Vegetation Dynamics and Seasonal Responses of North and South America from EOS-MODIS Vegetation Indices

Consistent and long-term measurements from satellite sensors are important in assessing the spatial and temporal variability of the earth's terrestrial vegetation and in studying how global ecosystems are changing and how the earth's vegetation is being transformed. In this paper we present and evaluate one year of vegetation index product availability from the Moderate Resolution Imaging Spectroradiometer (MODIS). Two MODIS vegetation indices (VI), the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) are produced at 1 km, 500 m, and 250 m resolutions and 16-day compositing periods. Multitemporal, seasonal profiles of the MODIS VI's are presented for numerous biome types in North and South America which depicted quite well their respective phenologies. The dynamic range of the MODIS VI's are presented and their sensitivities in discriminating vegetation differences are evaluated over sparsely vegetated areas as well as high biomass, densely vegetated areas. We found the NDVI to asymptotically saturate in high biomass areas while the EVI remained sensitive to vegetation variations in higher biomass regions such as in the Amazon. Validation campaigns were made at test sites representing semi-arid grass and shrub, savanna, and tropical forest biomes. Results show a good correspondence between airborne-measured, top-of-canopy, reflectances and VI values with those from the MODIS sensor over the test sites. Simultaneously derived field biophysical measures also demonstrated the science utility of the VI's.