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J.T. Lundholm and E.A. Walker
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Evaluating the Habitat-Template Approach Applied to
Green Roofs
Jeremy T. Lundholm1,* and Emily A. Walker1
Abstract - The habitat-template concept is meant to help select plant species for use in
artificial ecosystems by seeking out locally occurring habitats that share environmental
conditions with the target artificial ecosystem. For green roofs with shallow growing media,
appropriate habitat templates might be found in local exposed, rocky, or otherwise infertile
habitats. Many studies report plant selection using this process, but recent research suggests
that it may fail for green roofs in extreme climates. In this review, we identify ecological
novelty as a potential challenge to the habitat-template approach, and demonstrate that the
tendency to focus solely on abiotic conditions may result in species selection biased toward
generalists. We propose that trait-based species-selection and attention to the finer details
of a given habitat template, including companion species, are worthwhile approaches to
successfully diversify plant assemblages on green roofs.
Introduction
Successful rooftop greening must strike a balance between conflicting design
constraints because features that create a supportive growth environment for plants
may exceed the structural capacity of a building. The placement of soil and vegetation
on top of a building adds to the weight loading of the roof. As a consequence,
many green roofs incorporate artificial substrates that are shallower than the soils
occupied by most native vegetation communities in the region. These “extensive”
green roofs thereby present challenges to plant survival due to the relatively harsh
abiotic conditions associated with shallow soil depths and rooftop exposure. For
over a century, designers of temperate-zone green roofs have focused on droughttolerant
species, especially succulents, to maximize plant survival, coverage, and
ecosystem services in challenging rooftop environments (Köhler 2006, Köhler and
Poll 2010). The original habitats of many of the species (e.g., Sedum spp. [stonecrops])
used successfully on extensive green roofs are rocky environments with
shallow soil (Lundholm 2006).
Originating in Germany, a green roof industry catering to the needs associated
with rooftop plant establishment has developed in the last 40 years, formed of companies
that supply specialized growing media, engineered components, and plants
for green roofs. Over the last 2 decades, green roofs have gained popularity in
North America, and several major European companies have set up North American
branches to take advantage of these new markets. The established firms and startups
primarily relied on a palette of plant species tested in Europe, dominated by
1Departments of Biology and Environmental Science, Saint Mary’s University, Halifax, NS,
B3H 3C3 Canada. *Corresponding author - jlundholm@smu.ca.
Manuscript Editor: Michael Strohbach
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2018 Green Roofs and Urban Biodiversity Special Issue No. 1:39–51
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succulent species native to Europe and Asia (Dvorak and Volder 2010). The industry
relies on succulents because these species survive better than other life forms under
the hot and dry conditions typically found on green roofs (Monterusso et al. 2005,
Rayner et al. 2016, Rowe 2015) and also recover better from severe drought (Bousselot
et al. 2011). Many of the green roof companies operating in North America
have developed regionally specific plant mixes to satisfy the demand for native
species of a variety of life forms (Butler et al. 2012).
While the increasing popularity of native plant species in ground-level landscaping
motivated the development of these green-roof native-plant mixes, plant
diversity also influences ecosystem service provisioning by green roofs. Green
roofs are multifunctional ecosystems, and, although succulents perform some functions
well, a greater diversity of plant life-forms may be important to provide a full
range of services. For example, grasses may perform stormwater-retention functions
better than succulents because they have high water-demand and their taller
canopies may intercept more rainfall (Dunnett et al. 2008, Nagase and Dunnett
2012). Consequently, plantings that combine life forms with different resource-use
strategies can promote improved provision of multiple ecosystem-services by green
roofs (Lundholm 2015a).
Choosing plant species that will survive on green roofs is essential to maintaining
aesthetics and performance because vegetation dieback leads to functional
impairment (Speak et al. 2013) and may result in additional economic costs for
installers or owners. Beyond their thermal and hydrological services, green roofs
provide habitat for many spontaneously colonizing organisms, including insects
that are rare or otherwise important for conservation (Jones 2002, Kadas 2006).
