title: Woody encroachment over 70 years in South African savannahs: overgrazing, global change or extinction aftershock?
abstract: Woody encroachment in 'open' biomes like grasslands and savannahs is occurring globally. Both local and global drivers, including elevated CO2, have been implicated in these increases. The relative importance of different processes is unresolved as there are few multi-site, multi-land-use evaluations of woody plant encroachment. We measured 70 years of woody cover changes over a 1020 km(2) area covering four land uses (commercial ranching, conservation with elephants, conservation without elephants and communal rangelands) across a rainfall gradient in South African savannahs. Different directions of woody cover change would be expected for each different land use, unless a global factor is causing the increases. Woody cover change was measured between 1940 and 2010 using the aerial photo record. Detection of woody cover from each aerial photograph was automated using eCognitions' Object-based image analysis (OBIA). Woody cover doubled in all land uses across the rainfall gradient, except in conservation areas with elephants in low-rainfall savannahs. Woody cover in 2010 in low-rainfall savannahs frequently exceeded the maximum woody cover threshold predicted for African savannahs. The results indicate that a global factor, of which elevated CO2 is the likely candidate, may be driving encroachment. Elephants in low-rainfall savannahs prevent encroachment and localized megafaunal extinction is a probable additional cause of encroachment.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.
authors: Botany Department, University of Cape Town, Private Bag, Rondebosch 7701, South Africa School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Natural Resources and the Environment, CSIR, PO Box 395, Pretoria 0001, South Africa nicolastvns@gmail.com.
authors: Nicola
authors: 0000-0002-0693-8409
authors: N
authors: Stevens
authors: Global Change and Sustainability Research Institute, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa.
authors: B F N
authors: BF
authors: Erasmus
authors: School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Natural Resources and the Environment, CSIR, PO Box 395, Pretoria 0001, South Africa.
authors: S
authors: S
authors: Archibald
authors: Botany Department, University of Cape Town, Private Bag, Rondebosch 7701, South Africa South African Environmental Observation Network, National Research Foundation, Private Bag X7, Claremont 7735, South Africa.
authors: W J
authors: 0000-0002-3441-2084
authors: WJ
authors: Bond
mesh_terms: D000822:Animal Husbandry; D057231:Climate Change; D003247:Conservation of Natural Resources; D053476:Extinction, Biological; D065948:Grassland; D013019:South Africa; D014197:Trees / Q000254:growth & development*
keywords: aerial photography; land use; savannah; tree cover; woody encroachment; woody thickening