Articles

Buildings change the flow of energy and matterthrough urban ecosystems often causing environmen-

tal problems These problems can be partially mitigated by altering the buildingsrsquo surficial properties Roofs can representup to 32 of the horizontal surface of built-up areas (Frazer2005) and are important determinants of energy flux and ofbuildingsrsquo water relations The addition of vegetation andsoil to roof surfaces can lessen several negative effects ofbuildings on local ecosystems and can reduce buildingsrsquo en-ergy consumption Living or green roofs have been shownto increase sound insulation (Dunnett and Kingsbury 2004)fire resistance (Koumlhler 2003) and the longevity of the roofmembrane (Porsche and Koumlhler 2003) They can reduce theenergy required for the maintenance of interior climates(Del Barrio 1998) because vegetation and growing plantmedia intercept and dissipate solar radiation Green roofs canalso mitigate storm-water runoff from building surfaces by

collecting and retaining precipitation thereby reducing thevolume of flow into storm-water infrastructure and urban waterways Other potential benefits include green-spaceamenity habitat for wildlife air-quality improvement and re-duction of the urban heat-island effect (Getter and Rowe2006) Architects have applied green-roof technology world-wide and policymakers and the public are becoming moreaware of green-roof benefitsAlthough green roofs are initiallymore expensive to construct than conventional roofs they canbe more economical over the life span of the roof because ofthe energy saved and the longevity of roof membranes(Porsche and Koumlhler 2003)

Although green roofs represent a distinct type of urbanhabitat they have been treated largely as an engineering or horticultural challenge rather than as ecological systemsThe environmental benefits provided by green roofs derivefrom their functioning as ecosystems The first goal of this

Erica Oberndorfer (e-mail ecoberndorfergmailcom) and Jeremy Lundholm work in the Department of Biology at Saint Maryrsquos University Halifax Nova Scotia

Canada Brad Bass works in the University of Torontorsquos Centre for Environment in Ontario Canada Reid R Coffman works in the Division of Landscape Architecture

at the University of Oklahoma in Norman Hitesh Doshi works in the Department of Architectural Science at Ryerson University in Toronto Nigel Dunnett is with

the Department of Landscape University of Sheffield Western Bank Sheffield United Kingdom Stuart Gaffin works at the Columbia University Center for Climate

Systems Research in New York Manfred Koumlhler works at Hochschule Neubrandenburg in Germany Karen K Y Liu is with Wolfgang Behrens Systementwicklung

Vancouver British Columbia Canada Bradley Rowe is with the Michigan State University Department of Horticulture in East Lansing copy 2007 American Institute

of Biological Sciences

Green Roofs as UrbanEcosystems EcologicalStructures Functions andServices

ERICA OBERNDORFER JEREMY LUNDHOLM BRAD BASS REID R COFFMAN HITESH DOSHI NIGEL DUNNETTSTUART GAFFIN MANFRED KOumlHLER KAREN K Y LIU AND BRADLEY ROWE

Green roofs (roofs with a vegetated surface and substrate) provide ecosystem services in urban areas including improved storm-water managementbetter regulation of building temperatures reduced urban heat-island effects and increased urban wildlife habitat This article reviews the evidencefor these benefits and examines the biotic and abiotic components that contribute to overall ecosystem services We emphasize the potential forimproving green-roof function by understanding the interactions between its ecosystem elements especially the relationships among growing mediasoil biota and vegetation and the interactions between community structure and ecosystem functioning Further research into green-roof technologyshould assess the efficacy of green roofs compared to other technologies with similar ends and ultimately focus on estimates of aggregate benefits atlandscape scales and on more holistic cost-benefit analyses

Keywords urban ecology biomimicry built environments habitat creation energy conservation

wwwbiosciencemagorg November 2007 Vol 57 No 10 bull BioScience 823

article is to describe the history and components of living-roofecosystems the second is to review the ways in which the struc-ture of a green roofmdashincluding vegetation growing mediumand roof membranemdashdetermines its functions

History of green roofsRoof gardens the precursors of contemporary green roofshave ancient roots The earliest documented roof gardens werethe hanging gardens of Semiramis in what is now Syria con-sidered one of the seven wonders of the ancient world Todaysimilarly elaborate roof-garden projects are designed forhigh-profile international hotels business centers and privatehomes These green roofs known for their deep substrates andvariety of plantings as ldquointensiverdquo green roofs have the

appearance of conventional ground-level gardens and theycan augment living and recreation space in densely populated urban areas (figure 1 table 1) Intensive green roofs typicallyrequire substantial investments in plant care Furthermorethey emphasize the active use of space and carry higher aesthetic expectations than ldquoextensiverdquo green roofs whichgenerally have shallower soil and low-growing ground cover

Extensive green roofs are a modern modification of theroof-garden concept They typically have shallower sub-strates require less maintenance and are more strictly func-tional in purpose than intensive living roofs or roof gardens(figure 1 table 1 Dunnett and Kingsbury 2004) In theirsimplest design extensive green roofs consist of an insulationlayer a waterproofing membrane a layer of growing medium

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824 BioScience bull November 2007 Vol 57 No 10 wwwbiosciencemagorg

Figure 1 Examples of (a b) intensive green roofs (with deeper substrate more elaborate vegetationand higher maintenance requirements) and (c d) extensive green roofs (with shallow substrate hardydrought-tolerant vegetation and low maintenance requirements) Photographs Brad Rowe

ba

c

d

and a vegetation layer This basic green-roof design has beenimplemented and studied in diverse regions and climatesworldwide

