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Functioning. Parameterization and Evaluation
Jean-Francoise Soussana, Vincent Maire, Nicolas Gross, Bruno Bachelet, Loic Pages, Raphaël Martin, David R.C. Hill, Christian Wirth
To cite this version:
Jean-Francoise Soussana, Vincent Maire, Nicolas Gross, Bruno Bachelet, Loic Pages, et al.. GEMINI:
a Grassland Model Simulating the Role of Plant Traits for Community Dynamics and Ecosystem
Functioning. Parameterization and Evaluation. Ecological Modelling, Elsevier, 2012, 231, pp.134-
145. �10.1016/j.ecolmodel.2012.02.002�. �hal-00707633�
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ContentslistsavailableatSciVerseScienceDirect
Ecological Modelling
j ourna l h o me pa g e:w w w . e l s e v i e r . c o m / l o c a t e / e c o l m o d e l
Gemini: A grassland model simulating the role of plant traits for community dynamics and ecosystem functioning. Parameterization and evaluation
Jean-Franc¸oisSoussanaa,∗,1,VincentMairea,1,NicolasGrossa,2,BrunoBacheleta,3,LoicPagèsb, RaphaëlMartina,DavidHillc,4,ChristianWirthd,5
aINRAUR874UREP,GrasslandEcosystemResearch,F-63100Clermont-Ferrand,France
bClermontUniversité,UniversitéBlaisePascal,LIMOS,BP10448,F-63000Clermont-Ferrand,France6
cINRAUR1115PSH,PlantesetSystèmesdeculturesHorticoles,F-84914Avignon,France
dMax-PlanckInstituteforBiogeochemistry,D-07745Jena,Germany
a rt i c l e i n f o
Articlehistory:
Received27October2010
Receivedinrevisedform31January2012 Accepted6February2012
Keywords:
Partitioning Growth Carbon Nitrogen Functionalbalance Coordinationtheory Speciesdiversity
a b s t r a c t
Astructure–function–diversitymodelofgrasslandecosystems(Gemini)hasbeendeveloped.Forapoten- tiallyunlimitednumberofclonalplantpopulations,itexplicitlysimulatescompetitionfortwokey resources(lightandnitrogen)alongverticalcanopyandsoilprofiles.Populationturnover,shootand rootmorphogenesis,photosynthesis,respiration,transpiration,Nacquisitionbyuptake,allocationof assimilatesbetweenstructuralcompartments,andreservestorageandremobilization,aresimulatedfor eachplantpopulation.Theobject-orientedstructureofthemodelingframeworkallowstocouple,or not,thesimulatedplantpopulationstoothersub-modelsdescribingclimatevariables,soilfunctioning, grazingbehaviorandgrasslandmanagement.Partitioningofgrowthbetweenshootstructures,leafpho- tosyntheticproteinsandrootsisbasedontwoassumptions:(i)functionalbalancebetweenrootandshoot activity,(ii)coordinationofleafphotosynthesis.Themodelwasparameterizedfromplantfunctionaltrait measurementsof13nativeperennialpasturegrassspeciesgrowninmonoculturesathighNavailability andlowcuttingfrequencyinafieldtrial.Predictedandmeasuredannualdry-matteryieldswerehighly correlatedwithoutbiasacrossspecies,Nsupplyandcuttingfrequencytreatmentsinmonoculturesand inmixturesofsixspecies.Resultsshowtheabilityofthismechanisticmodeltosimulatewithoutbias nitrogenanddisturbanceresponsesofnetprimaryproductivityandofplantcommunitystructure.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Howglobalchangesmayinfluencethedynamicsofbiodiversity acrossvariouslevelsofbiologicalorganizationisakeychallenge to predict future ecosystem functions and services. Ecosystem responsetoanyenvironmentalchangeisnotonlydrivenbythe direct effectsof abiotic controlsbut alsoindirectly affected by changesin thephysiology and morphologyof individualplants
∗Correspondingauthor.Tel.:+33473624423.
E-mailaddress:[email protected](J.-F.Soussana).
1 JFSandVMhavecontributedequallytothiswork.
2 Currentaddress:INRA,USC1339(CEBC-CNRS),F-79360,BeauvoirsurNiort, France;CEBC-CNRS(UPR1934),F-79360,BeauvoirsurNiort,France.
