Effect of aggregating spatial parameters on modelling forest carbon and water fluxes

Estimatingspatialvariabilityofcarbonandwaterfluxesisanessentialtaskinecologicalmodelling.Inthisarticle,thesensitivityof carbonandwaterfluxestothespatialvariabilityofbiochemicalandstructuralpropertiesofcanopiesisassessedinbeechforestsusinga process-basedmodel(CASTANEA).Firstly,asensitivityanalysiswascarriedoutbyvaryingsimultaneouslyacombinationofsixkey parameterswithinarealisticrange:theabovegroundwoodbiomass(B),thesoilwaterreserve(SWR),thecanopyclumpingfactor(CF), the leaf area index (L), the leaf mass per area of sunlit leaves (Msun) and the leaf nitrogen content (N). Secondly, three spatial scales of variabilitywere consideredusingthreestudy sites whose areasrangedfrom0.8to1000 ha.Thefirstareastudiedwasa heterogeneous standlocatedinold-growthforestinFontainebleau(south ofParis, France). The spatialvariabilityofthebiophysicalandbiochemical ecosystemcharacteristicsin80 m 2 outof100 m 2 wasdetermined. Forthe twoothercasestudies,weselecteda sample ofnine plotsin which the key input parameters were measured. Sensitivity analysis indicated that photosynthesis and ecosystem respiration show a moderate non-linear response to L, SWR and B. In spite of these non-linear responses, the three case studies revealed that using parameters averaged over the whole area, induces only a slight bias in the estimation of carbon fluxes and almost no bias in the estimation of water fluxes. The implication of the low sensitivity of carbon and water fluxes to parameter aggregation is discussed in relation to the general problem of the scaling up fluxes from ecosystems to large forest regions. # 2006 Elsevier B.V. All rights reserved.