The modern green roof originated at the turn of the 20thcentury in Germany where vegetation was installed on roofsto mitigate the damaging physical effects of solar radiation onthe roof structure Early green roofs were also employed as fire-retardant structures (Koumlhler 2003) There are now several com-peting types of extensive green-roof systems which providesimilar functions but are composed of different materialsand require different implementation protocols (figure 1)

In the 1970s growing environmental concern especially inurban areas created opportunities to introduce progressiveenvironmental thought policy and technology in GermanyGreen-roof technology was quickly embraced because of itsbroad-ranging environmental benefits and interdisciplinaryresearch led to technical guidelines the first volume of whichwas published in 1982 by the Landscape Research Devel-opment and Construction Society (FLL 2002) Many Germancities have since introduced incentive programs to promotegreen-roof technology and improve environmental stan-dards Building law now requires the construction of greenroofs in many urban centers (Koumlhler and Keeley 2005) Suchlegal underpinnings of green-roof construction have had a major effect on the widespread implementation and successof green-roof technology throughout Germany Green-roofcoverage in Germany alone now increases by approximately135 million square meters (m2) per year Haemmerle (2002)calculates that approximately 14 of all new flat roofs inGermany will be green roofs the total area covered by greenroofs is unknown The market for sloped green roofs is alsodeveloping rapidly and accessible green roofs have becomea driving force in neighborhood revitalization

Green-roof vegetationRooftop conditions are challenging for plant survival andgrowth Moisture stress and severe drought extreme (usually

elevated) temperatures high light intensities and high windspeeds increase the risk of desiccation and physical damageto vegetation and substrate (Dunnett and Kingsbury 2004)Plants suitable for extensive green roofs share adaptations thatenable them to survive in harsh conditions These plantshave stress-tolerant characteristics (sensu Grime 2001)including low mat-forming or compact growth evergreen foliage or tough twiggy growth and other drought-toleranceor avoidance strategies such as succulent leaves water stor-age capacity or CAM (crassulacean acid metabolism) phys-iology (figure 2 Lee and Kim 1994) However frequentdrought-related disturbance to green-roof vegetation alsofavors some ruderal species (sensu Grime 2001) that canrapidly occupy gaps Green-roof communities are dynamicand with time vegetation is likely to change from the origi-nal composition (Koumlhler 2006)

Since the 1980s researchers have tested many herbaceousand woody taxa in different rooftop conditions (Heinze 1985Boivin et al 2001 Koumlhler 2003 Durhman et al 2004 Mon-terusso et al 2005) Heinze (1985) compared combinationsof various Sedum species grasses and herbaceous perenni-als planted at two substrate depths in simulated roof plat-forms Sedum species outperformed the other taxa except inconsistently moist substrate deeper than 10 centimeters (cm)In these conditions a taller grass and herbaceous canopylayer created shaded conditions that proved unfavorable to the Sedum species Other studies support the suitability oflow-growing Sedum species for use in green roofs because oftheir superior survival in substrate layers as thin as 2 to 3 cm(VanWoert et al 2005a) Physical rooftop conditions suitabilityfor plant growth and the cost of various substrates have alsobeen examined (Dunnett and Nolan 2004 Rowe et al 2006)

The composition and character of green-roof vegetationdepend on many factors To a large extent substrate depth dictates vegetation diversity and the range of possible speciesShallow substrate depths between 2 and 5 cm have morerapid rates of desiccation and are more subject to fluctuations

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wwwbiosciencemagorg November 2007 Vol 57 No 10 bull BioScience 825

Table 1 A comparison of extensive and intensive green roofs

Characteristic Extensive roof Intensive roof

Purpose Functional storm-water management thermal Functional and aesthetic increased living spaceinsulation fireproofing

Structural requirements Typically within standard roof weight-bearing Planning required in design phase or structural parameters additional 70 to 170 kg per m2 improvements necessary additional 290 to 970 (Dunnett and Kingsbury 2004) kg per m2

Substrate type Lightweight high porosity low organic matter Lightweight to heavy high porosity low organic matter

Average substrate depth 2 to 20 cm 20 or more cm

Plant communities Low-growing communities of plants and mosses No restrictions other than those imposed by selected for stress-tolerance qualities (eg substrate depth climate building height and Sedum spp Sempervivum spp) exposure and irrigation facilities

Irrigation Most require little or no irrigation Often require irrigation

Maintenance Little or no maintenance required some weeding Same maintenance requirements as similar or mowing as necessary garden at ground level

Cost (above waterproofing membrane) $10 to $30 per ft2 ($100 to $300 per m2) $20 or more per ft2 ($200 per m2)

Accessibility Generally functional rather than accessible will Typically accessible bylaw considerationsneed basic accessibility for maintenance

in temperature but can support simple Sedumndashmoss com-munities Substrate depths of 7 to 15 cm can support morediverse mixtures of grasses geophytes alpines and drought-tolerant herbaceous perennials but are also more hospitablefor undesirable weeds

Green-roof substrates tend to be highly mineral based withsmall amounts of organic matter (approximately 10 byweight) The mineral component may come from a varietyof sources and can be of varying weight depending on theload capacity of the roof Light expanded clay granules andcrushed brick are two common materials There is increas-ing interest in the use of locally derived lightweight granu-lar waste materials as sustainable sources for green-roofsubstrates

Climatic conditions especially rainfall and extreme tem-peratures may restrict the use of certain species or dictate theuse of irrigation Native plants are generally considered idealchoices for landscapes because of their adaptations to local climates and the native stress-tolerant floras (particularlydry grassland coastal and alpine floras) of many regions offer opportunities for trial and experiment Furthermorepolicies for biodiversity and nature conservation may favorthe establishment of locally distinctive and representativeplant communities Unfortunately many native plants appearto be unsuitable for conventional extensive green-roof systemsbecause of the roofsrsquoharsh environmental conditions and typ-ically shallow substrate depths In a study at Michigan StateUniversity only 4 of the original 18 native prairie perennial

