Carbon Emissions at Stake

Asner’s results from the analysis of satellite data held some surprises as well. According to Hyperion, anthocyanin pigments in the drought plot were 60 percent higher than the control plot at the end of the rainy season in July. Then by the end of the dry season, they were 40 percent lower. “If anthocyanin is an indicator of leaf turnover and growth, why would we see an increase in the drought plot compared to the normal plot at the end of the wet season?” Asner wonders. He is eager to get to work on additional experiments that will help him understand the question.

Asner is both surprised and incredibly pleased that the pigment and water absorption indicators were so successful in detecting the impacts of drought from space. As an unfunded—but, in his mind, absolutely critical—side project, Asner and some of his colleagues have been analyzing every single Hyperion image collected over the Amazon to document just how variable drought is across the region. “Drought in the Amazon is amazingly variable,” he says.

That variability could be one factor in why studies of the carbon cycle in the Amazon sometimes report the forest as a carbon dioxide sink while in another location, it will seem to be a source. The issue is more than just an interesting puzzle for scientists to solve. Asner’s and Nepstad’s results suggest that even if mature Amazon forests are a net sink for atmospheric carbon, the process is extremely sensitive to even moderate drought stress.

“What’s at stake here is about 70-80 billions tons of carbon--about a decade’s worth of human emissions—in the trees in the Amazon. Our results show that as temperatures and drought increase, much of that carbon may return to the atmosphere because growth slows way down and the biggest trees experience mortality for years afterward,” says Nepstad.

The early signs of drought stress are not likely to be detectable with the current generation of multi-spectral satellite sensors, whose observations of vegetation are confined mostly to the visible part of the electromagnetic spectrum. The 220 wavelengths observed by Hyperion provide an almost mind-boggling number of new options.

Hyperspectral sensors are the future of remote sensing,” says Asner, his voice filled with excitement. “It might sound unreal, even like science fiction,” he continues, “but we are talking about observing the vibrations, rotations, and interactions of molecules—like water molecules or pigments in tree leaves—from space. I think the most significant thing about this study is that it opens a whole new path for understanding how the forest is responding to the climate system.”

• Link:
• Amazon Ecology Page at Woods Hole Research Center

• References:
• Asner, G.P., Nepstad, D., Cardinot, G., and Ray, D. (2004) Drought stress and carbon uptake in an Amazon forest measured with spaceborne imaging spectroscopy. Proceedings of the National Academy of Sciences, 101(16), 6039-6044.
• Nepstad, D.C., Moutinho, P., Dias-Filho, M.B., Davidson, E.A., Cardinot, G., Markewitz, D., Figueiredo, R., Viana, N., Chambers, J., Ray, D., Guerreiros, J.B., Lefebvre, P., Sternberg, L., Moreira, M., Barros, L., Ishida, F.Y., Tohlover, I., Belk, E., Kaliff, K., and Schwalbe, K. (2002) The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest. Journal of Geophysical Research, 107(53), 1-18.

 Artificial El Niño Gets Underway

Scientists used the amount of light reflected from the control and drought plots at the beginning and end of the 2001 dry season to identify satellite-based signals of drought. The top graph shows reflectance measured by Hyperion at wavelengths between 450 and 1,250 nanometers in July (blue lines) and November (orange lines).

The forest reflected very little visible light (450 to 700 nanometers) because that is the type of light plants absorb for photosynthesis. The lower graph shows the visible reflectance in greater detail.

Dashed lines show wavelengths used for satellite-based estimates of carbon uptake: green for vegetation greenness, purple for light-use efficiency, and red for the activity of anthocyanin, a chlorophyll-helper pigment. (Graph by Robert Simmon, based on data from Greg Asner et al.)