Browsing by Author "Stock, William D"
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- ItemOpen AccessA comparative study of the distribution of C₃ and C₄ species of Cyperaceae in South Africa in relation to climate(1999) Chuba, David; Stock, William DIn this study the contribution of the climatic factors such as temperature, altitude, and mean annual precipitation as well as phylogeny to the determination of the distribution of C₃ and C₄ genera of Cyperaceae in South Africa has been investigated. Multiple regression analysis was used to determine the climatic factor that best predicted percent C₄ distribution. To see if the C₄ syndrome was influenced by taxonomy, the percentages of C₄ species was determined for each of the subtribes and genera. The results show that the relationship between the climatic factors and percentage of C₄ Cyperaceae species is similar to what has been found for the grasses (Vogel et al., 1978). However C₃ species of Cyperaceae seem to be abundant in most parts of South Africa. Only minimum growing season temperature showed significant correlation (P = 0.000391) with percentage C₄. The presence of the C₄ syndrome is shown to have some consistency within subfamilies, tribes and genera.
- ItemOpen AccessFactors possibly inhibiting growth of herbaceous plants in the understories of Eucalyptus cinerea(1989) Tshivhandekano, Thivhulawi Robert; Bond, William J; Stock, William DEucalyptus baxteri has been reported to produce a zone of suppression beneath its canopy when growing in coastal heath (De Moral, Willis and Ashton 1978). In their studies, investigations of ecophysiological parameters of soil water potential, soil nutrient levels and shading failed to account for suppression of understorey species. Their studies have shown that suppression of herbaceous layer species beneath the canopy of E. baxteri appear to be of an allelopathic rather than a competitive nature. Although the suppression of growth of herbaceous layer species under the understoreys of so many different Eucalyptus species has been attributed to allelochemicals produced by these trees, it is possible that suppression is due to other factors rather than toxicity of soil beneath Eucalyptus stands. The effects of plant competition for light, water and nutrients cannot totally be ignored when factors affecting the growth of herbaceous layer species under Eucalyptus understories are considered. For example, Lamont (1985) demonstrated that allelochemicals produced by leaves of Eucalyptus wandoo were not responsible for the suppression of herbaceous plants in the understorey of this plant species. From his study, it was found that competition for water at depth between the extensive lateral root system of E. wandoo trees and roots of adult shrubs was a more likely explanation for genesis of a suppression zone and location of its boundary. Therefore, to determine the meaning of Eucalyptus undergrowth effects, careful chemical detective work may be necessary. One need to establish first that the effect is in fact chemical and not one of competition between plants for light, water or nutrients. If it is possible, one must show that under natural conditions the quantitative relations of the chemical agents identified as they occur in the soil are adequate to produce the observed degree of inhibition of other plants which can be difficult to prove (De Moral et al. 1970). In Cape Town (South Africa), below Table Mountain, there is a bare area that resulted from the removal of an Eucalyptus cinerea stand. The strange thing about this bare area, is that from the time the Eucalyptus were clear-felled, it has been very difficult for plant species to regenerate in that bare area. The bare area has been apparent for about 5 years. The main objective of this study was to try and isolate these factors which might have been responsible for the suppression of growth under Eucalyptus stands, and their subsequent effect on regrowth after clear-felling. Therefore, this study was undertaken to test the following hypotheses. (1) Allelochemical Hypothesis. Since the genus Eucalyptus has been shown to produce several volatile terpenes, of which several have been shown to be toxic to seed germination and seedling growth, Eucalyptus trees might have been produced allelochemicals that were leached into the soil. These allelochemicals might be the ones that are responsible for inhibiting regeneration of plant species, even after the Eucalyptus cinerea trees were removed. (2) Topsoil Erosion Hypothesis. Since the understories of Eucalyptus were not rich in vegetation cover, the top soil, which is rich in nutrients and good for seedling establishment, was eroded down the slope. As a result, only the sub-soil was left. Plants were therefore unable to establish themselves in soils of such poor nutritional status with such low seed banks. (3) Soil Nutrient Depletion Hypothesis. Eucalyptus might have depleted most of the soil nutrients before they were clear-felled. Therefore, even after their removal, the nutrient status of the soil was poor in such a manner that very few plant species could establish themselves.
