Theassessmentofherbaceousplantcoverinwetlandsasani

EcologicalIndicators2(2002)287–293TheassessmentofherbaceousplantcoverinwetlandsasanindicatoroffunctionCharlesAndrewColeCenterforWatershedStewardship,ThePennsylvaniaStateUniversity,227EastCalderWay,UniversityPark,PA16801,USAAbstractIntheUnitedStates,wetlandsareoftencreated(ascomparedwithrestored)asmitigationfordamagedonetonaturalwetlandsbydevelopmentorotheractivities.Thereisincreasingconcernthatthesecreatedsitesdonotfunctionasdonaturalwetlands,evenafteraperiodofyears.Monitoringofthesecreatedwetlandsoftenconsistsofanassessmentofthepercentherbaceousplantcoverassomeindicatorofthefunctionalsuccessofthewetland.However,itisnotatallclearthatassessmentofherbaceouscovertranslatesintoanaccurateindicatorofwetlandfunction.InthispaperIreviewseveralfunctionscommonlyascribedtowetlandsandassessthereportedrelationshipofpercentherbaceouscovertothosefunctions(ifany).Ofsixfunctionsreviewed,onlyonehasaprobable(thoughindirect)positiverelationshipwiththepercentherbaceousplantcoveronasite.Moreusefulassessmentsofwetlandfunctionmightbemadewithotherstructuralindicators,suchasbasinmorphometry,treedensity,orbasalarea.©2002ElsevierScienceLtd.Allrightsreserved.Keywords:Function;Mitigation;Structure;Wetlands1.IntroductionIntheUnitedStates,wetlandsarefrequentlycreatedasmitigationfordamagedonetonaturalwetlands.TheNationalResearchCouncil(2001,p.13)defineswetlandcreationas“...theconversionofapersistentuplandorshallowwaterareaintoawetlandbyhumanactivity.”Wetlandrestoration,bycontrast,isdefinedas“thereturnofawetlandfromadisturbedoralteredconditionbyhumanactivitytoapreviouslyexistingcondition”(NRC,1992).Onceintheground,thepresumptionisthatcreatedwetlandsarethenassumedtobefunctionallyequiv-alenttonaturalwetlands.Assessingthatequivalencyisdifficult,atbest.IntheUnitedStateswetlandmit-igationisessentiallyalegalprocess(Section404oftheCleanWaterAct),withdetailsontheproposedE-mailaddress:[e-mail](C.A.Cole).mitigationspelledoutinapermitissuedbytheU.S.ArmyCorpsofEngineers(andsometimesbystateagenciesaswell).Mitigationissuestobeaddressedvarywidely,withsomepermitsbeingquitespecificaboutmitigationrequirements,andothersbeingverygeneral(Streever,1999).Onemightexpectthatthepermitwouldlistthecriteriawherebythecreatedwet-landwillbejudgedasasuccess(i.e.functional),butthisisnotalwaysthecase.Whensuchcriteriaareenumerated,theyareoftenverysimpleandtypicallyrelyuponvegetationstructuralcharacteristics,suchaspercentcover,orplantdominance(NRC,2001).2.DefiningandmeasuringfunctionItisnotatallclearthatmeasurementofvegetationstructure(e.g.percentcover)translatesintoanymean-ingfulindicatoroffunction(SimenstadandThom,1470-160X/02/$–seefrontmatter©2002ElsevierScienceLtd.Allrightsreserved.PII:S1470-160X(02)00064-X288C.A.Cole/EcologicalIndicators2(2002)287–2931996;Streever,1999).Forthatmatter,itisnotalwaysclearwhatactuallyconstitutesfunction.Manyassess-mentsofthefunctionofcreatedwetlandsare,inreal-ity,assessmentsofbiophysicalstructure(e.g.WilsonandMitsch,1996;FennesseyandRoehrs,1997).Itisnecessarytodefinewhatwemeanbyfunctionbeforewecanassesstheutilityofusingstructureasafunc-tionalindicator.Functionsaretypicallyconsideredasratesandpro-cesses(SimenstadandThom,1996;Hruby,1999).Atthescaleofecosystems,someoftheseprocessesin-cludehydrodynamics(WinterandRosenberry,1995;Coleetal.,1997),plantproductivity(Cole,1992;HooperandVitousek,1997;Tilmanetal.,1997),nitro-genfixationandcycling(HooperandVitousek,1997;Tilmanetal.,1997;Wardleetal.,1997),andcarbonsequestrationanddecomposition(Wardleetal.,1997).TheNationalResearchCouncil(1995,p.34)definedfunctionas“...allprocessesandmanifestationofprocessesthatoccurinwetlands.”Hammer(1993,p.69)stated“Functiondescribeswhatawetlanddoes,irrespectiveofanybeneficialworthassignedbyman.”Simplystated,functionsarewhatwetlandsdo(Smithetal.,1995).Table1listswetlandfunctionsasdefinedbyseveralauthors.Mostwetlandecologistsdonothaveadifficulttimedefining(insomemanner)justwhatconsti-tutesfunctioninawetland(Table1).However,veryfewattemptsaremadetodirectlymeasurefunction.Table1ExamplesofwetlandfunctionsasdefinedbyseveralauthorsHammer(1992)Smithetal.(1995)NationalResearchCouncil(1995)CronkandFennessey(2001)LifesupportShort-termsurfacewaterstorageShort-termsurfacewaterstorageHydrologyHydrologicmodificationLong-termsurfacewaterstorageLong-termsurfacewaterstorageBiogeochemistryWaterqualitychangesStorageofsub-surfacewaterMaintenanceofhighwatertablePlantandanimalhabitatErosionprotectionModerationofgroundwaterflowTransformationandcyclingofelementsOpenspaceandaestheticsDissipationofenergyRetention,removalofdissolvedelementsGeochemicalstorageCyclingofnutrientsAccumulationofpeatRemovalofelementsandcompoundsAccumulationofinorganicsedimentsRetentionofparticulatesMaintenanceofcharacteristicplantcommunitiesExportoforganiccarbonMaintenanceofcharacteristicenergyflowMaintenanceofcharacteristicplantandanimalcommunitiesNotallwetlandtypesexhibitallfunctions.Kentulaetal.(1992)suggestusingindicators,orthosevariablesseenascloselyrelatedtoaparticularfunction.However,withwetlandmitigationmonitor-ing,Kentulaeta