The Council verge as the next wetland: TREENET and the cities of Mitcham and Salisbury investigate

Abstract

Spatial pressures in many contemporary cities restrict the construction of surface wetlands of the scales necessary to deliver storm water management and ecosystem services. Alternative means are able to deliver some of these services by harvesting storm water at source and storing it in soils beneath road verges where it can irrigate urban trees and other vegetation. By incorporating vegetation into this storm water management infrastructure both water harvesting rates and storage capacities can be increased. If integrated with street infrastructure on a large scale, water sensitive urban design features and associated vegetation have the potential to make major contributions to improved human health, environmental quality and flood risk mitigation. Infrastructure established to support preliminary investigations in the cities of Mitcham and Salisbury has demonstrated problem-free function of storm water harvesting for street tree irrigation at the local level over seven years.

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Acta Hortic. 1108. ISHS 2016. DOI 10.17660/ActaHortic.2016.1108.8
XXIX IHC – Proc. Int. Conf. on Landscape and Urban Horticulture and International
Symposium on Sustainable Management in the Urban Forest
Eds.: G. Groening et al.
63
The Council verge as the next wetland: TREENET and
the cities of Mitcham and Salisbury investigate
T.Johnson1,D.Lawry2andH.Sapdhare3
1CityofMitcham,131BelairRoad,TorrensPark,5064,SouthAustralia,Australia;2TREENETInc.,WaiteCampus,
University of Adelaide, PMB 1 Glen Osmond, 5064, South Australia, Australia; 3University of South Australia,
MawsonLakesBoulevard,MawsonLakes,5095,SouthAustralia,Australia.
Abstract
Spatialpressuresinmanycontemporarycitiesrestricttheconstructionof
surfacewetlandsofthescalesnecessarytodeliverstormwatermanagementand
ecosystemservices.Alternativemeansareabletodeliversomeoftheseservicesby
harvestingstormwateratsourceandstoringitinsoilsbeneathroadvergeswhereit
canirrigateurbantreesandothervegetation.Byincorporatingvegetationintothis
stormwatermanagementinfrastructurebothwaterharvestingratesandstorage
capacitiescanbeincreased.Ifintegratedwithstreetinfrastructureonalargescale,
watersensitiveurbandesignfeaturesandassociatedvegetationhavethepotentialto
makemajorcontributionstoimprovedhumanhealth,environmentalqualityand
floodriskmitigation.Infrastructureestablishedtosupportpreliminaryinvestigations
inthecitiesofMitchamandSalisburyhasdemonstratedproblem-freefunctionof
stormwaterharvestingforstreettreeirrigationatthelocalleveloversevenyears.
Keywords:watersensitiveurban design,sustainableurban drainage system,storm water
management,permeablepavement,streettrees,hydraulicredistribution
INTRODUCTION
The global rate and scale of urbanisation is stressing ecologicalsystemsandthe
infrastructure on which cities depend (Andersson et al., 2014; Childers et al., 2014; Wu
2008),withmost ecosystemson Earthnowbeingaffectedbyurban demandsand outputs
(Elmqvistetal.,2013).Urbandevelopmentislargelyimpervious torainfallsoit canaffect
local hydrological cycles (Ferguson, 2010). Accompanying vegetation loss compounds the
hydrological consequences. These impacts together contribute towatershortagesand
flooding(Sheng and Wilson, 2009; Lee andHeaney,2003), urban heatislands (Australian
Government,2013;Ewenzetal.,2012)andgeneratesignificantnegativeimpactsonhuman
andenvironmentalhealth(Connell,2013;Weinstein,2013;Tarran,2006).
Greeninfrastructureandwatermanagement
LocalgovernmentsinAustraliaarelargelyresponsiblefortheprovision and
maintenanceofcivilinfrastructure.Theyarealsoincreasinglyresponsibleforhumanhealth
mattersthroughpreventivemeanssuchastheprovisionof‘green’urban environmentsfor
passiverecreation,includingsportingandactiverecreationfacilities and vegetated
streetscapes(HealthyEnvironsPtyLtd.,2014).
Green urban designs promise increased delivery of ecosystem services from within
cities as well as reduced impacts on surrounding environments. However, increasing
pressureonspace canresult inconflicts betweenacity’sbuilt andnaturalassets.Current
trendsingreenurbanism(Lehmann,2011)andgreeninfrastructure(ElyandPitman,2012;
Colding,2011)seeaunitingofthehorticulturalandcivilcomponentsofurbanpublicspace
todelivergreaterbenefitmorecosteffectivelyandtoprogresscitiestowardssustainability.
Waterisacriticalresourceofgreeninfrastructure.‘Intermsofsheermass,waterisby
far the largest component of urbanmetabolism’(Kennedyetal., 2007). Massive
infrastructureisneededtoprotectcommunitiesfromfloodingandtosupplywatertocities
(Brownetal.,2008).Predictionsofmoreextremeandfrequentdroughtsandfloods(IPCC,
2007)callintoquestionthepracticalityofcontinuingtosupplywaterandmitigateflooding

