Scanning for biomass

Dr Robert Washington-Allen (centre) preaches the value of ground-based laser scanning, while his students set up a Leica Scan Station 2. SAEON Ndlovu Node field technician Patrick Ndlovu looks on (right).   View larger version A sample of the output of the scanner, showing the fine branching structure of a Mopane tree, and surrounding vegetation.   View larger version A laser scan of the visiting American researchers and some staff of the SAEON Ndlovu Node, showing the high degree of detail that a TLS can reproduce. From left: Thabo Mohlala (SAEON), Rob Taylor (SAEON), Dr Barend Erasmus (WITS), Dr Tony Swemmer (SAEON), Dr George Chirima (SAEON), Dr Robert-Washington Allen (Texas A&M), Alfredo Delgado (Texas A&M) and Desmond Mabaso (SANParks)

Dr Robert Washington-Allen from the USA recently visited the SAEON Ndlovu Node in Phalaborwa and brought one of his favourite toys with him, a terrestrial laser scanner (TLS).

A TLS is a high-tech laser device originally designed to create three-dimensional images of construction sites for use by architects and engineers. It shoots out thousands of laser pulses a second, which reflect off surrounding objects and bounce back to the scanner, a fraction of a millisecond later. The position and return time of hundreds of thousands of these laser returns are recorded as the TLS rotates, creating a high-resolution, 3D image of whatever surrounds it.

Pioneering new methods

In recent years a few creative and tech-savvy ecologists have begun exploring this technology for collecting ecological data, pioneering new methods for measuring vegetation structure that may revolutionise certain fields of ecology.

Some of the Ndlovu Node’s vegetation monitoring sites were scanned to compare the accuracy and efficiency of using a scanner with conventional field methods. Detailed manual measurements of vegetation structure already exist for these sites, making them an ideal location for those with the technological expertise, such as Dr Washington-Allen, to test this new technology under “working conditions”.

Much of the scanning work was done at the Letaba Exclosure in the Kruger National Park, a large research site that includes a fenced area that keeps elephants and other large herbivores out. Big differences in the height and spread of the trees and the quantity of grass exist inside the exclosure thanks to years of protection from mega-herbivores. A comparison of scans done inside and outside therefore provided an excellent opportunity to test how well a TLS performs at sites varying greatly in vegetation structure.

Testing TLS on African ecosystems

While laser scanners have been used successfully in the northern Hemisphere for trees ranging from forest giants to shrubs, they are only now being tested in African savanna environments. The use of a TLS for measuring grass biomass is also something new, and Dr Tony Swemmer is particularly hopeful that this will produce useful results, as the conventional method for measuring grass biomass includes many long hours in the hot sun, clipping grass tufts by hand. The use of a TLS may ultimately allow for much faster and cheaper measurement of the productivity of African ecosystems.

Dr Washington-Allen is an Associate Professor of Environmental Monitoring and Assessment, based in the Ecosystem Science and Management Department of Texas A&M University. He was accompanied by an MSc student, Alfredo Delgado on his brief visit to South Africa in August.