Doctoral Degrees (Grassland and Rangeland Science)

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An assessment of Coastcross II Bermudagrass and kikuyu for growing out young beef animals.
(1983) Bransby, David Ian.; Tainton, Neil M.
The immediate aim of this study was to relate herbage availability, liveweight gain and stocking rate to one another under continuous and rotational grazing so that management recommendations could be formulated for Coastcross II Bermudagrass and kikuyu. In addition, these data would be used for an economic evaluation of growing out young beef animals on these two pasture species. The broad long term goal was to use this information to persuade farmers to make more use of planted pastures and thereby reduce the stocking pressure on the veld. Besides pursuing the primary objectives, the study offered an opportunity to make an observational assessment of the put-and-take technique for use in grazing trials. In relation to fixed stocking, variable stocking appeared to have no distinct advantage in this study. This conclusion is drawn from the fact that little success was achieved in applying put-and-take at a high level of precision. It was therefore recommended that future trials should make use of fixed stocking, except when variation in herbage availability is very large and predictable. In such cases the use of put-and-take would be warranted. Liveweight gain of animals was linearly related to herbage availability. In two seasons liveweight gain of animals on Coastcross was higher for continuous grazing than for rotational grazing at equivalent herbage availability, but in other seasons there was no difference between the two methods of grazing. At equivalent levels of herbage availability liveweight gain was higher in early summer than in late summer. Herbage availability decreased linearly witilin each season and on average, rotational grazing resulted in increasingly greater herbage availability than continuous grazing, as stocking rate was increased on Coastcross. However, no difference between the two grazing methods was evident on kikuyu. Finally, the relationship between liveweight gain and stocking rate was also described by a linear function. On average, the stocking rate at which maximum liveweight gain per ha occurred (SRmax) was higher for rotational grazing than continuous grazing on Coastcross, but not on kikuyu. However, no difference was evident between the two grazing methods in each season. During the study period annual rainfall varied from 506 mm to 990 mm. This offered a unique opportunity to examine the relation between some of the pasture production parameters measured and annual rainfall. The length of the grazing season and SRmax increased, but liveweight gain of animals at SRmax declined as annual rainfall increased. This caused seasonal liveweight gain at SRmax to increase initially, but then to reach a maximum and decrease as annual rainfall increased from 500 to 1000 mm. From this information it was possible to build two models which can be used to predict liveweight gain per ha and profit per ha from stocking rate and annual rainfall. These models can either be built into farm planning programmes or used directly by agricultural advisors. In tl1is thesis they have been used to show that there is a wide range in stocking rates and levels of annual rainfall within which it is possible to make substantial profits by grazing young beef animals on dryland pastures. This study has therefore provided forceful information which can be used to persuade farmers to make more use of planted pastures, and in so doing, conserve the veld.
Backing the burn: carbon sequestration in South african mesic grasslands through sustainable fire management.
(2024) Nicolay, Robyn Elizabeth.; Kirkman, Kevin Peter.; Tedder, Michelle Jennifer.; Mkhize, Ntuthuko Raphael.
Emphasising the ancient origins of the grassland biome in South Africa, much research has supported the role of grasslands in biodiversity, ecosystem services, and economic benefits for local populations. Despite this, the grassland biome faces challenges. It is highly transformed, poorly conserved, and urgently in need of preservation and restoration. Fire-adapted mesic grasslands are distinct ecologically from arid-adapted climatic climax grassland communities in that they are adapted to frequent defoliation, necessitating burning to maintain productivity and biodiversity. To fully understand the role of grasslands in the global climate change arena, it is necessary to quantify the impacts of grassland management, including fire and livestock grazing, on carbon exchange (source versus sink). Research in this thesis looks particularly at management with prescribed fire as a form of defoliation in mesic grasslands. To do this, I quantified soil total carbon stocks, fractionated stable pyrogenic carbon stocks, and soil carbon sequestration rates in various fire regime treatments at the Ukulinga Grassland Fire Experiment (UGFE), Pietermaritzburg. Additionally, I examined the impact of fire frequency on grazed grassland at Wakefield research farm, quantifying carbon stocks and release while exploring potential mechanisms behind observed patterns in this grazed system. Lastly, I monitored Eddy Covariate carbon flux data over a four year period at research catchment six, Cathedral Peak, KwaZulu-Natal, to understand seasonal and interannual flux within mesic grassland and observe patterns in source versus sink dynamics in these ecosystems. Research has emphasised the necessity of frequent fires to maintain grassy biomes and sustain the role of biotic and abiotic factors in this biome, through biochemical soil alteration in the form of ash deposition, and the alteration of above and below ground biomass. The complexity of managing this grassland is emphasized by the need to balance the impact of herbivory, prescribed fires, and the nature of biomass accumulation in these biomes, all of which influence carbon cycling. Differences were observed between different prescribed burning regimes. Substantial differences in soil organic carbon (SOC) and total nitrogen (TotN) stocks at different soil depths were observed, with the highest stocks observed in the top 5 and 10 cm of soil across all treatments. Annual winter and spring burns exhibit the highest SOC stocks and wider C:N ratios. Triennial burns display the lowest sequestration rates in the top 0 to 5 cm of soil, with negative rates within the 5 to 10 cm horizon. Over a period of 20 years, SOC sequestration increased in a 70-year-old experiment with no signs of stabilization within the 0 to 5 cm soil horizon, but SOC loss is noted below 5 cm in areas burnt triennially. Increased fire frequency in grassland also caused a reduction in the stable fraction of black carbon (BC), and contrastingly - increased levels of BC quantified in grassland burnt infrequently or excluded from fire. This pattern may be due to reduced alkaline ash deposition and subsequently greater soil acid saturation, suggested to result in increased pyrogenic carbon particulate size and reduced breakdown of this carbon in the soil. When considering the inclusion of livestock and grazing into grassland managed with prescribed fire, findings showed no significant differences in SOC and TotN levels between annually burned grasslands and those excluded from fire. Grassland managed with annual burning showed greater soil respiration rates compared to unburned sites, indicating greater soil microbial activity and root turnover. Annual burning and heavy grazing were both associated with reduced aboveground biomass accumulation compared to the adjoining unburned grassland. Additionally, annually burnt grasslands exhibit reduced aboveground biomass lignin and fibre percentage relative to adjacent unburned areas. Findings highlighted that increased fire frequency in grazed grassland influences livestock grazing behaviours through improvement of forage palatability and available biomass, contributing to greater belowground carbon turnover. Considering the mechanisms governing carbon dynamics in fire-dependent grassland, four years of flux data showed that following rainfall events, increased soil water content is linked to a rapid rise in soil respiration, aligning with heightened biological and photosynthetic activities during warmer growing seasons. These processes determine the rate and variability of grassland uptake and release of CO2. The findings support evidence that mesic grasslands managed with regular long term prescribed fires consistently act as carbon sinks, absorbing more carbon than they emit over periods exceeding 70 years. Findings from this research advocate for management practices utilizing a frequent burn regime, suggesting that such practices maintain persistent carbon sinks in South African mesic grasslands. This approach enhances the resilience and capacity of mesic grasslands to act as effective and consistent carbon sinks, even in the face of potential future climate change impacts. The evidence from these studies shows that prescribed fire during late winter / spring in South African mesic grasslands should enhance carbon sequestration and the role of these grasslands as a carbon sink.
Beef production from kikuyu and Italian ryegrass.
(1985) Bartholomew, Peter Edward.; Tainton, Neil M.
