High-resolution map of the height of Earth's forests developed by NASA-led team could throw light on forests' influence on climate change and wildlife habitats, help scientists better monitor stored carbon

LUXEMBOURG , February 21, 2012 (press release) – Climate change shows no signs of slowing down. Getting a better handle on forests' influence on climate change and the wildlife habitats within them will give scientists the information they need to monitor carbon stored in vegetation more effectively. Enter a team of researchers led by the American National Aeronautics and Space Administration (NASA) that has devised an accurate, high-resolution map of the height of the planet's forests. The findings are presented in the Journal of Geophysical Research.

The scientists used 2.5 million carefully screened, globally distributed laser pulse measurements from space. The Geoscience Laser Altimeter System instrument on NASA's Ice, Cloud and land Elevation Satellite (ICESat) collated light detection and ranging (lidar) data in 2005.

'Knowing the height of Earth's forests is critical to estimating their biomass, or the amount of carbon they contain,' explained lead author Marc Simard of NASA's Jet Propulsion Laboratory at the California Institute of Technology in the United States. 'Our map can be used to improve global efforts to monitor carbon. In addition, forest height is an integral characteristic of Earth's habitats, yet is poorly measured globally, so our results will also benefit studies of the varieties of life that are found in particular parts of the forest or habitats.'

The map, which has a 1-kilometre spatial resolution, shows the highest points in the forest canopy. The team validated the map against data generated from almost 70 ground sites around the world.

For the most part, the higher the elevation, the smaller forest heights become, and the lower the latitude, the taller the heights become. Their height in particular drops off the farther they are from the tropics, according to the researchers. Researchers noted an exception in southern tropical forests of Australia and New Zealand, where stands of eucalyptus top the 40-metre mark.

In their study, the researchers added more data to the ICESat data to compensate for the sparse lidar data, the effects of topography and cloud cover. Part of the data package included elevation data from NASA's Tropical Rainfall Measuring Mission and the WorldClim database, estimates of the percentage of global tree cover from NASA's Moderate Resolution Imaging Spectroradiometer on NASA's Terra satellite, and temperature and precipitation maps from NASA's Tropical Rainfall Measuring Mission. The team pointed out that WorldClim's high-resolution global climate data used for mapping and spatial modelling are available free of charge.

The new map highlighted that forest heights were taller than in a previous ICESat-based map. This is especially the case in both the tropics and in boreal forests. Heights decreased in mountainous regions.

The researchers said the accuracy of the novel map differs across major ecological community types in the forests. The impact of human activity on the distribution of forests as well as variability in the natural height of the forests affects the accuracy, they added.

'Our map contains one of the best descriptions of the height of Earth's forests currently available at regional and global scales,' Dr Simard said. 'This study demonstrates the tremendous potential that space-borne lidar holds for revealing new information about Earth's forests. However, to monitor the long-term health of Earth's forests and other ecosystems, new Earth observing satellites will be needed.'

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