacity provide the greatest opportunity for diversity by permitting a greater range of vegetation type and size, including trees and shrubs as well as ponds or wetlands. The manipulation of substrate depths created microhabitats that led to greater species diversity in the studies completed by Brenneisen in Switzerland. Another technique that has been used to create microclimates and microhabitats has centred on the addition of large objects such as branches, stones, sand piles and rubble (Grant et al., 2003). In Switzerland, solar panels on roofs created a shaded, damp area and led to an increase in invertebrate diversity (Gedge and Kadas, 2005). The placement of additional objects on green roofs as design features may encourage species to use the roofs for a variety of activities yet to be discovered. Branches can serve as resting sites for birds to perch and nesting structures, such as bird or bat boxes, can encourage species to nest and breed (Dunnett and Kingsbury, 2008). Similarly, branches or snags can provide physical connections and shady habitats for invertebrates on green roofs. Studies at the University of Sheffield have examined the role of plant diversity as a central feature in promoting invertebrate diversity, and found it not to be statistically significant. Rather, structural diversity or variance in height as well substrate depth was the most important factor in encouraging biodiversity (BUGS, 2007). 2.5 Proximity to Existing Urban Landscapes The proximity of green roofs to other naturalized zones or landscapes has been shown to have a positive influence on biodiversity. Features such as parks, greenways, fields, other vegetated
roofs were examined in a long- term study in Berlin from 1985 to 2005 (Kohler, 2006). Green roofs located in proximity to other grade level vegetated areas demonstrated higher levels of species richness compared to those that were not. Roofs that were located in urban centers but proximal to green areas were better candidates for natural dispersal and colonization from adjacent habitat than roofs located farther away. The TRCA study at the York University green roof has reported a similar finding from a case study on their extensive green roof. The York University study found six species of birds and a similar diversity of bees as were observed in the surrounding habitat at ground level (Miller, 2008). Research is lacking on the ability of green roofs to act as stepping stones for species conservation, bird feeding and nesting or habitat creation in an urban landscape. Researchers also question whether the city is the appropriate spatial scale from which to implement a landscape approach to green roof design (Canzonieri, 2007). European and North American research has provided evidence that green roofs can offer suitable habitat for targeted species in the urban world. The question emerges - will a handful of green roofs, if well designed for biodiversity, make a significant impact on regional biodiversity? For example, can green roofs act as â€œstepping stonesâ€? to connect isolated fragments of habitats for mobile and wind dispersed species? A landscape ecology approach to the design of green roofs would advocate for planning beyond an individual roof and moving to a framework of green roof aggregations where these networks of living roofs can effectively facilitate the movement of species. The advantages of a well coordinated green roof network could be measured in a larger geography in the creation of the synergistic effects created when a certain number of roofs were clustered in an area or in a corridor. 2.6 Summary of Important Green Roof Design Considerations A few general principles can be gleaned from the review of available research on biodiversity and green roofs. Regardless of where a green roof is placed in an urban area, biodiversity will increase because the green roof will eventually provide habitat for an unpredictable range of plants and animals. While many researchers support the use of native plants, there are broad disagreements on this issue. Green roof biodiversity does seem to increase if the installation is proximal to other naturalized spaces or habitats, although research on the potential synergistic effects of using green roofs to expand natural habitats or create corridors that link up natural areas is nonexistent at this time. Nonetheless, studies show that even isolated green roofs in urban areas achieve some level of biodiversity over time as seeds disperse land and respond to open disturbed growing media on roof tops. As plants grow and respond to this newfound space, a green roof may provide resting, feeding or breeding grounds for birds and other invertebrate species.
3.0 Bird, Insect, and Plant Diversity on Green Roofs 3.1 Evidence from Europe and America A search of existing literature on green roofs and biodiversity is limited by the fact that while green roofs have become more prominent in the North American marketplace, few rigorous scientific investigations have been conducted for this locality. Biodiversity research is now a growing area of interest and researchers in North America and Europe are beginning to collaborate in order to examine matured green roof installations in both continents. Most studies are currently European in origin; however, many have not been translated into English at this time. The available research has demonstrated that both extensive and intensive green roofs can provide temporary habitat for a range of migratory birds and invertebrate species. 3.2 Birds Response to Green Roofs Surveys of bird sighting and activities in Switzerland have found that the primary activity on green roofs was nesting and feeding. Among the 1,844 sightings 1,304 (70%) involved activities such as preening, searching for nesting material, feeding, nesting and singing (Brenneisen, 2003; 2004). The most common bird species on Swiss roofs included black redstarts (Phoenicurus ochuros), house sparrows (Passer domesticus), rock doves (Columba livia) and wagtails (Motacilla spp.). Baumann (2006) reported that certain migratory ground nesting birds, such as the Northern Lapwing (Vanellus vanellus) and Little Ringed Plover (Charadrius dubius), were using Swiss vegetated roofs as nesting sites. These species are listed as endangered and have been awarded high levels of protection under European biodiversity programs. Their habitat is threatened due to development pressures. Observations of migrant birds visiting the 20,300 square food green roof over Chicagoâ€™s City Hall was generated by Jerry Garden in 2002 - 2003 (Millett, 2004). In 2002, Garden observed numerous migrants including Flycatchers (Empidonax spp.), Kinglets (Regulus spp.), Blackcapped Chickadees (Poecile atricapillus), Cape May Warblers (Dendroica tigrina), Dark-eyed Juncos (Junco hyemalis), and Field and Song Sparrows (Passer spp.). By 2003, even more species were recorded including Woodpeckers (Picidae), Olive-sided Flycatchers (Contopus cooperi), Philadelphia Vireos (Vireo philadelphicus), Brown Thrashers (Toxostoma rufum), Common Yellowthroats (Geothylpis trichas) and six species of sparrows (Passer spp.) (Millett, 2004). Many of these species are long distance neotropical migrants known to be declining in numbers throughout North America. Neotropical migrant birds are the songbirds that represent over 50% of North American bird species. As spring begins, more than 300 species of Neotropical migratory birds head north to breed and raise young in the United States and Canada. In the fall, they return to warmer climates in tropical regions of Mexico, Central America, South America, and the Caribbean (Neotropical Migrant Birds, 2009). This survey helps to demonstrate the potential value of larger green roofs in support of migrating birds. Although green roof habitat may not be ideal for longer term breeding or immediate foraging requirements, green roofs may be a good temporary stopover destination useful for exhausted migrant birds. Other potential hazards to migrating birds include glass windows in urban tall buildings. Green roofs located in urban settings near tall buildings with extensive reflective (e.g., glass) surfaces may increase the incidence of bird strikes against these buildings by attracting them and serving as a location from which the birds take-off. Dr. Daniel Klem (2009) of Muhlenberg College has done studies over a period of 20 years that have examined bird collisions with windows and concludes that glass kills more birds than any other human related factor. It is estimated that at least 1 billion birds are killed by flying into windows every