Four grazing trials to characterise cultivated pastures, in terms of beef production, were conducted in Bioclimate 3 of Natal. Dual purpose and British beef type cows were run on kikuyu at stocking rates from 2,81 to 7,30 cows plus calves per ha. During the eight seasons of the trial the seasonal rainfall varied from 580 to 933 mm. There was a positive linear relationship between rainfall and pasture yield with maximum yield of kikuyu being recorded during February - March. Stocking rate affected pasture yields only during favourable rainfall seasons. Crude protein (CP) and crude fibre (CF) of kikuyu fluctuated markedly within and between seasons. However, CP increased and CF decreased as stocking rate increased. There were significant relationships between stocking rate and (a) calf performance, (b) calf livemass gain, (c) period required to attain maximum mass, (d) period on pasture for the cows, and (e) cow mass change: Weaners were run on irrigated Italian ryegrass at 5, 7 and 9 weaners per ha for four seasons. Stocking rate had little effect on the growth pattern of the pasture but affected dry matter yields. Reducing the stocking rate resulted in increased pasture yields and CF content but reduced CP levels of material on offer. Steers exhibited higher gains than heifers but lower carcass grades and stocking rates for maximum gain per ha (SRmax). Livemass gains of 1315 and 1224 kg per ha can be expected at SRmax of 6,85 and 9,54 for steers and heifers respectively. Yearling heifers run at four stocking rates on kikuyu for one season showed a negative linear relationship between stocking rate and gain and a positive linear relationship between pasture height and gain. A SRmax of 8,85 allows for a livemass gain of 1 040 kg per ha. The effect of feeding concentrates on foggaged kikuyu was evaluated. Foggaged kikuyu can be used as a source of roughage for fattening steers. However, as the steers became adapted to the concentrate the intake of kikuyu declined from 39 to 19% of their daily intake. Regressions derived from the characterisation trials allow for developing beef systems for different situations.
The bioeconomic implications of various stocking strategies in the semi- arid savanna of Natal.
(1994) Hatch, Grant Peter.; Tainton, Neil M.; Ortmann, Gerald Friedel.
Climatic and market uncertainty present major challenges to livestock producers in arid and semi-arid environments. Range managers require detailed information on biological and economic components of the system in order to formulate stocking strategies which maximise short-term financial risk and minimise long-term ecological risk. Computer-based simulation models may provide useful tools to assist in this decision process. This thesis outlines the development of a bioeconomic stocking model for the semi-arid savanna of Natal. Grazing trials were established at two sites (Llanwarne and Dordrecht) on Llanwarne Estates in the Magudu area of the semi-arid savanna or Lowveld of Natal. The Lowveld comprises a herbaceous layer dominated by Themeda triandra, Panicum maximum and P. coloratum and a woody layer characterised by Acacia species. The sites differed initially in range composition. Llanwarne was dominated by Themeda triandra, Panicum maximum and P. coloratum, while Dordrecht with a history of heavy stocking was dominated by Urochloa mosambicensis, Sporobolus nitens and S. iocladus. Three treatments were stocked with Brahman-cross cattle at each site to initially represent 'light'(0.17 LSU ha-'), 'intermediate' (0.23 LSU ha-') and 'heavy' (0.30 LSU ha-') stocking. Data collected at three-week intervals over seven seasons (November 1986 to June 1993 or 120 measuring periods) provided the basis for the development of a stocking model LOWBEEF - OWveid BioEconomic Efficiency Forecasting) which comprised two biological sub-models (GRASS and BEEF), based on step-wise multiple linear regression models, and an integrated economic component (ECON). The GRASS model predicted the amount of residual herbage at the end of summer (kg ha-') and the forage deficit period (days) over which forage supplementation would be required to maintain animal mass. Residual herbage mass at the end of summer (kg ha-') was significantly related (P < 0.01) to cumulative summer grazing days (LSU gd ha-'), rainfall (mm) (measured 1 July to 30 June) and range condition (indexed as the sum of the proportions of T. triandra, P. maximum and P. coloratum). The forage deficit period (days) over which herbage mass declined below a grazing cut-off of 1695 kg ha-' was significantly related (P < 0.01) to residual herbage mass at the end of summer. The BEEF model predicted the livemass gain over summer (kg ha¯¹) which was significantly related (P< 0.01) to rainfall (mm) stocking rate (LSU ha¯¹) but interestingly not to condition. The economic component (ECON) reflected the difference between gross income (R ha¯¹) and total costs, which were based on fixed and variable cost structures (using 1993 Rands), including demand-related winter costs, to reflect net returns to land management (R ha¯¹). A conceptual model of range dynamics based on three discrete states, was to developed to summarise the effects of rainfall and stocking rate in semi-arid savanna. State 1, characterised by iocladus and S. nitens, was associated with heavy stocking. Movement towards State 2, characterised by T. triandra and P. maximum, was associated with periods of above-average rainfall. Drought conditions, which comprised a major system disturbance led to stability at State 3, dominated by U. mosambicensis. Post-drought recovery was influenced by predrought composition and stocking levels where tuft numbers, basal cover and seedbank were significantly reduced by increased stocking within a sward dominated by species of low stature such as Aristida congesta subsp Urochloa mosambicensis, Sporobolus nitens, Sporobolus iocladus and Tragus racemosa. It was suggested that extensive soil loss may lead to stabilisation across an irreversible threshold at a forth state characterised by shallow species such as Tragus racemosa Aristida congesta subsp. congesta. Sensitivity of optimum economic stocking rate net return to price and interest rate fluctuations, and wage and feed cost increases were examined for various rainfall and range condition scenarios. Net return and optimum economic stocking rate increased as rainfall and range condition increased through the effect of increased residual herbage mass at the end summer, decreased forage deficit periods and reduced supplementary feed costs. Net return was highly responsive to changes beef price where an increase in beef price led to an increase in optimum economic stocking rate and net return. The effect of reduced prices may be compounded by dry where supply-driven decreases in price may occur. This suggested that for dry seasons the optimum stocking rate was the lightest within the range of economic stocking rates. Although an increase in interest rates would increase variable costs and lead to reduced returns, the influence of interest rates on enterprises will vary in relation to farm debt loads. Increased labour costs would result in a corresponding decline in net return although optimum economic stocking rate would remain unaffected. Increased supplementary feed cost had little influence on net return relative to the effect of demand-driven increases in feed costs as rainfall decreased. The distribution of net returns for stocking strategies of 0.20, 0.30 and 0.40 LSU ha¯¹ and climate-dependent stocking (where stocking levels were varied in relation to rainfall and hence forage availability) and range condition scores of 10, 50, 80 and a dynamic range model were examined for a 60 year rainfall sequence (1931-1991). While a range score of 10 would see residual herbage mass decline to below a grazing cut-off of 1695 kg ha¯¹ before the end of summer, a range score of 80 suggested that, irrespective of stocking strategy within the range investigated, herbage would not become limiting. This suggested that irrespective of stocking strategy a range score of 10, established across an irreversible soil loss threshold, would reflect accumulated losses over the 60 year period. In contrast, a range score of 80 would lead to positive accumulated returns. A dynamic range model (where range composition was related to previous seasons rainfall) and a climate-dependent stocking strategy, suggested that herbage would not become limiting by the end of summer and forage deficit periods would be restricted to an average of 88 days per year. Such an approach would yield a higher accumulated cash surplus than fixed stocking strategies. Incorporation of stochastic rainfall effects allowed the development of cumulative probability distributions based on 800-year simulations to evaluate the risk associated with various stocking strategies. Range condition played a major role in determining the risk of financial loss where decreased range condition was associated with enhanced risk. An increase in stocking rate resulted in increased variability in returns. Although the risk of forage deficits and financial losses may be reduced with lighter stocking, this may be at the cost of reduced returns during wetter seasons. Increased stocking may increase the probability of higher returns during wetter seasons although this may at the cost of increased risk of forage deficits and highly negative returns during dry seasons. Importantly, ecological risk may increase as stocking is increased. A flexible or climate-dependent strategy, where stock numbers are adjusted according to previous seasons rainfall, combine financial benefits of each approach and reduce financial risk. Although errors may carry high ecological costs where, for example, the effect of an above-average rainfall season would be to increase stock numbers into a subsequent dry season, the probability of incurring such error was low. Current livestock production systems in the semi-arid savanna of Natal based on breeding stock may not be appropriate in a highly variable environment where low rainfall may require extended periods of upplementary feeding or force the sale of breeding stock. A change in emphasis from current systems to a mixed breeding system, where the level of breeding stock would be set at the optimum economic stocking rate for drier seasons, may decrease both financial and ecological risk. Growing stock may either be retained or purchased during wetter seasons to reach the optimum economic stocking rate for such seasons. although growing stock may display a greater tolerance to restricted intake (during dry seasons) than would breeding stock, additional growing may be rapidly sold in response to declining rainfall with no influence on the breeding system. Integration of wildlife into current cattle systems may be an important means of reducing financial risk associated with variable rainfall and profitability and ecological risk associated with woody plant encroachment.