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solelythroughatraditional,centralised,engineering-basedapproach.
Waterisfundamentaltosustainingvegetationinurbanareas.Thispointiskeytothe
sustainablemanagementofbothurbanstormwaterandvegetation.Wetlands,swalesand
rain gardens have demonstrated the benefits achievable through retaining and detaining
waterintheenvironmentatornearitssource,butthesesolutionsrequireadequatespace.
For functional, financial, human and environmental reasons water storage solutions are
needed which require little space. Technologies such as the TREENET Inlet, permeable
pavements,infiltrationtrenches and‘leakywells’can reduce pressureontraditionalstorm
waterinfrastructureandpotablewatersuppliesbymoreeffectivelyusingroadreserves.
Byintegratingwaterandvegetationmanagementintoroadandfootpathengineering,
thecities of Mitcham and Salisbury have demonstratedthat costsaving,lowmaintenance
solutionsareavailablewithinmanyexistingroadverges.Bysustainingurbanvegetation,an
integrated approach to water management supports other local government initiatives
including climate change adaptation,protectingthehealthandwell-being of citizens and
reducing environmental impacts. Increased evapotranspirational cooling, shading and
thermalmassattributedtoagreaterabundanceofwaterinthelocalenvironmentwill,asan
example,provideamoretemperature-stablecitywhichwillfurther reduce energy
consumption, greenhouse gas emissions and costs to the community (McPherson et al.,
2005). Similarly, benefits will accrue from ecosystem services provided by trees, such as
improvements in social and psychological health resulting in reduced anxiety and fatigue
levelsandlowercrimerates(Tarran,2006).
To investigate and demonstrate the integration of horticulture and storm water
engineering, purpose-built infrastructure has been established in Salisbury Council’s
suburbanstreetstothenorthofAdelaideinSouthAustraliaand inMitcham tothe south.
Designedtoallowobservationandassessmentofsynergiesbetweenurbanhorticultureand
civilengineering(Levettetal.,2014;Johnsonetal.,2011a),thisinfrastructureishelpingto
providedataandquantifysolutionsthatwillincreaselocalgovernment’scapacitytoaddress
compoundingdevelopment,ecologicalandfinancialpressures.Bydeliveringstormwaterto
soilnearstreettrees,theCouncilsaimtoretainandusethisresourceatitssourceand,by
minimisingconflictsbetweentheinfrastructureandtreeroots,toimprovepublicsafetyand
reducemaintenancecosts.
Water harvesting strategies used by the cities of Mitcham and Salisbury include
wetlands,swales,detentionbasinsandstreetscape-scaleinfiltrationdevices,thelatterbeing
the subject of this paper. At the streetscape scale, devices used include permeable
pavements,soakagetrenches,andTREENETinletscoupledtoleakywellsofvariousdesigns.
Allofthesesystemsharvest‘firstflush’stormwaterrunoffto mitigate pollution loads
(Beecham,2010;Denmanetal.,2006;Leeetal.,2004)anddelayandreducefloodpeaks.
Permeablepavements
Permeable pavements have been developed over several decades (Ferguson, 2010;
ScholzandGrabowiecki,2007)andarebeingincreasinglyutilizedinternationally,yettheir
application in Australia remains relatively limited in comparison to impermeable
pavements.Reasonsforlowrateofapplicationmayincludemarginallyhigherconstruction
costsandalackofdataregardingeffectsofwaterinfiltrationintoreactivesoilsadjacentto
roadsandotherinfrastructure.
Construction details and aspects of the two different permeablefootpathpavement
designsunder investigationare detailed in Johnsonet al. (2011a). Permeableinterlocking
concrete brick footpath surfaces are supported by a layer of crushed rock (20 mm
screenings)whichisfreefromfinerparticles.The40%air-filledporosityofthescreenings
providesstormwaterdetentionadequatetocontaina42-mmrainfalleventinonedesign
and68mminanother;thewaterthensoaksintounderlyingsoils. The smaller capacity
designhasalevelinterfacebetweenthescreeningsandunderlyingsoil(designated‘perm-
level’); the larger capacity design has the screenings formed into a swale (‘perm-swale’)
beneath the footpath to encourage deeper root growth beneath its center (Johnson et al.,
2011b).Soilmoistureandoxygenlevels,treegrowthandsoilreactivityhavebeenmonitored

