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Calibration of a vegetation index to monitor Eucalyptus plantation leaf area index with MODIS reflectance time-series

Le Maire G., Marsden C., Ponzoni F.J., Verhoef W., Lo Seen D., Bégué A., Stape J.L., Nouvellon Y.. 2011. In : INPE; SELPER. XVth SBSR (Symposio Brasileiro de Sensoriamento Remoto) Conference, April 30th to May 5th, 2011, Curitiba, Brazil. s.l. : s.n., 8 p.. Simposio Brasileiro de Sensoriamento Remoto. 15, 2011-04-30/2011-05-05, Curitiba (Brésil).

Leaf area index (LAI) of Eucalyptus plantations is an important variable, linked to different biophysical processes: carbon assimilation, water consumption, litter production, etc. Eucalyptus LAI is highly variable in time throughout the stand rotation. After planting, LAI increases rapidly to reach a maximum value at the age of about 2 years. Then LAI fluctuates seasonally, with a small decrease with stand age, till the clear-cut which occurs at age 6-7. In addition to these variations with time, differences in space may arise from local soil and meteorological conditions. In this study, we propose a method to retrieve spatial and temporal values of Eucalyptus stand LAI during entire rotations based on Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared and red reflectance time-series. This work capitalizes on a recently published methodology based on radiative transfer model (RTM) inversion, which achieved good LAI retrieval results (r2=0.80, RMSE= 0.41) but was computationally intensive. We propose here to use the radiative transfer model results to calibrate a dedicated Eucalyptus plantation vegetation index which would give a similar result to RTM inversion, but in a simpler way. This vegetation index takes the form of a soil adjusted vegetation index. When using only red and NIR reflectance, r2 is 0.68 and RMSE is 0.49 on an independent validation dataset of destructive LAI measurements. The additional use of stand age and day of year increases the performance of the index (r2=0.75 and RMSE=0.43). This simple index opens the way to the operational retrieval of Eucalyptus LAI from MODIS data.

Mots-clés : eucalyptus grandis; eucalyptus urophylla; indice de surface foliaire; plantations; modélisation environnementale; réflectométrie; spectrométrie; sao paulo

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