Browse : quantity and nutritive value of evergreen and deciduous tree species in semi-arid Southern African savannas.
(2012) Penderis, Caryn Anne.; Kirkman, Kevin Peter.
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.
Bushclump-grass interactions in a south-east African savanna : processes and responses to bush control.
(1996) Jarvel, Leander Clement.; O'Connor, Timothy Gordon.
The objective of this study was to investigate woody-grass interactions and the initial response of vegetation to bush control in the mesic Eastern Cape bushclump savannas. The occurrence of multi-species bushclumps, rather than single-trees, presented an interesting variation to an otherwise well-studied interaction. The effect of bushclumps on their local environment was characterized. Since all woody-grass interactions involve competition for irradiance, nutrients and moisture, a factorial experiment was designed to discriminate these individual and interactive effects. Mechanical and chemical bush control measures were investigated in a formal, replicated experiment. The herbaceous, woody and soil responses to bush control treatments, for the first two seasons, are reported. Bushclumps had a moderating effect on their microclimate when compared with the open grassland. Lower maximum and higher minimum temperatures, and higher humidity were the result of an 80-90% reduction in the irradiance regime. Soils beneath bushclumps were more fertile than grassland soils. The importance of bushclumps on sandier soils was discussed. Bushclumps were characterized by a sparse shade-tolerant herbaceous layer which contributed little to grazing capacity. An aspect effect increased grass production in the grassland on the south-facing side of bushclumps. Initial results suggest that the lateral spread of woody roots could be as far as 25 m. The factorial experiment tested the individual and interactive effects of irradiance (normal sunlight, 40% and 80% shade), nutrients (normal nutrient level, low and high nutrient addition levels) and moisture (low, normal and additional moisture levels) on the herbaceous layer. The interaction of 80% shade and high nutrients had a detrimental effect on herbaceous production. Deep shade did not affect herbaceous production, but Themeda triandra showed etiolated growth, aerial tillering, an increase in the number of leaves, and an increase in the proportion of stem under deep shade. The root mass of the herbaceous layer also decreased. This suggested that below-ground biomass production was impaired at the expense of maintaining aboveground biomass. The addition of nutrients significantly increased herbaceous production and resulted in a change in sward composition. Moisture was not an important factor in this experiment. Mechanical clearing in the bush control experiment resulted in a significant increase in herbaceous production. Panicum maximum colonized the ex-bushclump zone and contributed significantly to the increased production. Oversowing with Chloris gayana significantly increased grass yields. The two contrasting seasons revealed the importance of rainfall in affecting herbaceous production. The second season was characterized by lower soil fertility and a decline in grass quality. This was attributed to high grass production in the above-average rainfall season. A four-fold increase in woody stem density after two seasons demonstrated the coppicing ability of the woody layer once mechanically cleared. Most of the coppice occurred within the first season. Exceptional coppice growth characterized the second season. Acacia karroo recruitment was mainly from seed. Woody plants showed their susceptibility to chemical poisoning by dropping their leaves within the first season. Many of these individuals succumbed during the second season. Mortality was greatest in woody plants with a smaller basal circumference. Owing to the difficulty of accessing all woody stems in a bushclump, mortality in bushclumps was lower than that in the open grassland. Grass production in the bushclump and its periphery were significantly increased in both seasons. This was attributed to the increased productivity of mainly Panicum maximum which took advantage of the increased irradiance regime. Both the mechanical and chemical treatments displayed significantly greater grass production in the open grassland zone. This demonstrated the extent to which the woody layer had competitively dominated the herbaceous layer.
Control of bush encroachment with fire in the arid savannas of Southeastern Africa.
(1983) Trollope, Winston Smuts Watts.; Tainton, Neil M.
No abstract available.
Determinants of community composition and diversity in KwaZulu-Natal mesic grasslands : evidence from long-term field experiments and pot and plot competition experiments.
(2003) Fynn, Richard Warwick Sinclair.; Kirkman, Kevin Peter.
A predictive understanding of plant community response to various environmental influences (e.g. type, timing and frequency of disturbance, site productivity, fertilization, etc.) is a general goal of plant ecology. This study sought to further understanding of mesic grassland dynamics in KwaZulu-Natal using long-term field experiments (> 50 years) and short-term pot and plot competition experiments. The specific objectives were to: 1) examine the effects of long-term burning of grassland on soil organic matter content because of its potential impact on nitrogen cycling and community composition, 2) examine patterns of community composition and species richness in response to different type, timing and frequency of disturbance (burning and mowing) in a long-term grassland burning and mowing experiment and to different type and amounts of fertilizer application in a long-term grassland fertilization experiment, 3) develop hypotheses concerning the response of different species to disturbance and fertilization, and test these hypotheses using pot and plot competition experiments, and 4) provide a general synthesis of the results of the various field, pot and plot experiments that may be used to develop a predictive theoretical framework for mesic grassland dynamics. Total soil nitrogen was lowest in sites burnt annually, intermediate in sites burnt triennially and highest in sites protected from disturbance and sites mown annually in the dormant-period (spring or winter). Winter burning reduced soil organic carbon and total soil nitrogen more than spring burning. Mineralizable nitrogen was reduced by burning. The different effects of type, timing and frequency of disturbance on total soil nitrogen appeared to be an important determinant of community composition and species richness. Short-grass species (Themeda triandra, Eragrostis capensis, Heteropogon contortus, Diheteropogon amplectens and Eragrostis racemosa) were most abundant in annually burnt sites (especially winter burnt sites), whereas medium and tall-grass species (Eragrostis curvula, Cymbopogon spp., Hyparrhenia hirta and Aristida junciformis) were most abundant in triennially burnt sites, sites protected from disturbance and sites mown annually in the dormant-period, all of which had higher total soil nitrogen than annually or biennially burnt sites. Species richness and short-grass species declined with increasing levels of nitrogen fertilization in the fertilizer experiment and declined with increasing productivity and nitrogen availability in both the fertilizer and burning and mowing experiments. Thus, it was hypothesized that the type, timing and frequency of disturbance resulted in different compositional states through different effects on soil resources (especially nitrogen), which affected the competitive balance between short and tall species. The hypothesis that composition was determined by disturbance-mediated soil nitrogen availability was supported by competition experiments, which revealed that shortgrass species were most competitive in low-nutrient/low-productivity treatments and tall-grass species most competitive in high-nutrient/high-productivity treatments. The fertilizer experiment and a competition experiment revealed that tall broad-leaved species were most competitive in sites of highest productivity, fertilized with both nitrogen and phosphorus, whereas tall narrow-leaved species were most competitive in sites of intermediate productivity, fertilized with nitrogen only. It was hypothesized that summer mowing increased the abundance of short-grass species and decreased the abundance of tall-grass species in the burning and mowing experiment by increasing the competitive ability of short-grass relative to tall-grass species, rather than the tall-grass species being less tolerant of mowing. A competition experiment revealed that tall-grass species (Hyparrhenia hirta and Panicum maximum) were as tolerant of cutting as a short-grass species (Themeda triandra). Themeda triandra was shown to become extremely competitive in cutting treatments, reducing the biomass of most other species relative to their monoculture biomass, showing that its dominance of mown sites in the burning and mowing experiment was a result of its superior competitive ability rather than greater tolerance of mowing. However, many tall erect herbaceous dicots appeared to be intolerant of summer mowing, probably because their meristems are aerial and easily removed by mowing, whereas short creeping herbaceous dicots were increased by summer mowing probably because their meristems were below the mowing height. Further, these short species would be vulnerable to shading in unmown sites. Thus, species with basal meristems (hemicryptophytes) or meristems near the soil surface (geophytes) appear to be more tolerant of mowing than species with aerial meristems (phanerophytes), but the tradeoff is that a low meristem height renders them vulnerable to shading in unmown sites. Very high litter accumulation in the sites protected from disturbance appeared to have a direct influence on community composition and species richness. Species that dominated these sites (e.g. Tristachya leucothrix & Aristida junciformis) initiated tillers below-ground and had sharp erect shoots that appeared to be an adaptation for penetrating litter. Species that initiate tillers below-ground are probably less vulnerable to the effects of shading by litter because their tiller initiation is not dependent on high light availability. The fact that Aristida junciformis was shown to have very low competitive ability in two competition experiments, suggests its dominance of protected sites was through tolerance of high litter levels rather than competitive exclusion of other species. Low grass species richness in these sites was probably a result of an inability of many species to tolerate these high litter levels. This study has revealed that inherent site productivity and its interaction with the effect of disturbance on soil resources and litter levels is a major determinant of community composition and species richness. The effect of type, timing and frequency of disturbance on soil nitrogen was able to account for the principal changes in community composition. Thus, the influence of disturbance on soil nitrogen is a unifying principle in plant ecological theory that enables greater understanding of disturbance-composition relationships. However, intolerance of certain forms of disturbance (e.g. mowing) by species with aerial meristems, or intolerance of accumulating litter in the absence of disturbance by species without sharp erect shoots, may also have important influences on composition. In addition, this study has revealed that plant traits (height, leaf width, position of tiller initiation, shoot morphology and position of meristems) were well correlated with the various effects of disturbance and fertilization on community composition, indicating that a plant trait-productivity-disturbance framework has great potential for understanding and predicting species response to disturbance and multiple limiting nutrients.