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sinceconstructionandplantinginSeptember2009.
TREENETinlets
Developed by not-for-profit tree research organisation TREENET Inc., the TREENET
inlet device harvests storm water from the road for watering tree root zones in adjacent
verges. Severalhundred TREENETinlets have been installedacrosseightlocalgovernment
authoritiesinthreestatesofAustralia.Thiscurrentresearch is providing data to better
inform the application of this technology in future by investigatinglifecyclecosts,
quantifyingtheeconomicbenefitsofrelatedgreeninfrastructureandmeasuringimpactson
soilmoistureandreactivity.
TREENETinletdevices(Figure1)arecastintoconcretekerbstodivertstorm water
fromtheroadtosoilintheverge.Waterexitsthekerbandgutterdrainagesystemthrougha
slottedfaceplatemountedinthekerbandthenflowsviaaPVCstormwaterpipeintoleaky
wells(Figure2)constructedinthevergeuntilthelimitofthe well and the infiltration
capacityofitssurroundingsoil arereached.Theshallowdepressionintheguttercreatesa
small pool which reduces flow velocity marginally, thereby establishinganeddywhich
depositssedimentshighinthegutterandawayfromtheentryslot,thusminimisingclogging
andallowingformaintenancethroughroutinestreetsweeping.Flowvolumeswhichexceed
thedesigncapacitysimplybypassthedeviceandcontinuedownstreamtothenextinletor
beyondviathestormwaternetwork.

Figure1.TREENETInletinstallation.
Leakywells
Several designs have been developed in the search for the optimalleakywellfor
Adelaide’sclimate.Primaryfactorsconsideredintheirdesignaretotalstoragecapacityand
adequacytoaccommodatethepollutantloadedfirstflushofstormwaterrunoff(Leeetal.,
2004).Otherconsiderationsincludehydraulicconductivityandreactivityofthesite’ssoils,
localrainfallpatterns, catchmentarea,vegetationproximityandtype,andaccessibilityfor
maintenancepurposes.
Roadsideleakywellsandinfiltrationtrencheswithcapacitiesranging from 10 L to
7 kL have been constructed. Those connected to TREENET Inlets range between 65 and
150L;thesmallerbeingusedinshallowsoils and the larger at sites wheredrainage isa
primaryconcern.Theleakywelldesignwhichformspartofthecurrentresearch(Figure2)
hasa capacityof 50 L. Optimising design capacity ona cost-benefit basis requiresfurther
research.

