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Diurnal and annual exchanges of mass and energy between an aspen-hazelnut forest and the atmosphere:… Grant, R. F.; Black, T. Andrew; den Hartog, Gerry; Blanken, P. D.; Yang, P. C.; Russell, C.; Nalder, I. A.; Berry, J. A.; Neumann, Herman H. Nov 30, 1999

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JOURNAL OF GEOPHYSICAL  RESEARCH, VOL. 104, NO. D22, PAGES 27,699-27,717, NOVEMBER  27, 1999  Diurnal and annual exchangesof mass and energy between an aspen-hazelnut forest and the atmosphere: Testing the mathematical model Ecosyswith data from the BOREAS experiment R. F. Grant,a T. A. Black,2 G. den Hartog,3 J. A. Berry,4 H. H. Neumann,3 P. D. Blanken,2 P. C. Yang,2 C. Russell,s and I. A. Nalder• Abstract. There is muchuncertaintyaboutthe net carbon(C) exchangeof boreal forest ecosystems, althoughthis exchangemay be an importantpart of global C dynamics.To resolvethis uncertainty,net C exchangehas been measuredat severalsitesin the boreal forestof Canadaas part of the BorealEcosystem-Atmosphere Study(BOREAS). One of thesesitesis the SouthernOld Aspen site at which diurnal CO2 and energy(radiation, latent, and sensibleheat) fluxeswere measuredduring 1994usingeddycorrelation techniquesat different positionswithin a mixed 70 year old aspen-hazelnutforest.These measurementswere usedto test a complexecosystemmodel "ecosys"in which massand energyexchanges betweenterrestrialecosystems and the atmosphereare simulatedhourly under diverseconditionsof soil,management,and climate.Thesesimulationsexplained between70% and 80% of diurnalvariationin ecosystemCO2 and energyfluxesmeasured duringthree 1 week intervalsin late April, early June, and mid-July.Total annual CO2 fluxesindicatedthat during 1994,aspenwas a net sink of 540 (modeled)versus670  (measured) g C m-2 yr-•, whilehazelnut plussoilwerea netsource of 472(modeled) versus 540(measured) g C m-2 yr-•. Theaspen-hazelnut forestat theBOREASsitewas thereforeestimatedto be a net sink of about68 (modeled)versus130 (measured)g C  m-2 yr-• during1994.Long-term simulations indicated thatthissinkmaybe largerduring cooleryearsand smallerduringwarmer yearsbecauseC fixationin the model was less sensitiveto temperaturethan respiration.These simulationsalsoindicatedthat the magnitudeof this sink declineswith forestage becauserespirationincreaseswith respect to fixationas standingphytomass grows.Confidencein the predictivecapabilitiesof ecosystemmodelsat decadalor centennialtimescalesis improvedby well-constrainedtests of thesemodels at hourly timescales. parisonto fixationand respiration[Blacket al., 1996],there is a needfor accuratecalculationof the responses of fixationand Boreal forestsare currentlythoughtto be an importantsink respirationto climate if reliable estimatesof climate change for atmosphericC [Keelinget al., 1996].However,the warming effectson net C exchangeare to be made. This need is most of theseforestscausedby risingCO2 concentrationsand tem- likely to be met by biophysicallybasedecosystemmodelsthat peratureshypothesizedin someglobalclimatechangestudies accuratelyrepresentthe basicprocessesby which climate af[e.g.,Sellerset al., 1996]havecausedconcernthat thissinkmay fectsC fixation and respiration. be reduced,leadingto increasedC accumulationin the atmoThe sensitivityof net C exchangeto hypothesized changesin sphere.The sensitivityof net C exchangeto climatechangeis climate will be primarily determinedby the effects of rising on massand energyexchangebetweenthe determinedby complexinteractionsbetween C fixation and CO2 concentrations respiration,each of which is in turn determinedby complex forestcanopiesand the atmosphere,andby the effectsof rising responsesto climate. Becausenet exchangeis small in com- temperature on evapotranspirationand respirationby forest canopiesand soils.The effectsof CO2 on massand energy •Department of Renewable Resources, University of Alberta,Ed- exchangearisefrom thoseon C fixation [Stitt, 1991] and stomonton, Canada. matal conductance[Curtis, 1996]. The effect of CO2 on C 2Department of SoilScience, University of BritishColumbia, Van- fixation is causedby its effect on rubiscospecificityfor CO2 couver, Canada. versus02 [Jordanand Ogren,1984].The magnitudeof the CO2 3Atmospheric Environment Service, Environment Canada,Downseffect on C fixationis influencedby temperature[Idso et al., view, Ontario. 4Department of PlantBiology, Carnegie Institution of Washington, 1987] through the effect of temperatureon rubiscokinetics Stanford, California. and on the solubilityof CO2 versus02. It is alsoinfluencedby 5Department of Land ResourceScience,Universityof Guelph, irradiance[Allen et al., 1990], becauseCO2 affectslight and Guelph,Ontario, Canada. dark reactionsto differingdegrees,and by nutrients[Kimball, Copyright1999 by the AmericanGeophysicalUnion. 1993] through constraintsthat nutrient availabilityimposes upon rubiscokinetics.The effect of CO2 on stomatalconducPaper number 1998JD200117. 0148-0227/99/1998JD200117509.00 tanceoccursbecauseCO2 affectsC fixationcomparativelyless 1.  Introduction  27,699  27,700  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  than it doesCO2 concentrationgradientsacrossstomata[Ball, 1988].Changesin conductance with CO2 causeCO2 effectson C fixationto be influencedby plant water stress[Chaudhuriet al., 1990;Rogerset al., 1986].The biologicalbasisfor eachof theseCO2 effectson C fixationand stomatalconductance must be representedin ecosystemmodels used to study climate changeeffectson net C exchange. Rising temperaturescause higher evapotranspirationby causinggreatervapor pressuregradientsto developbetween the soilandplant surfacesandthe atmosphere. Actualchanges in evapotranspiration under hypothesizedchangesin climate will dependupon the extentto whichhigherevapotranspiration due to highertemperaturewill be offsetby lower transpiration due to lower stomatalconductanceunder higher CO2. Sustainedincreasesin temperature will eventually lead to lower soil and plant water statusand hence lower stomatal conductanceand C fixation. Rising temperaturesalso cause highersoiland plant respirationwhichif not offsetby higherC fixationwill causenet lossesof soil C [Jenkinson et al., 1991; Kirschbaum,1995;Partonet al., 1995].The biologicalbasisfor eachof theseeffectsof temperatureon evapotranspiration and respirationmustbe representedin ecosystem modelsusedto studyclimatechangeeffectson net C exchange. Ecosystemmodelsusedto estimateclimatechangeimpacts on net C exchangeshouldbe basedupon algorithmsin which complexinteractionsamong temperature,irradiance, nutrients,CO2 andwater on C, andwater exchangeare represented usingbasicbiophysicalprinciples.Theseinteractionsare highly dynamicand nonlinear,with pronouncedtemporaland spatial variationat a subdaily(e.g., hourly) timescaleand at a subcanopy(e.g., leaf) spatialscale.Some of the current efforts [e.g.,Bonanet al., 1997;Kimballet al., 1997;Frolking,1997]to model these interactionsuse temporally aggregatedclimate data (irradiance,temperature,humidity,wind, and precipitation), usuallyat a daily timescale,and spatiallyaggregated vegetation,usuallyat a communityscale,to calculateclimatic effectson C and water exchange.However, the complexand nonlinear  nature  of climate  interactions  on C and water  ex-  changemay reduceconfidencein the resultsof suchaggregation. Results from these models cannot be tested directly againstfluxesof massand energymeasuredat subdailytimescalesbut rather againstindicesof thesefluxesthat havebeen aggregatedto the samespatialand temporalscalesas thoseat which the model functions.These indices(e.g., daily net C exchange)are the net resultsof severaldifferent interacting processes (e.g., transientplant C fixationand plant plus soil respiration),somodeltestingcannotbe resolvedto the process level at which it is better  constrained.  We proposean alternativeapproachto the modelingof net C exchangein whichthe major processes of C transformation are first simulatedand testedat temporaland spatialscalesof the hourandthe organ(e.g.,leaf) or population(e.g.,species). Resultsfrom thesetestsare then aggregatedto and testedat temporaland spatialscalesof the decadeand the community [Grant,1996a,b]. This approachbenefitsfrom the better constrainedtestingpossibleat higher temporal and spatialresolutionwhile enablingthe examinationof modelbehaviorat the lower temporal and spatialresolutionat which ecosystemresponseto climatechangemustbe evaluated. Mass and energyexchangebetweenthe boreal forestsand the atmospheresuitablefor high-resolution modeltestinghave been measured at several sites in the Boreal EcosystemAtmosphereStudy(BOREAS) [MargolisandRyan,1997].Ex-  Rn,LE,H  CO2  overstow  Rn,LE,H  CO2 understow  Figure 1. Mass and energy exchangesbetween the atmosphereand the complexsoil and plant surfacesrepresentedin "ecosys."Rn, net radiation; LE, latent heat; H, sensibleheat; C, carbon;N, nitrogen;P, phosphorus.  