year in the United States alone (Klem, 2009). Further research is required to assess the nature and severity of this hazard in Toronto, although the Fatal Light Awareness Program in Toronto would corroborate some of Dr. Klemâ€™s findings (Dougan & Associates and North-south Environmental, 2009). While many authors agree that green roofs provide opportunities to improve biodiversity among avian species, the notion of targeting a green roof for a particular bird requires more careful analysis, design and monitoring. An urban extensive green roof was designed to attract the Lapwing (Vanellinae) and Plover (Charadriinae) by Brenneisen (2003; 2004) using moss, grasses and an herb mix - all plants and materials determined to have nesting and habitat potential for these species. Five pairs of Northern Lapwings (Vanellus vanellus) were able to nest and breed on this urban green roof suggesting that breeding birds can use green roofs as nesting sites in urban areas (Baumann, 2006). Brenneisen has since reported in a recent telephone interview that all but one of the chicks died on this roof (S. Brenneisen, personal communication, 2008). The surviving chick was supported by the researchers who established collection plates to capture rainwater on the green roof. These researchers decided to make this intervention because it was observed that if hollowedout areas in the substrate had been incorporated in the green roof design that puddles may have saved the lives of the other chicks as well. Lapwing parents do not fetch foodstuffs for their young, hence increasing the vulnerability of this targeted species on green roofs. Other researchers would argue that autecology is essential when designing for biodiversity on green roofs especially when designs aim to attract a particular target species (S. Benvie, personal communication, 2008). Baltimoreâ€™s National Aquarium in Maryland has attracted a nesting mallard duck amongst its 4,000 square foot sedum green roof system installed above the new Australia Exhibit at Pier 3. This species constructed its nest from the abundant sedum plant materials found growing on the roof system (Smith, 2007). The Chicago City Hall survey (Millett, 2004) also demonstrates the potential value of green roofs as temporary stop over destinations for migratory birds. To date, there are no studies in Canada that compare and examine the feeding, nesting, fledgling or brood successes on different types of green roofs. Baumann (2006) suggests that the value of enhancing the design of green roofs to provide habitat and food sources for breeding birds and their early young should be considered, given the number and potential for green roof applications particularly on commercial and industrial buildings that border residential, riparian or forested zones in urban areas. 3.3 Invertebrate Response to Green Roofs The importance of focusing on measuring and designing green roofs that support opportunities for invertebrates has also been expressed by green roof researchers (D. Gedge, personal communication, 2008). According to Gedge (2008), invertebrates have become a particular target species for the UK Biodiversity Action Plan. Traditionally, habitat type for green roofs have been viewed from a botanical or horticultural perspective alone, however; research by Brenneisen (2003; 2004) and Kadas (2006) have helped to focus the UK on measuring biodiversity by the number and variety of invertebrates including spiders, beetles, wasps and bees, but particularly the number and variety of spiders. It has been suggested that because spiders are a predatory species, they occupy a range of niches within plant communities and can be a biological indicator of the success of any one plant community on a green roof (D. Gedge, personal communication, 2008).
Kadas (2006) categorically concludes that it is not the quantity of habitat that is important but more so the quality of the habitat that supports successful invertebrate assemblages. Kadas (2006) suggests that spiders can be separated by some distance from each other as long as the quality of their habitat is strong and physical connections are available such as logs, sticks, and other plant structures. Spiders need access to moisture, shade, dampness and a consistent supply of prey insects in order to establish a sustainable community. Kadasâ€™ (2006) research would suggest that diverse invertebrate assemblages both rare and interesting, have adapted to conditions on a variety of green roofs in both the UK and Switzerland however, some substrate depths and types and vegetation are more conducive than others. Kadas (2003) investigated invertebrates on a sedum (monoculture) green roof in the UK. The researcher found more species on sedum roofs compared to a green roof built specifically to mimic brownfield habitat. Of the species collected, 10% of the invertebrates were designated rare and five had never been recorded previously in London (Gedge and Kadas, 2005). A species of spider that had never been observed in Southern England before that time was identified in the study. Similarly, in sampling seventeen UK-based green roofs Gedge and Kadas (2005) recorded 78 spider species, 18% of which were endangered, and 254 beetle species, 11% of which were rare. Clark and MacArthur (2007) have argued that green roofs could support a substantial arthropod (spider) community. In turn, they predict that a spider community is predictive of plant community longevity and resilience and that a decrease in green roof maintenance and plant replacement costs may follow. These authors suggest that a green roof with several types of spiders may be predictive of the overall ecological function of a green roof. Hence, increased diversity of spider species on a green roof may suggest greater biodiversity. Coffman (2007) compared two different green roofs in the mid-western USA, for biodiversity at a community and taxa level. The two roofs were an extensive sedum roof and an intensive green roof planted with a mixture of shrubs, grasses and forbs. The intensive roof supported a greater diversity at the community level, but no distinction could be made at the taxa level. Specifically, the intensive roof supported a higher diversity of rare spiders and rare birds species while the extensive roof was visited by more common birds. Both roofs supported a similar number of insect species. According to European researchers, green roofs can be successfully designed to provide sustainable habitat for rare species of invertebrates that are particularly at risk due to land-use changes associated with urban expansion (Jones, 2002; Brenneisen, 2003; Kadas, 2003; 2006). A comparison of colonization rates found on green roofs designed for biodiversity was compared to other conventional green roofs in Basel. The number of species of beetles and spiders on the biodiversity green roof increased over a period of 3 years whereas it took 3-5 years on a conventional green roof to support the same number of species (Brenneisen, 2003). The Cook + Fox Architects building (2006) in New York City provides a brief case study of an extensive green roof, planted primarily with sedum, that provides habitat for a range of invertebrates including migrating butterflies. This installation employed a mixed planting strategy to minimize monoculture patches to help foster biodiversity (Green Roofs for Healthy Cities, 2006). Almost immediately after installation, insects were observed on the green roof. Dragonflies were the first to arrive, followed by moths, gnats, and flies. In early September 2006, the first bird was spotted and later, two hawks were observed using the roof railing as a perch. New York occupies a strategic stopover in the semi-annual migration pathway for monarch
butterflies, and throughout the fall, butterflies were frequently foraging on the green roof. In late September, small, orange grub-shaped insects were observed on the leaves of Russian Stonecrop (Sedum kamtschaticum) which were identified as immature ladybugs (Coccinellidae spp.). A few studies to date report observations of butterfly activity on green roofs. Butterflies are potentially useful ecological indicators of urbanization because they are readily surveyed by environmental groups and are vulnerable to changes in microclimate, temperature, solar radiation, and the availability of specific host plants for ovipositing (laying eggs) and larval development and feeding. Jenrick (2005) found that Horseshoe Vetch (Hippocrepis comosa) and Kidney Vetch (Anthyllis vulneraria), which are reliable green roof plants, provide important food sources for UK-based butterflies and rare invertebrates. This researcher reports that butterfly species richness and diversity was directly correlated with plant species diversity (Jenrick, 2005). Certain types of plants might also assist with preserving or conserving butterfly species that require specific microclimates and or thin substrates. For example, the Lycaenid butterfly (Lysandra bellargus), in the UK, requires short turf and a warmer microclimate including a warmed upper plant canopy, specific host plants beneath and warmed soil. However, reductions in the intensity of animals that graze in the UK (e.g. (bovine, equine, and ovine species)has led to increases in vegetation heights and cooler temperatures with concomitant changes in biodiversity and species structure in some landscapes (Parmesan, 2005). For example, the Nymphalid butterfly (Argynnis pahia), in the UK, requires open woodland, where the sun can penetrate to the forest floor. Changes in woodland management have increased the canopy cover, increased shading, thus cooling this butterfly’s local microclimate to a potentially deleterious level (Thomas, 1993). Both of these examples illustrate the potential for using extensive green roofs to provide habitat for migrating butterflies in the UK. In San Francisco, rare and threatened invertebrate species such as the Checkerspot Butterfly (Euphydryas editha bayensis) have been identified as species which may benefit from the creation of suitable green roof habitats (P. Kephart, personal communication, 2008). By studying the composition of native plant communities and foraging pollinators, including butterflies, Kephart has translated ground level conditions to green roofs. The Checkerspot Butterfly has been observed to react positively to green roof installations which use amended substrates (serpentinite, i.e. blueish-green stone) and native grasses indigenous to the San Francisco Bay area. Several such installations have been designed by Kephart, including the 2.5-acre green roof of the California Academy of Sciences in San Francisco. 3.4 Green Roofs Biodiversity Research in Toronto 3.4.1 History of Toronto Green Roofs Green roofs are not a new technology for the City of Toronto as they were installed as roof gardens on private balconies, beach houses and alley garages back in the 1970’s and 1980’s (T. McGlade, personal communication, 2008). Over the past 5 years, green roof installations in Toronto have become more dominated by pre-planted, extensive, sedum-based systems. These extensive installations addressed a need in the market for light-weight, pre-established systems for clients who would otherwise not consider retrofitting older buildings with a green roof due to load restrictions. Observations on older green roof types across Toronto reveal that early designers were experimenting with biodiversity by varying substrate depths, substrate mixes, species composition (including blends of non-native and native species) that may have inadvertently supported biodiversity over time.
