A century of woody plant encroachment in the dry Kimberley savanna of South Africa

Abstract

Woody plant encroachment is frequent in dry savannas. Grazing is often considered to be a major cause of encroachment in dry savannas because grasses are removed by livestock, leaving bare areas for trees to colonise in wetter years. Earlier experiments conducted in the Kimberley area of the Northern Cape showed that neither fire nor grazing was important for woody plant encroachment. We used aerial and fixed-point repeat ground photographs, including historical photographs taken at the time of the Second Anglo-Boer War of 1899–1902, to assess the scale and timing of woody plant encroachment in the dry savannas near Kimberley in South Africa (mean annual rainfall = 300–400 mm). There were large increases in woody plant encroachment in most areas. Even at the battlefield of Magersfontein, where grazing has been virtually absent since its protection in 1960, we found that encroachment by trees and shrubs has occurred. Using aerial photographs, we found that the rate of encroachment has increased substantially since 1993. However, repeated photographs at certain sites indicate that encroachment produced cohorts of trees. We show that global drivers are perhaps of greater importance than local drivers such as heavy grazing and absence of fire.

Keywords:

• aerial photography
• fixed-point photographs
• global climate change
• semi-arid
• shrub encroachment