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Remote Sens. 2012, 4(12), 3766-3780; doi:10.3390/rs4123766

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

1
Eco&Sols, 2 Place Viala - bât 12, 34060 Montpellier cedex 01, France
2
SupAgro, UMR Eco&Sols, 2 Place Viala - bât 12, 34060 Montpellier cedex 01, France
3
Departamento de Ciências Atmosféricas, IAG, Universidade de São Paulo, Cidade Universitária-SP, 05508-090, Brazil
4
Department of Forestry and Environmental Sciences, North Carolina State University, Raleigh, NC 27695, USA
5
Remote Sensing Department, National Institute for Space Research (INPE), São José dos Campos-SP, 12227-010, Brazil
*
Author to whom correspondence should be addressed.
Received: 10 October 2012 / Revised: 20 November 2012 / Accepted: 21 November 2012 / Published: 27 November 2012
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Abstract

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 (R2 = 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 R2 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 (R2 = 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.
Keywords: remote sensing; eucalypt; EucVI; MOD13Q1; radiative transfer model; PROSAIL remote sensing; eucalypt; EucVI; MOD13Q1; radiative transfer model; PROSAIL
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

le Maire, G.; Marsden, C.; Nouvellon, Y.; Stape, J.-L.; Ponzoni, F.J. Calibration of a Species-Specific Spectral Vegetation Index for Leaf Area Index (LAI) Monitoring: Example with MODIS Reflectance Time-Series on Eucalyptus Plantations. Remote Sens. 2012, 4, 3766-3780.

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