Most green roofs have been installed on a client-by-client basis in new construction or on retrofit buildings on an ad hoc basis. Many were established on commercial buildings when roofing membranes or mechanical upgrades were completed or on residential home as owners learned more about the technology and expressed a willingness to give something back to the environment (T. McGlade, personal communication, 2008). Currently, hundreds of green roof installations exist in Toronto (J. Welsh, personal communication, 2009). As more people choose to install green roofs, continued monitoring of the number, distribution and total area of green roofs in Toronto is needed. 3.4.2 Existing Green Roof Biodiversity Research in Toronto Studies on green roofs and biodiversity are just beginning to be undertaken at the University of Toronto and other centres across the City of Toronto. In a few years, much more will be known about plant survivability and biodiversity on green roofs in Toronto. Local studies are important because green roofs require an understanding of population ecology (the dynamics of species populations as well as knowledge about how these populations interact with the environment) to ensure species selection and design promotes long term plant survivability and regeneration. Several local studies have focused on butterflies on green roofs. The Norfolk Field Naturalists annual Butterfly Count is conducted in July at Long Point, Ontario (Norfolk Field Naturalists, personal communication, 2009). Results indicated that overall butterfly density was lower than last year, however total counts were higher and even hit a record number for 5 out of the 45 species. Itâ€™s not completely clear what role the green roofs played in this region, however, observations of butterflies on a green roof within Long Point were included. Other observations of butterfly activity on green roofs reveal that plant species such as Swamp Milkweed (Asclepais incarnata), Tiger Lily (Hemerocallis spp.), and Coneflowers (Rudbekia spp.) have attracted butterflies to a 5 story green roof location downtown Toronto (T. McGlade, personal communication, 2008). In a study undertaken in Peterborough County, it was noted that the inclusion of ornamental and exotic plants in naturalization efforts could provide butterflies with new potential host plants to forage (Hogsden and Hutchinson, 2004). This study found that species richness is equally as high or higher in moderately disturbed compared to undisturbed sites, although a slight majority of butterflies (58%) preferred undisturbed sites. Some of the butterfly species that appeared well adapted to disturbed areas were observed feeding from a greater diversity of host plants, suggesting that they may be able to use a wider range of host plants as resources. The observations also suggest that if habitat and resource requirements are made available, even in a disturbed environment, butterflies will use the site. Finally, this study also suggests that a number of small sites in the urban core might provide a matrix of butterfly habitats leading some to conclude that increasing green roofs may provide solid opportunities for butterfly habitat (D. Gedge, personal communication, 2008). Other case studies have explored biodiversity on existing installations. The green roof located at York University, Toronto, was seeded with non-native grasses and forbs into low nutrient growing media in 2003 (TRCA, 2006). Green roof observations in 2005 isolated 91 vascular plant species, of which 29 (32%) were native. It was noted that from 2004 to 2005, 11 new native plant species were found on the roof. A faunal survey in 2004 and 2005 revealed that six bird species had visited the greenroof including a breeding pair of Canada Geese (Branta canadensis) and House Sparrows (Passer domesticus). The European Starling (Sturnus vulgaris) was the most frequently observed species at the site, although no migrating bird
activities were recorded at that time. However, migrant bird utilization is predicted to increase as plant and community structure evolves on the roof. The study examined the roof as for potential bee habitat site and reported that given its current plant community and structure, bee nesting and foraging may occur in the future. In terms of plant species diversity and proliferation, discussions with local Toronto green roof researchers, observers and academics as part of the research for this report note that when sedum species were planted in a variety of substrate depths, those that were supported in deeper substrates were more established, hearty and thick. Additions such as occasional (sporadic) irrigation and intermittent fertilizer applications were also noted to have encouraged species on the deeper substrate (R. Sage, personal communication, 2008). Observations taken from the Ryerson University School of Engineering green roof would suggest that monoculture plant species and designs (in this case, hybridized lily species) can provide favourable habitat support for the arrival of a variety of wind and animal dispersed varieties of plant species (K. Hahn, personal communication, 2008). In ongoing research, Hahn has quantified as many as 40 new plant species on the Ryerson roof that were not part of the original plant design. 3.4.3 Emerging Green Roof Biodiversity Research in Toronto Ongoing biodiversity research is currently being lead by Dr. Mart Gross' Lab in Biodiversity and Conservation Biology at the University of Toronto. In the fall of 2008, five research projects examined the biodiversity of birds, insects and plants at four urban habitats including green roofs in downtown Toronto. Thirteen sites were used in the urban core (Hasnain and Gross, 2009; Haile, 2009; Revinskaya, 2009; Varatharajan, 2009). Each site contained a green roof, bare roof, green ground and bare ground. Bare roofs were covered by traditional roofing materials such as tar paper, bitumen, and gravel; green grounds were lawns, parks and boulevards with typical vegetative cover; bare grounds included parking lots and other paved areas with gravel and concrete surfaces that were not intentionally planted with vegetation. These studies are among the first to examine green roof biodiversity in comparison to currently available alternative urban landscapes. The diversity of bird species seen on green roof sites was roughly equivalent to that on green ground sites. Bare ground sites and bare roofs were also used by birds but attracted different species. There was no use of green roofs by migrants. The research suggests that urban green roofs provide a superior foraging habitat for birds than do bare roofs in the urban core, but cannot replace green ground sites due to limitations in size, vegetation structure and levels of anthropogenic disturbance (Haile, 2009). Green roofs and green ground areas were found to contain similar insect abundance and diversity. These areas contained four times more insects than comparable bare habitats and significantly more diverse insect communities. Green roofs also hosted unique insect taxa not found elsewhere in urban Toronto (Hasnain and Gross, 2009). In addition, important pollinators were found to preferentially use green habitat over bare habitat independent of vertical height (Revinskaya, 2009). Finally, biodiversity of vegetation was examined through a collaboration between students at the University of Toronto and Ryerson University (Hahn, 2009; Varatharajan, 2009). A subset of eight green roofs was used to census vegetation. A total of 112 plant species belonging to 30 families were observed, with an average of 25 species found on each roof (Hahn, 2009). Almost 40% of observed species were asters (Asteraceae) or stone cops (Sedum). Approximately twothirds of species on green roofs were found to be colonists, meaning they were not intentionally