3 Currentaddress:ClermontUniversité,UniversitéBlaisePascal,LIMOS,BP10448, F-63000Clermont-Ferrand,France.
4 Currentaddress:CNRS,UMR6158,LIMOS,F-63173Aubière,France.
5 Currentaddress:UniversitätLeipzig,Institutfür BiologieI,04103Leipzig, Germany.
6 WebsiteoftheGEMINIproject:https://www1.clermont.inra.fr/urep/modeles/
gemini.htm.
andbythestructureofplantpopulationsandcommunities(Suding etal.,2008;KlumppandSoussana,2009).Howindividualresponse scaleintoecosystemlevelissometimeswell-known,e.g.photosyn- thesisscalesfromleaftoecosystem(Fieldetal.,1992).Incontrast, many population(e.g. self-thinning, plasticity) and community processes(e.g.speciesinteractions,speciesreplacement)arenot wellunderstood(McGilletal.,2006;VanWijk,2007;Grossetal., 2009).Anurgentgoalforglobalenvironmentalchangeresearchis thustobetterunderstandtheconsequencesofthesecomplexpro- cessesatthepopulationandcommunitylevelsandhowtheymay impactecosystemfunctioning(Sudingetal.,2008).
Plantfunctionaltraits,whicharemorphological,chemical,phys- iologicalandphenologicalplantcharacteristics(Lavoreletal.,1997;
Violleetal.,2007),havebeenproposedasausefultooltoupscale individual response into ecosystem level (Lavorel et al., 2007;
Sudingetal.,2008).Functionaltraitsarelinkedwithindividualtol- erancestosimpleabioticfactors(Sudingetal.,2003;Louaultetal., 2005;Grossetal.,2007a)andbiotic interactions(Soussanaand Lafarge,1998;Grossetal.,2007b,2009;Lavoreletal.,2007).More- over,functionaltraitsareusedtotranslateindividualresponses intothecommunityandtheecosystemlevels(Schymanski,2008;
0304-3800/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ecolmodel.2012.02.002
Suding etal., 2008; Grosset al.,2009).Atthe ecosystemlevel, thesametraits(communitymean-trait,Violleetal.,2007)deter- mineecosystemfunctioningthroughadditive(communityeffectis determinedbymass-ratioprocesses,KlumppandSoussana,2009) andnon-additive(communityeffectisdeterminedbycomplemen- tarityprocesses,Villegeretal.,2008)effects.
Nevertheless,sincealmostallempiricalstudieswithplanttraits arecorrelative,thecausallinkagesbetweendifferentlevelsoforga- nizationsremainunclear(seereviewofLavoreletal.,2007;Savage etal.,2007).Amechanisticapproachcouldhelpimproveourunder- standingoftheroleofplanttraitsforcommunitydynamicsand ecosystemfunctioning(Loreauetal.,2002).Ideally,thisapproach would assemble withinthe same framework‘the explicitinclu- sionoforganismaltrade-offs,ofenvironmentalconstraints,andofthe basicmechanismsofinterspecificinteractions’(Tilman,1990).Such requirementsimplydevelopinga novelmodelingframeworkby assemblingatleastthree components:(i)biophysical lawsthat simulatetheenergy,carbonandwaterexchangesbetweenvegeta- tionandatmosphereatecosystemscale(e.g.Wohlfahrtetal.,2000), (ii)carbon,nitrogenandwatercyclesinthesoil–plant–atmosphere continuum(e.g.ThornleyandJohnson,1990),and(iii)plantpopu- lationdynamicsbasedonresourcecompetition(e.g.Tilman,1988).
Anindividualbased approach mayoffera valuable perspec- tivetobuilddynamicalstructure–function–diversity modelsfor severalreasons(Grimmetal.,2006).Acrossthehierarchyofbio- logicalorganizationlevels,selectionoccursattheindividuallevel (MarksandLechowicz,2006).Inaddition,individualplantsarewell defined,measurableandtheycanbecharacterizedbyparameters derivedfrommeasuredfunctionaltraits,thathavebeenrecently used for new modeling development in ecology (Lehsten and Kleyer,2007;Savageetal.,2007;VanWijk,2007).Moreover,the individualbasedapproachoffersthepossibilitytosimulateplas- ticadjustmentsofplantformandfunction(YinandSchapendonk, 2004;Hoglindetal.,2001)inresponsetoresourcelevelsmediated byinteractionswithneighbors.
