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Lettheconceptoftraitbefunctional!CyrilleViolle,Marie-LaureNavas,DenisVile,ElenaKazakou,ClaireFortunel,Ire`neHummelandEricGarnierAbstractInitssimplestdefinition,atraitisasurrogateoforganismalperformance.Wethereforegiveanunambiguousdefinitionofplanttrait,withaparticularemphasisonfunctionaltrait.Ahierarchicalperspectiveisproposed,extendingthe‘‘performanceparadigm’’toplantecology.‘‘Functionaltraits’’aredefinedasmorpho-physiop-henologicaltraitswhichimpactfitnessindirectlyviatheireffectsongrowth,reproductionandsurvival,thethreecomponentsofindividualperformance.Wefinallypresentanintegrativeframeworkexplaininghowchangesintraitvaluesduetoenvironmentalvariationsaretranslatedintoorganismalperformance,andhowthesechangesmayinfluenceprocessesathigherorganizationallevels.IntroductionFollowingDarwin’s(1859)proposal,traitswereinitiallymainlyusedaspredictors(proxies)oforganismalperformance.Overthelastthreedecades,developmentsincommunity(Grime1974,PetcheyandGaston2002,McGilletal.2006)andecosystem(Chapin1993,Grime1998,LavorelandGarnier2002,EvinerandChapin2003)ecologyhaveforcedtheconceptoftraitbeyondtheseoriginalboundaries,andtrait-basedapproachesarenowusedinstudiesrangingfromtheleveloforganismstothatofecosystems.Withineachdiscipline,diversetypesoftraitsarethususedtoassessinherentcomponentsinvolvedintheexplanationofcomplexprocessesdefinedathigherorganizationallevels(Fig.1)Afurthercomplicationhasarisenwiththeintroductionof‘‘functionaltraits’’,anexpressioncoinedwiththeemergenceoffunctionalecologyasadiscipline(Calow1987,Keddy1992a).Currentattemptstodevisefunctionalclassificationsoforganisms(NobleandGitay1996,WoodwardandCramer1996,Lavoreletal.1997),definethefunctionaldiversityofcommunities(Walkeretal.1999,PetcheyandGaston2002,Masonetal.2005)andassessspecieseffectsonecosystems(LavorelandGarnier2002,EvinerandChapin2003),alladvocatetheuseof‘‘functionaltraits’’.Theaimsofthispaperaretoproposeunambiguousdefinitionsof‘‘traits’’and‘‘functionaltraits,’’andtosuggestaframeworkonhowtraitscouldbeusedtoaddresscurrentissuesincommunityandecosystemecology.Planttraits:definitionsInthefirstone,Petcheyetal.(2004)testedhowdifferentindicesoffunctionaldiversitycouldexplaindifferencesinbiomassproduction.Theseindiceswerecalculatedusinganumberofso-called‘‘traits’’:eightweremeasuredattheindividualplantlevel(fourleaftraits,threetraitsdescribingthestatureoftheplantsandseedmass),whilethreeweremeasuredataplotlevel(standingbiomass,vegetationcoverandcanopyheight).Inthesecondone,Eviner(2004)characterizedtheinfluenceofplantstandingbiomass,soilconditionsandmicrobialphosphorus,alldefinedas‘‘planttraits’’.Inmanycases,atraitisdefinedattheleveloftheindividual(Table1,2).Thisdefinitionofatraitrequiresfurtherprecisions:(1)theparticularvalueormodalitytakenbythetraitatanyplaceandtimeiscalledan‘‘attribute’’(Lavoreletal.1997);(2)withinaspecies,thetrait,eithercontinuousorcategorical,mayshowdifferentattributesalongenvironmentalgradients(Fig.2)orthroughtime;(3)theattributeforatraitisusuallyassessedforonepopulation(averageofattributesofasetofindividuals)inspaceandtime(i.e.forasinglevalueofthex-axisofFig.2).Thishastwoconsequences.First,thereisnotasingletraitattributeforanyparticularspecies(Fig.2).Second,informationonthelocalenvironmentwherethetraithasbeenmeasuredisessentialtointerprettheecologicalmeaningoftra...