planted within the installations (Hahn, 2009). Most of these colonist species were found to be perennials that were wind-dispersed in origin (Varatharajan, 2009). Green roofs had the highest diversity of plant species in comparison to the other urban habitat types (Varatharajan, 2009). Thus, the ongoing research from the Gross lab is showing that green roofs make a significant contribution to the biodiversity of plants and animals in urban Toronto. As several researchers have noted, the importance of arthropods in creating a healthy ecosystem in the long term cannot be underestimated. As demonstrated by studies of brownfields in London and prairie grassland in Alberta, spider populations are best established on a green roof that has been designed to mimic a highly diverse ground level environment. Most researchers agree that substrate depth and variation is the most important design component for creating sustainable, biodiverse green roof habitats. These design features will be reflected in some of the discussion below. 3.5 Future Directions for Biodiversity Research Quantification of the presence and use of green roofs by bird, insect and plant species is in its infancy in North America. Long-term monitoring is necessary to further understand and resolve the dynamics within these unique ecosystems. There is a multitude of opportunities for research on this topic, especially in the following areas as little empirical research has been directed at examining: Coexistence of species on mature green roofs in urban areas; Patterns of species' survival and immigration / emigration; Alterations in species' life cycles associated with green roof habitats in urban areas; Occurrence of invasive or unwanted species; Influence of neighbouring ground-level habitat on species composition; Effect of green roof placement and aggregations, and, Impact of varying biodiversity and other green roof benefits
4.0 Opportunities and Challenges for Toronto 4.1 Toronto’s Biodiversity in Context Within the City of Toronto, the main reservoir of biodiversity is found within the natural heritage system illustrated on Map 9 of the Official Plan. The components of the City’s natural heritage system and strategic directions for improving the natural ecosystem and increasing biodiversity are described in a 2001 report (City of Toronto and Toronto and Region Conservation Authority, 2001). An overview of Toronto’s ecological context and natural heritage is provided below. The City of Toronto is part of Ontario’s Mixedwood plains ecozone, one of three ecozones found in Ontario. The Mixedwood Plains extends along the Quebec City -Windsor corridor, including the densely-populated region of southern Ontario and makes up about 10% of Ontario’s entire geography. The smallest of the ecozones, the Mixedwood Plains is nonetheless home to half of Canada's population. Its cool winters (average temperature -5ºC) and warm summers (average temperature 17ºC) are prone to highly changeable weather, as the ecozone is in one of the major storm tracks of North America. This ecozone has been heavily impacted by intensive agriculture and urbanization such that native habitat has been reduced to isolated remnants of the original landscape. The remaining fragments of original forest account for Ontario’s largest diversity of tree species. The City of Toronto is ecologically bounded by several kilometres of Lake Ontario shoreline in the south, the Oak Ridges Moraine in the north and two major river watersheds, the Humber and the Rouge River, on its western and eastern boundaries, respectively. The City encompasses approximately 63,551 ha of land where urban development and extensive engineering have heavily fragmented the pre-existing natural environment. While only a fraction of the original meadows, forests, wetlands and riparian habitats remain, as a whole Toronto maintains a reasonable coverage of terrestrial natural habitat for an dense urban area, primarily as a result of the extensive valley land and river networks where development has been restricted. The most robust biodiversity can be found within Toronto’s river valley networks (Humber, Don, Highland and Rouge). However, vulnerability from adjacent land use changes, invasive species and general degradation continue to impact negatively on biodiversity and species survivability in the long term. The total amount of natural ground cover available in the City (based on 1999 information) is just over 8,595 ha (13.5% of the total city area) not including streetscapes, residential yards, urban trees or manicured park areas. Forest areas occupy 4,384 ha (6.9%) of the City of Toronto’s land base and are found predominantly in the valley’s, with the largest forest blocks located in Rouge Park and Morningside Park. The Great Lakes Remedial Action Plan (as cited in the 2001 City of Toronto Natural Heritage Study) suggests a minimum of 30% forest area cover would be necessary to establish and maintain a sustainable urban ecosystem. However, this target may be too large for an urban area such as the City of Toronto given that the City is essentially developed. Forest interior habitat exists within the Rouge and Morningside Park areas with smaller fragments in the Don and Humber watersheds. Other rich and biologically diverse habitats across the City of Toronto include the Leslie Street Spit area, High Park, parts of the Lake Ontario shoreline and the Toronto Islands. Meadows and wetlands in Toronto also support area-sensitive species. The largest meadow fragment can be found over the Beare Road Landfill (81.1 ha) and two others are contained in the Rouge. Wetlands occupy a small amount of area but receive a higher biodiversity weighting because they are smaller and can support crucial biodiversity within their constituent species. These wetland ecosystems are essential to ecosystem health and biodiversity in Toronto.
