Tracking Vachellia erioloba dynamics over nearly four decades in the Kalahari Gemsbok National Park

By Dr Helga van der Merwe (SAEON) and Dr Hugo Bezuidenhout (SANParks) Tweet

Vachellia erioloba (camel thorn) is a keystone tree species in the southern Kalahari.

A long-term study over nearly four decades tracks two populations in different landscapes of a large conservation area and offers valuable data on this species under different environmental conditions (interior sandy dune field versus clayey Nossob Riverbed).

In the southern Kalahari, Vachellia erioloba is regarded as an important species as it provides food, shelter, perches, nests and roost sites to many types of animals, and harbours a distinct assemblage of plant species below its canopy.

In South Africa, V. erioloba is protected by legislation. It is long-lived and not easily affected by short-term climatic variations, possibly because individuals of this species have some of the deepest roots of any known species (the maximum recorded was 68 m), allowing them access to deep groundwater sources.

There is a growing concern that this species is declining in the southern Kalahari due to factors such as an increase in fire frequency, increasing groundwater abstraction, alien invasive species as well as harvesting.

Prof. Gretel van Rooyen (UP, SAEON Research Associate), Tshililo Ramaswiela (SAEON), Dr Charlene Bissett (SANParks) and Dr Noel van Rooyen (previously from UP, SAEON Research Associate) measure Vachellia erioloba trees in the Nossob Riverbed (Photo: Marco Pauw) Vachellia erioloba, a keystone tree species in the southern Kalahari (Photo: Helga van der Merwe)

Long-term study continued

Quantitative information on growth rates and lifespan of the species is however, still lacking. In 1978, surveys were initiated in the Kalahari Gemsbok National Park to gather information on the growth rate, life span and population dynamics of V. erioloba in the interior dune field and in the Nossob River habitats.

The key questions in current study were to (a) determine the mean annual increase in stem diameter and tree height of 18 permanently marked V. erioloba trees; and (b) to determine the mean age of these trees when they died in the interior dune field of the park; additionally (c) to follow the changes in the size class distribution of a V. erioloba population in the Nossob Riverbed.

Mortality and coppicing were recorded for the 18 trees marked in the interior dune field. Stem circumference measurements enabled the calculation of growth rates for these trees. Results from the population in the Nossob Riverbed indicated that over the 38-year period, this population has progressed from a young and growing to a mature-to-old population with limited recruitment.

The valuable collaboration between the University of Pretoria (UP), SANParks and SAEON has enabled the continuation of a 38-year-old dataset, thereby greatly improving our understanding of this long-lived tree species.