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826 BioScience bull November 2007 Vol 57 No 10 wwwbiosciencemagorg

Figure 2 Typical extensive green-roof vegetation (a) Sedum species and mosses (b) Rhodiola rosea a succulent alpine androck outcrop species of northeastern North America and northern Europe in one of its native habitats (limestone barrens inNewfoundland) (c) Sedum species on a typical extensive green roof Photographs (a) Erica Oberndorfer (b) Jeremy Lund-holm (c) Karen Liu

ba

c

species growing in 10 cm of sub-strate persisted after three years Incomparison all 9 nonnative speciesof Sedum used in the study thrived(Monterusso et al 2005)

In theory almost any plant taxoncould be used for green-roof appli-cations assuming it is suited to theclimatic region grown in appro-priate substrate at an adequatedepth and given adequate irriga-tion Wind stress resulting frombuilding height and form may affectthe selection of plants and visibil-ity and accessibility are other selec-tion criteria Although Sedumremains the most commonly usedgenus for green roofs the scope forgreen-roof vegetation is wide andmany possibilities have yet to be realized

Ecosystem services providedby green roofsThe green-roof benefits investigatedto date fall into three main cate-gories storm-water managementenergy conservation and urbanhabitat provision These ecosystemservices derive from three maincomponents of the living roof sys-tem vegetation substrate (grow-ing medium) and membranes (figure 3) Plants shade the roofsurface and transpire water cooling and transporting waterback into the atmosphere The growing medium is essentialfor plant growth but also contributes to the retention ofstorm water The membranes are responsible for water-proofing the roof and preventing roof penetration by roots

Storm-water management Urban areas are dominated byhard nonporous surfaces that contribute to heavy runoffwhich can overburden existing storm-water managementfacilities and cause combined sewage overflow into lakes andrivers In addition to exacerbating flooding erosion and sed-imentation urban runoff is also high in pollutants such as pes-ticides and petroleum residues which harm wildlife habitatsand contaminate drinking supplies (Moran et al 2005)

Conventional storm-water management techniques in-clude storage reservoirs and ponds constructed wetlandsand sand filters however these surface-area intensive tech-nologies may be difficult to implement in dense urban centers (Mentens et al 2005) Green roofs are ideal for urbanstorm-water management because they make use of existingroof space and prevent runoff before it leaves the lot Greenroofs store water during rainfall events delaying runoffuntil after peak rainfall and returning precipitation to the

atmosphere through evapotranspiration (figure 4 Mentenset al 2005 Moran et al 2005) The depth of substrate the slopeof the roof the type of plant community and rainfall patternsaffect the rate of runoff (Dunnett and Kingsbury 2004Mentens et al 2005 VanWoert et al 2005b) Studies in Port-

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wwwbiosciencemagorg November 2007 Vol 57 No 10 bull BioScience 827

Figure 3 Types of conventional extensive green-roofing technology (a) Complete sys-tems each component including the roof membrane is installed as an integral part ofthe roof (b) Modular systems vegetation trays cultivated ex situ are installed above theexisting roofing system (c) Precultivated vegetation blankets growing medium plantsdrainage mats and root barriers are rolled onto the existing roofing Inverted systems(not shown) which are increasingly popular feature waterproofing membranes belowthe insulation layer Many proprietary systems with variations on these strategies areavailable throughout North America and Europe Graphics Jeremy Lundholm

Figure 4 Storm-water runoff retention in a green-rooftest plot in Ottawa Ontario Canada in 2002 Values are sums of total runoff retained The green roof had 15centimeters of growing medium and was planted withlawn grasses (Liu and Baskaran 2003) it was comparedwith an adjacent conventional roof of the same size

land Oregon and East Lansing Michigan showed that rainfall retention from specific green roofs was 66 to 69for roofs with more than 10 cm of substrate (Moran et al2005) Rainfall retention varied from 25 to 100 for shal-lower substrates in other studies (Beattie and Berghage 2004)Green roofs can reduce annual total building runoff by asmuch as 60 to 79 (Koumlhler et al 2002) and estimatesbased on 10 green-roof coverage suggest that they can reduce overall regional runoff by about 27 (Mentens et al2005) In general total runoff is greater with shallower substrate and steeper slopes (Mentens et al 2005 Villarrealand Bengtsson 2005)Although green roofs can reduce runoffthey do not solve the problem of reduced recharge of ground-water in urban areas

Improving roof membrane longevity Waterproofing mem-branes on conventional dark roofs deteriorate rapidly in ultraviolet (UV) light which causes the membranes to becomebrittle Such membranes are consequently more easily dam-aged by the expansion and contraction caused by widelyfluctuating roof temperatures By physically protecting againstUV light and reducing temperature fluctuations green roofsextend the life span of the roof rsquos waterproofing membraneand improve building energy conservation Temperature sta-bilization of the waterproofing membranes by green-roof cov-erage may extend their useful life by more than 20 years(USEPA 2000) some green roofs in Berlin have lasted 90 yearswithout needing major repairs (Porsche and Koumlhler 2003)In Ottawa Canada Liu (2004) found that an unvegetated reference roof reached temperatures higher than 70 degrees Celsius (degC) in summer while the surface tem-perature of the green roof only reached 30degC The mem-brane on the reference roof reached 30degC on 342 of the660 days of the study whereas the membrane underneaththe green roof only reached that temperature on 18 days(figure 5)

