Tracking subtle environmental changes in the Amazon Basin, home to the largest contiguous area of tropical forest in the world, is extremely difficult. Making matters more challenging are the area's large seasonal fluctuations in precipitation that affect climate and land use. A report published online this week by the Proceedings of the National Academy of Sciences outlines a new method of quantifying the effects of drought over the vast forest--from space.

"The Amazon is simply too big and complex to study on the ground alone," study author Gregory P. Asner of the Carnegie Institution in Stanford, Calif., says. "Thus far its been impossible to determine some of the most basic properties of the forest that we need in order to understand what happens during common climatic events--such as the El Nio dry periods--and what that means to forest growth and the amount of carbon that is locked up in the forest via photosynthesis." Conventional remote-sensing approaches are not well suited to studying drought in tropical forests because the effects may be too subtle to detect or the biophysical mechanisms of the plants are not well understood, the researchers note. In the new work, Asner and his colleagues investigated a novel way of tracking conditions on the ground from space using NASAs Earth Observing-1 (EO-1) satellite. The technique, known as imaging spectroscopy, measures the amount of sunlight that is reflected off the earth. The amount of scattering occurring in the near-infrared region is then related to the water content of the forest canopy.

To test the satellite measurements against ground-based ones, the scientists set up a 10,000-square-meter site inside a forest that was experiencing drought. Essentially, they covered a large swath of the rain forest in plastic, depriving it of precipitation, and monitored soil moisture and other characteristics over time. According to the report, the satellite data are highly sensitive to drought conditions and the observations allowed the team to estimate the forests carbon content and productivity with greater accuracy than current models do. Comments Asner, "The fact that we confirmed that satellite imaging can be used to measure something as detailed as the physiology of the rain forest canopy means that this technique might be useful for understanding other types of ecological phenomena at both large and small scales."