Overall, Toronto maintains a strong coverage of terrestrial natural habitat for an urban area, primarily as a result of the extensive valley land networks where development is restricted coupled with the presence of the Rouge Park. Natural habitat cover is not distributed evenly across the City and decreases substantially from east to west and from north to south across the City. The lowest scoring habitats in terms of biodiversity are described as small, narrow remnant habitat patches that are completely isolated from other natural areas. Many of these fragmented habitats are small (less than 5 ha in size) and are convoluted in shape with a limited capacity to support a viable animal population. Toronto is home to several rare animal and plant species, many located in the eastern Rouge valley watershed while other at-risk species are scattered across minor fragments. While numerous and rare flora exist in the City, many are threatened by encroachment of urbanization, loss of habitat, invasive species such as Emerald ash borer (Agrilus planipennis), garlic mustard (Alliaria petiolata), and purple loosestrife (Lythrum salicaria) coupled with an overall decline in ecosystem health. 4.2 Site Scale Opportunities Site scale opportunities are designed to encourage biodiversity and natural colonization of specific species and plant communities over time at a specific and suitable location. 4.2.1 Biodiversity and Natural Colonization Conceptually, green roofs can be viewed as a strategy to shift lower diversity habitats (conventional bitumen roofs) into higher diversity ones (green roofs). Houghâ€™s (2004) observations on terrestrial habitats, suggest a phased-in method to restore fuller diversity. He recommends starting with fast-growing pioneer plant species that provide initial ground cover, ameliorate existing soil damage, add nitrogen and bacteria, provide niches for fungi and create a variety of microclimates for other species to intercept. This would be followed by an intermediate phase where different plants are introduced and are followed by a climax phase (forest groves) where slow-growing, long-living perennials and trees become part of the established plant community. Based on 8-10 years of observations on research plots, has been noted that this process is more successful if the three phases are implemented at different times (Hough, 2004). Not all practitioners agree with Houghâ€™s phased approach when applied to green roofs. One researcher recommends designing the green roof landscape immediately for climax phase (W. Amelung, personal communication, 2008). For example, green roofs on fortified (cement) roof structures could be considered for climax phase natural colonization designs where slow growing native plants and wetlands would give way to shrubs and small trees over time â€“ as long as substrate choices and depths were suitable. While there are very few buildings where it would be feasible to establish forest groves (climax phase) on green roofs, some existing or new buildings can accommodate the biomass of small to medium trees in mixed plantings along with other hardy perennial herbaceous species which help buffer and bolster adjacent forested areas (W. Amelung, personal communication 2008). 4.2.2 Sensitive and Rare Plants Rare and sensitive vegetation communities found in Toronto include tall grass prairie, savannah remnants and several coastal plant communities (City of Toronto and TRCA, 2001). These types of rare and sensitive plants may benefit from locations on roofs as they may be less likely to be
disturbed by wildlife, humans or development as they can be designed to mimic conditions similar to the natural habitats of these plant communities. Habitat that is suitable for rare or sensitive species may be modeled on green roofs and then studied over time to capture adaptations and changes in community structure. Alvar4 species (vegetation adapted to seasonal flooding, extreme summer drought and limestone/chalky conditions) such as Sedum spp. have been successful adapters in the shallow, low-nutrient substrates on green roofs. The open gravel and bedrock conditions typical of natural alvars in Ontario, support mosses, algae, liverworts, lichens, grasses and some rare herbaceous plants that have been recommended for some green roofs in Toronto. According to recent research by Hahn (2009) communities of sedum species on green roofs may set beneficial conditions for natural colonizations and habitat extension for fuller biodiversity on green roofs. Other rare and/or highly sensitive plants in Toronto are located within larger habitat patches or in isolated and inaccessible areas. Examples of such species include sea-rocket (Cakile edentula) and seaside spurge (Euphorbia polygonifolia) at Bluffers Park and fringed gentian (Gentiana crinita) at East Point and on Toronto Island Park. Green roofs placed adjacent to or near these locations plants may benefit from wind and wildlife dispersal of these rare and sensitive plants. However, there is little to no research with these plants on green roofs in the City of Toronto. 4.2.3 Migratory and Breeding Birds The City of Toronto has a number of policies and programs directed at migratory and breeding bird conservation and these are described in more detail in the Migratory Birds reports for the City of Toronto (Dougan & Associates and North-South Environmental, 2008) and Birds of Toronto (2007). While habitat created by green roofs will typically not provide the same quality of food or shelter found in a natural area, green roofs do provide vegetation where there would otherwise be none and thereby create potential habitat for local and migratory birds. Green roofs could be used as part of a strategy to provide or enhance stopover habitat for migratory birds and foraging, nesting and mating needs of breeding birds Urban development and loss of habitat have impacted travel distances, expended energies, and reduced the availability of food sources for migratory birds passing through Toronto. A matrix of well-distributed aggregations of diverse green roof habitats may become attractive for migratory birds that view green roofs as possible â€œstepping stonesâ€? in a search for more suitable and larger habitat patches at ground level (D. Gedge, personal communication, 2008). Diverse green roofs established with grasses and herbaceous plants mature each season to produce numerous seed heads that can provide invaluable energy sources for newly arriving migratory birds particularly those who are exhausted by a lengthy migratory journey over Lake Ontario (Stutchbury, 2007; S. Brenneisen, personal communication, 2008). The following examples illustrate how breeding and migratory birds in the City could benefit from habitat opportunities on green roofs: â€˘
Species such as the Northern Cardinal pubescens), Black-Capped Chickadee carolinensis), Rock Pigeon (Columba House Sparrow (Passer domesticus)
(Cardinalis cardinalis), Downy Woodpecker (Picoides (Poecile atricapillus), White-Breasted Nuthatch (Sitta livia), European Starling (Stumus vulgaris) and the may benefit from habitat provided by green roofs
Alvars are naturally open habitats with either a thin covering of soil or no soil over a base of limestone or dolostone. North American Alvars support a distinctive set of flora and fauna, and almost 75% are located in Ontario; Ontarians have a responsibility to conserve these globally significant habitats and their specialized species communities. http://www.natureconservancy.ca/site/News2?page=NewsArticle&id=5961&news_iv_ctrl=0&abbr=on_ncc_
particularly if aggregations of biodiverse green roofs provide habitat and food for breeding pairs. Some of these species were recorded on the York University green roof and the downtown Toronto green roofs featured in the University of Toronto study (TRCA, 2006; Haile, 2009). •
Temperate migrants, including the American Robin (Turdus migratorius), the Red-winged Blackbird (Agelaius phoeniceus) and the Song Sparrow (Melospiza melodia) build nests on or near existing buildings in Toronto and have been observed on the York University green roof among other downtown Toronto roofs (TRCA, 2006; Haile, 2009). Increases in the number and diversity of available green roofs, might provide habitat and feeding opportunities for these nesting and migrating temperate species.
Grassland birds found in Toronto include the Eastern Meadowlark (Sturnella magna) and Bobolink (Dolichonyx oryzivorus). These species are often observed in the unmowed grasses along hydro corridors in the City (Dougan & Associates and North-South Environmental, 2008) and may benefit from green roofs that are designed as meadow or grassland habitats on buildings (where technically feasible) adjacent to or near hydro corridors or Lake Ontario (D. Gedge, personal communication, 2008). 4.2.4 Butterflies
Toronto and area support a rich diversity of butterfly species with mixed habitat preferences that include trees, grasses, shrubs, native plants and environmental conditions. Some species prefer habitats like deciduous woodlands, fields, roadsides, pine barrens, wooded swamps, parks, moderately disturbed open environments, weedy areas, gardens, or roadsides and require a range of plant species like alfalfa, clovers, milkweeds, thistles, tickweed, sunflowers, peppermint, dogbane, asters and others. While little to no empirical research has been conducted on butterfly activity in response to green roofs in the Toronto area, informal sightings of Monarch butterflies have occurred on downtown green roofs (J. Spring, personal communication, 2008). These sightings are encouraging, especially given that some local green roof designs have been shown to incorporate plant choices that attract and support butterfly larvae of the Monarch (Currie and McGlade, 2005). Green roofs may have the potential to support the reintroduction of threatened species like the Karner Blue butterfly (Lycaeides Melissa samuelis) that occurs naturally in the northeast U.S. and in Wisconsin although there is no large continuous habitat where it does occur. A study in southern Ontario found that habitat fragmentation is stalling the Karner Blue’s migration northward into Ontario from the United States (Chan and Packer, 2005). It has been suggested that all of the Karner Blue butterfly’s habitat is fragmented, with sites often several hundred kilometres apart. The Karner Blue larvae feed only on wild Lupine (Lupinus perennis L.) that may be available along a few roadsides, gardens and disturbed environment Ontario. The Karner Blue is also a weak flier, (weaker than other commonly occurring eastern "blues") and is made vulnerable because it usually flies close to the ground. Species such as butterflies need more than a host plant in order to thrive including a host of environmental factors such as a complex network of organisms above and below ground. There may be a way to shift green roof design toward plant species communities modelled from ground level studies that support the autecology of this species. It should be noted however, that green roofs designed to attract this species of butterfly may require a very high level of complexity and aggregation to overcome the current fragmentations within their habitat.