Summer cooling During warm weather green roofs re-duce the amount of heat transferred through the roofthereby lowering the energy demands of the buildingrsquoscooling system (Del Barrio 1998 Theodosiou 2003)Wong and colleagues (2003) found that the heat trans-fer through a green roof in Singapore over a typical daywas less than 10 of that of a reference roof Research inJapan (Onmura et al 2001) found reductions in heat fluxon the order of 50 per year and work in Ottawa (Liu2004) found a 95 reduction in annual heat gain Astudy in Madrid showed that a green roof reduced thecooling load on an eight-story residential building by 6during the summer (Saiz et al 2006) In a peak demandsimulation the cooling load was reduced by 10 forthe entire building and by 25 9 2 and 1 for thefour floors immediately below the green roof For a typ-ical residential house in Toronto the cooling load for themonth of July was reduced by 25 for the building andby 60 for the floor below the green roof (Saiz et al

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828 BioScience bull November 2007 Vol 57 No 10 wwwbiosciencemagorg

Figure 5 Heat flux across roof membranes for different roofing systems in Toronto Ontario Canada on (a) a summer day (26 June2003) and (b) a typical winter day with light snow coverage (23 Janu-ary 2003) indicating that the green roof reduces the temperature fluc-tuations within the roofing system Positive heat flux values indicatethat net heat flux is from the outside into the building negative valuesindicate that the net flux is from the inside to the outside The greenroof had 10 centimeters (cm) of light-colored growing medium(c) Comparison of the average daily heat flow through the green roofsand reference (conventional) roof over two years Green roofs were installed in late July 2002 Green roof A has 10 cm of light-coloredgrowing medium green roof B has 75 cm of dark-colored growingmedium Both were installed in Toronto Abbreviations AI after installation BI before installation kWh kilowatt-hour W wattGraphs are redrawn from Liu and Minor (2005)

b

a

c

2006) Green roofs will have the greatest effect on energyconsumption for buildings with relatively high roof-to-wallarea ratios

In the summer green roofs reduce heat flux through theroof by promoting evapotranspiration physically shadingthe roof and increasing the insulation and thermal massGaffin and colleagues (2005 2006) applied energy-balancemodels to determine how effectively green roofs evaporate andtranspire water vapor compared with other vegetated surfaces(table 2) During the summer of 2002 experimental greenroofs at Pennsylvania State University performed equiva-lently to irrigated or wet habitats indicating that evapotran-spiration may be the most important contributor towardreducing summer building energy consumption under greenroofs Of course green roofs are not the only technologythat can provide summer cooling enhanced insulation maybe able to provide equivalent energy savings and can be com-bined with green roofs to further advantage Evaporativeroofs are another example of such a technology water issprayed on the roof surface to induce evaporative cooling(Clements and Sherif 1998) Rigorous comparisons of mul-tiple roofing systems are necessary to evaluate prospects foroptimal building energy savings

Urban heat island In urban environments vegetation haslargely been replaced by dark and impervious surfaces (egasphalt roads and roofs) These conditions contribute to anurban heat island (Oke 1987) wherein urban regions aresignificantly warmer than surrounding suburban and ruralareas especially at night This effect can be reduced by in-creasing albedo (the reflection of incoming radiation awayfrom a surface) or by increasing vegetation cover with suffi-cient soil moisture for evapotranspiration A regional simu-lation model using 50 green-roof coverage distributedevenly throughout Toronto showed temperature reductionsas great as 2degC in some areas (Bass et al 2002)

Urban habitat values Green-roof habitats show promise for contributing to localhabitat conservation Studies have documented invertebrateand avian communities on a variety of living-roof types in sev-eral countries (Coffman and Davis 2005 Brenneisen 2006Kadas 2006) Green roofs are commonly inhabited by vari-ous insects including beetles ants bugs flies bees spidersand leafhoppers (Coffman and Davis 2005) Rare and un-common species of beetles and spiders have also been recordedon green roofs (Brenneisen 2006 Grant 2006) Species rich-ness in spider and beetle populations on green roofs is pos-itively correlated with plant species richness and topographicvariability (Gedge and Kadas 2004) Green roofs have also been used by nesting birds and native avian communities(Baumann 2006) Rare plants and lichens often establishspontaneously on older roofs as well (Brenneisen 2006Koumlhler 2006) These findings have mobilized local and nationalconservation organizations to promote green-roof habitatparticularly in Switzerland and the United Kingdom Further-

more these results have encouraged discussion of green-roof design strategies to maximize biodiversity (Brenneisen2006)

Living roofs also provide aesthetic and psychological ben-efits for people in urban areas Even when green roofs are onlyaccessible as visual relief the benefits may include relaxationand restoration (Hartig et al 1991) which can improve human health Other uses for green roofs include urban agriculture food production can provide economic and educational benefits to urban dwellers Living roofs also re-duce sound pollution by absorbing sound waves outsidebuildings and preventing inward transmission (Dunnett andKingsbury 2004)

Community and landscape properties How important is the living portion of green roofs to theirfunctioning Although plants are an important componentof green roofs recent work shows that the growing mediumalone can greatly reduce runoff from a green roof (VanWoertet al 2005b) The medium alone reduced runoff by approx-imately 50 in comparison with a conventional gravel roofadding vegetation to the medium resulted in negligible fur-ther reductions Other research shows that the depth of thegrowing medium is the main determinant of runoff retention(Mentens et al 2005) However water availability and seasonaffect the ability of the growing medium to retain waterWhen water is readily available evapotranspiration rates aremuch greater on vegetated roofs than on roofs with growingmedium alone especially in the summer (Farzaneh 2005)Complicating our understanding of green-roof functions is