changes measuredat the SouthernOld Aspensite[Blacket al., 1996]were selectedfor initial model testingbecausemeasurementswere conductedat different spatialscales(organ,species, community)which allowed concurrentmodel testing. Measurements  at other  sites in BOREAS  will be used for  model testingat a later date.  2. 2.1.  Model Development Ecosystem-AtmosphereEnergy Exchange  The accuratesimulationof energy exchangebetween the atmosphereand the terrestrialsurfacesis a key requirementof ecosystem modelssuchas "ecosys,"becausethis exchangeis believedto affectthe exchangeand transportof energyin the atmosphereand hence to affect climate. Ecosystem-atmosphere energy exchangeis stronglycontrolledby ecosystem water and nutrient status, so that these controls must be accu-  rately representedin ecosystem models. Energyexchangebetweenthe atmosphereand the terrestrial surfacesis resolvedhourly in ecosysinto that betweenthe atmosphereand the canopyof eachplant species,asdescribed by Grantet al. [1998,equations(1)-(15)] andelsewhere[Grant and Baldocchi,1992;Grant et al., 1993e,1995c]and that between the atmosphereand each of snow, residue,and soil surfaces[Grantet al., 1998,equations(16)-(23)], Grant[1992], Grantet al. [1995b](Figure1). Total energyexchange between the atmosphereand the terrestrialsurfacesis calculatedasthe sum of the exchanges with each plant canopyand each snow, residue, and soil surface.  Canopy energy exchangein ecosysis calculatedfrom an hourlytwo-stageconvergence solutionfor the transferof water and heatthrougha multispecific, multilayeredsoil-root-canopy system.The first stageof this solutionrequiresconvergence to a canopytemperatureat which the first-orderclosureof the energybalanceis achievedfor the canopyof eachplantspecies. The energybalancerequiresfirst that the absorption,reflection andtransmission of both shortwaveandphotosynthetically active radiation  be calculated  for each leaf and stem surface in  a multilayeredcanopy.Radiation includesdirect and diffuse sources,definedby solarand skyanglesaswell asforwardand backscattering within the canopies.Leaf and stemsurfacesare definedby species, height,azimuth,inclination,exposure(sunlit versusshaded)and opticalproperties.Nonuniformityin the  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  27,701  horizontal distributionof leaf surfaceswithin each canopy soil water potential gradients,which are also determinedby layer (clumping)is representedby a species-specific intercep- soil-root and root-canopy hydraulic conductancesin each tion fraction between zero and 1 which describes the fractional rootedsoil layer.Soil-rootconductanceis calculatedfrom root exposureof leaf and stem surfacesto direct and diffuseirra- length, given by a root growth submodel[Grant, 1993a, b, diance (versusself-shading).The fraction of photosynthetic 1998a;Grant and Robertson,1997],and from soil-roothydrauphotonflux densityabsorbedby eachcanopyis usedto parti- lic conductivitycalculatedaccordingto Cowan[1965]. Roottion the exchange of longwave radiation emitted by sky, canopyconductanceis calculatedfrom radial and axial conground,andcanopysurfaces,the net valuesof whichare added ductances[Reid and Huck, 1990] of primary and secondary to total shortwaveradiation absorbedby all leaf and stem roots,as describedby Grant [1998a].If the convergence critesurfacesto calculatecanopynet radiation. rion for differencebetweencanopytranspirationand uptake The energybalancethen requiressolutionsfor latent and versuschangein canopywater contentis not met, the energy sensibleheat fluxesat the temperatureof eachcanopy.If free balanceis solvedagain usingan adjustedvalue for canopy water is present on leaf or stem surfaces,latent heat flux is water potential.The convergence is then repeatedusingnew calculated from evaporation determined by canopy-atmo- valuesfor transpiration,uptake,and changein canopywater spherevapor densitygradientand aerodynamicconductance. contentcalculatedfrom the adjustedwater potential. If free water is not present,latent heat flux is calculatedfrom transpirationfrom leaf surfaces,which is also determinedby 2.3. Canopy C Fixation stomatal conductance. Sensible heat flux is calculated from Becauseleaf C fixationrate partly determinesleaf conduccanopy-atmosphere temperature gradient and aerodynamic tanceto water vapor,the accuratesimulationof leaf C fixation conductance.Canopyheat storageis calculatedfrom changes is an important requirementof ecosystemmodels.Leaf C in canopytemperatureand from the massesand water con- fixationis determinedby carboxylation, whichis controlledby tents of leaves,twigs,and stems. irradiance,temperature,and leaf CO2 concentration,and by Aerodynamicconductancein energy balance solutionsis diffusion,whichis controlledby the atmosphere-leaf CO2 concalculatedfrom zero plane displacementand surfacerough- centrationgradient and leaf conductance.The couplingof nessheightsof eachcanopyderivedfrom their heightsand leaf carboxylationand diffusionin ecosysallowsthe calculationof areas [Perrier,1982]. Aerodynamicconductanceof nondomi- a leaf C fixationrate, whichis then aggregatedto the canopy nant canopiesis reducedaccordingto the differencesbetween level. their heightsand that of the dominantcanopy,asproposedby This couplingoccursafter successful completionof the secChoudhuryand Monteith[1988]. Stomatalconductanceis cal- ond stage of the convergencesolution for heat and water culatedfor eachleaf surfaceof eachcanopyfrom leaf carbox- transfer,whena convergence solution[Grantet al., 1998,equaylationrate and from the CO2 concentrationgradientacrossits tion (54)] is usedto calculategaseousCO2 concentrationand stomates[Grant et al., 1998, equations(48)-(53)]. Leaf con- its aqueousequivalentin the mesophyllof eachleaf surfacein ductancesare aggregatedto the canopylevel for energybal- eachcanopy.These are the concentrations at whichthe diffuancecalculations [Grantet al., 1998,equations(13)-(15)]. Two sion rate of gaseousCO2 [Grant et al., 1998, equation(48)] controllingmechanismsare postulatedfor stomatalconduc- equalsthe carboxylationrate of aqueousCO2 within eachleaf tance:(1) leaf carboxylation rate at nonlimitingwaterpotential surface[Grantet al., 1998,equation(47)]. The diffusionrate is as determinedby ambientirradiance,temperature,and CO2, calculatedfrom the concentrationgradientacrossthe stomates and (2) canopyturgorpotentialat ambienttotal and osmotic dividedby the stomatalconductance [Grantet al., 1998,equawaterpotentials[Grantet al., 1998,equations(24) and(25)] as tions (49) and (50)] givenfrom the convergencesolutionfor determinedby canopywater potential.The calculationof leaf heat and water transfer describedabove. The carboxylation carboxylationrate and canopyturgor potential is described rate is the minimum of that from dark and light reactions below.A hypothesis that stomatalconductance is controlledby [Grantetal., 1998,equations(38)-(46)] calculatedaccording to root-derivedsignals[e.g.,Gollanet al., 1986]is not yet included Farquharet al. [1980].Thesereactionsare drivenby the prodof rubisco in the modelbecauseof uncertaintyin its parameterization. uct of the specificactivitiesand areal concentrations or chlorophyllat each node. These concentrationsare deter2.2. Canopy Water Relations minedby the growthof eachleaf as affectedby environmental The simulationof water statuseffectson energyexchangeis conditions(CO2, radiation,temperature,water,nitrogen).The basedon couplingthe uptake of water from the soil through photosyntheticphoton flux densityby which the light-limited the root to the canopywith the evaporationof water from the carboxylationrate is controlledis givenby the absorptionof canopyto the atmosphere. This couplingdeterminesthe water photosynthetically activeradiationdescribedabove.The CO2 statusof the canopyandhenceits conductance to watervapor. fixation rate of each leaf surface is added to arrive at a value The secondstageof the hourlysolutionfor heat and water for grossCO2 fixationby eachbranchof eachcanopy. transferthereforerequiresconvergenceto a water potential for eachcanopyat whichthe differencebetweencanopytran- 2.4. Canopy C Respiration spirationand root water uptake [Grantet al., 1998,equations Plantsboth fix and respire C, so net C exchangebetween (32)-(37)] equalsthe differencebetweencanopywatercontent plantsand the atmosphereis the differencebetweenthe two. If at its previouswater potential and that at its currentwater ecosystemmodels are used to calculate net C exchangeand potential. Canopy water potential controlstranspirationby henceecosystem contributions to atmosphericCO2 concentradeterminingcanopyturgor (calculatedas the differencebe- tions, the accuratesimulationof C respirationis essential. tweencanopywater and osmoticpotentials)whichaffectssto- Respirationhastwo components: maintenancerespiration,rematal conductanceand thereby canopy temperature,vapor quired to maintainthe biologicalintegrityof the plant, and pressure,and conductanceto vapor transfer. Canopywater