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Lidar reveals elephant impact

Oct 2012

Scientists and conservation managers have long suspected that elephants browsing for food are the main culprits behind toppled trees in the South African savannas – and airborne lidar and 3-D mapping have helped prove this.

Widespread tree loss is a concern to wildlife managers because it is known to have a negative effect on the habitat for many other species. To gain a better understanding of the problem, scientists from the Washington-based Carnegie Institution for Science collaborated with managers of South Africa’s Kruger National Park to produce a detailed analysis of treefall in the park.

They mounted the lidar equipment on the fixed-wing Carnegie Airborne Observatory so that its laser pulses could sweep across the savannas to provide detailed 3-D images of the vegetation canopy at tree-level resolution. The observatory’s vast coverage far surpasses previous field-based and aerial photographic evaluations, the scientists say.

In 2008 and 2010, the researchers identified and monitored 58,000 trees from the air, inside and outside of the exclosures and across the landscape. The lidar can detect even small changes in a tree’s height, and the scientists found that nearly 9 percent of the trees decreased in height in two years. They also found that the mapped changes in treefall were linked to different climate and terrain conditions.

Lidar and 3-D mapping data in South Africa’s savannas have revealed that elephants browsing for food trample trees, with possibly negative consequences for other species in the habitat. Courtesy of Gregory Asner, Carnegie Institution for Science.

The trees that elephants prefer – found in lowland areas with more moisture and high-nutrient soils – experienced most of the loss, they noted.

The study, published in Ecology Letters, showed conclusively that elephants, rather than other herbivores and fire, were the major contributors to tree loss over the two-year period. The lidar-acquired data showed that trees were toppled at a rate averaging six times higher than were those in elephant-free areas. The scientists also determined that elephants prefer trampling trees in the 16- to 30-ft range, with annual losses of up to 20 percent in this range.

“Our maps show that elephants clearly toppled medium-size trees, creating an ‘elephant trap’ for the vegetation. These elephant-driven tree losses have a ripple effect across the ecosystem, including how much carbon is sequestered from the atmosphere,” said researcher Gregory Asner. “Previous field studies gave us important clues that elephants are a key driver of tree losses, but our airborne 3-D mapping approach was the only way to fully understand the impacts of elephants across a wide range of environmental conditions found in savannas.”

Park officials now can use the data to help preserve trees and wildlife.

“Knowing where increasing elephant impacts occur in sensitive landscapes allows park managers to take appropriate and focused action,” said Danie Pienaar, head of scientific services of the South African National Parks.

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