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wwwbiosciencemagorg November 2007 Vol 57 No 10 bull BioScience 829

Table 2 Typical Bowen ratios reported for a range ofnatural and agricultural vegetated land surfaces

Land system Bowen ratio

Desert 1000Urban areas 500Mopane woodland in South Africa (dry season) 400ndash500Irrigated field (winter) 290ndash360Pine forest (July) 200Forest floor (July) 120ndash450Mopane woodland in South Africa (wet season) 100Pine forest in Siberia (July) 100Douglas fir stand 066Wheat field (summer) 060Forest in Indiana (annual average) 059Above forest canopy (summer) 050ndash300Soybean field (summer) 030Irrigated field (April) 028ndash030Irrigated field (August) 020ndash025Rainforest in Amazonia (wet season) 017Huaihe River Basin (ruderal) 014Tropical ocean 010Huaihe River Basin (paddy) 006

Note The Bowen ratio a ratio of sensible heat flux to latent heatflux allows for comparisons between different processes of surfacecooling Sensible heat flux occurs with cooling by convection and isdominant in arid climates latent heat flux occurs through evapo-transpiration and is greatest in vegetated or aquatic environmentsThe roofs consisted of Sedum spurium growing in 10 centimeters of growth medium

the shading of the roof surface by vegetation which may reduce evaporation from the soil surface

With respect to thermal benefits simulation models showthat taller vegetation leads to greater thermal benefits in trop-ical environments but these models do not separate the ad-ditive effects of soil and vegetation (Wong et al 2003)Experiments on green roofs suggest that most of the summercooling benefits from green roofs are attributable to evapo-transpiration (Gaffin et al 2005 2006) but the relative con-tributions of vegetation and substrates cannot be separatedout by these analyses A study using small-scale constructedmodels showed that reductions to heat flux through the roofat peak daily temperatures were greater in vegetated soil roofsthan in soil roofs alone with 70 of the maximum reductionattributable to the soil and the remainder to the vegetation(Takakura et al 2000) Therefore transpiration from livingplants is most likely responsible for a substantial proportionof the cooling benefits of green roofs and that proportioncould be boosted further by selecting species with high leafconductivity or large surface areas

Two properties of plant communities can influence green-roof performance the ability to resist and recover from en-vironmental fluctuations or disturbances and the rate atwhich resources can be consumed Using vegetation types thatrecover more rapidly from disturbance should increase the duration of functions made possible by living plants such astranspiration Greater resource use in green roofs should reduce runoff of water and nutrients High species diversityis expected to encourage more complete resource use (Tilmanet al 1997) and greater biomass constancy within the grow-ing season (Cottingham et al 2001)

Very simple communities of low species diversity may bevulnerable to environmental fluctuations but the notionthat more species are inevitably better is not always tenableMost of the functions of vegetation are dictated by the per-formance of dominant plant species and these are likely tobe relatively few in number (Grime 1998) Although the pro-motion of native species and communities may be importantfor conservation experimental evidence indicates that thefunctional structural and phenological properties of vege-tation are more important than ldquonativenessrdquo in promoting invertebrate biodiversity (Smith et al 2006) and other com-munity attributes in level-ground urban gardens In an ex-periment involving vegetation similar to that of extensivegreen roofs there was no relationship between species diversityand water retention (Dunnett et al 2005) but a diversity of functional types (eg rosette formers and grasses as opposed to monocultures of either) was crucial to maximiz-ing performance Work by Kolb and Schwarz (1993) indicatesthat vegetation including diverse functional types has a greaterpositive influence on the thermal properties of green roofs thanmonocultural types of vegetation

The limited size of green roofs as habitats has implicationsfor the biodiversity and landscape properties of areas inwhich green roofs are installed (Koumlhler 2006) Little is knownabout the relationships between roof area which may range

from approximately 1 to 40000 m2 for an individual roofand the habitat occupation rates of different taxa At least twoquestions still need to be addressed What are the relationshipsbetween other green-roof ecosystem services and roof areaand how do regional benefits relate to the landscape config-uration of green-roof patches in urban areas In summarygreen-roof benefits are partially derived from the living components of the system but more research is needed in determining the relationships between biotic communityparameters and ecosystem functioning with a view toward selecting biotic components that can improve green-roofperformance

Future research directionsMuch green-roof plant selection depends on lists from German research Further research is needed to identify suit-able plant species for living roofs in many other climatic re-gions Furthermore most green-roof plant combinations areselected for full sun exposure using species that originate inpermanently open (nonforested) habitats such as rock out-crops (eg most Sedum) cliffs dunes and heathland (Lund-holm 2006) New selections are being investigated to identifyplants suitable for shaded roof conditions Researchers are also investigating plants that provide other services such asremoving contaminants from storm water and providing resources (eg pollen) for native insects and other animals

Water quality The role of green roofs in storm-water reten-tion is well understood but some research demonstrates thatgreen-roof runoff includes increased levels of nitrogen andphosphorus due to leaching from the substrate (Dunnettand Kingsbury 2004 Moran et al 2005) Organic matternutrients and contaminants in the growing medium or roofmembranes can cause discharged water to be a new source ofsurface-water pollution Research on more inert substratesand on integrated gray-water reuse systems may lead to mit-igation of these effects Reducing the fertilization of green-roofvegetation should also improve runoff water quality but mayreduce plant growth or survival Selecting plants that optimizethe uptake of nutrients and contaminants may help to reducepollutants in runoff while promoting plant survival