growthrespiration,requiredto form new plant material. potentialcontrolsroot water uptakeby determiningcanopyThe hourlyproductof CO2 fixationis addedto a C storage  27,702  GRANT ET AL.: MODELING  MASS AND ENERGY  EXCHANGE  pool for eachbranchof eachcanopyfrom which C is oxidized hourly to meet maintenanceand growth respirationrequirements [Grant et al., 1998, equations(27) and (28)]. Low C storagemay causeC oxidation to be less than maintenance respiration,in which casethe differenceis made up through respirationof remobilizableC (consideredto be equalto the protein fraction) of leaf and twig C. Remobilizationstartsat the lowest node at which leaves and twigs are present and proceedsupward.Upon exhaustionof the remobilizableC in eachleaf or twig, the remainingC is droppedfrom the branch  the resourceconsumption rate of eachplant part. StorageN or P concentrationand turgor potential may constrainC oxidation for growthin differentpartsof the shootand root, causing storageC to migrate toward zones of lower C concentration where C oxidationis more rapid. Low storageN or P concentrationsmay alsoreduceleaf N and P concentrations, thereby reducingleaf carboxylationrates and hence leaf conductance. For perennialplant species,solubleC, N, and P are withdrawn from storagepoolsin shootbranchesinto a long-termstorage pool in the crown during autumn, causingleaf senescence. and added to the soil surface. Environmental constraints such SolubleC, N, andP are remobilizedfrom thispool to driveleaf asN, heat, or water stresswhichreduceC fixationwith respect and twig growththe followingspring.The timing of withdrawal to maintenancerespirationwill thereforeacceleratethe lossof and remobilizationis determinedby duration of exposureto leaf and twig C from the plant. Net CO2 fixationis calculated cooltemperatures(between3øand 8øC)undershorteningand for each branch as the differencebetween grossfixation and lengtheningphotoperiods,respectively. the sum of maintenance,growth,and senescence respiration. 2.7.  2.5.  Nutrient Uptake  The simulationof nutrientstatuseffectson energyexchange is basedon the couplingof nutrient(nitrogenN andphosphorusP) uptakefrom the soilthroughthe root to the canopywith nutrient assimilationin the root and canopy.This coupling determines  nutrient  concentrations  in the leaf which in turn  Soil Microbial Activity  The aqueousconcentrationsat which nutrients are maintained in the soil and hencethe rates at which they are taken up by plantsare stronglycontrolledby microbialactivityin the soil.This activityis coupledto the oxidationof soil C and the reductionof 02 and other electronacceptorsand henceto the exchangeof C with the atmosphere.The accuratesimulationof soilmicrobialactivityis thereforean importantrequirementof ecosystemmodels. Microbial activityin ecosysis representedasa parallelsetof substrate-microbe complexes[Grant et al., 1993a, b] which includethe rhizosphere[Grant, 1993c],plant residuesand animal manure [Grant and Rochette,1994], and native organic matter [Grantet al., 1993a,b]. Within eachcomplexthe activities of obligately aerobic, facultativelyanaerobic,and obligatelyanaerobicheterotrophsare simulatedhourlyat the temperaturesand water contentsof plant surfaceresidueand of a spatiallyresolvedsoilprofile [Grant, 1997;Grant and Rochette, 1994;Grant et al., 1997]givenfrom the energybalancecalculationsdescribedabove.Aspen and hazelnutresiduesare partitioned into carbohydrate,protein, cellulose,and lignin fractions accordingto Trofymowet al. [1995], each of whichis of differing vulnerabilityto hydrolysisby heterotrophicdecom-  determineleaf carboxylationratesand henceleaf conductance. Nutrient(N andP) uptakeiscalculatedhourlyfor eachplant speciesby iteratively convergingtoward values for aqueous concentrationsat its root and mycorrhizalsurfacesin eachsoil layer at whichradial transportby massflow and diffusionfrom the soil solutionto the surfacesequalsactiveuptake by the surfaces[GrantandRobertson, 1997,equation(14)]. The aqueousconcentrationsof nutrientsin eachsoil layer are controlled by precipitation,adsorption,and ion-pairingreactions[Grant and Heaney,1997],solutetransport[Grantand Heaney,1997], and microbial activity [Grant et al., 1993a, b]. Mass flow is calculatedfrom root water uptake describedabove,and diffusion is calculatedfrom root length densitiesgivenby the root growthsubmodel[Grant,1993a,b, 1998a;GrantandRobertson, 1997]. Root uptake is calculatedfrom root surfacearea [Itoh and Barber,1983], givenby the root growthsubmodel,and is constrainedby root oxygenuptakeand nutrientstorage[Grant, posers(Table 3). Soil organicmatter is alsopartitionedinto fractionsof differingvulnerabilityto hydrolysis.All heterotro1998a]. phic populationsconductC oxidationto supportgrowth and 2.6. Plant Growth maintenanceprocesses, the total of whichdrivesCO2 emission Growth respirationdrives expansivegrowth of vegetative from the soil surface.This oxidationis coupledto the reducand reproductive organs at different nodes of each shoot tion of 02 by all aerobicpopulations(includingpopulationsof branchthroughmobilizationof storageC, N, and P according N 2 fixers),to the sequentialreductionof NO•-, NO•-, andN20 to phenology-dependentpartitioning coefficients and bio- by heterotrophicdenitrifiers[Grantet al., 1993c,d; Grant and chemicallybasedgrowthyields[GrantandHesketh,1992].This Pattey,1999] and to the reductionof acetateby heterotrophic growthis usedto calculatethe lengths,areas,and volumesof methanogens[Grant,1998b].In addition,autotrophicnitriflers individualinternodes,sheaths,and leaves[Grant,1994a;Grant conductNH•- and NO•- oxidation[Grant, 1994b]and N20 and Hesketh,1992]from whichheightsand areasof leaf and evolution[Grant, 1995], and autotrophicmethanotrophsconstem surfacesare calculatedfor irradiance interceptionand duct CH 4 oxidation[Grant, 1999]. The ratesof all heterotrophicand autotrophicoxidationsare aerodynamicconductancealgorithmsdescribedabove.Growth respirationalsodrivesextensionof primaryand secondaryroot drivenby changesin the free energiesof reactantsand prodaxesand of mycorrhizalaxesof eachplant speciesin eachsoil ucts.All solubleand gaseousreactantsand productsundergo transportthroughthe soil profile [Grant layer throughmobilizationof storageC, N, and P as described convective-dispersive by Grant [1993a].This growthis usedto calculatelengthsand et al., 1993c;GrantandHeaney,1997]. areasof root and mycorrhizalaxesfrom whichroot uptake of water [Grant et al., 1998] and nutrients[Grant and Robertson, 3. Field Experiment 1997] is calculated.Transfersof storageC, N, and P among different shootbranchesand root axesare drivenby concen- 3.1. Site Description trationgradients[Grant,1998a]whicharisefrom the proximity The Southern Old Aspen site of BOREAS is located at of eachplant part to the site of resourceacquisitionand from 53.7øN106.2øW(PrinceAlbert National Park, Saskatchewan,  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  27,703  Table 1. Physicaland BiologicalPropertiesof the Orthic Gray Luvisolat the SouthernOld Aspen Site Reported by D. Anderson(TE-01) and usedin "Ecosys" Depth, m 0.01  BD, Mg m-3 0_.03MPa , m3 m-3 0_l.5MPa , m3 m-3 Sand,kgkg-1 Silt,kg kg-1 pH  CEC, cmolkg-1 Org.C, g kg-1 Org.N, mgkg-1 Org.P, mgkg-1  0.16 0.40 0.20 0.0 0.0 6.7  110 397 21246 1741  0.03  0.16 0.40 0.20 0.0 0.0 6.7  110 397 21246 1741  0.06  0.16  0.45  0.61  0.81  1.01  1.43  1.79  1.95  0.16 1.24 1.53 1.53 1.53 1.42 1.53 1.53 1.53 0.40 0.149 0.211 0.196 0.175 0.194 0.177 0.245 0.257 0.20 0.061 0.130 0.113 0.106 0.125 0.089 0.138 0.157 0.0 610 484 576 514 486 484 550 364 0.0 281 222 190 263 281 276 312 396 6.8  6.4  67.0 331 20596 1379  8.2 6.5 483 216  6.5  19.1 5.1 491 237  6.6  15.0 3.0 351 324  8.2  11.6 4.2 289 494  8.3  11.4 4.0 280 454  8.4  8.3 3.5 257 505  8.5  11.7 3.6 249 529  8.9  13.3 4.6 195 511  Org, organic.  