N. van Rooyen

Analysing shrub encroachment in the southern Kalahari: a grid-based modelling approach

Shrub encroachment is reducing the carrying capacity of arid grasslands in southern Africa for cattle. Although shrub-encroachment is known to occur as a result of the selective overgrazing of grasses by cattle, the interactions between rainfall and grazing are not well understood. Both the quantity and sequence of rainfall events are likely to influence the growth rates and competitive abilities of shrubs and grasses. Shrub encroachment is a slow process and animals are stocked at low densities in arid regions. Thus, field experiments for determining stocking rates that avoid shrub encroachment under various rainfall scenarios are almost impossible to replicate. We used a grid-based simulation model to investigate the shrub-grass dynamics of the southern Kalahari under various realistic rainfall scenarios and stocking rates of domestic livestock. The simulation experiments addressed the following questions: Does simulated cattle grazing lead to shrub encroachment? Over what time scale does the process take place? Are the dynamics of vegetation-change continuous in relation to grazing pressure or do they show a threshold behaviour? Simulation results indicated that the answers to all three questions depended on the quantity and sequence of rainfall. Simulated cattle grazing led to shrub encroachment under all rainfall scenarios, once stocking rates exceeded a threshold determined by long-term mean annual rainfall. The stocking rate threshold for shrub encroachment was less distinct (i.e. shrub cover in different simulation experiments had a higher coefficient of variation) under xeric than mesic climatic scenarios. This is because either competition from the herbaceous layer or rain may limit shrub establishment. In relatively mesic scenarios, where shrub encroachment was limited mainly by grass competition, the grazing of grasses beyond a certain threshold led to an almost deterministic increase in shrub cover. However, under xeric climates, where rainfall was lower and more stochastic, the rate of shrub encroachment in response to a given intensity of grazing became less predictable. The most significant finding of the simulation experiments was that, although the stocking rates currently recommended by pasture scientists are unlikely to lead to shrub encroachment within 20 years, they have a high probability of bringing about shrub encroachment within a century. These findings applied to most of the rainfall scenarios found in the southern Kalahari and are therefore of particular interest to rangeland policy makers in this semi-arid region.

Vegetation of the Tembe Elephant Park, Maputaland, South Africa

A hierarchical classification, vegetation map, description and ecological interpretation of the plant communities of the Tembe Elephant Park and surrounding areas are presented. The study area falls within the Maputaland Centre of Endemism, which is part of the Maputaland-Pondoland Region, a centre of plant diversity rich in endemic plants and animals. Sixty-four sample plots were distributed in a stratified manner throughout the study area. A TWINSPAN classification, refined by Braun-Blanquet procedures, revealed eight distinct, mainly woodland plant communities. Much of the vegetation distribution can be ascribed to vegetation dynamics and the geomorphological evolution of the region. Dynamics of water in the landscape either directly or indirectly, through its role in moisture levels and soil formation, plays a role in the determination of plant communities at the higher levels. The proposed classification and general descriptions can be used for nature reserve management, land-use planning and further research. Ordinations based on floristic data give an indication of floristic and associated habitat and environmental relationships. Soil character and moisture differences are usually slight, hence the sand-based communities do not have distinct boundaries, except for Sand Forest. Indications are that in the case of Sand Forest the distinct boundary zone of nearly bare soil may well be the result of allelopathy Eighty-four Maputaland Centre plant endemics/near-endemics were recorded for the study area, with Sand Forest being the most prolific habitat for such taxa, many which appear to be neoendemics. Because of its hypothesised origin (especially lack of specific historic events) and vegetation dynamics, the inclusion of Sand Forest in conservation areas is no guarantee of the future persistence of this rare vegetation type.

Mysterious circles in the Namib Desert: review of hypotheses on their origin

Abstract Circular, slightly concave depressions, devoid of vegetation and often surrounded by a fringe of tall grasses occur in a broken belt in the pro-Namib zone of the west coast of southern Africa. Several hypotheses have been proposed to explain the origin of these so-called fairy circles. The most important of these relate to areas of localized radioactivity; termite activity; or allelopathic compounds released by dead Euphorbia damarana plants. No evidence of increased radioactivity could be detected in soil samples collected from these sites. Although termites occur at these localities no evidence was found to link termite activity directly to the formation of barren patches. Bioassays conducted on soil collected from the centre of the barren patch clearly demonstrated an inhibition of plant growth, while soil collected from the edge of the barren patch had a stimulatory effect on plant growth. No indication of growth inhibition was found in soil collected beneath E. damarana plants. At this stage none of the proposed hypotheses can satisfactorily explain the origin of the fairy circles.

