Deciphering the components of regional net ecosystem fluxes following a bottom-up approach for the Iberian Peninsula

[摘要] Quantification of ecosystem carbon pools is a fundamental requirement forestimating carbon fluxes and for addressing the dynamics and responses of theterrestrial carbon cycle to environmental drivers. The initial estimates ofcarbon pools in terrestrial carbon cycle models often rely on the ecosystemsteady state assumption, leading to initial equilibrium conditions. In thisstudy, we investigate how trends and inter-annual variability of netecosystem fluxes are affected by initial non-steady state conditions.Further, we examine how modeled ecosystem responses induced exclusively bythe model drivers can be separated from the initial conditions. For this, theCarnegie-Ames-Stanford Approach (CASA) model is optimized at set of Europeaneddy covariance sites, which support the parameterization of regionalsimulations of ecosystem fluxes for the Iberian Peninsula, between 1982 and2006.

The presented analysis stands on a credible model performance for a set ofsites, that represent generally well the plant functional types and selecteddescriptors of climate and phenology present in the Iberian region – exceptfor a limited Northwestern area. The effects of initial conditions oninter-annual variability and on trends, results mostly from the recovery ofpools to equilibrium conditions; which control most of the inter-annualvariability (IAV) and both the magnitude and sign of most of the trends.However, by removing the time series of pure model recovery from the timeseries of the overall fluxes, we are able to retrieve estimates ofinter-annual variability and trends in net ecosystem fluxes that arequasi-independent from the initial conditions. This approach reduced thesensitivity of the net fluxes to initial conditions from 47% and 174% to−3% and 7%, for strong initial sink and source conditions, respectively.

With the aim to identify and improve understanding of the component fluxesthat drive the observed trends, the net ecosystem production (NEP) trends aredecomposed into net primary production (NPP) and heterotrophic respiration(R_H) trends. The majority (~97%) of the positive trends inNEP is observed in regions where both NPP and R_H fluxes showsignificant increases, although the magnitude of NPP trends is higher.Analogously, ~83% of the negative trends in NEP are also associatedwith negative trends in NPP. The spatial patterns of NPP trends are mainlyexplained by the trends in fAPAR (r=0.79) and are only marginallyexplained by trends in temperature and water stress scalars (r=0.10 andr=0.25, respectively). Further, we observe the significant role ofsubstrate availability (r=0.25) and temperature (r=0.23) in explainingthe spatial patterns of trends in R_H. These results highlight therole of primary production in driving ecosystem fluxes.

Overall, our study illustrates an approach for removing the confoundingeffects of initial conditions and emphasizes the need to decompose theecosystem fluxes into its components and drivers for more mechanisticinterpretations of modeling results. We expect that our results are not onlyspecific for the CASA model since it incorporates concepts of ecosystemfunctioning and modeling assumptions common to biogeochemical models. Adirect implication of these results is the ability of this approach to detectclimate and phenology induced trends regardless of the initial conditions.

[学科分类] 地球化学与岩石