No abstract available. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1996.

Considerable understanding of the functioning of semi-arid systems is still needed to enable range managers to formulate management policies, with a degree of confidence. Long term data sets that encompass a wide range of interactions between the various major components of a semi-arid system (vegetation, herbivory, animal performance, landscape and rainfall), are unfortunately rare but essential to provide sufficient depth of data to adequately test various hypotheses about rangeland dynamics. This study comprises an analysis of a ten year data set derived from two cattle stocking rate trials in the semi-arid savanna of KwaZulu/Natal. Statistical analysis revealed that the most pronounced and rapid compositional change was due to rainfall, but that stocking rates between 0.156 and 0.313 AU ha ¯¹ had an important effect as well. Sites on steeper slopes with heavy stocking rates, exhibited the greatest amount of compositional change between 1986 and 1996 (40 Euclidean points in heavy stocking rate treatments on slopes vs 21-24 Euclidean points in heavy stocking rate treatments on flatter land, or 11-24 Euclidean points in low stocking rate treatments). Heavy stocking rates in conjunction with low rainfall tended to cause decreases in densely tufted perennial grasses and increases in annuals and weakly tufted perennials. Multiple regression analysis revealed that seasonal peak grass production (measured as disc height) declined between 1986 and 1996 only at those sites on steeper slopes with heavy stocking rates. The camps that declined in productivity also underwent the greatest degree of compositional change. The decline in grass productivity in certain high stocking rate camps did not translate into a decline in cattle performance. Depending on rainfall, cattle gained on a seasonal basis between 112 and 241 kg at low stocking rates, 82 and 225 kg at medium stocking rates and 84 and 217 kg at high stocking rates Rainfall, compared with stocking rate, accounted for the greatest amount of variance in seasonal peak grass production and cattle performance. Cattle performance had a strong curvilinear response to rainfall, which also proved to be a better predictor of cattle performance than grass biomass. There were no clear trends in soil physical and chemical characteristics between low and high stocking rates that could provide convincing evidence that loss of soil nutrients was an important mechanism of range degradation. The total standing crop of plant nitrogen but not of phosphorus tended to decline at high stocking rates. Plant nutrient and van Soest analyses suggested that forage quality was higher at heavy stocking rates. The results of this study generally supported traditional concepts of rangeland dynamics with regard to rainfall and grazing effects on compositional change and seasonal grass production. The results were important in being able to show quantitatively that heavy stocking rates result in a decline in grass production and that this effect is dependent on an interaction between stocking rate and landscape position or slope, and that there is a link between a decline in seasonal grass production and compositional change. The results also highlighted areas for future research that would be useful for furthering our understanding of various aspects of rangeland dynamics and mechanisms of degradation. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.

The structure and composition of savanna vegetation is influenced by resource availability and disturbance. Grasses, a major component of savannas, influence this resource availability by competing directly with trees for light, water and soil nutrient resources. The direct causes of bush encroachment are not always apparent, but are commonly ascribed to overgrazing and consequent decreased grass competition. The interaction, both above and belowground, between tree and grass seedlings and the surrounding grass sward is dependant on many factors, such as soil depth, seedling species and sward composition. These factors, as well as the presence or absence of defoliation, in the form of grazing or fire dictate whether the system will remain in a transition state as savanna or move towards a stable woodland state. The major competitive effects experienced by the tree seedlings were dependant on grass species and nutrient level. A. nilotica was affected by aboveground competition while A. karroo was affected by belowground competition. E. capensis caused the greatest decrease in A. karroo plant biomass. Both E. capensis and H. hirta had large competitive effects on the aboveground biomass of A. nilotica, while S. africanus had the greatest effect on belowground biomass. Increasing nutrient availability resulted in an increase in the competitive effect exerted on A. karroo, while little to no change was seen in the competitive effect exerted on A. nilotica. Soil depth constrained plant size in both tree species. The intensity of belowground interactions on tree biomass was unaffected by soil depth, while aboveground competition had a significant effect on shallow soils. Belowground competition was also of greater importance than aboveground competition in dictating tree seedling height. Grass seedlings growing on all three soil depths differed in mean mass, with E. racemosa having the least mass and T. triandra having the greatest. Simulated grazing by cutting the surrounding sward resulted in biomass increases in all three grass species. Changes in savanna composition and structure are thus likely to be influenced by initial species composition and soil depth and soil nutrient composition. While grazing creates niches for grass seedling establishment, heavy grazing has been observed to increase grass seedling mortality. Encroachment is thus more likely to occur on intensively grazed shallow and deep soils than on medium depth soils. This highlights the importance of ensuring the grass sward remains vigorous by resting and monitoring stocking rates to ensure veld is not over-utilized. It is then possible to maintain some form of tree-grass coexistence at a level where available grazing is not compromised. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.