4.3 Landscape Scale Opportunities Green roofs provide habitat in locations where otherwise there would be none. They also provide new and relatively accessible horizontal extensions to existing habitat fragments across the City (Hahn, 2009; Halie, 2009; Hasnain and Gross, 2009; Revinskaya, 2009; Varatharajan, 2009). According to a Toronto green roof study, the total flat roof area (greater than 350 sq. m.) that could accommodate roof greening is approximately 5,000 hectares or 50 million m2 of roof landscape (Banting et al., 2005). This study found that if just 8% of this roof landscape were greened, it would contribute significant improvements to urban air quality and storm water runoff quality. As well, more green roof applications could incrementally support urban biodiversity with plant communities, habitat, microclimates and unique ecosystems that would benefit flora and fauna including those that are rare and vulnerable. 4.3.1 Connecting Existing Habitat Green roofs can provide connections between existing habitat fragments. When observed from the sky, green roofs appear as green islands that have been described as ecological stepping stones in urban areas which can be used by migrating, feeding or breeding birds, insects and plant species. By intentionally designing diverse green roofs, more species may have opportunities to find nutrients, forage, nest, and fledge young. Examples where green roofs connect with existing habitats are the three proximal green roofs located in downtown Toronto on the Mountain Equipment Coop roof, the 401 Richmond street roof and the 215 Spadina Avenue roof. Each green roof is geographically close and relatively low (3-4 stories) which helps to connect habitat from the existing mature tree-lined streets in China town, the small park on Spadina south and the clustered trees in Grange Park to the Toronto waterfront itself. 4.3.2 Supporting Edge Habitats Green roofs can enhance or add to existing edge environments and buffer zones throughout the City. The goal is to enlarge existing naturalized habitats, by adding similar or complementary habitats in proximity to them. Green roofs could expand naturalized buffer zones and edge communities by adding native plants for conservation that perform other ecosystem services such as provide food for pollinators and resting, feeding, and breeding space for migratory birds. Green roofs may also be suited to preserve and increase the diversity between habitat fragments located at grade-level that are particularly susceptible to further disturbance from urban environments. For example, tree species within small forest fragments expose a large proportion of their edges (and the communities of flora and fauna that live on these edges) to vulnerabilities that take the form of excess wind exposure, temperature fluctuations, soil erosion, salt spray and small mammal predation by generalists such as raccoons, skunks, and foxes that feed on eggs and young birds. Green roofs on tall buildings may be less vulnerable than these edge habitat communities as they are more protected from invasion by terrestrial generalists lacking the means to scale walls and are not susceptible to salt spray, but also have to be designed to minimize the risk from wind, heat and exposure to flash rainfall events. Green roofs may also enhance the survivability of desired species within fragmented natural habitats in urban areas. Green roofs installed within short distances of targeted habitats may benefit plant and animal species on several scales, including providing a destination for natural colonization and supporting much needed protected foraging and nesting space in both the short and long term. Interestingly, research that compares green roofs and ground level biodiversity
supports the finding that where habitat fragments interface edge to edge they exhibit higher scores on the diversity of species within both habitats (Hough, 2004; Dunnett, 2006). Existing grade level fragments may also benefit from close aggregations of diverse green roofs placed strategically on buildings that align or lie parallel to designated fragment communities. Square-shaped fragments score highest in terms of habitat strength and biodiversity at grade level (Hough, 2004). It is purposed that green roofs may act as a proxy for ground level habitat, given that green roofs are mostly rectangular and/or square in shape. While the shape of green roofs may strengthen some biodiversity scoring, a typical green roof is small and often isolated from other natural habitat. Consideration to placing a number of green roofs (aggregations) within several hundred meters of each other may assist plant and animal species and enhance biodiversity scores over time. The City of Torontoâ€™s Natural Heritage study (2001) ranked fragmented habitats in terms of their shape and their biodiversity strength. Accordingly, fragment shapes that provide a higher biodiversity score had a lower ratio of exposed edge to non-exposed edge. Exposed edges add risk for further fragmentation, invasive species, wind damage, salt spray and predation. Lower scoring fragment sites are those with small, narrow remnant habitat patches that are completely isolated from other natural areas. Smaller fragments appear to support common species, but if maintained and improved they may become more substantial contributions to natural heritage as stepping stones or stopover areas for animals on the move â€“ another potential asset for green roofs designed for biodiversity across the City (City of Toronto and TRCA, 2001). 4.3.3 Supporting Conservation Source-Sinks Green roofs may provide positive habitats or habitat sources for the conservation and preservation of rare or threatened species. Habitat source-sink dynamics describe how variations in the quality of habitat affect populations of living organisms. When considering how green roofs might affect the survivability of a population, studies must be directed at that population over time. A green roof with high quality habitat may enhance population growth and be described as a source. A green roof with low quality habitat may be a sink â€“ or one that does not support a given population. However, if excess individuals from the source habitat green roof move to the sink habitat green roof, the sink population may be strengthened indefinitely. While conservation is a key driver for green roof infrastructure in the Technical Report on Green Roof Policy in London, UK (Design for London, 2008), conservation and preservation have not gained status in North America and require more study and ecological analysis. 4.3.4 Island biogeography An alternative option for the spatial arrangement of green roofs is based on the concept of island biogeography (MacArthur and Wilson, 1967). Small contained areas, for example, tropical islands, contain a large share of endemic and rare species since isolation from other land areas discourages migration and homogenization of communities. Over time, isolation may have played a role in protecting these endemic species as they are excluded from competition with other species that may be present in other areas. Urban development and changes across urban landscapes have also created islands of isolated habitats within and across large urban tracts (Earn et al., 2000). For example, species available in High Park in the west lack connectivity to species in the Don Valley due to roads, streets, buildings, neighbourhood development, industry, vehicles and many other obstacles. Earn and colleagues (2000) suggest that a lack of connectivity may in fact be important in protecting certain plant or animal communities. Green roofs offer a degree of isolation from other ecosystems across the City and may be effective in preserving some populations. Using green roofs to connect fragmented
natural area in urban settings is important since existing fragments may not meet minimum size and resource requirements for many species. However, connectivity should only be facilitated to a point since too much aggregation may allow aggressive species to invade and damage delicate habitats. 4.4 A Strategy for Toronto This section describes strategies that can be used to enhance biodiversity in the City of Toronto. Table 4.1 illustrates several design strategies that are predicted to enhance biodiversity and natural colonization on green roofs in Toronto. The strategies can be applied to green roofs in any area of the City. Table 4.1: Design Strategies to Enhance Biodiversity and Natural Colonization on Green Roofs in the City of Toronto Characteristics
Design Strategy • • •
• Substrate •
Emphasize native species Any non-native species used should be noninvasive Review suitable plant species including end points and life cycle needs for targeted species including soil substrates, plant community needs, microbes, food sources, moisture temperature, and relationships with microbes) Select grasses and herbaceous plants that produce numerous seed heads that can provide invaluable energy sources for migratory birds
Position substrate near building sites before elevating to the roof if practical (species can inoculate substrate at grade level) Incorporate local materials in substrate blends (compost/porous materials) (Refer to City of Toronto Green Roof by-law supplementary guidelines; FLL (2002) also provides ratios and blending recommendations) Use compost liberally where practical (composting may occur on roof or other gradelevels where possible) Vary substrate depths and drainage regimes to create a mosaic of microhabitats on and below the soil surface that? can facilitate colonization by a more diverse flora and fauna Vary substrate depths by adding berms/mounds, bare areas, and physical substrate connections (such as limbs or tree snags…) to enhance species movement (promotes heterogeneity)
Add bird boxes, bat boxes, and trap nests for bees as desired
Add snags (tree limbs) and stones for terrain variation and moisture retention
Add depressions to collect rain water for short periods
• • •
Maintain green roof 2-3 times per year to ensure unwanted trees and/or invasive species are removed (see Appendix A for partial list of invasive species) Irrigate to enhance survivability in first two years Uphold seasonal maintenance schedules Save seed pods in autumn;
Check load-bearing status of roof/building with structural engineer Avoid use of fertilizers where possible (phosphate loading can be an issue on green roofs)
Table 4.2 provides suggestions for green roofs that are located near or adjacent to specific locations in the City of Toronto including forested habitat, the Lake Ontario shoreline or hydro corridors (Figure 4.1). The locations were identified based on the overall direction for naturalization from the 2001 Natural Heritage Study (City of Toronto and TRCA, 2001) and the report on Migratory Birds in the City of Toronto (Dougan & Associates and North-South Environmental, 2008) Table 4.2: Design Strategies to Enhance Biodiversity on Green Roofs in Specific Locations in the City of Toronto Green Roof Location Areas adjacent to forested habitat (e.g., river valleys, Rouge Park).
