ETD Collection
Permanent URI for this collectionhttps://wiredspace.wits.ac.za/handle/10539/104
Please note: Digitised content is made available at the best possible quality range, taking into consideration file size and the condition of the original item. These restrictions may sometimes affect the quality of the final published item. For queries regarding content of ETD collection please contact IR specialists by email : IR specialists or Tel : 011 717 4652 / 1954
Follow the link below for important information about Electronic Theses and Dissertations (ETD)
Library Guide about ETD
Browse
3 results
Search Results
Item Investigating the life history strategy of an African savanna tree, Sclerocarya birrea subsp. caffra (marula)(2011-11-18) Helm, Chantal Vinisia1 Investigating the life history strategy of an African savanna tree, Sclerocarya birrea subsp. caffra (marula) Chantal Vinisia Helm Abstract Lack of understanding of the life history attributes and responses of savanna woody plants to disturbances, as well as the observation of unstable population structures in a keystone, savanna tree, Sclerocarya birrea subsp. caffra (marula), prompted this study. This study employed a combination of empirical, experimental and model formulation techniques, aimed at achieving its ultimate purpose of understanding the life history strategy of marula in the face of disturbance. Four main population structures were identified for marula in the low altitude savannas of South Africa: 1) adult dominated, 2) juvenile dominated, 3) with a “missing size class” and 4) stable (negative J-shaped). Spatial variability in structure indicated different drivers affecting different populations. High annual mortality rates of up to 4.6% in adult trees, no recruitment out of the fire trap and little regeneration were observed in the Kruger National Park (KNP) between 2001 and 2010, and consequently even greater instability in the structure of these populations already observed earlier in the decade. Growth rates of saplings between 2 and 8 m in height and 2 and 30 cm in stem diameter in the field were monitored between 2007 and 2010. Annual growth rates of up to 11 mm in diameter and up to 22 cm in height were observed. Annual relative growth rates ranged between 1.9 and 4.8% across sites. Growth rates were positively linked with rainfall and plant size. Growth rates, biomass allocation patterns, as well as storage and defence allocation in 3 to 28 month old marula seedlings were assessed under glasshouse conditions. Relative growth rates were highest directly after germination (20%), but did not exceed 5% thereafter. Allocation to roots (already 65% of the overall biomass at 3 months of age and >80% when older) was high regardless of soil type or provenance. Provenance affected height gain, and plants germinating from seeds collected at higher rainfall sites had faster height growth rates than those from seeds collected at lower 2 rainfall sites. Allocation to storage in the form of root starch peaked at 35%, while allocation to defence in the form of phenolics in the leaves peaked at 18%, being relatively higher than other species. No trade-off between growth and defence allocation was observed. However, in the second growing season, growth at the start coincided with a 50% decrease in starch reserves in the roots. Reproductive maturity was found to occur after 46 years and escape from the fire trap after 12 years in a disturbance free environment. Marula trees appear to be able to live for up to 300 years of age. High temporal variability in fruit production was observed, marginally linked to rainfall. Only 2% of seeds persisted for more than one year, and hence marula relied mostly on the current season’s fruit crop for input of new germinants. Fruit production was highly synchronous across trees at a site. Very high levels of seed predation were observed. Marula seeds can remain dormant for at least 10 years when stored in the laboratory. Germination takes place after 3 mm of rainfall every four days for two weeks and is enhanced by acid digestion and high temperatures. Germination percentages are relatively low (<50% of the endocarps). Marula seedlings appear highly adapted to fire, with high allocation to below-ground biomass and starch storage, as well as very thick bark from very small stem diameters, including a well developed resprouting response from very young. Marula stems were able to resist fire from 3.4 cm in stem diameter, and were completely resistant above 7 cm. Stem diameter growth was prioritised above stem height growth, indicating that in marula, diameter gain is more important than height gain in escaping the fire trap. Topkilled marula saplings are able to regain their prefire height within one season. However, rainfall patterns may have an overriding effect on these growth patterns. Adult trees appear to be made vulnerable to fire through bark stripping, toppling and pollarding and the subsequent invasion of the soft wood by borers. On nutrient-poor granite soils, marula has a resistant strategy to herbivory, however on nutrient-rich basalt soils, marula overcompensates for herbivory even at very low levels. This may explain why marulas are more vulnerable on basalt soils in the KNP, having 3 already been extirpated from the northern arid basaltic plains. Marula seedlings are extremely drought resistant through fast root penetration rates and high root: shoot ratios. A simple demographic model was developed which predicted that marula populations are unlikely to survive given the current elephant impact in the KNP and if the fire interval is less than once every seven years. Even though marula is highly resilient to damage from herbivory or fire alone, the combination of frequent fire and heavy