Satellite observation of El Nino effects on Amazon forest phenology and productivity

Climate variability may affect the functioning of Amazon moist tropical for ests, and recent modeling analyses suggest that the carbon dynamics of the region vary interannually in response to precipitation and temperature anom alies. However, due to persistent orbital and atmospheric artifacts in the satellite record, remote sensing observations have not provided quantitativ e evidence that climate variation affects Amazon forest phenology or produc tivity. We developed a method to minimize and quantify non-biological artif acts in NOAA AVHRR satellite data, providing a record of estimated forest p henological variation from 1982-1993. The seasonal NDVI amplitude (a proxy for phenology) increased throughout much of the basin during El Nino period s when rainfall was anomalously low. Wetter La Nina episodes brought consis tently smaller NDVI amplitudes. Using radiative transfer and terrestrial bi ogeochemical models driven by these satellite data, we estimate that canopy energy absorption and net primary production of Amazon forests varied inte rannually by as much as 21% and 18%, respectively. These results provide la rge-scale observational evidence for interannual sensitivity to El Nino of plant phenology and carbon flux in Amazon forests.