Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998

Changes in the cover and density of shrubs and trees were assessed from aerial photographs (1940, 1974 and 1998) as well as from fixed‐point photographs taken in 1984 and 1996 in the Kruger National Park, South Africa. Woody cover (trees and shrubs combined) increased by 12% on granite substrates but decreased by 64% on basalt substrates over the past 58 years. Both these figures are expressed in terms of the initial values, respectively. The density of the large tree component of woody vegetation decreased on both substrates. Woody vegetation cover declined as fire return periods became shorter, but the relationship was weak. The increases in woody plant density and cover on granite are thought to be the result of decreased competition from grasses, which in turn is a result of overgrazing by wild herbivores whose numbers have been kept high through the provision of surface water. These effects were not seen on the relatively nutrient‐rich basalts, where grasses can recover rapidly even after heavy grazing. The decline in overall woody cover on basalts is interpreted as a result of regular, short‐interval prescribed burning over the past 40 years, while the universal decline in large trees seems to result from an interaction between regular, frequent fires and utilization by elephants. The implications for management are discussed. Résuné On a évalué les changements du couvert et de la densité des arbustes et des arbres à partir de photos aériennes (1940, 1974 et 1998) et de photos prises d'un endroit fixe en 1984 et en 1996 dans le Parc National Kruger, en Afrique du Sud. Le couvert boisé (arbustes et arbres pris ensemble) a augmenté de 12% sur les substrats granitiques mais diminué de 64% sur les substrats basaltiques, au cours des 58 dernières années. Ces deux chiffres sont exprimés en termes de valeurs initiales, respectivement. La densité de la composante de grands arbres de la végétation boisée a diminué sur les deux substrats. Le couvert végétal boisé a diminué alors que les périodes de récupération entre les feux raccourcissaient, mais la relation est faible. On pense que l'augmentation de la densité et du couvert boisés sur le granite est le résultat d'une diminution de la compétition exercée par les herbes qui elle, résulte d'un surpâturage des herbivores sauvages dont le nombre a été maintenu élevé par l'apport d'eau de surface. On n'a pas observé ces effets sur les basaltes relativement riches en nutriments, car les herbes peuvent y repousser