Browse selection, intake, utilisation, palatability, quality and production are tightly linked and need to be considered together in trying to improve our understanding of browsing dynamics and the interactions between browsers and vegetation. Such an understanding is necessary in order to re-evaluate determinations of browser carrying capacities and evaluating actual and potential impacts of browsing animals on vegetation composition and diversity. Browser carrying capacity is determined by both the quantity and the nutritive value of forage. The measurement of browse quantity and nutritive value and the matching of browse supply to browser demand are central to sustainable utilisation and the monitoring of vegetation health. South African savannas are poorly studied with respect to tree canopy growth and browse production making it difficult to quantify the available browse biomass on which browsing capacity estimations are based, and consequently difficult to estimate levels of browsing that are sustainable. This study addressed these issues by investigating browse dynamics, broadly aiming to (1) explore factors affecting browse production, biomass and nutritive value; (2) develop models to assess and monitor these parameters across seasons and properties; (3) use the resultant models in improving our understanding of how to determine browser carrying capacities. More specifically, our study sought to examine the effects of plant physiognomy, forage nutritive value, canopy stratum, defoliation, temperature, rainfall and soil nutrient status on the browse production of evergreen (Carissa bispinosa, Euclea divinorum, Gymnosporia senegalensis), semi-deciduous (Spirostachys africana, Ziziphus mucronata) and deciduous (Acacia nilotica, Dichrostachys cinerea) savanna tree species from June 2003 – June 2005 in three sites along the northern Zululand coastline of KwaZulu-Natal. Available browse biomass, during the dry season, of four key savanna tree species (A. nilotica, E. divinorum, G. senegalensis, and S. africana) was estimated through the development of allometric regression equations. Non-linear regression was used to investigate the relation between the leaf dry mass (LDM) and canopy volume (CVol) of each of the four tree species. Exponential regression (y = a + brlnx) of the natural logarithm of CVol data provided the most accurate and precise description of the tree CVol – LDM relation. A study was undertaken to determine which factors may influence browse production in a southern African savanna. Regression tree models for the browse production identified that the dominant factors influencing browse production were CVol (m3), season, species and height to the lowest leaves of the tree canopy (HL) (m). The length of the growing season had a marked effect on the production potential of savanna tree species, suggesting that improved conditions for growth, i.e. greater rainfall, soil moisture content and improved soil nutrient availability result in a longer period of rapid sustained growth. Species was identified as an important contributing factor to differences in browse production rates, suggesting the need for the development of species or species group models. Mean annual browse production of evergreen trees was greater than that of deciduous and semi-deciduous trees. Mean quarterly (three monthly) browse production was highest, for all trees, during the wet season, with the greatest difference between wet and dry season production being observed in deciduous forms. Evergreen forms showed continuous growth over the whole study, with enhanced growth over the wet season. Deciduous forms, on the other hand, concentrated growth in spurts, when environmental conditions became favourable, with most production occurring during a short growing season. Browse nutritive value was found to be greatest during the wet season, when growth and photosynthesis are at their greatest. Further, browse nutritive value was greatest in deciduous species. Evergreen trees were found to have greater acid detergent fibre (ADF) concentrations than both the deciduous and semi-deciduous trees. By contrast, crude protein (CP) concentrations were greater in semi-deciduous and deciduous species than in evergreen species. The daily CP requirements for maintenance for an adult impala (45 kg) were met by all species over all three study areas and all seasons. Daily CP requirements for growth and lactation, however, were only ever met by deciduous and semi-deciduous species, though this result was not consistent over study areas and seasons. Predictive models for the production of browse on deciduous, semi-deciduous and evergreen trees in northern Zululand were developed using multivariate adaptive regression spline functions. The best predictors of growing season browse production in all three tree guilds (defined here as a group of trees having a characteristic mode of living) were primarily measurable tree dimensions, while the prevailing environmental conditions had little impact. Differences in the production, nutritive value and available browse biomass between the different tree forms and seasons have a profound effect on the determination of browser carrying capacities and need to be incorporated into any game or conservation management plan. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

