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Researchers use satellite data to develop forest mortality map

US: Scientist Jeffrey Chambers and colleagues at the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have devised an analytical method that combines satellite images, simulation modeling and fieldwork to detect forest mortality patterns and trends. This new tool will enhance understanding of the role of forests in carbon sequestration and the impact of climate change on such disturbances.

“One quarter of CO2 emissions are going to terrestrial 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,” he added.

Chambers, in close collaboration with Robinson Negron-Juarez at Tulane University, Brazil’s National Institute for Amazon Research 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 per cent 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.

Trees and other living organisms are key players in the global carbon cycle, a complex biogeochemical process in which carbon is exchanged among the atmosphere, the ocean, the biosphere and earth’s crust. Fewer trees mean not only a weakening of the forest’s ability to absorb carbon, but the decay of dead trees will also release carbon dioxide back into the atmosphere. Large-scale tree mortality in tropical ecosystems could thus act as a positive feedback mechanism, accelerating the global warming effect.

The Amazon forest is hit periodically by fierce thunderstorms that may bring violent winds with concentrated bursts believed to be as high as 170 miles per hour. The storms can blow down many acres of the forest; however, Chambers and his team were able to paint a much more nuanced picture of how storms affected the forest.

By looking at satellite images before and after a storm, the scientists discerned changes in the reflectivity of the forest, which they assumed was due to damage to the canopy and thus tree loss. Researchers were then sent into the field at some of the blowdown areas to count the number of trees felled by the storm. Looking at the satellite images pixel by pixel (with each pixel representing 900 square meters, or about one-tenth of a football field) and matching them with on-the-ground observations, they were able to draw a detailed mortality map for the entire landscape, which had never been done before.

Source: SciTech Daily