To date, most dynamic structure–function models based on planttraitsconcernsingletrees(LeRouxetal.,2001;Allenetal., 2005;MarksandLechowicz,2006).Recently,theoreticalmodels havebeendevelopedtodemonstratetheroleoftraits,theirdiver- sity,theirdegreeofcorrelationandtheirplasticityforecosystem functioning(Norbergetal.,2001;Savageetal.,2007).However, thesemodelsincludeafewtraitsonlyanddonottakeintoaccount biophysicallawsthatconstrainmassandenergyexchangeinplant canopies.Dynamicmodelsofpasturegrasses(Hoglindetal.,2001;
GrootandLantinga, 2004;Lafargeetal.,2005;Tomlinsonetal., 2007)weredevelopedformonocultures.However,thesemodels cannotbeusedformixturesandaresometimesrestrictedtothe simulationofasinglegrowingseason(GrootandLantinga,2004).
Finally, non individual-based models simulating mixed grasses (SchippersandKropff,2001)andgrass-clovergrowth(Lazzarotto etal.,2009)werepreviously developed,buttheyarenotbased onplanttraitsframeworkandtheydonotincludeshootandroot morphogenesis.
We have developed from previous works (Soussana et al., 2000a,b) a dynamic structure–function model which simulates average individual plants for each population of a multi- species canopy consisting of perennial C3 grass species. This model called Gemini (Grassland Ecosystem Model with INdi- vidual centred Interactions) is parameterized from a large number of shoot and root traits in each plant population (Table1).Theaimsofthisstructure–function–diversitymodelare tounderstandhowplanttraitsinteractwithabioticfactorstocon- trolplantpopulationdynamics,plantcommunitydynamicsand ecosystemfunctioning.Inthepresent paper,wereview thekey conceptsofthismodelanddiscussitsparameterizationfromplant traitmeasurements.Weprovideevidenceofthemodelabilityto
simulate:(i)withinandbetweenspeciesvariationinabove-ground productivity along environmentalgradients for grass monocul- tures,(ii)speciesdominanceingrassmixtures.
2. Methods
2.1. Experimentaldata
The experimentalsite usedfor model evaluationwasestab- lishedinspring2002inanuplandareaofcentralFrance(Theix, 45◦43N,03◦01E,870ma.s.l.)onagraniticbrownsoil(Cambisol, FAO).Thelocalclimateissemi-continental,withameanannual temperature of 9◦C and a mean annual rainfallof 760mm. 13 nativeperennialC3grassspeciesthatco-occurinmesicpermanent grasslands were studied in monocultures: Alopecurus pratensis, Anthoxanthumodoratum,Arrhenatherumelatius,Dactylisglomerata, Elytrigiarepens,Festucaarundinacea,Festucarubra,Holcuslanatus, Loliumperenne,Phleumpratense,Poapratensis,Poatrivialis,Trisetum flavescens.ALoliumperennecultivar(Clerpin)wasaddedasacon- trol.Henceforth,inthetext,speciesarereferredtobytheirspecies name.Inaddition,threemixturesofsixspecies,whichweredrawn amongthe13species,werealsostudied.Themixtureswere:(i) D.glomerata,F.arundinacea,F.rubra,L.perenne,P.pratensisandC.
cristatus;(ii)A.pratensis,A.odoratum,A.elatius,E.repens,H.lanatus andT.flavescens;(iii)A.elatius,D.glomerata,E.repens,F.arundi- nacea,F.rubraandH.lanatus.C.cristatuscouldnotbestudiedin monocultureasitsufferedfreezingdamageatthetimeofseedling implantationinmonocultureduringthewinter.Thisspeciesrep- resentedverylowabundance(<2%)withinmixtureandwasnot consideredinthisstudy.
The experimentaldesignhas3 complete randomized blocks each crossingtwo factors:cuttingfrequency (3and6cutsyr−1, C− and C+, respectively) and N fertilizer supply (120 and 360kgNha−1yr−1, N− and N+, respectively). Phosphorus and potassiumweresuppliedinspringatnon-limitingratesforgrowth.
