Parameterization of an ecosystem light-use-efficiency model for predicting savanna GPP using MODIS EVI.

Accurate estimation of carbon fluxes across space and time is of great importance for quantifying global carbon balances. Current production efficiency models for calculation of gross primary production (GPP) depend on estimates of light-use-efficiency (LUE) obtained from look-up tables based on biome type and coarse-resolution meteorological inputs that can introduce uncertainties. Plant function is especially difficult to parameterize in the savanna biome due to the presence of varying mixtures of multiple plant functional types (PFTs) with distinct phenologies and responses to environmental factors. The objective of this study was to find a simple and robust method to accurately up-scale savanna GPP from local, eddy covariance (EC) flux tower GPP measures to regional scales utilizing entirely remote sensing oservations. Here we assessed seasonal patterns of Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation products with seasonal EC tower GPP (GPP EC ) at four sites along an ecological rainfall gradient (the North Australian Tropical Transect, NATT) encompassing tropical wet to dry savannas. The enhanced vegetation index (EVI) tracked the seasonal variations of GPP EC well at both site- and cross-site levels ( R 2 = 0.84). The EVI relationship with GPP EC was further strengthened through coupling with ecosystem light-use-efficiency (eLUE), defined as the ratio of GPP to photosynthetically active radiation (PAR). Two savanna landscape eLUE models, driven by top-of-canopy incident PAR (PAR TOC ) or top-of-atmosphere incident PAR (PAR TOA ) were parameterized and investigated. GPP predicted using the eLUE models correlated well with GPP EC , with R 2 of 0.85 (RMSE = 0.76 g C m − 2 d − 1 ) and 0.88 (RMSE = 0.70 g C m − 2 d − 1 ) for PAR TOC and PAR TOA , respectively, and were significantly improved compared to the MOD17 GPP product ( R 2 = 0.58, RMSE = 1.43 g C m − 2 d − 1 ). The eLUE model also minimized the seasonal hysteresis observed between green-up and brown-down in GPP EC and MODIS satellite product relationships, resulting in a consistent estimation of GPP across phenophases. The eLUE model effectively integrated the effects of variations in canopy photosynthetic capacity and environmental stress on photosynthesis, thus simplifying the up-scaling of carbon fluxes from tower to regional scale. The results from this study demonstrated that region-wide savanna GPP can be accurately estimated entirely with remote sensing observations without dependency on coarse-resolution ground meteorology or estimation of light-use-efficiency parameters. [ABSTRACT FROM AUTHOR]