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First forest height map released

July 23, 2010
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Written by Catriona Davis for CNN

Tree Canopy Height Map (Photo courtesy of NASA Earth Observatory & CNN)

(CNN) — Scientists have produced the first worldwide map showing the height of forests using data from NASA satellites.

The map will help scientists work out how much carbon is locked up in forests and how quickly that carbon cycles through the eco-system and back into the atmosphere.

This can be used to calculate whether the planet can continue to soak up so much of our annual carbon emissions and whether it will continue to do so as climate changes.

Aside from tracking carbon, other uses of the map include producing models that predict the spread and behavior of fires, and ecological models that help biologists understand the suitability of species to specific forests.

The map shows that the tallest forests are in the Pacific Northwest of North America and parts of southeast Asia.

Assistant Professor Michael Lefsky, of Colorado State University, collected the data for the map from laser technology, known as LIDAR, that measures the canopy height by recording how much longer it takes for light to bounce back from the ground than the top of the canopy.

He based his map on data from more than 250 million laser pulses collected during a seven-year period.

Even these 250 million pulses were only able to cover 2.4 percent of the Earth’s surface, so Lefsky combined the LIDAR results with information from another instrument on board the satellites that is able to cover much broader areas but without the same depth.

Lefsky told CNN: “It is certainly a milestone to demonstrate that this can be done and it will be a technique we can use to go forward. There are already people using the data to do things that could never be done before.

“This has given us a better understanding of the pattern of trees in the Amazon, as all previous studies had disagreed on it.”

The map shows the height that 90 percent of trees reach, or are taller than, within 5 square kilometers (1.9 square miles) regions — not the maximum heights of individual trees.

The tallest canopies, reaching more than 40 meters (131 feet) are temperate conifer forests. Tropical rain forests reach around 25 meters (82 feet), a similar height to the oak, beech and birch forests common in Europe and the United States. Boreal forests dominated by spruce, fir, pine and larch usually had canopies of less than 20 meters (82 feet).

Humans release about 7 billion tons of carbon annually, mostly in the form of carbon dioxide, of which 3 billion tons go into the atmosphere and 2 billion tons into the oceans. The remaining 2 billion tons is suspected to be captured by forests and stored as biomass, although this has not yet been proven.

Ecologists are just beginning to work out which types of forests and soils store most carbon and whether they can continue to absorb our carbon emissions.

Lefsky said: “We know there’s a relationship between height of trees and biomass, so we can use it to calculate biomass.

“My next step is to make observations about how much of this biomass is living and how much is dead and decomposing. As it decomposes it releases its carbon dioxide again. I have a team of researchers working on those observations in the Amazon now.”

For more information visit the NASA Earth Observatory website!

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