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
Figure2.Leakywelldesign.
TwentyeightTREENETinletshaverecentlybeeninstalledinthecityofMitchamusing
variousbackfillmaterialstoinvestigatepotentialpollutionmitigationbenefits.Stormwater
sampleswill be collected from thebasesoftheleakywellsduring late2014 and through
2015, to be analysed for pollutants including phosphates, nitrates,heavymetalsand
suspended solids. Soil samples collected from the sites during construction of the leaky
wells, and additional samples to be taken in 2016, will be analysed and compared to
determinethepollutionmitigationcapacityofthethreebackfillmaterials.
RESULTSANDDISCUSSION
Infiltrationtrenchesandleakywells
Streetscape-scalestormwaterharvestingsystemshavebeensuccessfullyfunctioning
inthecitiesofMitchamandSalisbury since 2007withnooperationalfailuresorproblems
observed.Atonesite,soakagetrenches600mmwide,1000mmdeepandtotaling80min
lengthcandetainupto20kLofstormwaterinthevoidsbetweenthecrushedrock
screenings which fill them; the water then percolating into surrounding soil. Percolation
rates after seven years of operation appear consistent with those immediately following
construction.DuringheavyraininJuly2014,flowobservedby-passingthesystemwas
minimal.Increasedsoilmoisturelevelshavenotresultedinanyobservabledeteriorationof
footpath,road,drainageorotherinfrastructure.
Similarly,ataseparatesiteadrainagepitdivertsstormwaterintoasoakagetrench
9minlengthwithadetentioncapacityof2.5kL.Thistrenchfillstocapacityapproximately
40timesperyear.Builtattheendofaseveredrought,waterrecharge wasanticipated to
causesubstantialsoilheave,potentiallydislodgingthe newkerbing.Noimpactshavebeen
observedatthissitesinceconstructionin2008.Inanenvironmentwithcopiousamountsof
leaflitter and fruit producedby a matureavenue ofWhite cedar (Meliaazedarach)street
treesandusingonlyascreenedagriculturaldrainagepipeinthestormwaterpittofilter
waterenteringthetrench,afterthreeyearsofoperationthesystem was measured to be
operatingwithaninflowrateof70Lmin-1.

67
Inbothofthesecasesitislikelythattheestablishedavenuesofriverredgum
(Eucalyptuscamaldulensis)andwhitecedar(Meliaazedarach)havedevelopedopportunistic
root systems which take advantage of the additional water, with hydraulic redistribution
(Prietoetal.,2012;NeumannandCardon,2012;Blebyetal.,2010)reducingwaterpotential
around the trenches and sustaining prolonged storm water harvesting. Several similar
systemshaveremainedfullyoperationaloversimilartimespanswithno maintenanceand
withnoobservedinfrastructureissues.
TREENETinletshavebeenprovidingdrainageandirrigationsolutionsatseveralother
locations for over two years. At one site a large river red gum(Eucalyptuscamaldulensis)
treehadlifted kerbingby approximately 100mm, creatingpoolingalongthestreetwhich
resultedindrivewayaccessdifficulties.Installationof5TREENETinletsatafractionofthe
costofkerbreplacementhasdrainedthesitethroughtwoyearswithnoilleffectsobserved
on surrounding infrastructure. As road-sweeper access is difficult at this site, sediments
regularly build up in the inlets’ pools. This has been observed to only marginally reduce
infiltration rates however. Adequate drainage is possibly sustained by the inlets in the
absence of maintenance due to high hydraulic conductivity of the built-up sediments. At
severalsiteswhereTREENETinletshavebeeninstallednearmaturetrees,rootgrowthhas
been observed to occupy screenings in the leaky wells within the first spring after
construction, suggesting that hydraulic redistribution of waterfromthewellsto
surroundingsoilscanbeexpectedtoestablishrapidly.ThecurrentresearchintoTREENET
inlets,theircostsandbenefits,isscheduledforcompletionin2016.
Permeablefootpathpavements
Datarelatingtopermeablepavementresearchiscurrentlybeinganalysed;anumber
of preliminary observations warrant reporting at this stage however. All of the average
annual rainfall of 546 mm incident upon the permeable research pavements has been
intercepted and managed in situ since September 2009, with no runoff and discharge to
receivingwatersresultingandnonegativeimpactsonthepavements themselves or
problemswithsurroundinginfrastructurebeingobserved.
Between2011and2014surfacelevelmovementduetomoistureinduced soil
reactivityappearedsimilar atpermeable andimpermeable controlpavements.Soil ‘heave’
recordedattheendofwinter2013(30August)rangedfrom+1to+30mmatcontrolsites,
+7 to +31 mm at ‘perm-level’ sites, and +10 to +30 mm at ‘perm-swale’sites(relativeto
February2011levels).LevelsrecordedonApril 8,2014of+18to-25mmatcontrolsites,
-4to-20mmat‘perm-level’sitesand+11to-22mmat’perm-swale’ sites (relative to
February2011levels)showedageneraltrendofshrinkageduetodryingcausingasurface
levelreductionofapproximately30mmacrossthedifferentpermeablepavementtypesand
controls.Theserelativelyminorsurface levelfluctuationsobservedonmoderatelyreactive
soilsalongthe320mlong researchsite,andtheabsence ofdiscontinuitiesinlevelsatthe
interchangesbetweenpermeableandimpermeablesurfaces,suggest that permeable
pavementsmaybeabletobesafelyusedinfootpathconstruction,particularlyinsemi-arid
climateswithmoderateorlowsoilreactivity.
ACKNOWLEDGEMENTS
The authors acknowledge the ongoing support of the project partners: city of
Mitcham,Adelaideand MountLoftyRanges Natural ResourcesManagement Board,city of
Salisbury,TREENETInc.,UniversityofMelbourne,UniversityofSouthAustralia.
Literaturecited
Andersson,E.,Barthel,S.,Borgström,S.,Colding,J.,Elmqvist,T.,Folke,C.,andGren,A.(2014).Reconnectingcities
to the biosphere: stewardship of green infrastructure and urbanecosystemservices.Ambio43(4),  445–453
http://dx.doi.org/10.1007/s13280-014-0506-y.PubMed
AustralianGovernment.(2013).StateofAustralianCities2013.DepartmentofInfrastructureandTransport.
Beecham,S.(2010).Treesasessentialinfrastructure:engineeringanddesignconsiderations.Paperpresentedat:
TREENET11thNationalStreetTreeSymposium(Adelaide,SouthAustralia).