utilisation is proving fatal for marula populations in the KNP and elsewhere. In terms of other savanna tree species, marula is an outlier in its life history strategy, being extremely well adapted to the effects of fire with very thick bark, extensive resprouting ability and fast growth rates, combined with very high allocation to root mass, and levels of storage and chemical defence, as well as having very drought tolerant seedlings. Its main weakness as an adult, appears to be its soft wood, which is susceptible to wood borer attack. The perplexing lack of recruitment at some sites in spite of the extraordinary ability of marula seedlings to resprout from an early age, withstand extensive drought, have fast root penetration rates, extremely high root reserve storage and resistance to fire at small stem diameters, combined with high levels of fruit production and low water requirements for germination, is probably due to a combination of the lack of a dense persistent seed bank, high inter-annual variability in fruit production, low germination percentages, high seed and /or seedling predation rates and possibly dispersal of seeds away from suitable habitats. Overall, the unstable population structures observed in the low altitude savannas of South Africa, specifically in the KNP, do not bode well for the future persistence of marula as a dominant canopy tree species. Keywords: elephant, fire, growth, mortality, recruitment, regenerationItem The long-term effects of fire frequency and season on the woody vegetation in the Pretoriuskop Sourveld of The Kruger National Park(2007-03-01T12:26:35Z) O’Regan, Sean PatrickO’Regan SP, 2005. The long-term effects of fire frequency and season on the woody vegetation in the Pretoriuskop sourveld of the Kruger National Park. MSc Dissertation, University of the Witwatersrand, Johannesburg. The role of fire in the management of conservation areas has historically been a contentious issue in which traditional agricultural principles and ever-changing conservation principles tend to collide. The Kruger National Park (KNP) in the early 1950s was no exception where the appropriate use of fire and its ecosystem consequences were hotly debated. The controversy surrounding the management of fire in the KNP highlighted the significant lack of understanding of fire and its role in the ecosystem and because of this controversy, the Experimental Burn Plot (EBP) experiment was established in 1954. The EBP experiment comprised 12 treatments, and a pseudo-randomised block design was used in which the 12 fire treatments were replicated four times each in four of the six major vegetation zones identified at the time. The EBP experiment originally comprised 192 experimental plots approximately 7 Ha in size each and covered approximately 12 km2 in the KNP. The twelve fire treatments were an annual burn in August, biennial and triennial burns in February, April, August, October, and December, and a control on which fire was excluded. Despite having been plagued with negative assessments from internal and external researchers from its inception, the EBP experiment was meticulously maintained, and it has now become a valuable research asset in the KNP. Four replicates of twelve plots each were located in the Pretoriuskop sourveld landscape of the KNP. These replicates were named Fayi, Kambeni, Numbi, and Shabeni after nearby landmarks. The Pretoriuskop region is a moist infertile mesic-savanna, which experiences on average 744mm of rain annually. The dominant tree species in Pretoriuskop are Dichrostachys cinerea and Terminalia sericea and the dominant grass species is Hyperthelia dissoluta. A baseline survey of the woody vegetation was done on all the Pretoriuskop plots in 1954 by HP Van Der Schijff. A second survey of the woody vegetation on all the Pretoriuskop plots was done in 1996 by SP O’Regan. This provided a 42-year period of treatment application over which the effects of fire frequency and season on the woody vegetation of the Pretoriuskop region were studied. The aim of this study was to investigate the long-term effects of the twelve fire treatments on the density, structure, and species composition of the woody vegetation in Pretoriuskop. The objectives of this study were: 1. To carry out a complete re-survey of the trees and shrubs on the Pretoriuskop EBPs using similar methods as those used in the baseline survey in 1954. 2. To capture into a digital format pertinent woody vegetation survey data from surveys that had been conducted on the Pretoriuskop EBPs between 1954 and 1996. 3. To compare the density, structure, and composition of the woody vegetation on the Pretoriuskop EBPs between 1954 and 1996, to determine the effects of fire on the woody vegetation of Pretoriuskop. 4. To investigate the history of the Kruger National Park Experimental Burn Plots experiment. The four replicates in the Pretoriuskop region were found generally to have very similar woody vegetation traits (density, species composition, and structural composition). However, the EBPs were established and surveyed in two distinct phases, the first phase comprised the control, August Annual, and the Biennial plots, and the second phase comprised the Triennial plots. The baseline structural composition of the plots established in the first phase was different from the structural composition of the plots in the second phase. Furthermore, the Pretoriuskop EBPs are located in two distinct vegetation types, namely the open and the closed Terminalia sericea \ Combretum woodlands of the Pretoriuskop region. The Numbi and Shabeni replicates are in the open Terminalia sericea \ Combretum woodlands, and the Kambeni and Fayi replicates are in the closed Terminalia sericea \ Combretum woodlands. It was found that