- ItemOpen AccessGrowth, water use efficiency and stable carbon isotopes in commercial clones of Eucalyptus(1993) Le Roux, Debbie Jeanette; Bond, William J; Stock, William DThe expansion of Eucalyptus plantations to supply timber for an increasing population in South Africa will result in a great reduction in the country's run-off water. If Eucalyptus continues to be the source of timber in South Africa, the selection of more water use efficient species for planting in existing and new areas has to be implemented. An understanding of the physiological factors ruling growth and water use efficiency in Eucalyptus is needed to develop selection criteria for improved water use efficiency and harvestable stem production under a limited water supply. This study investigated the effects of soil moisture availability on the growth and water use efficiency of 6 commercial clones of Eucalyptus commonly grown in South Africa with the aim of determining the following: (i) The extent of clonal variation in growth, dry mass allocation patterns, water use efficiency and the water cost of wood production at 16 months after planting. (ii) The influence of plant physiological traits such as patterns of dry mass allocation, canopy leaf area, leaf canopy density, specific leaf area, foliar nitrogen concentration and instantaneous rates of photosynthesis and transpiration, on growth, water use efficiency and the water cost of wood production. (iii) The complications associated with sampling for stable carbon isotope ratios (δ¹³C) within a eucalypt canopy and the potential use of δ¹³C in plant tissues as a tool for ranking clonal water use efficiencies.
- ItemOpen AccessA nitrogen budget for the Cape Metropolitan area : is nitrogen enrichment occurring in the soils of remnant patches of lowland fynbos?(2000) Kathawaroo, Deshika; Bond, William J; Stock, William DAnthropogenic activities create nitrogenous pollutants which threaten the existence of lowland fynbos, comprising 92 Red Data Book species and 14 Cape Flats endemics. A regional Nitrogen budget was constructed through the collation of existing data for the Cape Metropolitan Area. An NOx inventory has revealed that vehicles emit 66 % of the total NOx emissions into the atmosphere. The maximum potential N deposition is 184 kg N ha⁻¹ yr⁻¹ for the lowlands. Air quality in an industrial area, Goodwood, has revealed that a large proportion of the emissions remain in the air and have the potential to deposit 33 kg N ha⁻¹ yr⁻¹. Atmospheric N deposited on unimpacted coastal fynbos is 1.99 kg ha⁻¹ yr⁻¹. Thus remnant patches of lowland fynbos are threatened by replacement by grasslands as a result of atmospheric N inputs. Direct measurements of soil N pools are required, since signals of leaching in rivers may be premature as N accumulating in soil pools may not yet have reached saturation point. NH₃ emissions, wet and dry N deposition, and atmospheric transport models are required in order to construct a N budget for the CMA. Thereafter, appropriate preventative strategies can be devised in order to prevent the replacement of remnant patches of lowland fynbos.
- ItemOpen AccessA preliminary study of the impacts of alien Acacia infestation (A saligna) on the relative rates of nitrogen and phosphorus cycling in Lowland Fynbos, southwestern Cape, South Africa(1988) Wienand, Karen Tania; Stock, William DThis study was carried out on Sand Plain Lowland fynbos at Pella from April to September 1988. Environmental factors, nitrogen and phosphorus pool sizes and mineralization processes were investigated in the surface soils (1-10cm) of 7-8 year old fynbos vegetation and an adjacent Acacia saligna (Labill.) Wendl. infestation. While there was no significant difference in soil temperature between fynbos and acacia sites, soil moisture and organic matter content was significantly higher in the acacia soils. This favoured decomposition so that soil nutrient analyses showed enrichment of the soils (higher N and P) by the acacia infestation. Soil N and P mineralization was assayed using in situ incubations. Due to their higher soil total N concentrations, acacia soils showed greater inorganic N concentrations. In both acacia and fynbos soils ammonium was the dominant N form. This was ascribed to the high soil moisture content, while the low temperatures appeared to be the factor most strongly influencing ammonium accumulation. The low nitrate accumulations even in the field incubations indicated that the nitrification process was inhibited, probably by the high soil moisture content and low temperatures. The variable patterns of inorganic P accumulation were ascribed mainly to fluctuations between microbial mineralization and immobilization. Contrary to the hypothesis that the higher soil organic matter and greater concentrations of total N and Pin acacia soils would result in higher mineralization rates, there was no significant difference in the rates of N and P mineralization between fynbos and acacia soils. Thus, it was concluded that the higher decomposition rates in acacia soils was not associated with greater mineralization rates during the wet season (period of study).