Green roofs that contain a greater variety of floral resources are expected to provide
habitat of greater value for pollinators and other invertebrates (Williams et al. 2014)
than roofs with lower species or life-form diversity. These functional considerations
provide further motivation for selecting plants in addition to succulents for use on
green roofs.
Given that North America has a diversity of habitats that feature shallow soils
and high plant-biodiversity, Lundholm (2006) recommended the habitat-template
model as an ecological approach to selecting a variety of plant species for use on
extensive green roofs. The habitat-template, or habitat-analog approach exploits
what urban ecologists have known for decades: that some species are preadapted
to urban or other artificial environments because they evolved in natural habitats
with similar characteristics (e.g., Gilbert 1989, Wittig 2004, Woodell 1979). Thus,
searching for local habitats that share similar abiotic conditions with green roofs
provides a potential method for finding suitable species for green roofs, representing
biomimicry at the ecosystem level (Pederson Zari 2016). Many researchers and
designers of constructed ecosystems have incorporated these ideas into green-roof
plant selection on several continents (Fig. 1; MacDonagh and Shanstrom 2015, Van
Mechelen et al. 2014a, Williams et al. 2010), and the concept applies to a range of
other urban and industrial ecosystems (Lundholm and Richardson 2010). The goal
of this paper is to evaluate this concept as it applies to green roofs 10 years after the
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publication of the original “habitat template” paper in Urban Habitats (Lundholm
2006). We also delve further into the concept of ecological novelty and outline
how this approach may inform the ecological engineering of green roofs and other
constructed ecosystems.
Empirical Tests of the Habitat-Template Approach
There is little doubt that the broad concept of a habitat template for green roofs
has validity: the succulents typically used on extensive green roofs have their origins
in relatively extreme habitats such as rock barrens, cliffs, or deserts. However,
the general applicability of the habitat-template approach in sourcing non-succulent
species suited to green roofs from local habitats is unclear. Many recent studies
encompassing different climate zones report the use of habitat-template or analog
concepts to identify suites of plant species suitable for inclusion in extensive green-
Figure 1. Examples of natural habitat templates and corresponding green roofs.
(a) coastal rock barrens, Izu Peninsula, Japan, with (b) corresponding green roof; (c) dry
grassland, Victoria State, Australia, with( d) corresponding green roof; (e) Mediterranean
scrub, Athens, Greece, with (f) corresponding green roof; (g) coastal heathland, Nova Scotia,
Canada, with (h) corresponding green roof. Photographs © J. Lundholm.
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roof designs (e.g., Caneva et al. 2015, Kinder 2009, Myers 2012, Van Mechelen et
al. 2014a) and emphasize regionally specific vegetation types as sources of plants
(e.g., Natvik 2012, Williams et al. 2010). This approach has generated several
regional lists of candidate green-roof species, but not all taxa have been experimentally
validated (Caneva et al. 2015, Myers 2012, Van Mechelen et al. 2014a).
Overall, the concept fits with the idea that shallow substrates place key abiotic
limits on the survival of plant populations in extensive green-roof systems. One of
the most detailed evaluations of the habitat-template concept for green roofs (Mac-
Donagh and Shanstrom 2015) indicates that careful attention to the details of species
requirements, with empirical testing on multiple roofs, results in a selection of native
species that survive on roofs for at least a decade and likely much longer. Van
Mechelen et al. (2014a, b) describe how the habitat template approach can be combined
with a functional-trait approach to derive useful plant lists for a given region.
However, recent work also indicates that sourcing plants from a local habitat characterized
by severe drought and shallow substrates may not be sufficient to ensure
survival of non-succulent taxa on green roofs. Rayner et al. (2016) evaluated a range
of plant species representing several life forms, all native to drought-prone habitats
from several regions and found that leaf succulence was the best predictor of survival
regardless of the source region or habitat of the species. The habitat-template
approach may work best as a coarse filter for selecting species, but it can be refined
through the use of complementary techniques like functional-trait analysis.