Objectives Enhance/buffer adjacent ecozones and link green roofs to forest ecosystems at grade level. Beneficial matrix influence through climate and hydrological mitigation to buffer adjacent forest ecosystems. Provide perching/ breeding/ feeding opportunities for migratory birds, butterflies and insects.
Areas adjacent to Lake Ontario (shoreline/ waterfront) and river valley corridors
Provide habitat for native plants. Extend perching/ breeding/ feeding zones for migratory birds, butterflies and insects.
Provide habitat for native meadow/praire plants and Areas adjacent to hydro corridors
Extend meadow grassland habitats and support zones for migratory birds, butterflies and insects. Provide habitat for native meadow/prairie plants
Design Strategies Create higher order â€œclimaxâ€? ecosystems; use small shrub and tree species Enhance property perimeter regions at grade level to scale up available shrubs and other forest constituents
Species for Conservation or Protection Forest Interior birds, rare plants, native shrubs/small trees, pollinators including butterflies (along with other trophic benefactors eg. microbial soil constituents)
Design for aggregations of green roofs on clusters of buildings.
Meadow grasses (native and non-native) perennials + tall grass prairie species; also try pre-vegetated mat systems with augmentations in substrate depths/ shapes/ mounds where practical. Include plants that produce abundant seeds to feed early spring migrants. Meadow, grasslands or prevegetated mats with augmentations to substrate depth as practical.
Migratory birds and butterflies, native plants, insects and other pollinators (along with other trophic benefactors) Alvar species
Meadow plants, grass and shrubland birds, butterflies and invertebrates Alvar species and possibly some meadow marsh species where water is retained more.
Figure 4.2: Location of buildings with green roof potential in relation to river valleys (forest habitat) and Lake Ontario (shoreline) in the City Toronto.
Green roofs placed adjacent to forested areas may capitalize on ecosystems strengths at grade level and within the canopy itself. Proximal green roofs afford songbirds and other migratory species including butterflies, a safe, green (cool and damp), ecozone for perching, resting, feeding and breeding. Green roofs become ecological sinks for seed dispersal from forest interiors (dropped by visiting birds, insects or wind) where natural succession may occur without vulnerability from human or predator intervention. Green roofs with open substrates may become host for succession species brought by wind or visiting fauna. While trees and shrubs require more substrate and structural integrity, some roofs may be equipped for this added weight through biomass and adapt quite well over time. Areas near the lakeshore and adjacent to river corridors have been identified as particularly important for migratory birds. Green roofs, particularly large roofs, positioned near the lakeshore, river valleys and ravines and designed with structurally diverse plant community may create new habitat opportunities and in these areas. Green roofs located adjacent to hydro corridors where meadowland habitats dominate may provide support to meadow species such as migratory birds, plants and winged invertebrates. Green roofs are ideal for supporting grassland/prairie habitat. Meadow, tall grass prairie and coastal plant species have been shown to survive reasonably well on existing green roofs throughout Toronto (TRCA, 2006; T. McGlade, personal communication, 2008). Numerous grass and shrubland birds including Bobolinks (Dolichonyx oryzivorus), Eastern Meadowlarks (Sturnella magna), Brown Thrashers (Toxostoma rufum) and Eastern Towhees (Pipilo erythrophthalamus) are experiencing population declines across eastern North America. Green roofs may enhance feeding and breeding opportunities for these migratory species. Where technically feasible, buildings with large roof surface areas are particularly suitable to increase migratory bird feeding opportunities.
4.5 Challenges for Green Roofs and Biodiversity Two major challenges for biodiversity on green roofs are extreme weather variations and invasive species. Climate change may exacerbate both of these conditions. 4.5.1 Extreme Conditions Plant and animal communities on green roofs are challenged by thin substrates, small habitat zones, exposure to intense sun and wind, building heights (especially non-winged species and poor climbers), lack of moisture or flooding (at times), quick freeze conditions (a lack of connection to geothermal earth can freeze water-clogged root systems), urban air pollution and disconnection from ground level habitats for renewal (seed banks, forests, wetlands etc). Earthworms, for one, are unable to survive the intense temperatures on green roofs as thinner substrates quickly transfer summer heat throughout the growing media (Brenneisen, 2006). At time of planting, green roofs are particularly vulnerable to moisture balance and are vulnerable to plant death if some form of moisture (rainfall) or irrigation is not available. Once plant roots are established, irrigation can be disconnected, however, adequate rainfall is often essential in the first and second growing season.
