Mapping world's trees

Using satellites and lasers, scientists have produced a unique map that details the heights of the world’s forests. For the first time, using a uniform method, the map spans the entire globe. Scientists from the Colorado State University and NASA say it will help them build an inventory of how much carbon the world’s forests store and how much is recycled back into the atmosphere.

“We’re interested in forests globally because as the forests change, or are cleared, a large amount of the carbon goes to the atmosphere and that is one of the greenhouse gases that impacts the climate,” says Sassan Saatchi of NASA’s Jet Propulsion Laboratory.Developing an understanding of the complexities of the carbon cycle is a key element of climate research.

“Humans produce seven billion tonnes of carbon annually, mostly in the form of carbon dioxide (CO2); about five billion tonnes of which end up either in the atmosphere or the oceans.According to researchers, it is unclear where the remaining two billion tonnes go, but it is suspected that the world’s forests play a major role in capturing and storing the “missing” carbon. Forests play a major role in mitigating the global climate so that’s why we’re interested in it,” explains Dr Saatchi.

“In order to quantify the carbon stores in the forest and how they change, we look at the satellite data and we look at a very high resolution satellite imagery to pin-point where the forests are, what type of forests, and how much carbon they have.”

Light work

The scientists used data from Light Detection and Ranging (LiDAR) technology, which is a powerful technique for accurately gathering height data from a great distance. It is capable of capturing vertical slices of surface features, such as forests, on Earth.

The height of the forest canopy is measured by shooting pulses of light at the surface and observing how much longer it takes for light to bounce back from the ground than from the top of the canopy. The pioneering research mirrors the work botanists have been doing on the ground. Their work involves the painstaking tagging and measuring of trees to establish an inventory of woodland areas in specific locations.

While it is a much slower process than gathering data from space, the results help to build up a complete picture of forest demography. “Satellite imaging can give indications of what’s going on a planetary scale and can maybe detect changes, but then we’re faced with understanding what has caused those changes,” says James Lutz, a researcher at the University of Washington.

Lutz studies large forest plots in minute detail. The aim is to detect changes and trends in the vegetation.

“It is here that we can relate what the satellite records to what’s really happening on the ground,” he says. One such plot is located on the Gifford Pinchot National Forest in south-west Washington.

Tall ambitions

The Pacific north-west of the US has some of the world’s tallest forests. At the Wind River Canopy Crane Research Facility, scientists from the University of Washington follow the life cycles of trees such as the Douglas Fir, many of which are more than 500 years old. The 25-hectare plot includes an industrial crane that lifts researchers over 60m, in a gondola, to above the tree canopy. The crane was installed in the forest in 1994 as part of an ambitious project to carry out research in the tree canopy. “The crane can access about 300 tree crowns and has helped us map and understand more about the structure of the forest and also about how this forest functions as a habitat,” says Ken Bible, the site director.

The main tower of the crane structure also houses sensors that measure carbon levels within the forest.

“The NASA mapping and the work that we have done here go hand in hand with understanding the forest structure,” says Bible. The Washington forest is an old growth ecosystem and is largely undisturbed by man. It is identified by NASA as one of the most densely covered areas of woodland on Earth - along with forests in portions of South-East Asia. “These forests do have a crucial role and it’s important to realise that in much of the world very little old growth forests remains,” says Lutz. Multiple factors such as rainfall levels, temperature and soil type, determine whether old growth forests or new plantations are the most efficient in storing carbon. Using data collected on the ground in conjunction with the NASA map, which will evolve and cover the Earth in more detail, the scientists plan to monitor the forests for decades to come.

“Right now, a new market is being developed – the carbon market – because of their storage of carbon and because of their ability to grow, sequester carbon, the forests come to the market and they have a value,” says Saatchi. “So this information directly goes to what the public need and the policymakers need,” he added.