Development and testing of a remote controlled oesophageal fistula valve for goats.
(1993) Raats, Jan Gabriël.; Tainton, Neil M.; Trollope, Winston Smuts Watts.
A remote control sampling technique was developed for the collection of oesophageal fistula samples from goats. Number and size of samples can be varied and collected throughout the day without disturbing the animal's normal feeding behaviour. The equipment developed and tested in this study consists of an oesophageal fistula valve which allows the fistula to be opened and closed, a rechargeable battery pack and motor to operate the valve, a portable radio and receiver to activate the valve motor, and a harness to attach the equipment to the body of the animal In addition, a closing device to effectively seal large oesophageal fistulae (> 1 050 mm²), which in turn is required to accommodate the valve, was developed. During field tests with the fistula valve, 10 % of I 027 sampling attempts failed due to blockage of the valve, and an average of 1.3 incidences of equipment failure were recorded per animal per sampling day, from an average of 9.9 extrusa collections per day. Observed feeding behaviour (grazing/browsing) as well as grass / bush ratio of fistula valve and standard fistula bag samples of four goats, formed the basis for the evaluation of this technique. In addition, extrusa recovery rates, measured under controlled conditions, were used in the evaluation of this sampling method. Differences in extrusa composition between the fistula valve and fistula bag techniques varied substantially during the browsing period within a camp and also between camps. Furthermore, during high frequencies of observed grazing, there were large differences between the fistula valve and fistula bag methods. During this study, the fistula valve technique provided a more realistic estimate (R²=.91) of the observed feeding behaviour of goats than the fistula bag technique (R²=.63). Under controlled conditions, the large oesophageal fistula, with or without the valve, enables high and consistent extrusa recovery rates (87 % recovery; SD 7.5).
The development of cultivated dryland grass pastures for livestock production in the high rainfall areas of Rhodesia.
(1979) Rodel, Malcolm George Wishart.; Booysen, Pieter de Villiers.; Elliott, R. C.
No abstract available.
The development of stocking rate models for three veld types in Natal.
(1990) Turner, John Robert.; Tainton, Neil M.
The overall objective was to develop stocking rate models for three veld types. namely the Lowveld. the Southern Tall Grassveld and the Natal Sour Sandveld. in Natal. Sub-objectives were to determine the 1) residual herbage mass at the end of the summer, 2) residual herbage mass at the end of winter and 3) individual animal performance under grazing conditions, and the effect of stocking rate on these three variables. Multiple linear regression component models were successfully developed to meet all three of the sub-objectives for each of the three veld types. Results show that veld condition is an extremely important factor determining animal production from veld, and that stocking rate on veld in good condition could possibly be double that on veld in poor condition. Stocking rate did not have the expected impact on individual animal performance in the summer. although it did have an important moderating influence under any particular set of environmental conditions. Stocking rate did. however. have a marked effect on herbage production and therefore on herbage availability in winter and so also on the ability to overwinter cattle without having to supply additional supplementary feed. Stocking rate in summer therefore had a major indirect effect on animal production in the winter. Carryover of residual herbage from one year to the next is probably not as important in these veld types as in some other parts of the country.
Drivers of vegetation change in the eastern Karoo.
(2019) Du Toit, Justin Christopher Okes.; Kirkman, Kevin Peter.
The Nama-Karoo Biome occupies much of the western central region of South Africa and transitions into the Grassland Biome along its eastern boundary along a gradual ecotone. The area is characterised by hot summers and long, frosty winters, relatively low rainfall, peaking in mid- to late-summer, with high inter-annual variability, and botanically by a co-existence of grasses and dwarf shrubs, with grass abundance positively related to average annual rainfall that increases from west to east. Biome shifts in response to changes in rainfall pattern and grazing have been suggested but never directly examined. Major drivers of botanical composition are rainfall and grazing by livestock. Fire is rare, occurring sporadically if high rainfall allows for good grass growth. This thesis focused on understanding the influence of rainfall, grazing, low temperatures, and fire on botanical composition at Grootfontein, a site in the ecotone between the Nama-Karoo and Grassland Biomes that is home to grazing trials up to 85 years old. The following specific questions were addressed: Question 1: Over the long term, has Grootfontein shown patterns of rainfall cyclicity or experienced directional change, and how might these have influenced composition and productivity? Using data from 1888 to 2012, cyclicity in rainfall was evident for periods of approximately 20 and 60 years. Rainfall has also consistently increased since the mid-1970s, and this increase corresponds with a general pattern of increased grassiness in the eastern Karoo. Question 2: How do rainfall and grazing, alone and in interaction, influence vegetation composition in the eastern Karoo? Compositional data from the 1960s and 2010s from various treatments at two sites at Grootfontein (Camp 6 and Seligman grazing trials) show a shift from dwarf-shrub- to grass-dominated vegetation, consistent with the increased rainfall over that time. An influence of grazing, both present and historical, was evident but secondary to the effect of rainfall. In some cases, there has been a shift to grassland. Data from the Camp 6 and Seligman grazing trials from the 1940s to the 1960s further indicate a combined influence of season of grazing and of rainfall. High-intensity summer-only grazing by livestock largely extirpates grasses and allows shrubs to thrive, while summer grazing in the form of rotational grazing or continuous grazing allow for a balance of grasses and dwarf-shrubs. Severe declines in both grasses and shrubs occurred apparently in response to drought, though the exact conditions required to cause such mortality remain unclear. Plant cover data from 2008 to 2015 from the Boesmanskop grazing trial showed that consecutive years of exceptionally high rainfall increased plant cover to nearly 100%, and increased the abundance of grasses. Competitive exclusion of dwarf-shrubs by grasses was not evident. Question 3: What have been the trends in minimum temperatures, frost, and potential growth season at Grootfontein, how might these have influenced botanical composition and productivity, and is there evidence of increasing temperatures consistent with global warming? This is addressed using minimum-temperature data from 1916 to 2014. Minimum temperatures were lower than are usually reported. Variability in minimum temperatures was high, including a cooling from the 1910s to the 1950s and a warming from the 1950s to the 2010s. The length of the growing season (last frost to first frost of the subsequent season) varied considerably, and may have the potential to influence botanical composition. Question 4: What is the influence of fire in Karoo vegetation? Based on the effects of a single fire on Grootfontein, fire killed some species while most species resprouted. Grasses appeared unaffected in terms of survival, several species of dwarf shrub (notably Eriocephalus ericoides and Ruschia intricata) were killed, and will need to re-establish by seed (termed nonsprouters), while most dwarf shrub species resprouted. This resprouter/ nonsprouter dichotomy was found to be evident at a range of other fire sites in the Karoo. Heavy grazing appeared to strongly impede the recovery of burnt veld, maintaining it as a sparse grassland dominated by annual species and occasional unpalatable shrubs. Should the grassiness of the Karoo continue to increase, then fire may become more frequent thereby maintaining a grassland state. The findings allow for greater understanding of interactions among rainfall, grazing, and fire in eastern Karoo ecosystems, and these are discussed in the context of an existing state-andtransition model of eastern Karoo vegetation dynamics. The importance of long-term rainfall trends, rather than short-term variability, are highlighted. Long-term increases in rainfall will likely induce a biome shift to grassland, concomitant with a drastic reduction in dwarf-shrubs. This will likely alter both long-term carrying capacity for livestock and the type of animals that may be optimally stocked. Increased grassiness will result in the increased likelihood of fire, and if post-fire grass fuel loads remain above a critical level, a fire/grass feedback loop may be initiated whereby dwarf-shrubs are largely eliminated owing to their slow rates of growth or re-establishment. Introduction of infrequent fire will likely result in resprouter-dominated vegetation proliferating. It is demonstrated that the resilience of Karoo veld may be higher than previously thought, with severe grazing, droughts, and fire not pushing veld beyond a threshold into a state of denudation. Thus the prospects of conserving Karoo landscapes despite historical management remain high. Some key future research efforts needed to improve our understanding of Karoo ecology include the life-histories of dwarf-shrubs, the conditions of drought and herbivory under which grasses and dwarf-shrubs die, and how and when perennial dwarf-shrubs and grasses regenerate. Based on historical trends, the continued existence of long-term research trials, such as those at Grootfontein, may be under threat and should receive attention.
