Calibration of a Species-Specific Spectral Vegetation Index for Leaf Area Index (LAI) Monitoring: Example with MODIS Reflectance Time-Series on Eucalyptus Plantations

European Integrated Project Ultra Low CO2 Steelmaking (ULCOS-contract) [515960]; CNPq [306561/2007]; EucFlux project; French Ministry of Foreign AffairsThis study was funded by the European Integrated Project Ultra Low CO2 Steelmaking (ULCOS-contract no. 515960), by CNPq 306561/2007, and by the EucFlux project. We thank International Paper of Brasil and in particular Jose Mario Ferreira, Augusto Miguel Nascimento Lima and Sebastiao Oliveira, for providing data and technical help for field campaigns and the French Ministry of Foreign Affairs for their financial support. We are also very grateful to Osvaldo Cabral (EMBRAPA), Marcos Ligo (EMBRAPA), Elcio Reis (IPBr), Cristiane Camargo Zani (ESALQ), Giampiero Bini Cano (Instituto Botanico), Renato Meulman Leite da Silva (ESALQ), and Reinaldo Camargo (ESALQ) for their help during field work.3

إنجليزي. The leaf area index (LAI) is a key characteristic of forest ecosystems. Estimations of LAI from satellite images generally rely on spectral vegetation indices (SVIs) or radiative transfer model (RTM) inversions. We have developed a new and precise method suitable for practical application, consisting of building a species-specific SVI that is best-suited to both sensor and vegetation characteristics. Such an SVI requires calibration on a large number of representative vegetation conditions. We developed a two-step approach: (1) estimation of LAI on a subset of satellite data through RTM inversion; and (2) the calibration of a vegetation index on these estimated LAI. We applied this methodology to Eucalyptus plantations which have highly variable LAI in time and space. Previous results showed that an RTM inversion of Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared and red reflectance allowed good retrieval performance (R-2 = 0.80, RMSE = 0.41), but was computationally difficult. Here, the RTM results were used to calibrate a dedicated vegetation index (called "EucVI") which gave similar LAI retrieval results but in a simpler way. The R-2 of the regression between measured and EucVI-simulated LAI values on a validation dataset was 0.68, and the RMSE was 0.49. The additional use of stand age and day of year in the SVI equation slightly increased the performance of the index (R-2 = 0.77 and RMSE = 0.41). This simple index opens the way to an easily applicable retrieval of Eucalyptus LAI from MODIS data, which could be used in an operational way.