G.K. Theron

Predicting competitive interactions between pioneer plant species by using plant traits

Abstract. A competitive effect hierarchy for 15 Namaqualand pioneer plant species was established by using the mean mass of the phytometer (Dimorphotheca sinuata) when grown in combination with itself and 14 other species. There were no clear groupings of species in the hierarchy. This competitive hierarchy (gradient) indicated which species are strong competitors (resulting in a low phytometer mass) with D. sinuata and which species are weak competitors (resulting in a high phytometer mass). Each plant species has a certain combination of plant traits which determines its life history strategy and competitive ability. Regressions of various plant traits (measured on plants grown singly) against phytometer biomass indicated which traits were significantly correlated. The traits, most being size-related, were: maximum shoot mass, total mass, stem mass, reproductive mass, leaf area, stem allocation, specific leaf area (SLA), vegetative height × diameter, leaf area ratio (LAR); and mean number of days to flower initiation. A forward stepwise multiple regression of the significant traits was used to determine an equation to predict competitive effect.

Dry season herbivore densities around drinking troughs in the Kruger National Park

Abstract Densities of large herbivores within distance annuli around drinking troughsin the Kruger National Park were determined by means of aerial counts. A relationship between herbivore use intensity and distance from drinking troughs was tested for and modelled with the logistic curve. A significant negative relationship was found. High herbivore densities within 0·5 km of the troughs dropped off steeply to a more or less uniform density beyond the immediate vicinity of the water. This has major implications for the rangeland condition in the immediate vicinity of troughs.

Classification of the vegetation of the B land type in the north-western Orange Free State

An analysis of the vegetation of the B land type, covering 928 000 hectares in the north-western Orange Free State is presented. More than 80% of the area has been ploughed. Releves were compiled in 104 stratified random-sample plots in the remaining natural vegetation. A TWINSPAN classification, refined by Braun-Blanquet procedures revealed seven plant communities which may be classified under four major communities. All communities are related to specific environmental conditions. Descriptions of the communities are given.

Impact of water provision on herbaceous community composition in the Kruger National Park, South Africa

Abstract We studied the relationship between herbaceous plant community composition and distance from water to test whether the provision of water in the Kruger National Park has an impact on community composition. Composition at watering points has shifted to disturbance related annual species (e.g. Urochloa mosambicensis, Tra‐gus berteronianus, Dactyloctenium aegyptium, Amaran‐thus thunbergii and Alternanthera pungens) whereas perennial species (e.g. Digitaria eriantha, Themeda triandra and Sporobolus fimbriatus) tended to increase in presence with distance from water. There was a significant relationship between the first axis coordinates of ordinations of community composition in transects and distance from water. Results show that permanent water troughs for large herbivores in the Kruger National Park have an impact on the herbaceous community composition. Community composition along transects radiating from watering points is satisfactorily described by the logistic equation and parameters derived ...

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

Syntaxonomy of the vegetation of the Fb land type in the western Transvaal grassland, South Africa

Relatively little is known about the vegetation of the western Grassland Biome in South Africa. The classification of the vegetation of the Fb land type forms part of a research programme on the synthesis of the vegetation of the western Grassland Biome. Using a numerical classification technique (T winspan ) as a first approximation, the classification was refined by applying Braun-Blanquet procedures. The result is a phytosociological table where two new alliances and seven new associations are recognized. The new syntaxa are ecologically interpreted as well as described. Associated gradients in habitat were identified by using an ordination algorithm (D ecorana ). This study should contribute to the present knowledge and ecological understanding of the vegetation of the western Transvaal grassland.

Fruit polymorphism in ephemeral species of Namaqualand. I. Anatomical differences between polymorphic diaspores of two Dimorphotheca species

Differences in germination behaviour between different diaspore types of Dimorphotheca sinuata DC. and Dimorphotheca polyptera DC. induced the morphological study of the above-mentioned species. The study reveals that the difference between the diaspore types lies in the structure of the pericarp. Pericarp characteristics such as presence of mucilage, tanniniferous substances and cuticles as well as extensive sclerenchymatous zones, are associated with those diaspore types with the poorest germination. Fruit polymorphism represents a survival strategy in these species from the arid Namaqualand.

Removal of sodium and chloride from a saline soil by Mesembryanthemum barklyi

Abstract The removal of sodium and chloride from a saline-mined soil by Mesembryanthemum barklyi N.E. Br. was investigated under laboratory conditions. The sodium, magnesium and chloride content of plants increased when grown on a saline soil, while the calcium, potassium, nitrogen and phosphorus content decreased. No significant difference in biomass production occurred between plants grown on saline and non-saline sandy soils.

The physical environment and major plant communities of the Heilbron–Lindley–Warden–Villiers area, northern Orange Free State

An analysis of the physical environment and associated major plant communities of the Heilbron–Lindley–Warden–Villiers area (northern Orange Free State) is presented. Releves were compiled in 263 stratified random sample plots. Care was taken to avoid severely degraded areas. A T winspan classification, refined by Braun-Blanquet procedures, revealed three distinct vegetation types. These vegetation types were subdivided into seven major plant communities. A hierarchical classification, description and ecological interpretation of the seven major plant communities are presented. The easily distinguishable major plant communities should be managed as separate ecological units in order to achieve optimal utilization and conservation of the vegetation of this part of the Grassland Biome.

Salinity-induced changes in anatomy, stomatal counts and photosynthetic rate of Atriplex semibaccata R. Br.

Anatomical changes in the roots and leaves of Atriplex semibaccata R. Br., induced by salinity stress, as well as photosynthetic and stomatal response to salinity, were investigated. As salinity increased, decreases were observed in root diameter and leaf size, as well as in the number of chloroplasts in the chlorenchyma and bundle sheath cells. Development of these two cell types was also inhibited. Net leaf photosynthetic rate and leaf stomatal conductance decreased with increasing salinity. while the intercellular CO 2 concentration increased. Both stomatal closure and inhibition of biochemical processes probably caused the reduced leaf photosynthetic rates. The stomatal indices suggest that the trend towards an increase in number of stomata per unit leaf area with an increase in salinity was not due to decreased epidermal cell size.