Dry woodland and savanna vegetation dynamics in the Eastern Okavango Delta, Botswana.
(2012) Tedder, Michelle Jennifer.; Kirkman, Kevin Peter.; Bonyongo, Mpaphi Casper.; Morris, Craig Duncan.; Trollope, Winston Smuts Watts.
The Okavango Delta is an extremely dynamic system with variable vegetation comprised of permanent swamps, seasonal swamps, dry islands, floodplains and dry grassland, savanna and woodland. The system is largely driven by the interaction between fire and the annual flood, which filters down from the Okavango River catchments in Angola. While extensive research has been conducted on the flood-driven vegetation little is known about the dry woodland and savanna regions bordering these flood-driven habitats. A taxonomic classification of woody species composition resulted in eleven vegetation types. These data were then reanalyzed in terms of woody species morphology allowing these eleven vegetation types to be grouped into four functional response groups in order to provide a platform for improving the understanding of how dry woodland and savannas interact with the environment. These four groups were the savanna group mixed thornveld and the three woodland groups; mixed broadleaf woodland, shrub mopane woodland and tall mopane woodland. Burning in mixed thornveld and mixed broadleaf woodland was found to decrease woody species density and grass fuel loads and could be used for grazing management to remove unpalatable growth and improve grass species composition, while burning in shrub mopane woodland and mixed mopane woodland merely decreased the woody understory and is not recommended. Utilization dominated by grazing livestock resulted in overutilization of the grass sward leading to bush encroachment in both mixed thornveld and shrub mopane woodland, while utilization by goats alone resulted in underutilization of the grass sward and a dominance of herbaceous annuals. Livestock utilization had no effect on the occurrence of Pecheul-loeschea leubnitziae, a shrubby pioneer previously thought to be an indicator of overgrazing, however extensive P. leubnitziae cover was associated with a sward dominated by shade-tolerant grasses with low forage quality. Shrub mopane woodland and tall mopane woodland appear to be more stable vegetation states than mixed broadleaf woodland and mixed thornveld being less vulnerable to colonization by pioneer species and alteration as a result of utilization or environmental factors. For this reason management and monitoring of mixed thornveld and mixed broadleaf woodland is essential to prevent vegetation degradation and to ensure optimal forage availability for both livestock and wildlife.
Ecological effects of fire in the montane grasslands of Natal.
(1985) Everson, Colin Stuart.; Tainton, Neil M.
Although controlled burning has been used to manage Highland Sourveld grasslands, little was known of its effects on the vegetation. This study examined the effects of past fire treatments on veld condition,species composition, dry matter production, quality and canopy recovery growth rates of these grasslands. Also, six techniques of estimating the species composition of grasslands were compared in order to decide on a standard technique for monitoring these grasslands. From this work it was concluded that the wheel point method is the most satisfactory. Veld condition scores were significantly lower in grassland protected from fire than where veld had been burnt or burnt and grazed at regular intervals. Frequent defoliation was found to maintain the grassland composition largely unchanged over a period of 30 years. Individual species were, however, found to react strongly to defoliation frequency. Plant demographic studies were therefore carried out to explain this differential response to burning. Three Decreaser and two Increaser I species were studied. In all species examined, recruitment of secondary tillers was stimulated by regular burning, each species being well adapted to a regular fire regime. Differential responses to burning were best explained by the combined effects of the different reproductive capacities and mortality rates of tillers of these species. A biennial spring burning regime was shown to be most suitable for maintaining the most important grass species at their present levels of abundance. Annual winter and biennial spring burning did not result in significant differences in dry matter production. Maximum net roductivity was approximately 230 g/m² in both treatments, placing them amongst the more productive areas of Southern Africa. Examination of canopy recovery growth rates showed that there is little difference in the percentage canopy cover at the end of the growing season when veld is burnt annually in winter or biennially in spring. However, differences in season of burn resulted in exposure to erosive forces at different times of the year. The results of this investigation have highlighted the importance of regular burning during the dormant period in the montane grasslands of Natal.
The effect of high-density, short-duration stocking on soils and vegetation of mesic grassland in South Africa.
(2018) Chamane, Sindiso Charlotte.; Kirkman, Kevin Peter.
High-density, short-duration stocking (HDG) is currently gaining popularity amongst farmers in South African mesic grasslands but little is known about its potential impact on soil properties and plant species composition, particularly the forbs (herbaceous dicotyledonous and non-graminoid monocotyledon) which contribute more to plant diversity than grasses. Under HDG, animals are stocked at higher stocking rates and densities than conventional grazing systems and burning is discouraged. This study used a fence-line contrast approach to compare the long-term impact of “real world” HDG systems with rotational grazing systems at a lower stocking density (LDG) on soils and vegetation composition including forb growth habits at two study sites, Kokstad and Cedarville. An experimental trial was set up at Ukulinga Research Farm to determine the short-term effects of HDG compared with no grazing on plant species composition and demography of the selected perennial forbs. Another field experiment was used to determine the response of three mesic grassland perennial forb species (Afroaster hispida, Gerbera ambigua and Hypoxis hemerocallidea) to intense defoliation and interspecific competition with a grass species (Themeda triandra). Soils were more compacted under HDG but soil chemical properties did not differ between HDG and LDG at both Kokstad and Cedarville. There was a low percentage cover of desirable palatable grasses and high forb species turnover under HDG at Kokstad and low grass and forb species responses at Cedarville. There was high litter accumulation under HDG over the long- and short-term period. High litter accumulation reduces irradiance for plants, and may lead to lower basal cover. The intense grazing and trampling due to the higher stocking rate and stocking density under HDG resulted in less erect forb growth habits and more prostrate growth habits at Kokstad. A study of demography revealed that HDG threatened future populations of the grazing-sensitive species Afroaster hispida, Agathisanthemum chlorophyllum and Gerbera ambigua through increased mortality or reduction in the recruitment of large from small individuals. Intense defoliation altered the competitive response of A. hispida, it had a high competitive response when undefoliated but when defoliated its competitive response was reduced. Gerbera ambigua and H. hemerocallidea were not affected by the interaction between defoliation and competition. Findings from this study has shown that HDG potentially has a negative impact on soil health and vegetation composition of South African mesic grassland.
Effects of climate and management on biomass, species composition and phenology and their regenerative responses in a mesic grassland.
(2024) Ojo, Theresa Abosede.; Tedder, Michelle Jennifer.; Kirkman, Kevin Peter.
