New boreal forest biomass maps produced from radar satellite data

PARIS - A new processing algorithm has been developed using stacks of images from the Advanced Synthetic Aperture Radar (ASAR) on ESA’s (European Space Agency’s) Envisat satellite that allows for the retrieval of boreal forest biomass well beyond the levels that have been previously reported.

Forests play a crucial role in Earth’s carbon cycle by absorbing carbon from the atmosphere and storing it in biomass.

Undisturbed forests are carbon sinks as they absorb carbon, while forests that are logged or burnt down become carbon sources as they release carbon into the atmosphere.

Boreal forests and woodlands are estimated to cover approximately 14.5 percent of Earth’s land surface, comprising an area of nearly 16 million sq km (about the size of the contiguous 48 states of the US).

The boreal forest ecosystem, which spans Russia, northern Europe, Canada and Alaska, comprises interrelated habitats made up of forests, lakes, wetlands, rivers and tundra.

Boreal forests - including forest soil - store a third more carbon stocks per hectare as tropical forests, making them one of the most significant carbon sinks in the world.

These regions are considered global hotspots, that is, areas of increased warming, which represent possible important tipping points for abrupt climate change.

“Biomass, one of the most essential climate variables defining the functions of the Earth system, is the big unknown factor in the carbon cycle. Since no biomass maps exist with a high level of accuracy, we do not know how much is changing and cannot do calculations with any certainty,” said Professor Christiane Schmullius with the Friedrich Schiller University Jena.

“With this new algorithm, it is the first time that we have something in hand that may be a first step to a global biomass map,” she added.

The BIOMASAR project, sponsored by ESA’s Support to Science Element (STSE), has fully validated the algorithm using ASAR data, which is capable of acquiring images regardless of darkness and cloud cover, with existing in-situ information.

To validate the algorithm, several test sites were chosen in Scandinavia, Siberia and Canada, where both extensive datasets of ASAR images, acquired in ScanSAR mode at mid-resolution, and in-situ measurements were available.

The results show that the retrieval of forest growing stock volume (GSV) - the amount of wood expressed in cubic metres per hectare - is possible from Envisat ASAR data in boreal forests well beyond the levels that have been reported previously, that is, up to 500 cubic metres per hectare.

Using this method, Envisat archives can be exploited to generate global maps of GSV yearly over the entire boreal ecozone with a resolution of 10 km and accuracies of 20 percent. (ANI)