Air quality Although extensive green roofs being low in bio-mass have little potential to offset carbon emissions from citiesintensive roof gardens that support woody vegetation couldmake significant contributions as an urban carbon sinkUrban vegetation is known to trap airborne particulates andto take up other contaminants such as nitrogen oxides Thepotential benefits of roof greening for air quality have yet tobe documented except for indirect assessments of the impactof energy savings on emissions (Bass and Baskaran 2003)

Ecosystem services and community properties The role of bio-diversity in living-roof performance has been little investigatedFundamental questions about the role of increased species diversity native versus exotic diversity and taxonomic versus

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830 BioScience bull November 2007 Vol 57 No 10 wwwbiosciencemagorg

functional diversity can be addressed using the living-roofecosystem With green roofs the system of interest is com-pletely artificial and thus experiments can be done on the sys-tem itself rather than on the simplified facsimiles that aretypically used in diversity studies While covariables such asbuilding height aspect and shading may affect vegetation andoverall green-roof performance the potential for replicatedexperiments with proper quantification or control of thesevariables across multiple rooftops is great (Felson and Pick-ett 2005) Although vascular plants are the most studiedcomponents of green roofs other groups may also contributeto green-roof benefits Bryophytes and algae warrant inves-tigation both as integral components of green-roof systemsand as potential facilitators or competitors for vascular plantsBelowground components of the living-roof communitymay also influence ecosystem services directly or throughtheir contribution to plant performance (eg mycorrhizae)Both vegetation and belowground biota probably play a rolein the stability and retention of the growing medium but noresearch has yet addressed the effects of green-roof commu-nity composition on substrate properties

An advantage of biodiversity or ecosystem function stud-ies on green roofs is that the ecological functions translatemore or less directly into economic savings for people Greenroofs are likely to become an important component of urbansustainability in the future provided that favorable public pol-icy measures encourage and enable their construction Al-though legislation and government incentives promoting(and requiring) green-roof technologies are common in partsof Europe such measures are lacking in other regions in-cluding North America Further research efforts public ed-ucation and communication of green-roof benefits topolicymakers will help remove institutional barriers to thistechnology

Cost-benefit models Robust cost-benefit models are neces-sary for evaluating whether green roofs are in fact the mosteffective technology for mitigating common urban environ-mental problems Alternate approaches to reducing buildingheat flux involve improving insulation and using living walls(vertical gardens) to enhance shading (Bass and Baskaran2003) Inverted roofing systems in which insulation layersshield the waterproofing membranes from UV radiation canincrease roof longevity in the absence of vegetation The useof high-albedo high-reflectance roofing materials can alsocontribute to reducing urban heat-island effects (Prado andFerreira 2005) Similarly storm-water problems can be man-aged using rainwater reclamation cisterns and gray-waterrecycling (Dixon et al 1999)

Although comparative testing of these technologies is crucial to the development and refinement of green tech-nologies and the consequent amelioration of urban envi-ronments it is equally important to consider the aggregatebenefits of these technologies High-reflectance roofmembranes for example may more effectively reduce urbanheat-island effects than green roofs but they do not offer a

solution to storm-water management issues or create urbanwildlife habitat Another little-investigated avenue is the com-bination of green roofs with other green building technolo-gies including solar thermal and photovoltaic applicationsOne of the key goals of industrial ecology is to move towardintegrated ecological-industrial systems that eliminate wasteproducts and maximize energy capture over the entire life cycle of the materials (Korhonen 2005) Green roofs can assist in meeting this goal by providing a sink for gray wateramong other integrated benefits but research has thus far em-phasized the benefits of individual green building technolo-gies rather than the synergistic effects of integrating them

Finally the cost-benefit analyses of any green buildingtechnology could be improved by more carefully consideringbenefits that are difficult to quantify Even simple visual con-tact with vegetation has been shown to improve health reducepostoperative recovery times increase employee satisfactionand reduce stress (Cooper-Marcus and Barnes 1999) Themarketing value of green roofs is also seldom considered butthe value of a ldquogreen buildingrdquo brand is potentially tremen-dous to private enterprise universities and cities in attract-ing clients students faculty and tourists Other lessquantifiable benefits of green roofs including increased community space and improved ldquolivabilityrdquo of cities are factors that would extend the evaluation of new technologiesbeyond bottom-line efficiency and cost-effectiveness

Green roofs as ecosystems Green roofs represent a class oftechnology that can be considered bioengineering or bio-mimicry the ecosystem created by a green roof rsquos interactingcomponents mimics several key properties of ground-levelvegetation that are absent from a conventional roof Greenroofs like other constructed ecosystems (eg sewage-treatment wetlands bioswales for storm-water managementor living walls) mimic natural ecosystems to provide ecosys-tem services In particular extensive green roofs represent thepotential for the establishment of shallow soil habitats andtheir accompanying biodiversity in temperate ecosystemssome of the highest rates of plant species diversity and en-demism occur in relatively unproductive habitats such asrock pavements scree slopes and cliff faces (Larson et al 2000)Plant selection is not limited to any particular habitat how-ever and the potential diversity of green-roof habitatsmdashas wellas their potential for supplying goods and services such asherbs and vegetables or other cropsmdashawaits further research

The beneficial functions of green roofs and their eco-nomic and environmental costs require more investigationTheir functioning as biological systems and the interactionof the organisms that inhabit them represents a frontier inapplied ecology and an opportunity to put interdisciplinaryresearch into practice at the interface between constructedecosystems and the greater urban environment

AcknowledgmentsWe would like to thank Green Roofs for Healthy Cities for supporting this initiative and also two anonymous review-

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wwwbiosciencemagorg November 2007 Vol 57 No 10 bull BioScience 831

ers for comments that improved the manuscript Financialsupport was provided by the Natural Science and Engineer-ing Research Council of Canada