Canada) near the southernlimit of the boreal forest on an 3.3. Leaf CO2 Fixation Orthic Gray Luvisol(Typic Haploboroll,Table 1) overlyinga On August 24, 1994, selectedleavesnear the tops of the glacial till. The overstoryat this site is 70 year old aspen aspenand hazelnutcanopieswere enclosedin the cuvetteof a (Populustremuloides Michx.), 21.5 m highwith a stemdensity portable gas exchangesystem(model MPH-1000, Campbell of 860ha-•. Theunderstory isdominated byhazelnut (Corylus Scientific,Logan,Utah) with an infraredgasanalyzer(model cornutaMarsh.), 2 m high with occasionalwild rose (Rosa 6262, LiCor Inc., Lincoln,Nebraska)and a dewpoint mirror woodsiiLindl.) and alder (AlnuscrispaPursh). (model Dew-10, General Eastern, Woburn, Massachusetts) whichenabledprecisecontrolof CO2, temperature,irradiance, 3.2. Canopy Mass and Energy Exchange and humidityat the leaf surface.The leaveswere subjectedto Fluxeswere measuredduring 1994 using eddy correlation incrementalchangesin either CO2 concentration,air temperwith sonicanemometersmountedabovethe aspenoverstoryat ature, or irradiance with all other environmental conditions 39 m (DAT-310, Kaijo-Denki, Tokyo) and abovethe hazelnut held constant.Responseof CO2 flux to CO2 concentrationwas understoryat 4 m (model 1012R2A Solent,Gill Instruments, measured at highirradiance (about1000/xmolm-2 s-j) and Lymington,England) and with closed-pathinfrared gas ana- high chamberhumidity(70-80%). Responseof CO2 flux to lyzers(model6262,LI-COR Inc., Lincoln,Nebraska).Instan- irradiancewas measuredafter preconditioningto 1000/xmol taneousmeasurements of verticalwind speedand scalarquan- m-2 s-•. Irradiance wasthenincreased in stepsof about300 tities at 20 Hz were usedto calculatehalf-hourlyaveragefluxes /xmolm-2 s-• untilsaturation wasevidentandthendecreased with corrections for air densityandsamplecellpressure.Vapor in stepsto zero. Responseof CO2 flux to temperaturewas densitiesmeasuredat 4 m were checkedagainstthosefrom an measuredby increasingchambertemperaturefrom 15øto 35øC open-pathH20 analyzer(model K20 hygrometer,Campbell in stepsof 2.5øCunder saturatingirradianceand constantdew Scientific,Logan,Utah). Net radiationwasmeasuredwith net point temperature. This protocol was chosen to mimic the radiometersmountedon the main flux tower at 33 m (model covariationof leaf temperatureandvaporpressuredeficitthat S-l, Swissteco Instruments,Oberriet, Switzerland)and on a occursnaturallyon warm, dry daysin this environment.Mea65 m mobile tram at 4 m (models S-1 and S-14 miniature, surementsof CO2 flux and stomatalconductancewere taken Swissteco Instruments). when steadystatevalueswere achieved(usually30 min after Daytime CO2 flux was measuredduring 1994 at the soil conditionswere changed). surfacewith a soil respirationchamber (model 6000-09 LICOR) attachedto PVC collarsand a portablephotosynthesis 3.4. EcosystemC Distribution system(model6200LI-COR). Meteorologicaldata,including Allometric equationsrelating diameter measuredat 1.4 m global shortwaveradiation, air temperature,wind speed,huabove groundto biomassof stem,branch,and foliage,to sapmidity and precipitation,were recorded as 15 min averages during 1994, 1995, and 1996 over the aspen canopyat the woodvolume,and to leaf area were developedby Goweret al. SouthernOld Aspen site, as describedby Shewchuk[1996]. [1997]for eachtree withineachof four 30 m x 30 m replicated Yearly-averagedclimate data are given in Table 2. Further plotsnear the flux tower at the SouthernOld Aspensite.Each detailsaboutsitemeasurements are reportedby Blankenet al. of thesetreeswas taken to representa number of other trees of similar stature on an areal basis in order to scale all mea[1997] andBlacket al. [1996]. surementsfrom the plot to the hectare.Understoryvegetation was removedfrom a 2 m x 2 m subplotrandomlylocatedin Table 2. Yearly AveragedClimate Data Recordedat the eachof the 30 m x 30 m main plotsand storedat 3øC.Samples SouthernOld Aspen Site During 1994, 1995, and 1996 of this vegetationwere separatedinto ephemeralplant material, and into new or old foliage and twigs from perennial 1994 1995 1996 plants.Samplecomponentswere then dried and weighed. 6.16 4.83 3.73 Maximumtemperature,øC Distancesbetweenadjacentringswere measuredby Gower -2.96 -3.49 -5.01 Minimum temperature,øC et al. [1997]to the nearest/am(Dual Axis OpticalMicrometer 11.03 11.45 11.10 Solarradiation,MJ m-2 d-1 and Spalding B5 Digital Position Display System,Gaertner 1.17 1.08 1.14 Precipitation, mmd-1 Scientific,Chicago,Illinois)in coresremovedfrom eachtree in  27,704  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  Table 3. BiologicalPropertiesof Aspen,Hazelnut, and Soil Microbial PopulationsUsed in Ecosys Variable  Value  Units  Aspenand Hazelnut  Maximumcarboxylation rate Maximum rubiscooxygenationrate Maximum electron transportrate Quantum efficiency M-M constantfor carboxylation M-M constantfor oxygenation Fractionof leaf protein in rubisco Fraction of leaf protein in chlorophyll Maximum  50 10.5 500  •mol CO2g- 1 rubiscos- 1 •mol 0 2 g-1 rubiscos-1 •mol e- g-1 chlorophyll s-1  0.5 12.5 500  •mol e- •mol quanta /.tM CO2 /.tM 02  N:C ratio in leaf  N:C ratio in twig and root N:C ratio in stem  Maintenancerespirationof plant Growth yield of leaf and twig Growth yield of stem Growth yield of root Interceptionfraction, aspen Interceptionfraction,hazelnut  0.25 0.05 0.15 0.0375 0.0025 0.016 0.64 0.84 0.64  g C g-1 C g C g-1 C g N g-1 C g N g-1 C g N g-1 C g C g-1 N h-1 at 30øC gCg -1C g C g-1 C gC g-1C  0.65 1.0  m 2 m -2 m2 m-2  Soil  Decompositionof residuecarbohydrate Decompositionof residueprotein Decompositionof residuecellulose Decompositionof residuelignin Decompositionof soil particulatematter Decompositionof soil humus Microbial specificrespirationrate M-M const.for microbial C uptake Maintenancerespirationof labile microbe Maintenancerespirationof resistantmicrobe Microbial growthyield on 02 Microbial growthyield on NOx Microbial N 2 fixationyield on 02  1.0 1.0 0.15 0.025 0.025 0.005 0.20 35 0.010 0.0015 0.60 0.25 0.16  the sampleplots.Annual incrementsin tree diameterscalcu- 4. lated  from  these differences  g res.C g-1 micr.C h-1 g res.C g-1 micr.C h-1 g res.C g-1 micr.C h-1 g res.C g-1 micr.C h-1 g res.C g-1 micr.C h-1 g res.C g-1 micr.C h-1 g C g-• micr.C h-1 at 30øC gC m-3 g C g-1 N h-• at 30øC g C g-1 N h-1 at 30øC gC g-1 C g C g-• C gNg-lC  Model Experiment  were used with the allometric  equations described aboveto estimateoverstory growth.Lit-  4.1.  Model  Initialization  and Run  The ecosystem modelecosys wasinitializedwith datafor the physicalpropertiesof the Orthic Gray Luvisolat the Southern Old Aspensite [Anderson,1995,Table 1], andwith parameters for the biologicalpropertiesof aspenandhazelnut(assumedto be identical),and of soil microbialpopulations(Table 3). All model parametersfor C fixationand respirationby plant and microbialpopulationswere the sameas thoseusedin earlier studies of C and energy exchangeover agricultural crops [Grantand Baldocchi,1992;Grant et al., 1993e,1995c,1999c] and soils[Grant, 1994a,1997;Grantand Rochette,1994;Grant et al., 1993a,b, c, d, 1995b,1997].Model parametersfor plant architectureand growthhabitwere thoseof a perennialdeciduoustree. No alteration of model parametersfrom thoseused in earlier studieswas conductedfor the aspen-hazelnutstudy reported here. The model was then run for 100 yearsunder randomyearly sequences of hourly-averaged meteorological data recordedin 1994, 1995, and 1996. During the first year of the run, aspen and hazelnutwere seededonto bare soil at 0.1 and 1.0 m-2, respectively. The aspenwasprovidedwith a smallC reserveto simulateregrowthfrom C storedin roots.At the beginningof from14Cdatingof subsurface ashlayers)at several othersites every tenth year of the model run, all hazelnut stemswere to surfaceresidue,sonew in PrinceAlbert National Park wherethe SouthernOld Aspen transferredfrom standingphytomass site is located. stemsregrewfrom the soil surfacethe followingspring.  terfall wascollectedeachfall and springfrom 1 x 1 m screens in eachplot and separatedinto leaf and nonleafcomponents. The leaf componentwas further separatedby species.This work is describedfurther by Goweret al. [1997]. Ten soilcores(10 cm diameterx 30 cm depth)were taken by Steeleet al. [1997] from each plot on April 28-30, 1994, before soil thawingwas complete.The soil was separatedby horizon,compositedby plot, mixed,and sievedto passthrough a 1 cm mesh screen.As many fine roots as possiblewere removed,after whichthe root-free soilwasreturnedby horizon into eachhole and the forestfloor wasreplacedon top. In June 1995 and 1996,five coresper plot were removedwith a corer (5 cm diameterx 30 cm depth),storedin plasticbagsat 3øC, and then elutriated(Gillison'sVariety FabricationInc., Benzonia, Michigan). Roots recoveredfrom the elutriationwere sorted and classifiedas herbaceousor woody, dried at 70øC, and weighed.Fine root growthwascalculatedfrom changesin fine root massover time by Steeleet al. [1997]. Allometric equationsrelating diameter measuredat 1.4 m abovegroundto aspenwood biomasswere usedto estimate wood biomassof differentlyaged aspenstands(as measured  GRANT ET AL.: MODELING 4.2.  Model  MASS AND ENERGY EXCHANGE  Results  During the seventiethyear of the model run, hourly mass and energyexchangeover aspenand hazelnut simulatedwith 1994 meteorologicaldata were comparedwith resultsobtained from the overstoryand understoryflux towersat the field site during 1994.SimulatedCO2 and energyfluxesover the aspen were calculated as the sum of those from the soil surface, the  surfaceresidue,the hazelnut, and the aspen.SimulatedCO2 and energyfluxesover the hazelnutwere calculatedas the sum of those from the soil surface, the surface residue, and the  hazelnut.Three 1 week periodswere selectedfor comparison. The firstwasin earlyspring(April 24 to May 1) after snowmelt and before leaf-out to observemodel behaviorwhen foliage was not present.The secondwas in late spring(June 7-14) during a transitionfrom clear to cloudyweather to observe model responseto changingatmosphericconditions.The third was in midsummer(July 15-22) when radiationand temperature were greatestto observemodel simulationof larger mass and energyfluxes. After completionof the model run, all statevariablesin the model were initializedwith the valuesthey had held at the end of August 23 of the year during which the massand energy exchangecomparisons describedabovewere made.The model was then run for 24 hoursduring which incrementalchanges were made in either atmosphericCO2 concentration,air temperature,or irradiancewith all other environmentalconditions held constantat valuesused in the leaf CO2 fixationstudy describedabove.CO2 fixation ratesand stomatalconductances  simulatedfor an individualleaf surfacein the upper part of both the aspenand the hazelnutcanopieswere comparedwith measured  values.  