The wood and the trees

IN AN isolated forest in the Sivalik hills of south-western Nepal, intense sun beats down through the treetops. A sweaty trek up a steep, rocky slope leads to a spot where a team of researchers is busy measuring the trees.

They are working for Forest Resource Assessment Nepal, a joint venture between the Nepalese and Finnish governments. Two global-positioning-system devices guide the researchers to their target. Once there, they use tape measures, callipers and a hand-held laser to measure the heights and girths of all the trees within a 500-square-metre plot. These measurements, and each tree's species, are recorded on a clipboard. One plot finished; 959 more to go.

A classic piece of forestry, then. Boots on the ground. Specimens duly counted. But this is a study with a twist, for its purpose is to calibrate a new approach to the subject—one that will gather information by the bucketload without the need to rely on quite so many boots. This new approach uses a technique called lidar.

Like its cousins radar and sonar, lidar (light detection and ranging), works by broadcasting electromagnetic waves towards a target and then building up a picture from the reflection. In the case of lidar, the waves are in the form of an infra-red laser beam. And in the case of the forests of south-western Nepal, the target is the trees. During a forest survey, an aircraft-borne lidar sweeps a beam that fires about 70,000 pulses a second over the canopy. A sensor on the aircraft records the time it takes to receive the backscattering of pulses, and that is used to compute distances to the forest canopy and to the soil beneath.

The result, when processed through the computers of Arbonaut, a Finnish natural-resource-management company, is a three-dimensional image of the forest that can be correlated with, and calibrated by, the efforts of the chaps with the tape measures. And that, in turn, can be used to estimate the amount of carbon stored in the plot examined, and extrapolated to calculate the carbon stored in larger areas of forest that have been scanned by lidar, but not measured with tapes.

The point of the project, which should be completed by 2014, is to allow Nepal to participate in international carbon-trading schemes that pay poor countries with lots of trees not to cut them down. The Reducing Emissions from Deforestation and Forest Degradation (REDD) scheme agreed at the United Nations' climate-change conference in Cancún last December may eventually be worth $30 billion a year. Nepal wants a slice of that. Lidar monitoring may provide a way of making sure it is delivering on its side of the bargain.

Until a few years ago, assessing the amount of plant matter in a forest in a cheap and accurate manner seemed an insurmountable problem, according to Eric Dinerstein, chief scientist of the World Wide Fund for Nature, a conservation group that is also involved in the Nepalese lidar project. Although a woodland's area can be worked out from satellite photographs, that gives only a hazy idea of the mass of the plants growing there.

If Forest Resource Assessment Nepal and projects like it are successful, that will change. It will then be possible, with reasonable confidence, to pay REDD money out only to those countries that deliver the goods—or, rather, the trees—in sufficient, measurable quantities.