Increasing temperatures, attributed mainly to increased greenhouse gas emissions, are a leading cause of climate change. Evidence shows that changing climatic conditions significantly affect terrestrial ecosystem structure and function. Fire, grazing, and recurrent drought are some of the more common disturbances that impact many grasslands. Two ongoing experiments (Ukulinga Grassland Fire Experiment (UGFE) and Drought-Net experiment at the Ukulinga Research Farm, University of KwaZulu-Natal, Pietermaritzburg, KwaZulu-Natal) were used to investigate the impacts of these disturbances on grassland community dynamics. This research assessed the effects of rainfall variation and warming and management factors (defoliation and fire) on phenology of grass species vis-a-vis their regenerative responses from bud banks, soil seed bank, and seed rain and aboveground species composition and biomass in the mesic grasslands of South Africa over two years. The Drought-Net experiment treatments include the combined effect of rainfall variation (drought (50%), ambient (100%), and wet (150%)) and temperature (warmed and ambient) manipulation. The warming treatment was applied using open-topped warming chambers, and the rainfall treatment was applied using rainout shelters to intercept 50% of the ambient rainfall. The intercepted rainfall was piped onto the neighboring plot to increase rainfall by 50%. In the first objective (Chapter 3), the combined effect of rainfall variation and temperature manipulation on the phenology of grass species vis-a-vis their regenerative responses from bud banks was conducted. Drought delayed the timing of budding and flowering and shortened the duration. There was no effect on the total number of buds per vegetative tiller per tuft compared to the reproductive tillers. Warming advanced the budding timing and extended the budding duration. This shows that the response of timing and duration of budding and flowering of these studied grass species was sensitive to the short-term effects of drought and warming. In the second objective (Chapter 4), the effect of burning frequency, winter defoliation type (fire/mow), summer defoliation with differing winter defoliation (defoliation time) applied over ~70 years and temperature manipulation on the phenology of grass species vis-à-vis their regenerative responses from bud banks was conducted. Across the two years of warming treatment application, the sensitivity and timing of species phenological responses and bud bank density relative to resource allocation to vegetative versus sexual reproduction were examined. Warming delayed the time of budding and flowering and increased duration slightly (p < 0.005). Annual winter mow delayed timing and extended duration of budding and flowering as compared to annual winter fire (p < 0.005). Summer defoliation frequency quickened the timing and shortened the duration of budding and flowering compared to winter defoliation frequency (p < 0.005). The flowering duration for Themeda triandra was shortened under annual burning and lengthened under the triennial burn plots (p < 0.005). In ambient plots, vegetative status had more buds per tiller and number of tillers per tuft compared to warmed plots. Annual winter defoliation by fire had the greatest number of tillers per tuft for Heteropogon contortus and the lowest recorded in the annual winter defoliation by mow plots. A greater number of buds per tiller was recorded under the summer defoliation plots as against the winter defoliation plots. Summer defoliation plots had a greater number of tillers per tuft for H. contortus than the winter defoliation plots. The results of this study indicate that the type, time, and frequency of defoliation and their interaction with the effect of warming have a notable influence on the budding and flowering periods of the species studied, ultimately impacting the length of their reproductive cycle and altering the ratio of vegetative to flowering reproductive status. The reason for the contrasting results where warming advanced the time of budding in chapter 3 and delayed in chapter 4 could be as a result of the insurance hypothesis whereby the variation of species responses to changes in environmental condition of an ecosystem can insure the system against reduction in its functioning. The third objective (Chapter 5), combined effect of management factors (defoliation and fire), and temperature manipulation on the species composition, soil seed bank, and seed rain in the Drought-Net experiment and UGFE. Plant community composition was examined to determine sensitivity to climate and management. On the Drought-Net experiment, the main and interacting effects of warming and the various rainfall regimes did not impact the soil seed bank and seed rain (p > 0.05). On the UGFE, the main effect of warming and its interactions with the burning and defoliation treatments had no significant effect on soil seed bank and seed rain (p > 0.05). The seed rain composition was only affected by defoliation frequency, while the main effects of fire and defoliation frequencies affected the soil seed bank (p < 0.05). The result of the study showed that the responses of the seedlings that germinated from the seed bank and seed rain were species-specific towards the effects of the management factors. The lack of effect of drought and warming shows that mesic grasslands are resilient to fluctuations in moisture and temperature regimes in the short term. The fourth objective (Chapter 6), examined the effect of different rainfall regimes and temperature manipulation on the species composition and biomass of a mesic grassland in the Drought-Net experiment. The results showed that the main effect of rainfall variation and their interactions with warming had an impact on the grass species composition, with just the main effect of warming being observed in the following year. No impact on the forb composition was seen in either year. While there was no evidence of an interaction impact, the main effects of rainfall variation and warming on biomass production were significant. Drought and warming reduced biomass production. This demonstrated how quickly drought reduced the grass community composition, which resulted in changes in productivity. Objective five (Chapter 7), this study examined combined effect of warming and management factors (defoliation and fire) on the aboveground species composition and biomass at UGFE. Plant community composition was examined to determine sensitivity to the impact of long-term burning and defoliation frequency under ambient and warmed conditions and whether shifts in plant community composition resulted in changes in productivity. The study showed that the main effect of warming, burning, and defoliation regimes resulted in a shift in plant community composition, which altered productivity. The overall conclusion of this study is that under different defoliation frequencies, types and timing, the species-specific seasonal patterns of species composition, phenological development and bud bank production were strongly influenced by the resultant effect of these disturbances on the availability of soil moisture and light intensity. This resulted in a reduced productivity of the grass community in our mesic grassland.
Effects of herbivores, fire and harvesting on the population dynamics of Acacia drepanolobium sjoestedt in Laikipia, Kenya.
(2007) Okello, Bell Dedan.; O'Connor, Timothy Gordon.
Effects of herbivory, fire, and tree harvesting on Acacia drepanolobium were studied using plant population dynamics as the philosophical basis of research. Specifically, growth rates, chrono-sequence of re-growth, biomass and charcoal yield, herbivory, flowering, seed production, germination, mortality and the ants of Acacia drepanolobium were studied in the black cotton ecosystem of Mpala Research Centre, Laikipia, Kenya, between September 1995 and December 2000. Acacia drepanolobium was the most abundant tree or shrub with densities ranging from 80% to 98% of all the overstorey species, but it was the least browsed of all the trees and shrubs in the black cotton ecosystem, ranging from a mean of 7.2% to 9% of the individuals browsed. The tree is inhabited by four Acacia ant species, Crematogaster mimosae, Crematogaster sjoestedti, Crematogaster nigriceps, and Tetraponera penzigi, which are believed to be obligate, and which probably play a role in the low browsing rates observed. Six herbivore treatments replicated three times (no herbivores - O; only cattle - C, all herbivores allowed - MWC {control}, mega-herbivores {elephants and giraffe} and wildlife {W} – MW only, wildlife – W - only {all wildlife except mega-herbivores} and wildlife and cattle only - WC) was the main experimental design used in understanding the dynamics of the tree species under influence of different herbivores. Mean annual height growth of A. drepanolobium trees was 24.9 cm yr-1, while the mean Relative Growth Rates ranged from 14.6 x 10 –3 cm cm-1 yr-1 to 18.7 x 10 –3 cm cm-1 yr-1. Growth rates were different among the herbivore treatments and between seasons. Shoots of the tree grew by a mean range of 6.8 cm to 9.1 cm, were similar among the treatments but differed among the seasons. Canopy volume increased over time although it fluctuated with seasons, suggesting an increase in bushiness of A. drepanolobium in the ecosystem. Trees occupied by different ant species showed differences in shoot density (number of new shoots per twig), being greater in Crematogaster nigriceps occupied trees compared with the other ant species. Swollen thorn (gall) density per unit of twig length was greatest in treatments with megaherbivores; these galls were significantly larger on trees occupied by the ant Crematogaster nigriceps. Treatments with herbivores were more spinescent than the total exclusion treatment. Spine lengths ranged from 0.8 to 2.4 cm, and recorded a progressive reduction of up to 36.36.7% in treatments without browsers suggesting a relaxation of induced defence in A. drepanolobium. Flowering in A. drepanolobium was low and staggered over the study period ranging from 0.8% to 2.0% of the trees with no differences among the treatments suggesting that the level of herbivory was not sufficient to influence reproduction of the tree in the experimental site. Consequently, seedling recruitment was very low within the experimental site. However, a nearby site recorded flowering of between 22.7% and 93.5%. Mean pod production, mean number of seeds per tree and mean weight of pods and seeds had a positive linear relationship with tree density (R2=0.77, 0.81 and 0.81 respectively). Trees occupied by Crematogaster mimosae were the most likely to flower (68%) compared with C. nigriceps (5.8%), again suggesting that ants had an effect on the tree’s reproduction. Mortality of A. drepanolobium trees averaged 0.9% to 4.2% over the study period, being significantly greater in treatments with mega-herbivores. Seedling survival ranged from 42% to 75%, being greatest in the cattle only treatment. Between 30% and 100% (mean 67.2%), of A. drepanolobium seeds were attacked by a bruchid beetle (Bruchus sp.). Seeds attacked by bruchid beetles had significantly lower germination rates. Similarly, seeds passed through a fire also recoded significantly low germination rates compared with normal seeds. Fire (3.4%) and bruchid beetles (20.7%) germination compared with (control) undamaged seeds (84%) play an important role in the population dynamics of A. drepanolobium. Fewer A. drepanolobium seeds (33%) were recovered from the surface compared with buried (72%) seeds after a fire, indicating seed loss from the effect of fire and predation. In the burn experiment, fire top-killed 16% of A. drepanolobium trees but no tree or seedling was killed. On the other hand, fire significantly reduced the density of non-A. drepanolobium trees by between 50% and 100%, with none of them showing signs of coppicing after the fire unlike top-killed A. drepanolobium trees. Woody biomass from A. drepanolobium was strongly related to stem diameter (Y = 3.77x + 1.17, R2 = 0.96, P < 0.001). Mean charcoal production from earthen kilns was 2.83 Mg ha- 1. Height and stem diameter in coppicing stands increased at a mean rate of 28.6 cm yr-1 and 0.7 cm yr-1 respectively. Biomass in coppicing stands accumulated at a mean rate of 1.3 Mg ha-1 yr-1 in a 14-year period, yielding dry biomass of 18.26 Mg ha-1 useable wood that can produce a minimum of 3.0 Mg ha-1 of charcoal. This study shows that Acacia drepanolobium populations are affected by several factors including herbivory, fire and ants. The population dynamics of this tree shows that it can be harvested for sustainable charcoal yield over a 14-year cycle.