Ecological Novelty
Many drought-prone environments host a variety of plant life-forms, including
forbs, graminoids, leaf succulents, shrubs, bryophytes, and lichens. Among vascular
plants, succulents are the most drought-tolerant and show superior survival and
recovery from drought in many green-roof contexts, outperforming the other life
forms (Bousselot et al. 2011, Durhman et al. 2006, Getter et al. 2009, MacIvor et al.
2013, Monterusso et al. 2005, Rayner et al. 2016, Thuring et al. 2010). This finding
suggests that the green-roof conditions are harsher than those experienced by many
species at ground level in natural habitats within a region. A particular ground-level
analog habitat may already represent the most extreme edaphic conditions in which
a species can survive; such a species might already be living at its niche margins
and may be unable to accommodate additional stress. Green roofs impose stresses
similar to those present in natural analog habitats, such as drought and high maximum
temperatures, but the magnitude of these stresses may be amplified or reduced
on the roof relative to the habitat analog; the character of both the “biophysical
envelope” and the “abiotic infrastructure” (Hobbs et al. 2009) of a given green roof
may be perceived as distinct from the conditions circumscribed by its local analog.
There also may be novel sources of stress in rooftop environments introduced by
artificial soil components, urban pests, pollution, or lack of symbionts (e.g., John
et al. 2014).
An ecosystem may be designated as novel if it features abiotic or biotic conditions
without natural historical precedents; this novelty is maximized following
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simultaneous and pronounced alteration of both living and non-living components
of a system (Hobbs et al. 2009). Recent articulations of the “novel ecosystem”
concept suggest that we should use the term “hybrid” for artificial or constructed
ecosystems such as green roofs (Hobbs et al. 2013). In that view, the term “novel
ecosystem” is reserved for ecosystems that have some historical continuity with a
natural or semi-natural habitat, such as those that have been heavily influenced by
disturbance by humans or invasive species but are still remnants of some “natural”
habitat like a forest or grassland. However, a focus on one level of ecological organization
(ecosystem) may obscure key ecological processes occurring at other
levels. The habitat-template concept implies a gradient of novelty (Lundholm and
Richardson 2010) such that an artificial ecosystem can present conditions similar
to those experienced by a plant population in its natural habitat. Although the
whole ecosystem may be considered “novel” or “hybrid” because it was created
by humans and lacks historical continuity with a natural habitat, the goal of the
habitat-template approach is to seek plant species that are suited to the conditions
on a green roof, thus reducing the novelty encountered by individuals of a given
species that we plant on a roof. This gradient of ecological novelty (Lundholm
and Richardson 2010) represents a way of conceptualizing differences between
the ecological context in which a plant population evolved and the current setting
in which it grows. The habitat-analog idea is intended to inform plant selection by
reducing the potential for plants to encounter a novel environment on a green roof
in which they are unable to survive. We re-examined the various studies reporting
the superiority of succulents over other life forms on shallow-substrate green roofs
using an “ecological novelty” lens and found that leaf succulents may show some
level of pre-adaptation to the extremes of temperature and soil moisture on green
roofs. In contrast, conditions may be too different from those typically experienced
in natural areas by plants with other life forms for them to survive on green roofs.
Thus, ecologically novel aspects of the green roof act as a strong filter to constrain
the plant species that can be grown in these created landscapes .