Whensoil water content(SWC) wasbelow 10%, allplots were irrigated(seePontesetal.,2007forfulldetails).Plantfunctional traits of monocultures (Table 1) weremeasured in 2003,2004 and 2006 in thehigh Nsupply (N+)and low disturbance(C−) treatment(Maireetal.,2009;Pontesetal.,2010).Thistreatment providednon-limitingconditionsformorphogeneticdevelopment andabove-groundproductivity;andwasusedformodelparame- terization.Othermonoculturetreatments(C+N+,C−N−,C+N−)as wellasallmixtureswereusedforanindependentevaluationofthe model.
2.2. Modelpurpose
ThemodelisdescribedfollowingtheODD(Overview,Design concepts and Details) standard protocol proposed by Grimm et al. (2006) for individual-based and agent-based models. It should be noted that Gemini is an individual-centred model, rather than being individual-based, since it simulates average individuals withineach plant(oranimal)population.A detailed list of all 132 equations, as well as the 187 variables and the 100 default parameter values and their units is available (atwww1.clermont.inra.fr/urep/modeles/gemini.htm)andwillbe sendonrequest.
ThemainpurposeofGeminiistounderstandthedynamicsand plasticityofplantspecieswithinacommunityandtheroleoftraits andtheirplasticityforecosystemfunctioning.Themodelconsid- ersclimatic(short-waveradiation,temperatureandprecipitation) andatmospheric(CO2concentration)abioticdrivers.Management conditionsconcernbothdisturbance(bycuttingandgrazing)and fertilization(inorganicandorganicNsupply).Themodelwasbuilt
Table1
PlanttraitsusedforthecalibrationoftheGeminimodel.Abbreviations,unitsandreferencesaregivenforeachtrait([1]Cornelissenetal.,2003;[2]Kazakouetal.,2007;[3]
Pontesetal.,2007;[4]Pontesetal.,2010;[5]Craineetal.,2002).Eachtraitwasusedtocalibrateoneorafewparametersbyspecies.Parameterdefinitionsareprovidedin Table2.
Plantfunctionaltrait Abbreviation Unit Linkedreference Modelparameter Modelmechanism
Leafmorphologicaltrait
Leaflength LL cm [3,4,5] Lleaf0,Fsheath,bAL,aAL,bWL,aWL Shootmorphogenesis
Sheathlength SL cm [3] Fsheath Shootmorphogenesis
Leafarea LA cm2 [1] bAL,aAL Shootmorphogenesis
Leaffreshmass LFM mg [5] bWL,aWL Shootmorphogenesis
Leafdrymass LDM mg [5] bWL,aWL Shootmorphogenesis
Leafdrymattercontent LDMC mgDMg−1FM [1,3,5] LDMC Shootmorphogenesis
Specificleafarea SLA m2kg−1 [1–5] bWL Shootmorphogenesis
Numberofgrowingleaves NG nbtiller−1 [4] ngleaf Shootmorphogenesis
Numberofmatureleaves NM nbtiller−1 [4] nmleaf Shootmorphogenesis
Rootmorphologicaltrait
Primaryrootlength RL cm [5] LLplast,Lroot1 Rootmorphogenesis
Rootmaximalorder RO nb [5] LLplast,rrplast,ordermax Rootmorphogenesis
Rootlifespan RLS Degreeday [5] Trootg1,Trootm1 Rootmorphogenesis
Primaryrootdiameter RD mm [1,5] rrplast,rrroot1 Rootmorphogenesis
Typeofrootreserveorgan TRO Rhizome,stolon... [1] WrrCmax,WrrNmax Rootmorphogenesis
Plantmorphologicaltrait
Vegetativeelongatedplantheight VE cm [1,5] C Shootmorphogenesis
Tillerdensity TD tillersm−2 [4] intcl,Tsen0 Populationdynamic
Plantphenologicaltrait
Earlinessofgrowth EG – [4] bLER Populationdynamic
Phyllochron PH Degreeday [2,4] ph0 Shootmorphogenesis,
Populationdynamic Leafandrootphysiologicaltrait
LeafNresorptionrate RE % [2] fns,RN,RP Physiology
Leafmasslossatsenescence Massloss % [2] fcs,RC Physiology
RootNuptakecapacity Imax mgg−1DMh−1 [1,5] Su0 Physiology
Leafandrootchemicalcompositiontrait
LeafNcontent LNC mgg−1 [1–5] fns Chemicalcomposition
LeafCcontent LCC mgg−1 [5] fcs Chemicalcomposition
RootNcontent RNC mgg−1 [5] fnr Chemicalcomposition
RootCcontent RCC mgg−1 [5] fcr Chemicalcomposition
withamodulararchitecture,whichpermitstoincludeornotdif- ferentbioticagents(plantspecies,soilmicrobialdecomposers,and herbivoresatgrazing)aswellasdifferentenvironmentandman- agementmodules(soil,vegetation,fertilizationandcutting).