4.5.2 Invasive Species Invading species are one of the greatest threats to the biodiversity of Ontario's waters, wetlands and woodlands. Originating from other regions of the world, and in the absence of their natural predators or controls, invading species can have devastating effects on Ontario native species, habitats and ecosystems in cities and regions across the province. Studies note that while green roofs serve as destination points and useful habitat for wanted species, some unwanted varieties may thrive and management strategies as well. A list of invasive terrestrial plant species species that threaten environments around Toronto is provided in Appendix A. One that is particularly relevant to green roofs is Purple Loosestrife (Lythrum salicaria), an invasive exotic wetland plant, that has had well-documented widespread impacts on biodiversity in wetlands. Green roofs could become a repository for purple loose-strife; however, few if any reports of purple loosestrife on green roofs have been noted in the City. Hand-weeding is one mechanism to control invading plants, including unwanted tree species that can be quickly employed on green roofs. Interestingly the types of green roofs that are best equipped to reduce invasive species are the extremely light-weight extensive roofs as their substrates become nutrient deserts over time. 4.5.3 Climate Change Climate change will exacerbate habitat loss and the threat from invasive species, currently the two largest threats to biodiversity in Canada (Lovejoy, 2005). Climate change is already affecting the physiology, phenology (migration time, budding and flower time etc) and biogeography (location) of plant and animal species globally (Hughes, 2000; Root et al., 2003). Plant phenology (budding and flowering time) is predicted to respond to increasing atmospheric CO2 concentrations, but the responses will be species-specific, making it difficult to provide a general statement (Murray and Ceulemans, 1998; Ward and Strain, 1999). Species of birds and butterflies have begun shifting pole-ward and upslope (Hannah et al., 2005). Birds are beginning to demonstrate altered ranges and Toronto may encounter species of birds that are typically found 100â€™s of kilometres south of the border within the United States. Green roof design will need to consider a changing species compliment in Toronto over time and design plant communities accordingly. The primary short-term response of plants may be phenomenological, especially on green roofs. Warmer winters and earlier springs are expected to affect the timing of budding, leafing and flowering on plants as these phases occur with accumulated temperature, total heat or growing degree days above a certain threshold (PeĂąuelas and Filella, 2001). Plant communities are expected to take much longer to migrate and react to climate change compared to birds. Butterflies are capable of accessing and exploiting small patches of habitat, especially if their requirements are fairly limited (Thomas, 2000). Butterflies may move quite rapidly with changes in temperature at grade level (Parmesan et al., 1999). However, some species respond at about one-half of the expected rate (Grabherr et al., 1994: Parmesan, 2005). Generally, insects are predicted to reach adulthood more quickly, and some species will undergo more generations per year (PeĂąuelas and Filella, 2001). This may help birds adapt and find food sources on green roofs particularly at migration times. Some migratory bird species will be vulnerable to a mismatch in timing between resource availability (insects and plants) and their life cycle. This lack of synchronicity with plant/insect life cycles is predicted to put further stress on migratory birds and pollinators that rely on this
balance. Day length is a migration cue for several species of bird (Coppack et al., 2001). These animals may arrive at their breeding grounds too late or too early to take advantage of the emergence of essential food plants and insects (Visser et al., 1998). Green roofs designed to support migratory birds will need to consider climate change variables when selecting plant communities in the short and long term. Climate change will present a challenge to soil biota and the ecosystem services provided by many soil organisms. For example, in a grassland ecosystem, the soil food web is composed of microflora, protozoa, arthropods, nematodes and annelids (Moore et al., 2004). Even small changes in the soil habitat will be seen as changes in abundance and biomass and/or biodiversity (Bongers and Ferris, 1999). Nonetheless, it is still not possible to generalize the impacts across all ecosystems, particularly green roofs. In grassland habitats, some of which may be models for green roofs in Toronto, the soil organisms have been found to be vulnerable to climate change which may affect the resilience of higher trophic levels as the soil food web plays a critical role in mediating plant allocation of nutrients and herbivores (Wall, 2005). Climate change will also impact mycorrhizal fungi and the soil bacteria that fix nitrogen for plants use. Mycorrhizal fungi are likely to play a key role in plant community transitions as diversity in mycorrhizal fungi promote diversity in a wide range of plant communities (Janos 1987; Borchers and Perry, 1990; Van der Heijden et al., 1998). On green roofs, successful establishment of plants is highly dependent on certain types of mycorrhizal fungi. Changes in fungi availability will impact green roof ecosystems services over time if these communities are hindered along their life cycles. Many species will alter their range to match changing climates, which will become more difficult with habitat fragmentation. Ecosystems will not move as one unit to a new habitat. Individual species will move at their own speed and not necessarily to the same new fragment areas. Fragmentation will also reduce the likelihood that new species will be capable of colonizing these habitats through seed dispersal. Climate change will open up new opportunities for invasive species and pioneering species, particularly those that are commonly termed 'weeds'. The most visible impact is likely to be on a speciesâ€™ range, where impacts that will involve colonization in some areas and population decline in other areas (Hewitt, 1996), a response that can be extremely rapid. Climate change may pose a challenge not only to green roofs but to all ecosystems in the City. However, green roofs, because of their small size, thinner substrates, height and low nutrient availability may be particularly more vulnerable than other systems. One strategy is to manage the out-migration and in-migration of plant communities quickly so as to minimize the risk associated with established new invaders in order to maintain similar degrees of diversity while maintaining the desired ecosystem services provided by the green roof. Another strategy is to use green roofs as part of a larger network of ecosystems to reduce habitat fragmentation. Monitoring and maintenance is important, both at the surface to assess the impacts of climate change on existing species, and below the surface to maintain communities of mycorrhizal fungi and other soil bacteria. Strategies for green roof design that target biodiversity and species conservation will require ongoing monitoring in order to observe how these habitats adapt to climate change. Green roofs may inspire unique adaptations that can assist researchers understand responses at grade level. Depending on the degree of change necessary, a green roofâ€™s adaptation may be its best medium to long-term strategy for survival climate change.
5.0 Summary and Conclusions Green roofs are becoming more common in North America and are being spurred on by municipal incentives, strong partnerships and collaborations between industry and research as well as improvements in technology, materials and performance. Green roofs are one tool for enhancing biodiversity in urban areas. Green roofs can help to bolster and extend existing grade level habitat by establishing habitat in areas where it would not otherwise exist. While green roof habitats may not be as abundant or as high quality as those at ground level, green roofs can provide suitable habitat for animal and plant species that are able to adapt to and develop survival strategies to cope with extreme weather and temperature conditions. Green roofs can be designed to maximize biodiversity particularly when biodiverse grade level habitats are adjacent or close by. Green roofs can be designed to mimic almost any grade level habitat although alvar and other adaptable plant species may be more suitable. There are several ways to deconstruct an urban green roof; one way would be to view it would be as a biophysical desert island within a sea of urban form. Another would be to view green roofs as functional ecological units within an ecological network. It is this latter view that will enable green roofs to benefit both the natural environment already established in the City and population health, well being and quality of life therein. As demonstrated in this report, green roofs not only reduce energy use, manage storm water runoff and improve air quality but also to preserve and increase biodiversity. Suggestions have been provided about how the City of Toronto might use green roofs to contribute to biodiversity from a regional perspective, within a landscape (i.e., larger geographic context) and within an ecological planning context, although challenges are presented by extreme weather and temperature conditions, shallow, low nutrient substrates, possible invasive species and climate change as predicted for Canada. City of Toronto green roof programs, incentives and by-laws are sending strong market signals to the development, construction, roofing, landscape design, and technical green roofing industry. Leadership has also stemmed from a variety of sources including international green building standards such as LEED, industry associations, including Green Roofs for Healthy Cities (Toronto) who have fostered North American networks, capacity in industry, market development, and hosted yearly conferences that coalesce expertise and research from across the globe. Building retrofits and a sophisticated residential market are contributing to the steady growth in green roof installations currently being experienced across the GTA. The City of Toronto also co-hosted the International Green Roof congress in 2009. Toronto is also claiming a place in green roof research as the University of Toronto and Ryerson University embark on collaborative green roof studies that are predicted to benefit and spur more research and knowledge transfer to industry in years to come. Green roofs may become a touchstone for visitors, citizens and politicians alike â€“ as ideas generated on roofs get conveyed back to communities, gardens, parks and backyards in places in and beyond the city.
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Appendix A Terrestrial Invasive Plant Species for Ontario include:
garlic mustard leafy spurge purple loosestrife (acquatic) Japanese siltgrass European buckthorn dog-strangling vine glossy buckthorn phragmites (acquatic) giant dogweed giant hogweed (acquatic) Japanese knotweed spotted knapweed Oriental bittersweet white mulberry marsh sow thistle mile-a-minute
Source: Stewardship for Ontario report www.stewardshipcentreontario.on.ca