27,705  plex interactions among several processes.These include changingCO2/O2 concentrations causedby declininggaseous solubilities,changingcarboxylation,oxygenationand electron transport rates causedby more rapid reaction kinetics, and decliningturgorpotentialscausedby increasing vaporpressure differences.Theseinteractionscausedsimulatedleaf CO2 fixation and stomatalconductanceto increasewith temperature below20øC,and to decreasewith temperatureabove20øCfor the conditionsof irradiance,CO2, and vapor pressureunder whichthe field measurements were taken (Figures2a, 2b). In the model, increasesat lower temperatureswere attributedto more rapid reactionkinetics,while declinesat highertemperatures were attributed to lower CO2:O2 ratios and to lower turgor potentials.Theselower potentialswere calculatedfrom the convergencesolution describedabove for equilibrating soil-root-canopy water uptake with canopy-atmosphere vapor diffusionunder canopy-atmosphere vapor pressuregradients that rosewith temperature.Hoggand Hurdle [1997] reported that stomatalconductanceof aspenat the field site declined when vapor pressure gradients exceeded 1 kPa, a value reachedat 18øCin this study.The simulatedresponseof leaf CO2 fixationto increasingtemperaturewasmore pronounced than that measured(Figure 2a) and that of stomatalconductance did not reproducethe higher values measuredbelow 20øC(Figure2b). The responsesof leaf CO 2 fixation and stomatal conductanceto increasingirradiancein the model are determinedby the interactionbetweenlight and dark reactionson CO2 fixation under ambienttemperature,CO2, and vapor pressure. The initial slopeof the irradianceresponsecurvesof aspenand hazelnutwere the same(Figure 2c), suggesting that the common value of quantumefficiencyusedin the model (Table 3) for irradiance-limitedCO2 fixation was accurate.The transition to irradiance-saturated CO2 fixation occurredat higher irradiance for aspenthan for hazelnut, due in the model to higher areal rubiscodensityand hence higher maximum dark reaction rates in the aspen. Stomatal conductancerose with CO2 fixation under increasingirradiance (Figure 2d) as required in the model to maintain a constantCO2 concentration  Model resultsfor annual net primary productivity(NPP), net ecosystem exchange(NEE), and aboveground phytomass growth of a 70 year old aspen-hazelnutforest under 1994 climatewere then comparedwith estimatesof NPP, NEE, and growth derivedfrom aggregatedflux data and from tree ring analysisduring 1994. Model results for C accumulationin different ecosystemcomponents(e.g., leaves, roots, forest floor) were also comparedwith data obtainedfrom the field ratio across the stomates. site and from other related sites.Long-termmodel resultsfor The responsesof leaf CO2 fixation and stomatal conducC accumulationin aspenstemsand brancheswere compared with resultsof allometricstudiesof aspengrowthin the same tance to increasingCO2 concentrationin the model are determined by the Michaelis-Mentenconstantsfor carboxylation ecologicalzone as that of the field site. and oxygenation(Table 3) and by the effectsof CO2 on the compensation point and therebyon the carboxylationefficien5. Results cies of the light and dark reactions.Leaf CO2 fixation rose hyperbolicallywith CO2 concentrationunder the irradiance, 5.1. Leaf CO2 Fixation and Stomatal Conductance temperature,andvaporpressureof the field study(Figure2e), The responses of CO2 fixationand stomatalconductanceby forcingstomatalconductance to decline(Figure2f) in orderto selectedleaf surfacesto increasingtemperature, irradiance, maintain a constantCO2 concentrationratio acrossthe stoand CO2 concentrationsimulatedon August 24 of the seven- mates in the model. tieth year of the model run are comparedin Figure 2 with responses measuredat the SouthernOld Aspen site on August 5.2. Canopy Mass and Energy Exchange 24, 1994.In the model theseresponses were greaterfor aspen 5.2.1. Early spring. During the April comparisonperiod, than for hazelnutbecauseaspenleaveshad lowerspecificleaf solar radiation and air temperaturewere rising (Figure 3a), areas and hence higher areal N concentrationsand thereby while vaporconcentrationand precipitationremainedlow (avrubisco densities. Leaf N concentrations simulated under the eragedaytimeRH • 50%) (Figure 3b). Snowmeltwasmostly soil and climatic conditions of the field site declined from completeby DOY 100 (April 10) in the model and DOY 102 --•0.09gN g C- • in earlyspringto -0.07 g N g C- • duringlate (April 12) in the field. In the absenceof leaf surfaces,most summer,which is consistentwith averagevaluesof 0.08 g N g radiantenergyreceivedby the aspen-hazelnut forestduringthe C-• reportedfromtheNorthernOldAspensitebyDangetal. week was returned to the atmosphereas sensibleheat over [1997]. The responsesof CO2 fixation and stomatalconduc- both the aspen(Figure4a) and the hazelnut(Figure4b) (simtance to increasingtemperaturein the model arisefrom com- ulatedversus measured sensible heatfluxes'overaspen, R2 =  27,706  GRANT ET AL.: MODELING MASSAND ENERGY EXCHANGE  (;.sz_LU IOWri) uo!lex!_-I ZOO  GRANT ET AL.: MODELING MASS AND ENERGY EXCHANGE  •'-'1000 i  ---.--  raoiadon  E 8oo-......temperature•  o  ?  '  60o ß-  400  J•  20o  27,707  0 114  ,,, 115  ",  .•,."'""', ' 'k '  15  10•  •,'• •;ø•. •-•.•.• • o 116  117  ]--.-- vapor conc.  118  /1/•,.,;•  -1 I precipitation  J •  119  ..... 120  Z'"".,  •., ,J ""'"t..  121  !, )1'  •  [-0.80  ,,•41 / '\ ' •'"' •W'J'"'"' i '•' 3].'...... '/ L. v,., .% ..,..,...,./' •'"" \.r.L0.4• •" 0 ' ' ' I ' ' ' I ' ' ' I ' ' ' II ' II ' I ,I, !....I  •  114  115  116  117  118  119  120  o.o 121  •  Day of Year Figure3. (a)Radiation, airtemperature, (b)vapor concentration andprecipitation during theAprilcomparisonperiod.  0.78, b = 0.94 +_0.04; overhazelnut, R2 = 0.63, b = ure 6a), andrainy(Figure6b). Data for net radiationwere onlyoverthe aspenfor thefirstandlastdaysof the 1.22 _+0.08). In themodel,largeupwardsensible heatfluxes available to simulated values (Figure7a).At this were simulatedfrom the recentlydry forestlitter layer.This periodbutwereclose time, total canopy leaf area in the model was 6 m2m-2 about layerwarmed to temperatures several degrees abovethoseof divided between aspenandhazelnut, whilethatin the the air, and as muchas 10øCabovethoseof the soil surface evenly underneath, becauseof its exposure to solarradiation,its low fieldwasabout5 m2 m-2, of whichhazelnutwasslightlylarger  of leaf-outcaused netraheatcapacity, anditslowthermalconductivity. Similarly large thanaspen.The recentcompletion diation and, consequently, mass and energy exchange to be temperature differences weremeasured at thefieldsiteat this time of the year [Blankenet al., 1997].Differencesin net much smaller over the hazelnut than over the aspen canopy preferential interception of irradiance radiationand sensibleheat flux simulatedover aspenand ha- (Figure7b),indicating aspencanopy. The ratioof net radiation zelnutwere due to interceptionof irradianceby aspenstem by the dominant simulatedoverhazelnutversusaspenin the modelwaspartly surfaces (Figure4a versus Figure4b). bytheinterception fraction usedforaspen (Table3). CO2fluxesduringlateAprilremained small(Figure5) be- affected with solaranglefrom 0.20in the morning causesoilrespiration wasconstrained bylowsoiltemperatures Thisratioincreased to 0.25at midday(Figure7a versus Figure7b) undertheforestlitterlayerandbecause plantrespiration and andevening fixationwereconstrained by the absence of leaves.A respira- which was close to one of 0.26 calculated for mid-June from tionfluxof about2/xmolm-2 s-• fromthehazelnut canopy in net radiation measurementsat 4 versus39 m [Chen et al., the modelwascausedby remobilization of storageC before 1997]. leaf-out. Small downwardfluxessimulated late in the compar-  Duringthisperiod,slightly moreradiantenergy wasparti-  isonperiodindicate thebeginning of CO2fixation byemerging tioned into latent heat than into sensibleheat over both the  canopies (Figure7). In the model, leaves, although leafemergence in thefielddidnotbeginuntil aspenandthe hazelnut energypartitioning wascontrolled bycanopy conductance aga few dayslater. fromleafconductances whichweredetermined by 5.2.2. Late spring. Air temperatures andvaporconcen- gregated ratesandturgorpotentials. LeafCO2fixation trations werehigherduringthesecond comparison periodthan leafCO2fixation by irradianceandtemperature at the duringthefirst(average daytime RH •- 60%).Duringthelast ratesweredetermined (Figure2), andleafturgorpotentialpotentials twodaysof thisperiodtheweather became cloudy, cool(Fig- leafsurfaces  27,708  GRANT ET AL.: MODELING MASS AND ENERGY EXCHANGE  (a)  600400-  200 ,'-"  13 ,,  -200  •..,,:,;..  -400  v  '  -600'! , , , , , , , , , , , , , , , , , , , , , , , , , , ,,  i'F  114  • 400-  115  116  117  18  119  120  .  121  (b)  20 i ß  -200'  ,•  ',  : ,,,•.• '  '  'i  :i • ß  ::i,  ,;  ,1'  ,.,'.,/' LE  •'  ,,  -400 . , , , , , ,'/ , , , , , ,,,, • SI ,....... , • , , , • , , , , , , , , 1' 4  115  116  117  118  119  120  121  Day of Year Figure4. Energy fluxes simulated (lines) andmeasured (symbols) over(a)theaspen overstory and(b)the hazelnut understory duringtheApril comparison period.  weredetermined bytranspiration fluxescontrolled bysoiland aggregated rootandmicrobial respiration (Table3) through atmospheric waterstatus.Thushighersolarradiationand air the soilprofile.Downward fluxessimulated overthe aspen temperaturesduring DOY 161 and 162 versusother DOY  were close to those measured under lower irradiance and  (Figure6a) caused greaterleafCO2fixation(Figure8), and coolertemperatures duringDOY 159and160butweregreater hencegreaterleaf conductance, leadingto greaterupward thanthosemeasured underhigherirradianceandwarmertemlatentheatfluxes(Figures 7a,7b).Greaterupwardfluxes were peratures(Figure6a) duringDOY 161and 162becausemeaalsomeasuredduringthesetwo days,so that modelfluxes suredfluxesdid not respondto improvedweatheron these  followed changes in diurnaltrendsof measured values during dates(simulated versus measured CO2fluxes: overaspen, R2 the week (simulatedversusmeasuredlatent heat fluxes:over  = 0.80, b = 1.23 +_0.05; overhazelnut,R 2 = 0.24, b = 0.32 _+0.05). Wind speedsremainedbelow1 m s-• on DOY 0.56, b = 0.68 _+ 0.05). Turgorpotentialsin the model 159, 160,and 163,possibly causingsomescatterin the mearemainedhighduringthisperiod,sostomatalconductance was suredvalues. Bothmeasured andsimulated downward CO2  aspen, R2 = 0.79, b = 1.19 +_0.05; overhazelnut, R2 =  littleaffected bysoilandatmospheric waterstatus. Coolrainy fluxeswerereduced by lowerradiationandcoolertemperaturesduringthelasttwodaysof thecomparison period.Fluxes  weatherduringDOY 164 and 165 causedmeasuredand simulated fluxes to remain small.  measured and simulated over the hazelnut varied from down-  CO2fluxessimulated andmeasured overtheaspenduring wardratesof lessthan5 /•molm-2 s-• duringthe days, thisperiodreacheddownward ratesof 20-25 /•molm-2 s- • reduced byshading fromtheaspenoverstory, to upward rates duringthedays,indicating activefixation byaspen plushazel- of5-10/•molm-2 s-• during thenights, indicating thathazelnutleaves, andupward ratesof 5-10/•molm-2 s- • duringthe nut net CO2 fixation was slightlyless than soil plus nights, indicating aspen plushazelnut plussoilrespiration (Fig- aboveground hazelnut respiration. GreaterupwardCO2fluxes ure 8). In themodel,overstory CO2fluxeswerethenetresults measured andsimulated duringthe nightsoveraspenversus of aggregated CO2fixationat leafsurfaces (responses to irra- hazelnutindicateaspencanopyrespiration above4 m. dianceandtemperature shownin Figure2), of aggregated 5.2.3. Midsummer.Therewasonecool,cloudydayin the branch respiration bybothaspen andhazelnut canopies, andof middleof the midsummer comparison period(DOY 199),  G•T  ET AL.: MODELING MASS •D  ENERGY EXC•GE  27,709  30ß o  overstory understor?  1  o  =1,  . ß  o  o  I  O  1'  i  i  115  I  I  i  i  i  116  I  i  i  i  117  I  !  i  i  118  I  i  i  119  i  I  i  i  !  120  121  Day of Year Figure5. CO2fluxes simulated (lines)andmeasured (symbols) overtheaspen overstory andthehazelnut understory duringthe April comparison period.  800.l__o__ ....... temperature ,'•'• radiation •..'•, ø. oi"• , ,  600 ;..½•.' •,.,.}" • ]'"•1  ::  ' a2:0  •  •  ._.., ,,.,,,'/ ,:,, /i1', / \ / • /V• ',..,;.'  .  .....,! .:  15 10  o, .... , , '•'--,-•nd' '---'--'' ' ' ' ' ' '.... ' ' ' ' ' ' ' ' '  ]•" ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '  ,•14• :11--i--;raePc•rpitCaøt•ø 0 .., r,, .... OI  _/--'  - 0 ' ' ' I  158 159  '  • % ,,,,','• ....  ' ' I ' ' ' I  160  "%)'x, ,,,,'  I  %,' ....  I  161 162 163  164 165  Day of Year Figure6. (a) Radiation, airtemperature, (b)vaporconcentration andprecipitation during theJunecomparisonperiod.  27,710  GRANT ET AL.: MODELING MASS AND ENERGY EXCHANGE  6OO  400  :  200  )  ø• A  o  0  /•  -200  ,•  ..  ,  -400  • •:,,,  "  -600,  , , , , , . ,,  158  159  ., ,  , ,,  , , , , . , . , , ,,  160  161  162  , , , ,, 163  ,.,,  164  165  200  (b)  lOO o  '  -lOO  -200,  , , , , , , ,,  158  159  , , , , , , , , , ,,  160  161  , , , , , .,,,  162  163  164  165  Day of Year Figure7. Energy fluxes simulated (lines) andmeasured (symbols) over(a)theaspen overstory and(b)the hazelnutunderstory duringtheJunecomparison period.  30  .  ß  ß  20  .  overstory  • understory---•--  '..•,.  lO  i  -20  '  158  '  '  I  159  '  '  '  I  '  '  160  '  I  161  '  '  '  I  162  '  mm  '  '  I  163  '  '  '  I  164  '  '  '  I  165  Figure8. CO2fluxes simulated (lines)andmeasured (symbols) overtheaspen overstory andthehazelnut understory duringthe Junecomparison period.  GRANT ET AL.: MODELING  •,  1 ooo  11111111  .  radiation o. E 800'--.-.....temperat  MASS AND ENERGY EXCHANGE  ,l,l,,,lll,lll,lll,  30 ..,.,. I  ,,  •: 600' ._o400' .o3 200' o3  '  """i' tt'" 1'"  .  (a)f (1) '25  ,  ?'  15•  ,1,  0  27,711  , , ,  o o .16  vapor conc.  ib)  precipitation  ß 'l  :.•,, ,,"..., %2',,,",,."'  196  . _  197  ,,,,, 198  •,"""'..' """ •  199  V  ,,,, ,,,,,,,,,, 200 201 202  ""  14 • 12 •  •o•'  203 ] v  Day of Year Figure 9. (a) Radiation,air temperature,(b) vapor concentrationand precipitationduringthe Julycomparisonperiod. followedby warm,clearweather(Figure9a) with stablevapor period,so canopyconductances were little affectedby soil and concentration (averagedaytimeRH • 55%) (Figure9b).Total atmosphericwater status. canopyleaf area and its distributionbetweenaspenandhazelDiurnal trendsin CO2fluxesmeasuredandsimulatedduring nut wassimilarto that in late springboth in the modeland at this period (Figure 11) were similarto thoseduringthe late the field site. The ratio of net radiation simulated over hazelspring(Figure 8), with smallerdownwardand upwardrates nut versusaspen(Figure 10bversusFigure 10a) followedthe occurringundercool,cloudyweatheron DOY 199 (Figure9a) same diurnal range of 0.20-0.25 as that during late spring, and greater downwardand upwardrates under warm, clear while the ratio measured at 4 versus 39 m declined from 0.26 weatherafterward.Thesefluxesare drivenby the aggregated to 0.23. Most radiant energyreceivedby both the aspenand responses of leaf CO2 fixationto irradianceand temperature, the hazelnutcanopiesat this time was returned to the atmo- as shownin Figure 2. As during the late springcomparison sphereas latentheat,fluxesof whichreached400 W m-2 period(Figure8), fluxessimulatedunderhigherirradianceand during the warm weather that followedrainfall on DOY 199 warmertemperatures(after DOY 200) werehigherthanthose (Figure 10a). Sensibleheat fluxesremainedlow, especially measured, while those simulated under lower irradiance and over the hazelnut(Figure 10b). In the model,high radiation coolertemperatures(DOY 197, 200) were closer(simulated measured CO2fluxes duringtheweek:overaspen, R2 ._ and air temperaturesfollowingrainfall on DOY 199 (Figure versus 9a) causedrapid leaf CO2 fixation(e.g.,Figure2), and hence 0.74, b = 0.94 _+ 0.04; over hazelnut,R 2 -- 0.04, b = largeleaf andcanopyconductances. Theseconductances, com- 0.17 +_0.06). Windspeeds remainednearor below1 m s-• bined with relativelyhigh canopy-atmosphere vapor pressure before DOY 200, possiblycausingsomescatterin the meadifferencesgeneratedby higher canopy temperaturesand sured values. Fluxes measured and simulated over the hazelnut moderatevaporconcentrations (Figure9b), causedmostradi- variedfrom downwardratesof lessthan 5 /•mol m-2 s-• ant energyto be partitioned to latent heat flux later in the duringthedaysto upward ratesof 10/•molm-2 s-• duringthe comparisonperiod (Figures10a, 10b). Measuredlatent heat nights,indicatingthat hazelnutnet CO2 fixationwas lessthan fluxeswere alsolargeat thistime (simulatedversusmeasured soil plus abovegroundhazelnutrespiration.More rapid uplatentheatfluxesduringtheweek:overaspen,R2 = 0.78, ward CO2 fluxes simulatedduring midsummerversuslate b = 0.97 +_0.04; overhazelnut,R 2 -- 0.72, b = 0.99 +_ springwere causedby the effectsof higher temperatureson 0.05). Canopyturgorin the modelremainedhigh duringthis growth and maintenancerespirationof plant and microbial  27,712  GRANT ET AL.: MODELING MASS AND ENERGY EXCHANGE  600-: o  •  200  •  ø  ;  -200  •-:  -400•  '  -600 •",,,  •  '•  196  •  '  (a)  '  ', •,,,  197  •  "  '•' •,,,  o•  ',,,  198  -ß  ,, •,,,  199  '" •,,,  200  ,'-' ,,• . , ,  201  202  I  203  2OO  100 ' .,,, •',;,t •,' (b ß  o  -100-1  -200],,, 196  '  .  ',,,, 197  o Rn --  z•  •  '  -,,-  S  ,, ,• ,s, .;-::..,,,,,,,',,, 198  199  200  ,", , , , 201  202  203  Day of Year Figure10. Energy fluxes simulated (lines)andmeasured (symbols) over(a)theaspen overstory and(b)the hazelnutunderstory duringtheJulycomparison period.  populations. Fluxes simulated over hazelnut were about 2  beenreduced byrefixation of respired CO2withinthecanopy, •mol m-2 s-• moreupward thanthosemeasured because soil althoughthe extent to which this occurredcould not be evalrespiration in themodelwasgreaterthanthatmeasured during uated.Respirationin the modelwaspartitionedmoreto the thisperiod(Figure12). aboveground aspen(271g C m-2) andlessto thesoilplus aboveground hazelnut (793g C m-2) thanwastheestimated 5.3. Annual Carbonand Water Exchange respiration (20and870g C m-2).Annualaboveground respiAnnualC exchange accumulated from hourlyvaluesin the rationof aspenandhazelnut in themodelisthesumof growth  modelis comparedwith that estimatedfrom measuredC fluxes  andmaintenance requirements for all leaves,twigs,branches, according toBlacketal. [1996]andfromallometric techniques stems,and reproductive biomass in eachcanopy.Aspenleaf according to Goweret al. [1997](Table4). Annualecosystem biomass in the model reached a maximum valueof 100g C grossCO2 fixationof 1132g C m-2 in the modelwas11% m -2 during the year of study, which was close to theaverage greaterthanthatestimated byBlacket al. [1996].Fixationby value of 90 g C m -2 measured at several sites in thefield.This aspenin the modelwasmorethanthatestimated (811versus biomass would require about 50 g C m -2 for growth respira690g C m-2), whilefixationby hazelnut in the modelwas respiration similarto thatestimated (321versus 330g C m-2). Thepar- tionandat least75g C m-2 yr-• formaintenance respiration coefficients. Aspensapwood biotitioningof CO2 fixationin the modelwasdue to canopy usingpublished after80yearsof themodelrunwas7.9kgC m-2, which dominance effectson the interception of photosynthetic irra- mass at the site.Annualrequirediancethatcausedhigherirradianceintensities at leaf surfaces wasthe sameas that measured respirationof thisbiomass andconsequently higherratesof leafCO2fixation(Figure2) mentsfor growthandmaintenance havebeenestimated byLavigneandRyan[1997]to be about by the talleraspencanopy(Figures8 and11). Annualecosystem respiration of 1064g C m-2 in themodel 60 g C m-2 and80 g C m-2 yr-•, respectively. Additional was20% greaterthanthat estimated by Blacket al. [1996] requirements for growthandmaintenance of twigsandrepro(Table4). Grossfixationandrespiration estimated fromag- ductivebiomass alsocontributed to themodeltotalof 271g C gregatedfluxesmeasured abovethe aspencanopymayhave m-2 for aboveground aspen respiration (Table4). A corre-  GRANT ET AL.: MODELING  'E 30-  ß  ß  MASS AND ENERGY  27,713  overstory  o understo  ß  EXCHANGE  .'q,  .  20  ß  ".  ." ß  d' %  ,  ß  o % oe.  ,'7' O -20 (D  : I  196  ,  'l  I  197  '  '  '  I  I  ,  198  I  I  I  I ' I  199  I  I  200  ,  I  I  201  '  '  '  •  202  '  "  '  I  203  Day of Year Figure11. GO2 fluxessimulated (lines)andmeasured (symbols) overtheaspenoverstory andthehazelnut understoryduringthe Julycomparison period.  sponding totalof 115g C m-2 foraboveground hazelnut res- model includedall biomassinvolvedin the turnoverof foliage, twigsand reproductive material.This senespiration, plus525g C m-2 soilrespiration and154g C m-2 plusassociated aspenplushazelnutroot respiration causedthe soilplusha- cencewasgreaterthan that measuredfrom litter traps(216  91g C m-2 and66versus 32g C m-2 fromTable4). zelnuttobea netsource of472gC m-2 inthemodel. Soilplus versus of aspenandhazelnutin the modelincluded root respiration followeda pronounced annualcyclein both Root senescence the modeland at the field site(Figure 12), risingto almost10  all biomass involved in the turnover of roots. This senescence  /xmolm-2 s- • bymidyearandthendeclining. In themodelthe of 57 + 24 = 81g C m-2 (Table4) wassimilarto thatof 46g fromrootgrowthmeasured in growthcores annualrespirationcyclewasdrivenby changes in soiltemper- C m-2 estimated atureandwatercontentandby changes in plantphenology and a fine root turnover index of between 1.5 and 2 calculated that determinedchanges in thepartitioningof plantC to roots. Thiscyclewasabout30 daysearlierthanthatmeasured at the field site,althoughtotal annualvaluesof modeledversusmeasuredrespiration were similar.The combination of the soil  from minirhizotronmeasurements [Steeleet al., 1997].Annual  changein bothaspenandhazelnutbiomass in the model(112 and41g C m-2) wasabouttwo-thirds thatcalculated fromtree rings.  by the asplushazelnut source of 472g C m-2 withtheaboveground A greaterfractionof annualevapotranspiration forestwasattributedto thesoilin themodelthan aspen sinkof 540g C m-2 gavea NEE in themodelof 68g C pen-hazelnut  m-2. Aggregation of measured C fluxesindicated a largersoil in estimatesfrom accumulatedlatentheat fluxesbyBlacket al. beplushazelnut source of 540g C m-2 combined witha larger [1996](Table5). The partitioningof annualtranspiration aspensinkof 670g C m-2 whichgavean estimated NEE of tweenaspenandhazelnutwassimilarin bothmodelandfield 130g C m-2 [Blacketal.,1996].Kimballetal. [1997]simulated estimates.Partitioningin the modelwasdeterminedby canopy a NEE of 180g C m-2 atthissiteduring1994withoutexplicitly dominanceeffectson irradianceinterceptionand aerodynamic accounting for the hazelnutunderstory. Abovegroundsenescence of aspenand hazelnutin the  conductance. During the periodof full leaf (June 15 to September7), modelversusfield estimates of soil,hazelnut,and aspencontributions to totalevapotranspiration were0.12versus0.05, 0.16 versus0.17, and 0.72 versus0.78, respectively, indicatingthat mostof the differencein the partitioningof evapotranspiration occurred whenleaf areawaslow.Modeled and measuredwater useefficienciesof hazelnut(4.7 and 3.5 g  C fixedkg- • watertranspired) werehigherthanthoseof aspen (3.3and2.4g kg-•), reflecting thelowerirradiance to which the hazelnutwas exposed. 5.4. Long-Term Net EcosystemExchange  Net ecosystem exchange varieswith climateandwith forest age.Annualvaluesof NEE in the model(e.g.,Table4) were Day of Year influenced by antecedent C storagein the aspen,hazelnut,and Figure 12. Soil CO2 flux simulated(line) and measured forestfloor andby annualweatherpatternsthroughtheir dif(symbols) nearmiddayduring1994at theSouthern OldAspen ferent effectson C fixationand respiration.More definitive site.  estimates of annual NEE in this forest should therefore be  27,714  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  Table 4. Annual CarbonBalanceof a 70 Year Old AspenHazelnutForestSimulatedby Ecosysand Estimatedby AggregatingShort-TermFlux Measurementsand by MeasuringChangesin C Storage gCm -2 Simulated  by Ecosys  Estimated From  Fluxes  Estimated From  C Storage  Aspen Gross fixation  811  Abovegroundrespiration Abovegroundsenescence Abovegroundnet exchange  271 216 540  Rootrespiration b  109  Root senescence Root exudation  57 102  Changein biomass Changein storage  91 a 670  45  Root senescence Root exudation  24 34  Changein biomass Changein storage  41 -4  525 793 -472 1132  Total respiration  1064  in Saskatchewan. 32  The  simulated  accumulation  of C in the  wood and soilwas sustainedby an averageN 2 fixationrate of  2.5g N m-2 yr- • in themodelwhichiswithintherangeof 0.35 to 3.25g N m-2 yr-• measured in soilof aspenstandsby Brouzeset al. [1969].  66  6.  