Using diet and plant resources to set wildlife stocking densities in African savannas

Abstract The concept of carrying capacity originally was developed for domestic grazers and does not make provision for the wide variety of diets found in wild African herbivores. In the African savannas, herbivores are classified into 4 basic dietary classes: low-selective grazers, high-selective grazers, mixed feeders, and browsers. Given these conditions, a more appropriate approach was needed than the agricultural method based on livestock units to determine stocking rates for wildlife. Consequently, a new approach was developed that recognizes plant resource variation at the plant community level and differentiates between the grazing and browsing component in the diet of herbivores in the African savannas. The model used to calculate the grazing and browsing capacity on wildlife ranches provides for rainfall variability, quality and quantity of available grazing and browse, dietary requirements of each type of wildlife, and availability of suitable habitat. In this model the conventional conversion of wildlife to a Large Stock Unit has been replaced by a Grazer Unit, which is the equivalent of a 180-kg blue wildebeest (Connochaetes taurinus), and a Browser Unit, which is the equivalent of a 140-kg greater kudu (Tragelaphus strepsiceros). By separating the grazing and browsing components in the diet of wildlife for stocking density calculation, the diversity in the vegetation resources is optimally utilized. This concept is exemplified with an actual case study of a wildlife ranch in a South African savanna. In doing so, quantitative and clearly defined parameters are provided on which to base wildlife management decisions.

VEGETATION OF THE SOUTH-WESTERN ARID KALAHARI: AN OVERVIEW

SUMMARY The south-western Kalahari, with rainfall of 150–450 mm p.a., can be described as an arid savanna or semiarid desert. This area includes the southern Kalahari dunefields which is considered by most authors to have phytogeographical affinities with the Sudano-Zambezian Region, while others classify the area as part of the Nama-Karoo Region. Extensive vegetation maps are available for the Kalahari areas of Namibia and Botswana. For the dunefield area of South Africa, a detailed syntaxonomical vegetation classification, description and mapping of plant communities has been completed. Rainfall is clearly the main determinant of forage production in this environment. Although the vegetation and animal numbers of the south-western Kalahari conservation areas show definite changes in the short to medium term, mainly in reaction to rainfall, a long-term ecological balance prevails at present and should continue if the present large sizes of these conservation areas are maintained. The introduction of bore...

Plant communities of the rocky outcrops of the northern Orange Free State, South Africa

An analysis of the plant communities of the rocky outcrops of the northern Orange Free State is presented. Relevés were compiled in 42 stratified random sample plots. Care was taken to sample vegetation which is representative of vegetation in pristine condition whenever possible. A TWINSPAN classification, refined by Braun-Blanquet procedures, revealed seven distinctive plant communities. A hierarchical classification, description and ecological interpretation of the plant communities are presented. One order, three alliances, six associations and two sub-associations are newly described. The restricted distribution of the plant communities, together with the species richness and high species diversity recorded within these communities, makes it imperative that the conservation of these plant communities receives high priority.

Vegetation diversity of the Brand-se-Baai coastal dune area, west coast, South Africa: a pre-mining benchmark survey for rehabilitation

Prior to the mining of heavy minerals, the vegetation diversity of the Brand-se-Baai coastal area was investigated to serve as a benchmark for the future rehabilitation of the area. The vegetation was surveyed using the Braun–Blanquet procedure to classify the different plant communities. Six plant communities, some of which include several variants, were identified, described and mapped. A revegetation goal of 30 per cent, rather than 60 per cent, of the number of plant species present prior to mining are recommended. Copyright

The plant communities of the undulating grassland of the Vredefort – Kroonstad – Lindley – Heilbron area, northern Orange Free State

An analysis of the plant communities of the undulating grasslands of the northern Orange Free State is presented. Releves were compiled in 107 stratified random sample plots. Care was taken to avoid sampling of severely degraded areas. A T winspan classification, refined by Braun-Blanquet procedures, revealed six distinct plant communities. A hierarchical classification, description and ecological interpretation of the plant communities are presented.

Die plantegroei van die Roodeplaatdam-natuurreservaat II. Die plantgemeenskappe

The vegetation of the Roodeplaat Dam Nature Reserve . The vegetation of the Roodeplaat Dam Nature Reserve is classified, by means of the Braun-Blanquet method, into six plant communities. One of the communities is divided into four and another into three variations. The presence or absence of differential species was used as the basis for the classification. A Braun-Blanquet table is provided which indicates the complete phytosociological classification including all floristic relationships between the distinguished communities.

PHYTOTAB — A program package for Braun-Blanquet tables

A computer program package of six programs for vegetation classification is described. The package includes raw field-data input, automatic and user sequencing modes, final table and synoptic table preparation which are ‘camera-ready’ and also allows the user the option of ordinating by means of DECORANA.