68
Bleby, T.M., McElrone, A.J., and Jackson, R.B. (2010). Water uptake and hydraulic redistribution across large
woodyroot systems to 20m depth. PlantCell Environ.33(12),2132–2148 http://dx.doi.org/10.1111/j.1365-
3040.2010.02212.x.PubMed
Brown,R.,Keath,N.,andWong,T.(2008). Transitioningto watersensitivecities:historical,currentandfuture
transition states. Paper presented at: 11thInternationalConferenceonUrbanDrainage(Edinburgh,Scotland,
UK).
Childers,D.,Pickett, S., Grove,J.,Ogden, L., and Whitmer, A.(2014).Advancingurban sustainability theoryand
action: challenges and opportunities. Landsc. Urban Plan. 125, 320–328 http://dx.doi.org/10.1016/
j.landurbplan.2014.01.022.
Colding,J.(2011).The role of ecosystemservicesincontemporaryurban planning.InUrbanEcology:Patterns,
ProcessesandApplications,J.Niemelä,J.Breuste,T.Elmqvist,G.Guntenspergen,P.James,andN.McIntyre,eds.
(NewYork,USA:OxfordUniversityPress).
Connell,S.(2013). Urban forest–helpingtoamelioratethe effectsofpollutantsenteringcoastalhabitat.Paper
presentedat:TREENET14thNationalStreetTreeSymposium(Adelaide,SouthAustralia).
Denman,L.,May,P.,andBreen,P.(2006).Aninvestigationofthepotentialtousestreettreesandtheirrootzone
soilstoremovenitrogenfromurbanstormwater.Aust.JournalofWaterResources10(3),303–311.
Elmqvist, T., Fragkias, M., Goodness, J., Guneralp, B., Marcotullio, P., McDonald, R., Parnell, S., Schewenius, M.,
Sendstad, M., Seto, K., and Wilkinson, C., eds. (2013). Urbanisation, Biodiversity and Ecosystem Services:
ChallengesandOpportunities,AGlobalAssessment(Springer).
Ely,M.,andPitman,S.(2012). GreenInfrastructure,LifeSupportforHumanHabitats.TheCompellingEvidence
forIncorporatingNatureintoUrbanEnvironments(Australia:DepartmentofEnvironment,WaterandNatural
Resources,GovernmentofSouthAustralia).
Ewenz,C.,Bennett,J.,Kent,C.,Vinodkumar,H.,andClay,R.(2012).UrbanheatislandmappinginAdelaide.Paper
presentedat:TREENET13thNationalStreetTreeSymposium(Adelaide,SouthAustralia).
Ferguson, B.K. (2010). Porous pavements in North America: experience and importance. Paper presented at:
NovaTechConference(Lyon,France).
HealthyEnvironsPtyLtd.(2014).LivingWell.RegionalPlanforHealthandWellbeingfortheCitiesofUnleyand
Mitcham. https://s3-ap-southeast-2.amazonaws.com/ehq-production-australia/c7dd2efb6e51b8f795e7f5610
027290d99a8efd8/documents/attachments/000/014/103/original/Unley_Mitcham_Living_Well_Plan_Draft_Jun
e_2014.pdf?1402641830(AccessedJuly21,2014).
IPCC.(2007).ClimateChange2007:SynthesisReport.ContributionofWorkingGroupsI,IIandIIItotheFourth
AssessmentReportoftheIntergovernmentalPanelonClimateChange,CoreWritingTeam,R.K.Pachauri,andA.
Reisinger,eds.(Geneva,Switzerland:IPCC).
Johnson, T., Cameron, D., and Moore, G. (2011a). Trees, stormwater, soil and civil infrastructure: synergies
towards sustainable urban design for a changing climate. Paper presented at: International Conferenceof the
InternationalSocietyofArboriculture(Parramatta,Sydney,Australia).
Johnson,T.,Cameron,D.,and Moore, G. (2011b).Canpermeablepavementsreduce conflictsbetweenfootpaths
andtreeroots?Paperpresentedat:PublicWorks,CapitalSolutions: International Public Works Conference
(Canberra,Australia).
Kennedy,C., Cuddihy, J., and Engel-Yan,J.(2007).Thechangingmetabolismof cities. J. Ind. Ecol.11(2), 43–59
http://dx.doi.org/10.1162/jie.2007.1107.
Lee,J.G.,andHeaney,J.P.(2003).EstimationofUrbanimperviousnessanditsimpactsonstormwatersystems.J.
WaterResour.Plan.Manage.129(5),419–426http://dx.doi.org/10.1061/(ASCE)0733-9496(2003)129:5(419).
Lee, H., Lau, S.L., Kayhanian, M., and Stenstrom, M.K. (2004). Seasonal first flush phenomenon of urban
stormwater discharges. Water Res. 38(19), 4153–4163 http://dx.doi.org/10.1016/j.watres.2004.07.012.
PubMed
Lehmann, S. (2011). The Principles of Green Urbanism, Transforming the City for Sustainability (Taylor and
FrancisLtd.).
Levett,P.,Schweiger,E.,andLawry,D.(2014).Sustainablemanagementoftheinterfacebetweentreesandroad
pavements.Paperpresentedat:SustainabilityinPublicWorksConference(TweedHeads,Australia).
McPherson,G.,Simpson,J.,Peper,P.,Maco,S.,andXiao,Q.(2005). Municipalforestbenefits andcostsinfiveUS
cities.J.For.103(8),411–416.
Neumann,R.B.,andCardon,Z.G.(2012).Themagnitudeofhydraulicredistributionbyplantroots:areviewand
synthesisof empirical and modeling studies.New Phytol. 194(2), 337–352 http://dx.doi.org/10.1111/j.1469-