Despite broad similarities between natural templates and green roofs, the devil
may lie in the details, such as soil temperatures on roofs that are more extreme than
in natural environments (Rayner et al. 2016). Detailed consideration of the abiotic
differences between green roofs and natural habitats may reveal important elements
of novelty built in to green-roof habitats. For example, there is a common assumption
that rocky habitats are dominated by shallow-rooted species, but recent work
shows that a key strategy of some species is to grow long roots that can penetrate
cracks in rocks and find water much deeper than suspected when researchers considered
average soil depths alone (Poot and Lambers 2008, Schenk 2008). Such
species would find green roofs inhospitable because the physical space provided on
a green roof does not include narrow cracks that can act as refugia for long roots,
and plants with long roots might exploit structural weaknesses that could compromise
roof integrity (Archibold and Wagner 2007). Likewise, persistence of plant
populations in a particular natural habitat may reflect very different adaptations by
different species. Monterusso et al. (2005) compared drought-tolerant succulents
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with tall-grass prairie forbs and grasses and found that the succulents were generally
superior in survival and growth in the green-roof environment. Although both
groups of species can be considered drought-tolerant, tall-grass prairie forbs and
grasses generally have deep roots, whereas succulents are shallow-rooted. Thus,
establishment and survival on a shallow-soil green roof favors a specific kind of
drought-tolerant species. The results of Rayner et al. (2016) may also reflect different
plant strategies: grasses and forbs from harsh rock-outcrop habitats may
require thin cracks in bedrock to survive drought, whereas succulents living in
the same habitat likely use stem and leaf morphological adaptations or distinct
physiology (e.g., CAM photosynthesis) to survive. Altering the design of artificial
ecosystems to render specific abiotic conditions more suitable for constituent species
may reduce the novelty experienced by plants and any associated fauna, giving
them a better chance of survival (Lundholm and Richardson 2010). Simple habitat
modifications, such as the addition of coarse woody debris, gravel piles, or soil
mounds, can greatly reduce maximum summer temperatures on a green roof and
slow water-loss rates (Walker and Lundholm, In press), possibly allowing species
with a greater range of traits related to drought or high-temperature tolerance to
persist on a given roof. However, for green roofs in hot, dry regions, some climatic
features, such as high substrate temperatures (Simmons 2015), may be difficult to
mitigate without irrigation or other measures that are economically or environmentally
costly.
Generalist species are likely to tolerate a broader range of abiotic conditions
(Simmons 2015) than specialists, and thus, might be expected to have a better
ability to tolerate novelty, so the coarse habitat-template approach may be more
likely to select appropriate generalist species. We should also acknowledge that
some specialist invertebrates can spontaneously colonize green roofs (Jones 2002,
Kadas 2006), but planning green roofs for a greater diversity of habitat specialists
will likely require much greater attention to the details of the particular resource
and nesting requirements of these species. Attempts to build habitat heterogeneity
into green roofs (Brenneisen 2006, Molineux et al. 2015, Olly et al. 2011) can be
viewed as attempts to increase the ecological similarity between natural habitats
and green roofs, such that a greater range of more specialized species can find appropriate
habitat on green roofs (Best et al. 2015, Dunnett 2015). Similarly, habitat
features, such as human-made “bee hotels”, may be designed to resemble natural
nesting substrates for a variety of solitary cavity-nesting bees (MacIvor and Packer
2015). At a coarse level, bee hotels are engineered to imitate the physical dimensions
of natural cavities used by various bee species, but if the materials used for
their construction do not functionally approximate the thermal or hydrological
performance of natural cavities, they may only support common urban generalists
or adaptable introduced species (MacIvor and Packer 2015). Just as bee hotels may
fail to adequately mimic natural nesting conditions for native bees, green roofs may
represent abiotic conditions that are too extreme, relative to their associated habitat
analogs, to support the same range of plant life -forms or diversity of invertebrates
and instead may favor weedy, common, or generalist taxa.
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Biotic Interactions
The habitat-template concept assumes that abiotic factors are the most important
in limiting the kinds of plants that will survive on extensive green roofs. This assumption
is generally consistent with broad ecological theory (e.g., Grime 1973),
which links low-productivity habitats with stress-tolerant plant species as a result
of strong selection pressures related to low resource availability and harsh conditions.
However, competition or facilitation between plants, as well as herbivory,
can also be important even in infertile environments (McGraw and Chapin 1989,
Reader 1998); hence, the habitat-template model may also ignore key features of
the biotic environment that place limits on plant success. Biotic interactions may
also be novel relative to what plants experience in their “home” environment, such
as when herbivores that normally consume substantial amounts of biomass are
absent from the system (Keane and Crawley 2002), or when pollinators or other
species that facilitate the success of plants are missing (Parker 1997), resulting in
positive or negative effects on plant growth and survival.
