Population characteristics of Vachellia erioloba woodland near Kathu, Northern Cape

Abstract

Vachellia erioloba occurs in arid and semi-arid areas. In the dry woodlands of the southern Kalahari, it is often the only large tree in the landscape, implying that it is a keystone species that plays an important role in maintaining ecological processes and species diversity. Deforestation to make room for mining activities, solar power plants and residential areas poses a threat to V. erioloba woodlands, especially because developments of this nature often go hand in hand with increased groundwater extraction and dust pollution. Furthermore, overutilisation of this species for firewood, unsustainable pod removal and seed predation, as well as fire events have been cited as factors that could cause a decline in the health of V. erioloba woodland populations. The Kathu Forest in the Northern Cape Province, predominately consisting of V. erioloba trees, was the first to be declared a protected woodland in South Africa in terms of Section 12(1)(c) of the National Forest Act (NFA) (Act 84 of 1998) in 2009. Vachellia erioloba has further been declared a protected tree under the NFA. Despite its protected status, a decline in the V. erioloba populations surrounding Kathu is still observed. Given that numerous entities own and, by inference, manage a woodland, this study set out to determine the population characteristics of the V. erioloba woodlands around Kathu with the intention to ultimately make recommendations for the sustainable management of these populations. Seven V. erioloba woodland sites were investigated over a period of five years to determine the density and size class distribution of each site and its population dynamics (i.e. production, recruitment, establishment and mortality) as well as to observe ecological interactions (i.e. predation by bruchid and wood-boring beetles). In addition, other possible environmental stressors such as dust pollution, past fire events and distance from disturbances and rivers were noted for each population. Significantly more die-back and mortality were observed at woodlands with fire impact and other disturbances (i.e. close proximity to developments and mining) compared to populations with hardly any disturbances. Furthermore, although present within all populations, the impact of wood-boring beetles on the mortality of these trees seems negligible. In terms of population dynamics, the variation in results indicates that each population should be managed according to its own impacts. For example, public access to woodlands could have a negative impact on pod removal, while high concentrations of game and/or livestock could have a negative impact on sapling establishment.

By understanding the population dynamics of individual V. erioloba woodlands, managing agencies would be in a better position to implement effective management strategies to secure their sustainability. It is recommenced that such strategies should include an effective fire management plan and controlling the stocking rate on a property. Furthermore, monitoring of these sites ought to be extended over a longer period of time, since five years can at best only provide short-term information on a long living tree species.