An evaluation of irrigated ryegrass/clover pastures for fat lamb production in the Highland Sourveld of Natal.
(1986) Du Plessis, Thomas Marius.; Bransby, David Ian.
The primary objective of this study was to increase net farm income in the Highland Sourveld of Natal by developing pasture based fat lamb production systems. Secondary objectives included (a) determination of the most suitable pasture species or species mixture; (b) determination of the optimum level of N fertilization; (c) establishment of biological and economic optimum stocking rates; (d) projection of expected profit and (e) verification of proposals on a farm scale. Midmar Italian ryegrass was compared to selected legumes alone, and in mixtures with ryegrass in a small plot trial conducted over two years at a 3- and 6-week harvesting frequency under irrigation. Ryegrass in combination with red and white clover provided the highest yields for both harvesting intervals in both years. When this pasture was subsequently compared to ryegrass alone under grazing, no significant difference could be detected in lamb gains at 20 and 28 ewe-lamb units/ha, but at 36 ewe-lamb units/ha the lambs on ryegrass started losing weight at 30 kg, while those on ryegrass + clover continued to gain until a market weight of 40 kg was achieved. In a small plot trial little response was detected to added N above 175 kg N/ha on ryegrass + clover, and under grazing, no significant difference was observed between 275; 375 and 4,75 kg N/ha over two single year periods. Relationships between average daily gain (ADG) and stocking rate, ADG and disc meter height, and stocking rate and disc meter height were mostly well described by linear functions. Expected pre-weaning AOG was 150 g higher (p ≤ 0,01) than post-weaning ADG (100 g) in the first year, and a similar difference was observed in the second year. For the period common to all stocking rates an average maximum gain/ha of 824 kg was achieved at a stocking rate of 33,5 lambs/ha and a disc meter height of 4,5 cm. By lambing in April, lambs could be marketed in September at 40 kg and more weaned lambs could be grazed on the pasture and marketed in December, leading to an expected gain/ha of 1400 to 2200 kg, and expected profit/ha of R 1254 to R 2771, depending on stocking rate. When evaluated under farm conditions, this system produced 2060 kg live weight/ha and a profit of R 3206/ha. It is concluded from the present study that the fat lamb production system based on irrigated ryegrass + clover pastures can be recommended with confidence to farmers in the Highland Sourveld of Natal.
Factors affecting savanna tree sapling recruitment.
(2013) Vadigi, Snehalatha.; Ward, David Mercer.
Savannas are globally important ecosystems characterized by the coexistence of trees and grasses. Woody plants, which are slow-growing dominant life forms, influence the physiognomic structure and function of savanna ecosystems. Their density and distribution provides sustenance to a vast and unique savanna biodiversity, by forming a major source of food material to large mammalian herbivores, sheltering them and through their facilitation of diverse plant species. Savanna tree existence is strongly affected by factors that determine their sapling recruitment. We defined „sapling‟ as a young tree, in the first season of its growth, which does not depend on cotyledonary reserves (=seedling stage) and relies on external resources to grow further. Sapling recruitment may strictly be defined as the progression of a young plant from seedling to sapling stage. However, we believe that savanna tree saplings, present within the grass layer in the initial years of their growth, are equally vulnerable to environmental stresses. This study examines the factors affecting tree sapling establishment in a humid savanna (1250 mm mean annual precipitation). Additionally, the effects of fire were tested in a greenhouse experiment. Dominant species from humid savannas (> 1000 mm MAP), Acacia karroo, Acacia sieberiana, Schotia brachypetala and Strychnos spinosa, and mesic savannas (approx. 750 mm MAP), Acacia nigrescens, Acacia tortilis, Colophospermum mopane and Combretum apiculatum, were studied. In this thesis I examined the effects of resource availability (water, nutrients and light), disturbances (fire and herbivory) and competition (grass) on the sapling ecology of these species. Sapling recruitment and growth were assessed in terms of survival and aboveground growth responses, i.e. total biomass, stem growth rates (used as proxy measures for assessing persistence) and leaf biomass proportion (important for producing root reserves necessary to resprout). I studied the effects of fire and a nutrient gradient on survival and growth of four Acacia species in the presence of grass competition, in a controlled greenhouse experiment. Generally, Acacias invest in defenses after herbivory. I also determined their physical and chemical defense investments in this experiment. Sapling survival was not influenced by nutrients but highly varied among the species due to fire, indicating that fires may have a differential effect on species composition at a landscape scale. Intermediate levels of nutrients were found to be beneficial for sapling growth than high and low levels. This may be due to an increase in grass competition at higher levels of nutrients. Fires did not have a positive influence on sapling defence investment. To evaluate the relative importance of resource availability on sapling tree recruitment and its interactions with grass competition, I tested the effects of water (frequent irrigation vs. rainfall), shade (presence vs. absence), nutrients (addition vs. no addition) and grass competition (presence vs. absence) on sapling survival and growth under controlled field conditions in a humid South African savanna. Treatments did not have an effect on sapling survival, indicating that mortality is not defined by resource availability and grass competition in humid savannas. Shade had the greatest negative effect on sapling growth, suppressing the beneficial effects of nutrients and absence of grass competition. Nutrient limitation and grass competition had a relatively small influence on savanna sapling growth. Frequency of water availability had no effect on sapling growth, perhaps owing to high rainfall experienced over the experimental period. Therefore, canopy shade can be considered to be an important driver of tree dynamics in humid savannas with some degree of influence by nutrient availability and grass competition. The effects of clipping (i.e. simulated herbivory of grass and tree saplings) as influenced by nutrient availability and grass competition were examined on sapling survival and growth of all study species in a humid savanna. None of the treatments had an effect on sapling survival. This signifies that herbivory alone cannot significantly decrease plant density in humid savannas. However, tree saplings grew taller with a reduction in diameter and overall biomass, implying that saplings may become more susceptible to fires after herbivory. Nutrient addition and grass competition in general had a positive and negative effect, respectively, on sapling growth. This response was prominent in the stem length growth rates of defoliated saplings of one humid and two mesic species. These results imply that clipping (or herbivory) is the major factor reducing sapling vigour to establish, but is affected by both grass competition and nutrient availability. This study shows that fire has a differential effect on sapling survival of different species, particularly between humid savanna species. Light interception among all other resources limits the recruitment of saplings into adult size classes. Clipping, nutrient availability and grass competition had a relatively small direct effect, but may interact with other factors to alter sapling establishment dynamics. Wet-season droughts in humid savannas are not a hindrance to tree establishment because sapling survival was not dependent on frequency of rainfall. Thus, in humid savannas, fires can have a major impact on tree species density and composition while canopy shade has a very high potential to alter tree distribution.