69
8137.2012.04088.x.PubMed
Prieto, I., Armas, C., and Pugnaire, F.I. (2012). Water releasethroughplantroots:newinsightsintoits
consequencesattheplantandecosystemlevel.NewPhytol.193(4),830–841http://dx.doi.org/10.1111/j.1469-
8137.2011.04039.x.PubMed
Scholz,M.,andGrabowiecki,P.(2007). Reviewofpermeable pavementsystems.Build.Environ.42(11), 3830–
3836http://dx.doi.org/10.1016/j.buildenv.2006.11.016.
Sheng,J., and Wilson, J. (2009).Watershedurbanisation and changingfloodbehaviouracrosstheLosAngeles
metropolitanregion.Nat.Hazards48(1),41–57http://dx.doi.org/10.1007/s11069-008-9241-7.
Tarran,J.(2006).Trees,UrbanEcologyandCommunityHealth.Paperpresentedat:TREENET7thNationalStreet
TreeSymposium(Adelaide,SouthAustralia).
Weinstein,P.(2013).Environmentalchange,diseaseandbiodiversity.Paperpresentedat:TREENET14thNational
StreetTreeSymposium(Adelaide,SouthAustralia).
Wu,J.(2008).Making the case for landscape ecology,an effectiveapproach tourbansustainability.Landscape
Journal27(1),41–50http://dx.doi.org/10.3368/lj.27.1.41.


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