Drought stress and carbon uptake in an Amazon forest measured with spaceborne imaging spectroscopy

doi:10.1073/pnas.0400168101

Drought stress and carbon uptake in an Amazon forest measured with spaceborne imaging spectroscopy

^*Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305; ^‡Woods Hole Research Center, Woods Hole, MA 02543; and ^§Instituto de Pesquisa Ambiental da Amazônia, 66035-170, Belém, Brazil

Edited by W. G. Ernst, Stanford University, Stanford, CA (received for review January 10, 2004)

Abstract

Amazônia contains vast stores of carbon in high-diversity ecosystems, yet this region undergoes major changes in precipitation affecting land use, carbon dynamics, and climate. The extent and structural complexity of Amazon forests impedes ground studies of ecosystem functions such as net primary production (NPP), water cycling, and carbon sequestration. Traditional modeling and remote-sensing approaches are not well suited to tropical forest studies, because (i) biophysical mechanisms determining drought effects on canopy water and carbon dynamics are poorly known, and (ii) remote-sensing metrics of canopy greenness may be insensitive to small changes in leaf area accompanying drought. New spaceborne imaging spectroscopy may detect drought stress in tropical forests, helping to monitor forest physiology and constrain carbon models. We combined a forest drought experiment in Amazônia with spaceborne imaging spectrometer measurements of this area. With field data on rainfall, soil water, and leaf and canopy responses, we tested whether spaceborne hyperspectral observations quantify differences in canopy water and NPP resulting from drought stress. We found that hyperspectral metrics of canopy water content and light-use efficiency are highly sensitive to drought. Using these observations, forest NPP was estimated with greater sensitivity to drought conditions than with traditional combinations of modeling, remote-sensing, and field measurements. Spaceborne imaging spectroscopy will increase the accuracy of ecological studies in humid tropical forests.

Footnotes

↵ † To whom correspondence should be addressed at: Department of Global Ecology, Carnegie Institution, 260 Panama Street, Stanford, CA 94305. E-mail: gpa{at}stanford.edu.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: NPP, net primary production; ENSO, El Niño/Southern Oscillation; LAI, leaf area index; NDVI, normalized difference vegetation index; EO-1, National Aeronautics and Space Administration Earth Observing 1; ha, hectare(s); LWP, leaf water potential; PAW, plant-available water; SR, simple ratio; fAPAR, fraction of photosynthetically active radiation absorbed; PRI, photochemical reflectance index; ARI, anthocyanin reflectance index; LUE, light-use efficiency; SWAM, spectroscopic water-absorption metric.