Host specificity in South African mistletoes.
(2013) Okubamichael, Desale Yosief.; Ward, David Mercer.; Griffiths, Megan Elizabeth.
Mistletoes intimately connect to their host trees with a haustorium that allows them to access nutrients and water. Mistletoes in South Africa vary greatly in their degree of host specificity. Most species occur on a wide range of host families, while others are restricted to a single host family or—at the extreme—to a single host species. Mistletoes that are host generalists at a larger spatial scale may become host-specific at a local scale. One of the challenges in mistletoe biology is determining the factors that maintain local host specificity. Birds potentially reinforce the mistletoe–host interactions by direct dispersal. However, many mistletoe species coexist while parasitising different co-occurring host species. This suggests that host trees may impose more selection pressure than birds in determining host specificity. Thus, my thesis examines the role of host trees as ecological and physiological filters that influence the infection patterns and determine host specificity of mistletoes in South Africa. The second chapter of this thesis synthesises the literature on host specificity in mistletoes. I then present the results of four field and laboratory experiments that were used examine the features affecting host specificity in representatives of two families of mistletoes (Loranthaceae and Viscaceae) in South Africa. My main research objectives focus on host abundance and morphology, host compatibility, host water and nutrient content, abiotic influences on mistletoe seedling survival and growth and mistletoe–host stomatal morphology in relation to water potential that affect nutrient acquisition by mistletoes from their host trees. The geographic mosaic approach was explored as a potential explanation for the mistletoe–host interactions that direct host specificity in mistletoes. I synthesised the available literature on the mechanisms and factors that direct mistletoe host specificity. This was supported by data analysed from South African herbarium collections, books describing the South African flora and field observations in South Africa. I suggest that host abundance (host availability through time and space) and host compatibility (as determined by genetic, morphological, physiological and chemical factors) play a primary role in determining host specificity in South African mistletoes, while differential bird dispersal strengthens or weakens mistletoe–host interactions. Analysis of the network structure of mistletoe–host interactions at different levels (e.g., at the level of population, species and genus) followed by genetic and reciprocal germination experiments may reveal the patterns and mechanisms of host specificity in mistletoes. I quantified the mistletoe–host composition, height of potential host trees and nutrient and water content of mistletoes and their hosts at Pniel Estates. Surveys of the study site revealed a single mistletoe species, Viscum rotundifolium, parasitising only Ziziphus mucronata and Ehretia rigida. Both parasitised host species were not the most abundant trees, were not the tallest trees and did not have the highest water or nutrient content of trees in the area, although these factors have been found to be good predictors for mistletoe parasitism in other studies. Subsequently, I tested mistletoe–host compatibility by conducting a germination experiment in the greenhouse by inculcating seeds of V. rotundifolium on freshly cut branches of nine available potential host trees. I found that mistletoe seeds had a greater chance of attachment and subsequent survival on branches of E. rigida and Z. mucronata as compared with seeds on co-occurring Acacia and other potential host species. This suggests that host compatibility plays a role in directing the host specificity of V. rotundifolium at Pniel Estates. I found that individuals of V. rotundifolium had more negative water potentials than their host trees and, by doing so, they passively maintain the flow of nutrients. In addition, I found evidence that the mistletoe uses active uptake to access nutrients from host phloem because the leaf tissue of a mistletoe had a nitrogen-to-calcium ratio (N:Ca) >1. Conventionally, a high N:Ca ratio (>1) in the leaf tissue of a mistletoe is taken as evidence of active uptake from host phloem because N is highly phloem-mobile while Ca is a large molecule and is phloem-immobile. This method has shortcomings discussed at greater length in the chapter but my findings suggest that the mistletoe V. rotundifolium uses a combination of passive and active nutrient uptake. I quantified the mistletoe–host community composition and host physical features (height and diameter at breast height) in two sites in KwaZulu-Natal, South Africa—Highover and Mtontwane. The mistletoe Agelanthus natalitius (Loranthaceae) is common at both sites, parasitising the most abundant host species—Acacia karroo—and the second most abundant host tree—Acacia caffra. Prevalence of mistletoe infection (percentage of trees parasitised) was positively correlated with tree size (height and diameter at breast height). The two host species did not differ significantly in height. At Highover the host species A. caffra and A. karroo had a similar prevalence of mistletoe infection but at Mtontwane a significantly higher percentage of A. caffra trees was parasitised in comparison with A. karroo. However, the intensity of mistletoe infection (mean number of mistletoes per tree) was lower for A. caffra (Highover: 0.66 ± 0.01, Mtontwane: 0.89 ± 0.04) than for A. karroo (Highover: 0.73 ± 0.04, Mtontwane: 1.03 ± 0.64). There were two highly infected big trees in Highover and one in Mtontwane where many mistletoe-dispersing birds were nesting which inflated the numbers for intensity of mistletoe infection in A. caffra, however. I tested mistletoe–host compatibility by conducting a reciprocal transplant experiment in the two study sites. I applied a paired design, using one local and one non-local mistletoe seed in each pair, with seed pairs placed on the two main host species at the different sites. Except in Highover where an unidentified pathogen retarded growth and survival, mistletoe seeds placed on the same substrate and in the same site as their source host grew a longer hypocotyl and had greater survival. Regardless of source, mistletoes placed on A. karroo had longer hypocotyls and greater survival than mistletoes on A. caffra. These results suggest that there may be adaptation of the mistletoe Agelanthus natalitius to the most frequently encountered host species, Acacia karroo. To simulate the conditions encountered by mistletoes during the dry and cold South African winter, mistletoe seedlings were monitored at different levels of microclimate (light, temperature and moisture) in a growth chamber. I found that higher light availability (20% and 40% shade versus 80% shade), cool temperatures (15°C and 20°C versus 25°C) and continuous moisture availability improved seedling development and subsequent survival of two mistletoe species (Viscum rotundifolium and Agelanthus natalitius). I studied the leaf stomata of two host–mistletoe pairs (Acacia karroo–Agelanthus natalitius and Vitex obovata–Erianthemum dregei) using a scanning electron microscope to investigate some of the underlying mechanisms that enable mistletoes to maintain more negative water potentials than their host trees and at the same time control water loss. In addition, I examined the response of mistletoes to the application of abscisic acid (ABA), a plant growth regulator that controls stomatal closure. I found that the mistletoes had a higher density of stomata and had larger stomata than their host trees. In addition, both mistletoe and host leaves closed their stomata during midday and in response to exogenous ABA. The ability of mistletoes to control water loss in this way may be one reason why mistletoes rarely kill their host trees, which would be maladaptive. The mistletoes used in my studies are known to be host generalists at a larger spatial scale but I found that they were host specific at a local scale. The results of my research suggest that host abundance and compatibility play a role in directing host specificity, while host nutrient and water status have little effect on host specificity at this local scale. The interactions between the generalist mistletoes used in my studies and their hosts are likely to vary over the geographic ranges of the mistletoe and alternate among different hosts. This may create multiple locally host–specific mistletoe populations and produce a complex geographic mosaic of mistletoe–host combinations across space and time. I suggest that mistletoe populations in South Africa may comprise numerous lineages incapable of parasitising the full range of host species, which could potentially lead to the formation of distinct host races over time. In the future, it would be interesting to document the infection patterns of these generalist mistletoe species across their entire geographic ranges in southern Africa, with particular focus on the patterns of mistletoe infection in places where the host abundance changes among sites. Host preferences may vary with changes in host frequency and host community composition. This could be paired with reciprocal transplant germination experiments in several sites to ascertain whether the mistletoe species have higher fitness on the most locally abundant hosts.

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