Abstract
Nine ecosystem process models were used to predict CO2 and water vapor exchanges by a 150‐year‐old black spruce forest in central Canada during 1994–1996 to evaluate and improve the models. Three models had hourly time steps, five had daily time steps, and one had monthly time steps. Model input included site ecosystem characteristics and meteorology. Model predictions were compared to eddy covariance (EC) measurements of whole‐ecosystem CO2 exchange and evapotranspiration, to chamber measurements of nighttime moss‐surface CO2 release, and to ground‐based estimates of annual gross primary production, net primary production, net ecosystem production (NEP), plant respiration, and decomposition. Model‐model differences were apparent for all variables. Model‐measurement agreement was good in some cases but poor in others. Modeled annual NEP ranged from −11 g C m−2 (weak CO2 source) to 85 g C m−2 (moderate CO2 sink). The models generally predicted greater annual CO2 sink activity than measured by EC, a discrepancy consistent with the fact that model parameterizations represented the more productive fraction of the EC tower “footprint.” At hourly to monthly timescales, predictions bracketed EC measurements so median predictions were similar to measurements, but there were quantitatively important model‐measurement discrepancies found for all models at subannual timescales. For these models and input data, hourly time steps (and greater complexity) compared to daily time steps tended to improve model‐measurement agreement for daily scale CO2 exchange and evapotranspiration (as judged by root‐mean‐squared error). Model time step and complexity played only small roles in monthly to annual predictions.
Highlights
Cand water balancescan be studieddirectlywith field mea- ing ecosystemC balance are consistent.For example,if a surementsof tracegasfluxes[e.g.,Gouldenetal., 1997;Sellers ground-basedestimateof annualecosystemNEP differsfrom etal., 1997].Temporalgapsin measuremenrtecordsare inev- anEC-basedestimate,shouldoneor the other(or neither)be itable,and only a small part of the boreal forest comparedto modelpredictions?
PossibleC. onsequentlym, odelsof ecosystemC andwaterbal- commonset of ecosystemmodel parametervaluesfor the ancesare neededto fill measurementgapsand to extrapolate overstory(blackspruce),mossgroundcover,andmineralsoil to future conditions.Models can alsobe usedto put under- at the Northern Study Area (NSA)-Old Black Spruce (OBS) sitewasderivedfrom publishedsitemeasurestandingof processinto a largercontext,andto conductret- ments.(3) Model predictionsof whole-ecosystemCO2 exrospectiveanalysesof ecosystemresponsesto past climatic changeandET were comparedamongthe modelsandto EC
NSA soils describedby Potterand Klooster[1999].NPP is estimatedby were derivedpredominantlyfrom Glacial Lake Agassizsediradiationuseefficiencyfunctions.NASA-CASA includescon- mentsandconsistedof clays,organicsa, ndsomesandydepostrols on metabolismbasedon soil nutrient availability,soil its
Summary
[Ajtayetal., 1979;Olsonetal., 1983;Botkinand Simpson,1990; Potter, 1999], 12-13% of global soil organicmatter (SOM) The boreal forest biome is estimated to cover 6-13% of [Postet al., 1982;Schlesinge1r,997;Potterand Klooster,1997], Earth'sland area [,zljtayetal., 1979;Olsonetal., 1983;Sellerset and 14-45% of global land-surfacelitter (i.e., dead organic al., 1997] and may contain 5-22% of global plant biomass matter abovethe mineral soil) [Schlesinge1r,997;Potterand. Cand water balancescan be studieddirectlywith field mea- ing ecosystemC balance are consistent.For example,if a surementsof tracegasfluxes[e.g.,Gouldenetal., 1997;Sellers ground-basedestimateof annualecosystemNEP differsfrom etal., 1997].Temporalgapsin measuremenrtecordsare inev- anEC-basedestimate,shouldoneor the other(or neither)be itable, ,and only a small part of the boreal forest comparedto modelpredictions?. Biomecanbe measureddirectly,sospatialdata gapswill Our approachinvolvedfive main steps,all appliedto the exist.,stand-scalexperimentasl tudiesof effectsof period1994-1996.(1) A commondatasetof hourlymeteorolarger(i.e.,future)environmentaclhangesaredifficultor im- logicalvariablesat the NSA-OBS site was generated.(2) A possibleC. Onsequentlym, odelsof ecosystemC andwaterbal- commonset of ecosystemmodel parametervaluesfor the ancesare neededto fill measurementgapsand to extrapolate overstory(blackspruce),mossgroundcover,andmineralsoil to future conditions.Models can alsobe usedto put under- at the NSA-OBS sitewasderivedfrom publishedsitemeasurestandingof processinto a largercontext,andto conductret- ments.(3) Model predictionsof whole-ecosystemCO2 exrospectiveanalysesof ecosystemresponsesto past climatic changeandET were comparedamongthe modelsandto EC variability. Part to what spatialscaleand dimension(e.g., one vertical dimensionor a three-dimensionarlepresentation)are needed
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