Geminicansimulateapotentiallyunlimitednumberofplant species(orplantpopulationsfromthesamespecies)fromcurrently twoplantfunctionaltypes(perennialgrassesandlegumes).The modelfocusesontheacquisitionandtheutilizationoftwomajor resources(lightandnitrogen)byplantsandtheirconsequencesfor thecarbonandnitrogencycles.
2.3. Statevariablesandscales
Geminiconsistsofvegetation,soilandherbivoresub-models, coupledwithenvironmentandmanagementsub-models(Fig.1).
Thevegetationsub-model,namedCanopt(Soussanaetal.,2000a,b) isanindividual-centredmodelofamulti-speciesstandcompris- ingclonalgrassesand/orlegumesandformingamulti-layerplant canopy.Eachclonalplantpopulationisdescribedasacollectionof identicalaxes(e.g.tillersforgrasses).Moreover,allplantspeciesare assumedtobeperfectlymixedinthehorizontalplane.Plantpopu- lationdemographyiscalculatedfromthevegetativemultiplication andmortalityofaxes.
Thevegetationsub-modelconsistsoffourmodules:(i)abio- chemicalmodule,which simulatestheCand Nbalanceandthe partitioningofgrowthamongshootstructures(WS),leafproteins (WP)androots(WR)ofacollectionofidenticalaxesforeachplant population.Thecorrespondingstatevariablesarethenumberof axesbypopulation(D),themasses ofthreestructuralcompart- ments,oftwoCandNsubstratecompartmentsandoffourCandN reservecompartments;(ii)ashootmorphogenesismodule,which computesthedemographyandsizeofleaves(twostatevariables, lengthandmassperleaf);(iii)arootmorphogenesismodule,which
computesthedemographyandsizeofroots(twostatevariables, lengthandmass,perroot);(iv)acompetitionmodulewhichcal- culatesshort-waveradiationandinorganicNpartitioningamong mixedplantspecies.
Theenvironmentsub-modelcalculatesthemicroclimatewithin thecanopyandtheinorganicNbalanceofthesoil(orofthesub- stratewhenthevegetationmodelisnotcoupledtothesoilmodel).
Themanagementsub-modelscheduleseventscausedbygrassland management(cuttingdates,grazingperiods,Nfertilizerapplica- tions).
2.4. Processoverviewandscheduling
ThecarbonbalanceisbasedonFarquharetal.(1980)equations forleafphotosynthesis whichwereupdated accordingtoMaire (2009).AlinearrelationshipofVcmax(themaximalcarboxylation activityofRubisco)andJmax(theelectrontransportcapacity)with theareabasedleafproteinconcentration(Field,1983;Nijsetal., 1995)isassumed.Theverticalprofileofleafproteinsissimulated accordingtoHiroseandWerger(1987).Respirationisdividedinto growthandmaintenancecomponents.Leafrespirationvarieswith leafproteincontent.
Nitrogenacquisitionisbasedonsoildiffusionprocessesfrom soiltorootssurface(Barber,1995).Themodelseparatesmetabol- ically activerootsandroots whichhave entereda firststageof senescence(seemorphogenesis)andhavelosttheiruptakecapac- ity.
Therelativegrowthrateofthethreestructuralcompartments (WP,WS,WR)iscalculatedthroughabi-substrateC–Ngrowthequa- tionusingthreepartitioningvariables,oneforeachcompartment (JohnsonandThornley,1987).Thepartitioningsub-modelspeci- fiesatargetroot/shootratio,accordingtothefunctionalbalance hypothesis(Brouwer,1962;Davidson,1969;HilbertandReynolds,