Discussion  Complexecosystemmodelssuchas ecosysare intendedto functionat levels of temporal and spatialresolutionwhich extend from the higher levels at which individualprocesses (e.g.,leaf C fixation,microbialC oxidation)occurto the lower levels at which changesin ecosystembehavior (e.g., NPP, NEE) occur.The sensitivityof individualprocessesin such models to defined changesin boundaryconditionscan be testeddirectly againstexperimentalresultsat high temporal  870 -540  1020  890  Net Ecosystem Exchange 68  yearsof the model run. Halliwellet al. [1995]reportedaccumulationsof surfacedetritus>5 mm diameterof about 1 kg C  m-2 usinga lineintersect methodundermatureaspenstands 330  Ecosystem Gross fixation  soil below. Part of  due,valuesof whichstabilized at about1.8kg C m-2 after40  Soil Plus Hazelnut  Soil respiration Total respiration Net exchange  aspenstandsin PrinceAlbert NationalPark near the Southern Old Aspen site (Figure 13). Carbonin the model also accumulated in the forest floor and the mineral  173  115 66 206  Rootrespiration b  productivesitesin the aspenparklandof centralSaskatchewan derivedfrom measurements of woodvolume[Kirbyet al., 1957] and bulk density[Campbellet al., 1985],and a rate of 98 g C  the accumulationin the forestfloor appearedas surfaceresi-  Hazelnut 321  Abovegroundrespiration Abovegroundsenescence Abovegroundnet exchange  planting to 100yearsof agein themodelwas96g C m-2yr-a. Thisrateis similarto a rateof 80 g C m-2 yr-a calculated at  m-2 yr-• calculated fromwoodbiomass of differentlyaged  690 20  112 -56  Gro•s fixation  versuswarmer climatesin the BOREAS studyarea. They suggestedthat C accumulationwas more influencedby climate effectson forest floor decomposition than by thoseon plant growth. The averagerate of C accumulationin aspenwood from  130  (hours)andspatial(cm2) resolution (e.g.,leaf C fixationin  aNonfoliarlitter wasdistributedbetweenaspenandhazelnutsources Figure 2; microbialC oxidation[Grantand Rochette,1994]). in proportionto their foliar litter. Such tests are well constrained because test results arise from  URootrespiration includes onlyC oxidized forrootmaintenance and  a singleprocess(e.g.,leaf C fixation)with uniqueresponses to  growth.C oxidizedin the rhizospherefrom root exudatesandsenesced independently controlledchanges in boundaryconditions(e.g., root material is includedin soil respiration.  irradiance,temperature,CO2). Testingat this level of resolution is of great importancein supportingmodel estimatesof derived from values simulatedover severalyears. Average changesin NEE causedby changesin atmosphericCO2 and NEE simulatedduring yearswith warmer 1994 climate data temperature.The resultsof thesetestsmaybe usedto support (Table 6) were lowerandmorevariable(i.e., more dependent testsin which the sensitivityof spatiallyaggregatedprocesses upon antecedentC storage) than those during years with to uncontrolledchangesin boundary conditionsare tested cooler1995or 1996climatedata becauseplant C fixationwas againstexperimentalresultsat comparabletemporal(hours) (e.g.,Figures4 to 11).Such raisedby temperaturecomparatively lessthan were plant and andlowerspatial(m2) resolution soil C respiration(Table 6). I. A. Nalder and R. W. Wein (unpublisheddata, 1998) measuredlarger accumulations of forest floor C in similarlyaged aspen standsunder cooler Table 6. AverageNet PrimaryProductivity(NPP) and Net Ecosystem Exchange(NEE) Simulated(_ Standard Deviationof InterannualVariability)in a Mixed AspenTable 5. Annual Evapotranspiration of an Aspen-Hazelnut Hazelnut Forest Between65 and 80 Years of Age During ForestSimulatedby Ecosysand Estimatedby Aggregating Years With 1994, 1995, and 1996 Climate Data Short-Term  Flux Measurements  ClimateData,g C m-2 y-1 Simulated, mm  Aspen Hazelnut  245  Estimated, Soil  mm  NPP Aspen  NPP Hazel  446 _+ 24 376 _+ 32 352 _+ 28  174 _+ 17 200 _+ 52 170 _+ 30  Respiration  NEE  284  68  95  Soil  122  22  Total  435  401  1994 1995 1996  514 _+ 57 316 _+ 22 362 _+ 12  105 _+ 84 260 _+ 17 161 _+ 24  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  1.4x10 4 ß this study [3  •  Kirby  1.2x10  0  •  1.0x10•-  E 8.0x10 3o ß  o o 6.0x10  -  ß  •. 4.0xl (/)  0 -  2.0x10 3• .  0  4'0  6'o  8'o'  Age of Stand (Years) Figure 13. Growth of aspenwood simulated(line) and derivedfrom species-specific allometricequationsbyKirby[1957] in centralSaskatchewan, and by Nalder (this article)in Prince Albert National Park, Saskatchewan.  testsare lesswell constrainedthan thoseconductedat higher resolutionbecausetest data are the net product of several interactingprocesses(e.g., leaf C fixation versusplant and microbialrespiration),each of which respondsdifferentlyto changesin boundaryconditions.Thesetestsare alsolesswell constrained becausechangesin individualboundaryconditions are correlatedrather than independent(e.g., diurnal changes in temperaturefollow those of irradiance).These tests are therefore lessable to discriminateamong alternativemodel hypotheses(e.g., the accuracyof alternativehypothesesfor sensitivityto irradianceversustemperaturemay not be distinguished).The resultsof these testsmay in turn be used to supporttestsin whichthe sensitivityof temporallyaggregated processes(e.g., NEE) to uncontrolledchangesin boundary conditionsare testedagainstexperimentalresultsat compara-  ble spatial(m2) andlowertemporal(years)resolution (e.g.,  27,715  diurnalvariationin CO2 exchangemeasuredover the aspen canopy(Figures8 and 11). Diurnal changesin leaf fixation drovethosein leaf conductance which,whenaggregatedto the canopylevel and usedin a first-ordersolutionto the canopy energybalance,explainedbetween 70% and 80% of diurnal variationin latent heat flux measuredover the aspencanopy (Figures 7 and 10). Except for a tendencyto overestimate downwardCO2 flux in late spring,there were no apparent biasesin the modeledfluxes(b valuesnot significantly different from 1). Without an independentestimateof scatterin the measuredfluxes,there is no objectivemethod to establishthe extent of agreementbetweensimulatedand measuredvalues. However,evidencepresentedin Figures6-11 suggests that the aggregationtechniquesusedin ecosysallowleaf-levelbehavior (Figure 2) to be representedat the canopylevel. Suchrepresentationis importantif canopy-levelresponses to changesin atmospherictemperatureand CO2 concentrationare to be modeled accurately.Some improvementin model accuracy mightbe achievedby extendingthe resolutionof temperature, humidity,and CO2 concentrationfrom the canopyto the leaf as is currentlydonefor irradiance,but suchimprovementmay be limited [Sinclairet al., 1976].Furthermore,the couplingof a spatiallyresolvedcanopyto the spatiallyresolvedsoilprofile in ecosyswould entail a considerablecomputationalcost. Algorithmsin ecosysfor the partitioning,accumulationand senescence of C, N, and P in different organsof each plant species(leaves,twigs,reproductivematerial, branches,main stems,primary,and secondaryroots)allow diurnalchangesin canopy mass and energy exchangeto be aggregatedfrom hourlyto yearlyand decadaltimescales.The resultsfrom such aggregationcan be testedagainstchangesin plant and soil C measuredover the sametimescales(Tables 4-6, Figure 13). Such long-termtestscan identify systematicbiasesin shortterm modelbehaviorthat mayrequireseveralyearsto become apparent in a model run, therebyprovidingimportant feedbackto modeldevelopment.Althoughthesetestsare of greatest relevanceto the ecologicalquestionsbeing addressedby the model (e.g., longterm changesin NEE), they are very poorly constrainedand have little scientificvalue exceptto supportmodel testingat higher temporal and spatial resolution.  Model resultsat yearlyand decadaltimescalessuggestthat averageNEE for a 50-80 year old aspen-hazelnutforestin the  BOREASsouthern studyareais 160to 170g C m-2 yr-• (Table 6). This value may be larger for a youngerforest in whichthe forestfloor is still developingand maybe smallerfor an older forestin whichNPP is declining.BecauseNEE is the differencebetweentwo muchlargerC exchanges, fixationand respiration(Table 4), comparativelysmall changesin either may causecomparativelylarge changesin NEE. Changesin fixation and respirationmay causeNEE to becomesmaller duringwarmeryearsand largerduringcooleryears(Table 6), although changesin NEE will be affected by intra-annual trendsin temperatureand precipitation[Frolking,1997].It is therefore important that model estimatesof changesin NEE continue to be tested againstexperimentalestimatesmade over severalyears of contrastingweather. Such testingwill supportmodel estimatesof long-termchangesin NEE caused by changesin atmosphericCO2 and temperature.  Figures 12 and 13, Tables 4-6). Such tests are of greatest ecologicalinterestbut are very poorlyconstrainedbecausethe test data is of low precisionand may be explainedby a wide rangeof alternativemodelhypotheses, not all of whichmaybe widelyapplicable.It is thereforeimperativethat suchtestsbe extensivelysupportedby better constrainedtestsconductedat higherlevelsof temporaland spatialresolutionbefore ecosystem modelsare used for predictivepurposes.The need for suchtestshasdriventhe developmentof morecomplexmodels suchas ecosys. Diurnal changesin leaf C fixationsimulatedunder diurnal changesin irradiance,temperature,humidityand wind speed, Acknowledgments.Ecosyswas run on a SGI/CRAY Origin 2000 when aggregatedto the canopylevel and combinedwith ag- provided through the Multimedia Advanced ComputationalInfragregatedorganrespiration,explainedbetween70 and 80% of structure(MACI) projectof the Universitiesof Alberta and Calgary.  27,716  GRANT ET AL.: MODELING  MASS AND ENERGY EXCHANGE  3 WheatNetwork,Model and ExperimentalMetadata,2nd ed., pp. 65-74, GCTE Focus 3 Off., NERC Cent. for Ecol. and Hydrol., Allen, L. H., Jr., R. R. Valle, J. W. Mishoe, J. W. Jones, and P. H. Wallingford,Oxon, England, 1996a. Jones,Soybeanleaf gas exchangeresponsesto CO2 enrichment, Grant, R. F., Ecosys,in GlobalChangeand Terrestrial Ecosystems Task Proc. Soil Crop Sci. Soc.Fla., 49, 192-198, 1990. 3.3.1 Soil OrganicMatter Network(SOMNET): 1996 Model and ExBall, J. 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