Abstract
Vachellia erioloba is a keystone tree species in the southern Kalahari. This long-term study over nearly four decades tracks two populations in different landscapes (the interior sandy duneveld versus the clayey Nossob riverbed) of a large conservation area and offers valuable data on this species under natural soil moisture conditions and with limited anthropogenic influences. In 1978, 18 trees were permanently marked in a 1 ha plot in the interior duneveld of the Kalahari Gemsbok National Park (Dankbaar site). In the Nossob riverbed all trees in a 1 ha plot were surveyed in 1979 (Grootkolk site). At both sites, tree height and stem circumference were subsequently measured at irregular intervals until 2016 in order to investigate growth rates and population structure. Of the 18 marked trees at Dankbaar, six died and three showed coppice regrowth following substantial dieback after a fire. A mean height increase of 60 mm/year was recorded and the mean height of the remaining uncoppiced trees was 6.8 m in 2016. Stem diameter growth rate per year varied widely between trees and between years with a mean value of 2.5 mm/year over the 38-year period. Growth rate calculated for three 10-year intervals varied. Using the mean growth rate derived in the current study and stem size of the dead trees, the mean age of the trees when they died was estimated. At the Grootkolk site, the position of the centroid in relation to the midpoint of the diameter class range suggests that this population is gradually becoming a mature to old population with limited recruitment. This was supported by the size class distribution curves. However, no differences between slopes or intercepts of the stem diameter size class distributions were found.Conservation implications: This study was conducted in a large conservation area, that is, a natural ecosystem excluding most of the anthropogenic threats that are present outside of the park. The study illustrated that in the duneveld the population studied was self-sustaining, with recruitment occurring and large individuals presumably dying of old age. Although fire caused a few individuals to coppice, no fire-related deaths were reported. In the Nossob riverbed, surveys started in a stand of predominantly young trees and the size class distribution at that stage already showed a lack of recruitment. This stand is ageing and will likely disappear at this site; however, new young stands are appearing at other sites in the Nossob riverbed. Under the current conditions with negligible anthropogenic influences, it therefore appears that some V. erioloba populations in the park are increasing in size while others are decreasing, but that overall the species will persist. The impact of global climate change on this species is, however, unknown.
Highlights
Ecological patterns generated from a single snapshot in time should be used carefully when trying to identify the underlying processes driving the system (Jeltsch, Moloney & Milton 1999), and longterm data can be very useful to assist with the interpretation of ecosystem patterns and processes
The objectives of this paper were (1) to report on the mean annual increase in stem diameter and tree height of the 18 marked V. erioloba trees at the Dankbaar site; and (2) to document the mean age of those trees that died in the interior duneveld of the park; and (3) at the Grootkolk site to describe changes in the size class distribution of a V. erioloba population in the Nossob riverbed
A mean growth rate was derived from individuals in the interior duneveld as well as in the Nossob riverbed
Summary
Ecological patterns generated from a single snapshot in time should be used carefully when trying to identify the underlying processes driving the system (Jeltsch, Moloney & Milton 1999), and longterm data can be very useful to assist with the interpretation of ecosystem patterns and processes. Appropriate long-term data are difficult to find in the case of long-lived organisms such as trees, whose lifetimes are usually considerably longer than those of researchers (Menges 2000) In arid systems, this is further exacerbated by low rates of biotic change (Cody 2000; Lawley et al 2013; Van Rooyen et al 2015).
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