References citedBass B Baskaran B 2003 Evaluating Rooftop and Vertical Gardens as an Adap-

tation Strategy for Urban Areas Ottawa (Canada) National ResearchCouncil Canada Institute for Research in Construction Report noNRCC-46737

Bass B Krayenhoff ES Martilli A Stull RBAuld H 2003 The impact of greenroofs on Torontorsquos urban heat island Pages 292ndash304 in Proceedings ofthe First North American Green Roof Conference Greening Rooftopsfor Sustainable Communities 20ndash30 May Chicago Toronto (Canada)Cardinal Group

Baumann N 2006 Ground-nesting birds on green roofs in SwitzerlandPreliminary observations Urban Habitats 4 37ndash50

Beattie D Berghage R 2004 Green roof media characteristics The basicsPaper presented at the Second Annual Greening Rooftops for Sustain-able Communities Conference Awards and Trade Show 2ndash4 June 2004Portland Oregon

Boivin M-A Lamry M-P Gosselin A Dansereau B 2001 Effect of artificialsubstrate depth on freezing injury of six herbaceous perennials grownin a green roof system HortTechnology 11 409ndash412

Brenneisen S 2006 Space for urban wildlife Designing green roofs as habitats in Switzerland Urban Habitats 4 27ndash36 (14 August wwwurbanhabitatsorgv04n01indexhtml)

Clements JA Sherif SA 1998 Thermal analysis of roof spray coolingInternational Journal of Energy Research 22 1337ndash1350

Coffman RR Davis G 2005 Insect and avian fauna presence on the Ford assembly plant ecoroof Paper presented at the Third Annual GreeningRooftops for Sustainable Communities Conference Awards and TradeShow 4ndash6 May 2005 Washington DC

Cooper-Marcus C Barnes M 1999 Healing Gardens Therapeutic Benefitsand Design Recommendations New York Wiley

Cottingham KL Brown BL Lennon JT 2001 Biodiversity may regulate thetemporal variability of ecological systems Ecology Letters 4 72ndash85

Del Barrio EP 1998 Analysis of the green roofs cooling potential in buildings Energy and Buildings 27 179ndash193

Dixon A Butler D Fewkes A 1999 Water saving potential of domestic water reuse systems using greywater and rainwater in combinationWater Science and Technology 39 (5) 25ndash32

Dunnett NP Kingsbury N 2004 Planting Green Roofs and Living WallsPortland (OR) Timber Press

Dunnett NP Nolan A 2004 The effect of substrate depth and supplemen-tary watering on the growth of nine herbaceous perennials in a semi-extensive green roof Acta Horticulturae 643 305ndash309

Dunnett NP Nagase A Booth R Grime JP 2005 Vegetation composition of green roofs and its influence on runoff and biodiversity Paper presented at the Third Annual Greening Rooftops for Sustainable Communities Conference Awards and Trade Show 4ndash6 May 2005Washington DC

Durhman A Rowe DB Ebert-May D Rugh CL 2004 Evaluation ofcrassulacean species on extensive green roofs Paper presented at the Second Annual Greening Rooftops for Sustainable Communities Conference Awards and Trade Show 2ndash4 June 2004 Portland Oregon

Farzaneh R 2005 Evapotranspiration rates from extensive green roof plantspecies Masterrsquos thesis Pennsylvania State University

Felson AJ Pickett STA 2005 Designed experiments New approaches to studying urban ecosystems Frontiers in Ecology and the Environment3 549ndash556

[FLL] Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (Landscape Research Development and Construction Society) 2002Guideline for the Planning Execution and Upkeep of Green-roof Sites(14 August 2007 wwwf-l-ldeenglishhtml)

Frazer L 2005 Paving paradise Environmental Health Perspectives 113457ndash462

Gaffin SR Rosenzweig C Parshall L Beattie D Berghage R OrsquoKeeffe GBraman D 2005 Energy balance modeling applied to a comparison ofgreen and white roof cooling efficiency Paper presented at the Third Annual Greening Rooftops for Sustainable Communities ConferenceAwards and Trade Show 4ndash6 May 2005 Washington DC

Gaffin S Rosenzweig C Parshall L Hillel D Eichenbaum-Pikser J GreenbaumA Blake R Beattie R Berghage R 2006 Quantifying evaporative cooling from green roofs and comparison to other land surfaces Paperpresented at the Fourth Annual Greening Rooftops for SustainableCommunities Conference Awards and Trade Show 11ndash12 May 2006Boston

Gedge D Kadas G 2004 Bugs bees and spiders Green roof design for rareinvertebrates Paper presented at the Second Annual Greening Rooftopsfor Sustainable Communities Conference Awards and Trade Show 2ndash4June 2004 Portland Oregon

Getter KL Rowe DB 2006 The role of green roofs in sustainable develop-ment HortScience 41 1276ndash1286

Grant G 2006 Extensive green roofs in London Urban Habitats 4 51ndash65Grime JP 1998 Benefits of plant diversity to ecosystems Immediate filter

and founder effects Journal of Ecology 86 902ndash910mdashmdashmdash 2001 Plant StrategiesVegetation Processes and Ecosystem Properties

Chichester (United Kingdom) WileyHaemmerle F 2002 Der Markt fuumlr gruumlne Daumlcher waumlchst immer weiter

Jahrbuch Dachbegruumlnung 2002 11ndash13Hartig T Mang M Evans GW 1991 Restorative effects of natural environ-

ment experience Environment and Behavior 23 3ndash26Heinze W 1985 Results of an experiment on extensive growth of vegetation

on roofs Rasen Gruumlnflachen Begruumlnungen 16 (3) 80ndash88Kadas G 2006 Rare invertebrates colonizing green roofs in London Urban