Differential presence of plant enemies may be a common form of ecological
novelty experienced by green-roof plants. Pests such as herbivorous insects and
fungal pathogens are likely affected by many aspects of the green-roof environment
including availability of alternate habitats at ground level, overall abundance
of hosts in the neighborhood, dispersal limitations, and availability of organisms
at other trophic levels that might control pest populations. It is easy to imagine
scenarios where certain pests are more abundant on rooftops than in natural environments.
On green roofs in Halifax, NS, Canada, aphid pests were more abundant
on native plants on the roof compared to their natural habitats, possibly due to lack
of salt spray usually present in coastal environments or absence of predators that
reduce aphid populations in natural areas (Grimshaw-Surette 2016). However, body
size and roof height may limit the dispersal of some insects to green roofs (MacIvor
2016) thus, reducing colonization rates of both pests and their parasites (or predators)
relative to ground level, as has been observed in a forb–leaf-miner parasitoid
complex (Quispe and Fenoglio 2015). Additionally, there is much evidence to suggest
that many invasive species benefit from enemy release. In their invasive range,
these species have escaped predators that would have controlled their population
growth in their native range (Keane and Crawley 2002). Given the possibility that
green roofs are relatively isolated from their analog habitats and may have other
novel features, both native and non-native plants might experience enemy release
on rooftops, possibly leading to greater productivity and performance of ecosystem
functions. This possibility highlights the potential of novelty in the biotic
conditions, but also abiotic conditions, to result in positive effects on plants. For
example, plants may perform better in the roof environment than in their typical
natural habitat due to enhanced resource availability or other differences (Lundholm
et al. 2015). Both negative and positive consequences of novelty should be
considered in attempting to understand plant performance on gre en roofs.
Vascular plants living in dry, low-nutrient habitats often coexist with other autotrophs
such as bryophytes and lichens, and these associated species might play
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a role in facilitation of vascular plants (Heim et al. 2014) by, e.g., reducing the
physical stress caused by harsh conditions (Bertness and Callaway 1994, Butler et
al. 2011), but other, more intimate interactions, such as root-microbe associations,
may be essential for the survival of some plant species. Commercial green-roof
substrates may initially be devoid of mycorrhizal fungi (John et al. 2014), which
is a condition that may represent another novel feature of green roofs based on
artificial substrates (Best et al. 2015). This absence may limit the success of some
native species (Schroll et al. 2011). DNA sequencing of samples from green-roof
substrates has revealed the presence of diverse fungal communities (McGuire et
al. 2013), but without further characterization of the ecological roles of constituent
species or the relative abundance of key functional groups (e.g., root mutualists,
litter decomposers, pathogens), the contribution of spontaneously colonizing soil
microbiota to green-roof ecology and performance remains uncertain. Soil microbes
play key roles in ecosystem functions, such as soil stabilization (McGuire
et al. 2015), that also indirectly influence plant survival. Thus, both abiotic and
biotic substrate characteristics are important in determining whether a particular
plant species can survive on a roof. Use of natural soils and plants from the same
site may promote better plant performance through provisioning of local soil symbionts,
pre-established seed banks, or favorable mineral compositions (Best et al.
2015, McGuire et al. 2015). Use of native soils may be one explanatory factor in the
long-term success of the sod roofs of northern Europe, which are created by moving
entire sods, including soil, soil organisms, and plants to roofs (Best et al. 2015,
Natvik 2012). For roof designs that cannot accommodate natural soils, substrate
inoculation with bacteria or fungi appears to be a promising option for promoting
soil symbionts (Molineux et al. 2014, Young et al. 2015). The increasing awareness
of the importance of plant–microbe interactions and their specificity suggests
that the habitat-template model is incomplete without consideration of the entire
biological community interacting with plants in a given habitat. Adding plants to a
green roof without companion species will lead to a biased selection of species that
do not require mycorrhizae, facilitation, or other positive interactions with other
species. Sedum spp. are usually non-mycorrhizal, which may help to explain why
these plants are often successful in typical temperate-zone extensive green roofs
with artificial substrates (John et al. 2014).
