April Flowers for redOrbit.com – Your Universe Online
Scientists have debated how big a role elephants play in toppling trees in South African savannas for years. Now, using some very high tech airborne equipment, they finally have an answer.
Tree loss is a natural process, but in some regions it is increasing beyond what could naturally be expected. This extreme tree loss has cascading effects on the habitats of many species. Studying savannas across Kruger National Park, Carnegie scientists have quantitatively determined tree losses for the first time.
The team found that elephants, as previously thought, are the primary agents of tree loss. Their browsing habits knock trees over at a rate averaging 6 times higher than in areas that are inaccessible to elephants. The study, published in Ecology Letters, found that elephants prefer trees in the 16 to 30 foot height range, with annual losses of up to 20% at that size. The findings of this study will bolster our understanding of elephant and savanna conservation needs.
“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,” commented lead author Greg Asner of Carnegie’s Department of Global Ecology. “Our maps show that elephants clearly toppled medium-sized 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.”
Previously, researchers used aerial photography and field-based approaches to quantifying the tree loss and the impact of elephant browsing. This team used Light Detection and Ranging (LiDAR), mounted on the fixed wing of Carnegie Airborne Observatory (CAO). The LiDAR provides detailed 3-D images of the vegetation canopy at tree-level resolution using laser pulses that sweep across the African savanna. Able to detect even small changes in individual tree height, CAO’s vast coverage is far superior to previous methods.
Using four study landscapes within Kruger National Park, and in very large areas fenced off to prevent herbivore entry, the scientists considered an array of environmental variables. There are six such enclosures, four of which keep out all herbivores larger than a rabbit, and two which allow herbivores smaller than elephants. The team identified and monitored 58,00 individual trees from the air across this landscape in 2008 and again in 2010.
They found that nearly 9% of the trees decreased in height in two years. They also mapped treefall changes and linked them to different climate and terrain conditions. Most of the tree loss occurred in lowland areas with higher moisture and on soils high in nutrients that harbor trees preferred by elephants for browsing. Comparison with the herbivore free enclosures definitively identified elephants, as opposed to other herbivores or fire, as the major agent in tree losses over the two year study period.
“These spatially explicit patterns of treefall highlight the challenges faced by conservation area managers in Africa, who must know where and how their decisions impact ecosystem health and biodiversity. They should rely on rigorous science to evaluate alternative scenarios and management options, and the CAO helps provide the necessary quantification,” commented co-author Shaun Levick.
Danie Pienaar, head of scientific services of the South African National Parks remarked, “This collaboration between external scientists and conservation managers has led to exciting and ground-breaking new insights to long-standing questions and challenges.
Knowing where increasing elephant impacts occur in sensitive landscapes allows park managers to take appropriate and focused action. These questions have been difficult to assess with conventional ground-based field approaches over large scales such as those in Kruger National Park.”
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