Habitats 4 66ndash86Koumlhler M 2003 Plant survival research and biodiversity Lessons from

Europe Paper presented at the First Annual Greening Rooftops for Sustainable Communities Conference Awards and Trade Show 20ndash30May 2003 Chicago

mdashmdashmdash 2006 Long-term vegetation research on two extensive green roofsin Berlin Urban Habitats 4 3ndash26

Koumlhler M Keeley M 2005 The green roof tradition in Germany The exampleof Berlin Pages 108ndash112 in Hoffman L McDonough W eds GreenRoofs Ecological Design and Construction New York Schiffer

Koumlhler M Schmidt M Grimme FW Laar M de Assunccedilatildeo Paiva VL TavaresS 2002 Green roofs in temperate climates and in the hot-humid tropicsmdashfar beyond the aesthetics Environment and Health 13 382ndash391

Kolb W Schwarz T 1993 Zum Klimatisierungseffekt von Pflanzenbestaumln-den auf Daumlchern Veitshoumlchheimer Berichte 4 28ndash36

Korhonen J 2005 Theory of industrial ecology The case of the concept ofdiversity Progress in Industrial Ecology an International Journal 235ndash72

Larson DW Matthes U Kelly PE 2000 Cliff Ecology Cambridge (UnitedKingdom) Cambridge University Press

Lee KS Kim J 1994 Changes in crassulacean acid metabolism (CAM) ofSedum plants with special reference to soil moisture conditions Journalof Plant Biology 37 9ndash15

Liu K 2004 Engineering performance on rooftop gardens through field evaluation Journal of Roof Consultants Institute 22 (2) 4ndash12

Liu K Baskaran B 2003 Thermal Performance of Green roofs throughField EvaluationmdashOttawa Ottawa (Canada) National Research Council Canada Institute for Research in Construction Report noNRCC-46412

Liu K Minor J 2005 Performance evaluation of an extensive green roofPaper presented at the Third Annual Greening Rooftops for SustainableCommunities Conference Awards and Trade Show 4ndash6 May 2005Washington DC

Lundholm JT 2006 Green roofs and facades A habitat template approachUrban Habitats 4 87ndash101

Mentens J Raes D Hermy M 2005 Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century Landscapeand Urban Planning 77 21ndash226

Articles

832 BioScience bull November 2007 Vol 57 No 10 wwwbiosciencemagorg

Monterusso MA Rowe DB Rugh CL 2005 Establishment and persistenceof Sedum spp and native taxa for green roof applications HortScience40 391ndash396

Moran A Hunt B Smith J 2005 Hydrologic and water quality performancefrom greenroofs in Goldsboro and Raleigh North Carolina Paper presented at the Third Annual Greening Rooftops for Sustainable Communities Conference Awards and Trade Show 4ndash6 May 2005Washington DC

Oke TR 1987 Boundary Layer Climates 2nd ed New York MethuenOnmura S Matsumoto M Hokoi S 2001 Study on evaporative cooling

effect of roof lawn gardens Energy and Buildings 33 653ndash666Porsche U Koumlhler M 2003 Life cycle costs of green roofs A comparison of

Germany USA and Brazil Proceedings of the World Climate and Energy Event 1ndash5 December 2003 Rio de Janeiro Brazil (22 October2007 wwwgruendach-mvdeenri03_461_u_porschepdf)

Prado RTA Ferreira FL 2005 Measurement of albedo and analysis of its influence on the surface temperature of building roof materials Energyand Buildings 37 295ndash300

Rowe DB Monterusso MA Rugh CL 2006 Assessment of heat-expandedslate and fertility requirements in green roof substrates HortTechnol-ogy 16 471ndash477

Saiz S Kennedy C Bass B Pressnail K 2006 Comparative life cycle assessment of standard and green roofs Environmental Science andTechnology 40 4312ndash4316

Smith RM Thompson K Hodgson JG Warren PH Gaston KJ 2006 Urbandomestic gardens (IX) Composition and richness of the vascular plant

flora and implications for native biodiversity Biological Conservation129 312ndash322

Takakura T Kitade S Goto E 2000 Cooling effect of greenery cover over abuilding Energy and Buildings 31 1ndash6

Theodosiou TG 2003 Summer period analysis of the performance of aplanted roof as a passive cooling technique Energy and Buildings 35909ndash917

Tilman D Lehman CL Thomson KT 1997 Plant diversity and ecosystemproductivity Theoretical considerations Proceedings of the NationalAcademy of Sciences 94 1857ndash1861

[USEPA] US Environmental Protection Agency 2000Vegetated Roof CoverPhiladelphia Pennsylvania Washington (DC) USEPA Report no EPA841-B-00-005D

VanWoert ND Rowe DB Andresen JA Rugh CL Xiao L 2005a Wateringregime and green roof substrate design impact Sedum plant growthHortScience 40 659ndash664

mdashmdashmdash 2005b Green roof stormwater retention Effects of roof surface slopeand media depth Journal of Environmental Quality 34 1036ndash1044

Villarreal EL Bengtsson L 2005 Response of a Sedum green-roof to individual rain events Ecological Engineering 25 1ndash7

Wong NH Chen Y Ong CL Sia A 2003 Investigation of thermal benefitsof rooftop garden in the tropical environment Building and Environ-ment 38 261ndash270

doi101641B571005Include this information when citing this material

Articles

wwwbiosciencemagorg November 2007 Vol 57 No 10 bull BioScience 833