Earth's forests perform a well-known service to the planet, absorbing a great deal of the carbon dioxide pollution emitted into the atmosphere from human activities. But when trees are killed by natural disturbances, such as fire, drought or wind, their decay also releases carbon back into the atmosphere, making it critical to quantify tree mortality in order to understand the role of forests in the global climate system. Tropical old-growth forests may play a large role in this absorption service, yet tree mortality patterns for these forests are not well understood. Now scientist Jeffrey Chambers and colleagues at the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have devised an analytical method that combines satellite images, simulation modeling and painstaking fieldwork to help researchers detect forest mortality patterns and trends. This new tool will enhance understanding of the role of forests in carbon sequestration（隔离，扣押） and the impact of climate change on such disturbances.

"One quarter of CO2 emissions are going to terrestrial（陆地的） ecosystems, but the details of those processes and how they will respond to a changing climate are inadequately understood, particularly for tropical forests," Chambers said. "It's important we get a better understanding of the terrestrial sink because if it weakens, more of our emissions will end up in the atmosphere, increasing the rate of climate warming. To develop a better estimate of the contribution of forests, we need to have a better understanding of forest tree mortality."

Chambers, in close collaboration with Robinson Negron-Juarez at Tulane University, Brazil's National Institute for Amazon Research (Instituto Nacional de Pesquisas da Amazônia [INPA]) and other colleagues, studied a section of the Central Amazon spanning over a thousand square miles near Manaus, Brazil. By linking data from Landsat satellite images over a 20-year period with observations on the ground, they found that 9.1 to 16.9 percent of tree mortality was missing from more conventional plot-based analyses of forests. That equates to more than half a million dead trees each year that had previously been unaccounted for in studies of this region, and which need to be included in forest carbon budgets.

Their findings were published online this week in the Proceedings of the National Academy of Sciences (PNAS).

"If these results hold for most tropical forests, then it would indicate that because we missed some of the mortality, then the contribution of these forests to the net sink might be less than previous studies have suggested," Chambers said. "An old-growth forest has a mosaic（马赛克，镶嵌） of patches all doing different things. So if you want to understand the average behavior of that system you need to sample at a much larger spatial scale over larger time intervals than was previously appreciated. You don't see this mosaic if you walk through the forest or study only one patch. You really need to look at the forest at the landscape scale."