Although these ecological interactions are important, interacting populations
also impose evolutionary selection pressures on one another. Many studies have
revealed that coevolution among competing plant species and between plants and
other organisms is extremely local and may lead to genotypic variation caused
by competition among different plant species within a single pasture (Turkington
1979, 1989). Plants are adapted to microhabitat conditions, including their biotic
neighborhood; thus, maximizing plant success on green roofs might best be done
at a community level, as opposed to the selection of species that might not usually
grow together. This trend toward marked local adaptation further suggests that
plants from specific populations are adapted to each other; selection of the same
plant species within a region may yield different results when 2 species are grown
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together on a green roof, depending on whether their source populations interacted
in their evolutionary history. Recent work on species introductions shows
that local adaptations can evolve very rapidly (Oduor et al. 2016), and effects of
coevolution on resource use and ecosystem functions, such as biomass production,
can also develop quickly (Martin and Harding 1981). There is much recent interest
in designing biodiverse roofs to improve certain functions related to resource
use such as stormwater and nutrient retention (e.g., Johnson et al. 2016, Lundholm
2015a, Nagase and Dunnett 2012); plant population origin may represent a
neglected component of such studies. Given the potential for rapid evolution in
novel environments, the habitat-template approach need not rely on purely “natural
templates”, but can include completely artificial habitats. Some biodiverse roofs in
the UK adopted rubble fields (“brownfields”) as a template (Baumann and Kasten
2010, Dunnett 2015, Kadas 2006) due to their ability to support urban biodiversity
(Gibson 1998, Lorimer 2008), while others have mixed natives and introduced species
or contain only introduced species. Finally, with the growing recognition that
interspecific coevolutionary processes are important to individuals, populations,
communities, and ecosystems, and given the rapidity at which evolutionary change
can occur, it could be argued that old green roofs (e.g., Köhler 2006, Köhler and
Poll 2010, Lundholm 2015b, Rowe 2015, Thuring and Dunnett 2014) themselves
may be the ultimate habitat template for new green roofs in a particular region
because they might now contain assemblages of plants, microbes, and other interacting
species that have coevolved to survive and prosper on gr een roofs.
Conclusions
The goal of this paper was to evaluate the habitat-template concept for green
roofs from the perspective of modern ecology. Plant selection is relevant to many
aspects of green-roof ecology. There are numerous cases where the habitattemplate
approach has been used to successfully select plants for extensive green
roofs in different environments; there are also cases where it has proved inadequate.
Within a framework of ecological novelty, the habitat template approach
seeks to minimize the novelty of the abiotic infrastructure to promote long-term
vegetation persistence. The habitat-template approach is probably best used as a
coarse filter for plant selection on green roofs; several factors imply that a more
nuanced approach is required in some settings. The abiotic and biotic environments
on a green roof may be too novel for a given plant population to succeed.
Trait-based protocols have been productively used to give a finer resolution
for selection of plant species, and it makes sense to combine both approaches.
Another method is to extend the habitat-template approach even further, by not
only identifying appropriate plant species based on local habitats, but by including
entire communities and providing replication of the abiotic components of
these habitats. While this protocol may not be feasible for green-roof designers
due to logistic constraints, it may be important to employ it if habitat creation for
specialist taxa is a priority for a particular roof project. The ecological novelty associated
with green-roof systems may also lead to positive effects on ecosystem
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functioning. Considering these features may lead to ecological insights and could
help engineer better-performing green roofs.
Acknowledgments
We thank 2 anonymous reviewers for helpful comments that improved the manuscript.
We are grateful to Amy Heim, Scott MacIvor, Ayako Nagase, John Rayner,and Nick Williams
for many useful discussions of these concepts.
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