The effects of fire and mechanical clearing were investigated for their potential in assisting with the eradication of Chromolaena odorata (previously Eupatorium odoratum). The study was divided into two focus areas, the first focused on mechanical clearing of dense stands of C. odorata on three sites and the second focused on the long term influences of a single burn on C. odorata plants in the different size categories. For mechanical clearing, two key issues were investigated; namely whether this type of clearing procedure was effective in dense C. odorata stands and whether rehabilitation was necessary in these cleared areas. The study was conducted from July 2002 to June 2004. The area was subject to a severe drought throughout the duration of the study. The severe drought had a large influence on the result in both focus areas. A bulldozer was found to be a very effective at clearing dense C. odorata stands. Results from the mechanical clearing study showed that there was still a large viable grass seed population in the areas that had been covered by a dense stand of C. odorata plants for over three years. Thus, indigenous plants were able to re-colonize the area after removal of C. odorata without human intervention, even thought the area was experiencing a severe drought. The density of C. odorata seedlings emerging in the cleared areas was far lower than expected. The C. odorata density in the permanent plots, for seedlings that germinated in the first season after clearing (SeptemberOctober 2002), was only 0.25,0.03 and 0.72 per 5 m2 in the three sites respectively by the end of the study in June 2004. For the C. odorata seedlings that germinated in the second season (September-October 2003) the density was, 0.5, 0.56 and 1.06 per 5 m2 in the three sites respectively by the end of the study in June 2004. It was suspected that the drought influenced seed germination. Unfortunately the number of C. odorata seedlings was so low, that no significant relationship could be found between grass and C. odorata seedling density. By the end of the study the grass fuel mass in all the rehabilitated sites was already over 3000 kg ha-1, even though the area was experiencing a severe drought. This grass fuel load, when burnt, will assist land managers in controlling C. odorata plants, especially seedlings. Very few other alien invasive plant species emerged in the cleared areas. At the Mhlosinga site, Senna pendula made up less than one percent of the herbaceous species composition and only a single Ricinus communis plant was recorded. No alien plant species were recorded on the other two sites. Results from the burning trials revealed that plants in all the size categories were affected by fire. Greater fuel masses and fire intensities were required to kill larger C. odorata plants relative to smaller ones. Fire was found to be very effective at eliminating small and medium size C. odorata plants. Fire applied as a once off treatment had a significant long-term effect on the C. odorata population. The following fuel loads were required to achieve 80% mortality in this 11 study: for small plants a fuel load of over 4000 kg ha-I, for medium plants a fuel load over 4200 kg ha-I and for large plants a fuel load over 4600 kg ha-I. Little difference could be detected between a head or a back burn, as both fire types had their own advantages and disadvantages. Although some of the C. odorata plants in the burnt plots had not perished by the time of the first investigation, following the burn (February 2003), by the time of the second investigation (June 2004), many of these plants had eventually succumbed. These results highlighted the fact that plants which are damaged by fire were more likely to persish during an extended droughts period, than plants which were not subjected to fire. Results from the control plots, in the burning trials, for medium and large plants showed dramatic increases in density over time. Tagged individuals from the control plots did reveal that some of the medium and large plants did die during the drought, although the amount was negligible when compared to the number of new plants growing into the new size categories. A large proportion of the small plants in the control plots also survived the drought with many of them even growing into the medium category. The difference between the control plots and the burnt plots was obvious and significant, especially once the fuel mass exceeded 3783 kg ha-I. Results from this study show that fire can be used as a very effective tool in assisting land managers to control C. odorata in open savanna bushveld. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.

The increase of woody vegetation (also known as bush or shrub encroachment) in savannas has become of global concern to conservationists and rangeland managers alike. Bush encroachment has been associated with a decrease in rageland palatability. In addition, the increase in woody biomass has consequences for climate change, carbon sequestration, rangeland hydrology and nutrient cycling. As a result of these large changes in ecosystem functioning with bush encroachment, biodiversity may be threatened. Fire is considered to be one of the most important management tools used to control woody biomass in savannas. However, despite the use of fire in Ithala Game Reserve, areas have become encroached. This thesis assesses the role of fire in bush encroachment in Ithala Game Reserve. I start this thesis with a discussion of the bottom-up (water, nutrients, and light) and topdown (fire and herbivory) ecosystem components in the literature review. This sets the foundation for an understanding of the factors that affect savanna tree:grass ratios for the rest of this thesis. In addition the review discusses the potential effects of climate change on savanna tree:grass ratios. Recently, it has been proposed that increasing atmospheric carbon dioxide concentrations result in an increased competitive ability for C3 woody plants against C4 grasses. Many models have been produced to explain savanna dynamics. By assessing the role of fire in Ithala Game Reserve, its functioning is assessed in light of the current issues of bush encroachment. Textural analysis is a remote sensing technique that has been used to detect changes in woody vegetation using aerial photographs. Textural analysis was used to assess changes in woody vegetation cover and density from 1943 (earliest period for which aerial photographs were available for the study area) to 1969, 1990 and 2007 in Ithala Game Reserve (IGR). Field surveys were performed to assess the effects of the fire regime in IGR on woody vegetation structure and composition. Transects were performed in areas with different fire frequencies. The effects of fire frequency were compared between similar vegetation communities. Textural analysis showed that woody vegetation cover (+32.5%) and density (657.9 indiv. ha-1) increased from 1943 to 2007. Importantly, in some areas of IGR, the suppression of fire led to the rapid invasion of woody plants from 1990 to 2007. Field studies demonstrated the importance of fire in controlling woody vegetation in IGR. The densities of the encroachers, Dichrostachys cinerea and Acacia karroo were resistant to annual burns. However, the height of these deciduous microphyllous woody encroachers was reduced by more frequent fires. Following the suppression of fire, these trees grew taller and their negative impact on the herbaceous layer increased. Consequently, fuel loads (grassy biomass) declined and prevented the use of frequent and intense fires by management. The reduction in fire frequency allowed the invasion of woody evergreen macrophyllous species. Continued development of fire-resistant patches of evergreen macrophyllous vegetation will further reduce the effectiveness of fire in controlling bush encroachment. To control bush encroachment in IGR and the consequential loss of biodiversity, an intermediate fire frequency (one burn every 2 to 4 years) is required. Burns also need to be hot enough to increase the current rate of topkill. Management should act to optimize the accumulation of grassy biomass to fuel fires. / Thesis (M.Sc.)-Unversity of KwaZulu-Natal, Pietermaritzburg, 2010.