Two decades of Normalized Difference Vegetation Index changes in South America: identifying the imprint of global change

Estimates of carbon uptake at the continental scale become urgently needed as the role of countries as net sinks or sources of carbon gains political and economic importance. Despite uncertainties related to radiation use efficiency, the amount of photosynthetically active radiation (PAR) intercepted by the canopy is a reliable estimator of primary production. Theoretical and empirical data support the relationship between the Normalized Difference Vegetation Index (NDVI) derived from the Advanced Very High Resolution Radiometer sensor on National Oceanic & Atmospheric Administration satellites and the fraction of PAR intercepted by green canopies. It is shown, for the period 1981–2000, that there is an overall increase in the radiation intercepted by the canopy over South America by 1.3%, with rainforests making the largest absolute contribution (45%), followed by savannas (23%). Under conditions of minimal agricultural use, disturbance and anthropogenic N deposition, humid temperate forests showed the highest proportional increase in NDVI during the last two decades (4.9%). Deserts showed a net reduction in NDVI relative to the 1981–1985 average (24.4%). The expansion of agriculture over the last two decades was associated with NDVI reductions over subtropical forests. NDVI trends in South American region highlight a biome-dependent imprint of major global change noticeable in only two decades.