- ItemOpen AccessThe relationships between forest and mountain fynbos communities in the southwestern Cape Province of South Africa(1991) Manders, Patrick Timothy; Cowling, Richard M; Bond, William J; Stock, William DEvergreen forests are believed to be restricted to moist sites in the southwestern Cape because of seasonal drought and high fire frequency in adjacent fynbos shrublands. However, soil moisture has not been demonstrated as the controlling factor. Succession to forest has been observed in some areas and not all forest trees in the fynbos are killed in some fires. This thesis determines whether forest and fynbos communities are separated by habitat factors or whether they are different successional stages of the same vegetation type. The effects of fire on forests, regeneration processes and habitat factors were studied in Swartboskloof, near Stellenbosch. Establishment requirements of forest seedlings were investigated in nearby Assegaiboschkloof. The effects of habitat factors on germination, establishment, growth and competitive abilities of forest and fynbos species were determined in controlled growth experiments. Established forest stands in Swartboskloof were stable in response to fire. Studies on the relationship between plant size and ability to survive fire showed fire intensity to be the overriding factor. Unlike regeneration in fynbos, regeneration of forest species is not coupled to disturbance. Forests had smaller soil seed banks than nearby fynbos but had the same set of species. Dispersal of ·forest species seeds into fynbos occurs immediately after fire for wind- and bird-dispersed species. Therefore colonization of fynbos by forest species soon after fire is not limited by seed dispersal. Evidence of facilitation of forest development was found in mature fynbos. Forest seedlings were associated with tall vegetation, high cover and deep litter layers as well as proximity to perches. Facilitation is enhanced in nuclei of forest species within fynbos communities. These studies refute the concept of forest and fynbos as distinct vegetation types. The relationships between forest and fynbos are explained on the basis of successional processes. Germination of forest species may occur in post-fire conditions, but establishment is unlikely until habitat amelioration occurs. Soil moisture and nutrients affect the rate of forest development. It is the frequency of fire, in relation to the rate of forest development, which determines the extent of forest development.
- ItemOpen AccessStable isotope composition of faeces as an indicator of seasonal diet selection in wild herbivores in southern Africa(2005) Botha, M Susan; Stock, William DWe used stable carbon isotopes and nitrogen contents of faeces to investigate diet selection differences among wild grazers, browsers and mixed-feeders at seasonal intervals across a year in the Hluhluwe–Umfolozi Park, South Africa. Faecal 13C values showed that wildebeest and warthog selected predominantly C4 plant material throughout the year. Impala ingested significantly more C3 plant material during the winter months than in all other months. Nyala also ingested more browse during winter. The nitrogen content of wildebeest faeces was significantly lower in winter than in summer, suggesting a possible decline in diet quality during the dry winter months. No significant seasonal trend in faecal nitrogen content was evident for nyala or warthog. Nitrogen contents of impala faeces were significantly higher in spring than in other seasons. Faecal isotopic and nutrient content analyses appear to be useful indicators of short-term diet selection and nutritional status of free-ranging herbivores. Analyses show resource partitioning among the different herbivores at finer time resolutions than can be obtained from bone collagen or isotopic analysis of tooth enamel.
- ItemOpen AccessTranskei grasslands : recent phenomenon or ancient?(1999) Foord, John; Bond, William J; Stock, William DAcocks (1953) suggested that Transkei was once covered in Forest and Scrub-Forest and has possibly been transformed by the Iron Age Farmers to grasslands. Feely (1985) suggested that the grasslands are ancient. I therefore used C isotope analysis to reveal whether there has been any change in the dominant vegetation during the Iron Age period. We measured the δ¹³C value of the soil organic matter taken at various depths These were taken from selected sites throughout Transkei that represented the grasslands, forests and grasslands suspected of once being forest or scrub-forest. The results show that the vegetation has remained stable. Those area that are presently covered in grasslands have remained grasslands for the time period represented by the depth of the samples taken. There is however a small shift in the forest and grassland sites to a lower isotopic value either as a result of changing vegetation or changes in the atmospheric [O₂]. There is also archaeological, ecological and historical evidence supporting these results. There is evidence of settlements scattered along the coast and in river valleys during the Early Iron Age (AD 300 to AD 1000). The earliest evidence of settlements during the Late Iron Age dates to AD 1400. The impact of the farmers appears to be localised and the vegetation type appears to have remained constant although impacted on.