the species composition of the plots was influenced by the location of the plots in the different vegetation types. The exclusion of fire in the Pretoriuskop sourveld results in an increase in the density of the overstorey and understorey woody vegetation, and an increase in the number of species, species diversity, and species evenness. This is because fire sensitive and fire intolerant woody species become more abundant as the period between fires increases. In Pretoriuskop, there is no evidence of relay floristic succession, because fire sensitive and fire intolerant woody species do not replace fire tolerant species. Instead, the floristic succession is accumulative and fire tolerant, fire sensitive, and fire intolerant woody species coexist as the period between fires increases. Woody species tolerant of frequent fires in Pretoriuskop are Albizia versicolor, Catunaregam spinosa, Lonchocarpus capassa, Pavetta schumanniana, Senna petersiana, Strychnos madagascariensis, and Turraea nilotica. Woody species that are sensitive or intolerant of fire in Pretoriuskop are Acacia swazica, Bauhinia galpinii, Combretum mossambicense, Commiphora neglecta, Croton gratissimus, Dalbergia melanoxylon, Diospyros lycioides, Diospyros whyteana, Euclea natalensis, Hyperacanthus amoenus, Kraussia floribunda, Ochna natalitia, Olea europaea, Psydrax locuples, Putterlickia pyracantha, Tarenna supra-axillaris, and Zanthoxylum capense. Dichrostachys cinerea and Terminalia sericea were found to dominate in areas that had been burnt frequently as well as areas where fire has been excluded. The change in the density of the woody vegetation as the inter-fire period increases is not linear but rather J shaped with an initial decrease in the density as the inter-fire period increases from 1 year to 3 years. This initial decrease in density is the result of a loss of very short (<1m tall) woody individuals. In contrast, there is no initial decrease in the number of tree equivalents (phytomass) of the woody vegetation as the inter-fire period increases. After the initial decrease in the density of the woody vegetation, the density increases as the inter-fire period increases beyond 3 years. Generally in Pretoriuskop, post fire age of the vegetation was found to be an important factor affecting the structure of the woody vegetation, and as the inter-fire period increases the number of structural groups, the structural diversity, and the structural evenness of the woody vegetation increases. As the inter-fire period increases the number of single-stem individuals relative to the number of multi-stem individuals increases, and the average height of the woody vegetation increases. The findings regarding the effects of fire frequency on the Pretoriuskop EBPs were similar to the findings on other fire experiments in mesic African savannas. The finding on the Pretoriuskop EBPs differed from the findings in other fire trials that were in arid savannas in Africa. Generally, the exclusion of fire in moist savannas (> 600 mm of rain annually) results in the woody vegetation becoming denser, while the exclusion of fire in arid to semi-arid savannas (< 600mm of rain annually) does not result in the woody vegetation becoming denser. In Pretoriuskop, fires occurring in summer between December and February have a different impact on the density, species composition, and structure of the woody vegetation than fires occurring in winter between August and October. Furthermore, fires occurring in April have a different impact on the density, species composition, and structure of the woody vegetation in Pretoriuskop. Woody vegetation burnt by summer fires is denser than woody vegetation burnt by winter fires. The number of species and species diversity of the woody vegetation is also higher in vegetation burnt by summer fires in comparison with vegetation burnt by winter fires. The density and species composition of woody vegetation in areas that have been burnt in summer fires is more similar to areas where fire has been excluded than to areas that have been burnt in winter fires. The woody species associated with vegetation burnt in summer fires and where fire has been excluded are Euclea natalensis, Antidesma venosum, Diospyros lycioides, Phyllanthus reticulatus, Grewia flavescens, Grewia monticola, Ochna natalitia, Peltophorum africanum, Rhus pyroides, Diospyros mespiliformis, Rhus transvaalensis, Securinega virosa, Putterlickia pyracantha, Rhus pentheri, Commiphora neglecta, Heteropyxis natalensis, and Olea europaea. Structurally the average height of the woody vegetation is taller in areas burnt by winter fires than in areas burnt by summer fires. The woody vegetation in areas burnt in summer fires have more single-stem individuals relative to multi-stem individuals than in areas burnt in winter fires. The structural composition of areas burnt in summer fires is more similar to areas where fire has been excluded than with areas burnt in winter fires. The structure of the woody vegetation in areas burnt in winter fires is generally dominated by multi-stem individuals that are 0-1m tall or 3-5m tall. The structure of the woody vegetation in areas burnt in summer fires or where fire has been excluded is dominated by both single-stem and multi-stem individuals of all heights and basal diameters. Findings regarding the effect of early dry season fires (April) in comparison with late dry season fire (August) on the woody vegetation are consistent with the findings on other fire trails in Africa. However, a comparison of all the fire-timing treatments between the Pretoriuskop and Satara EBPs in the KNP reveals that the timing of fires affects the woody vegetation differently in different areas even when the affects at certain times appear similar. The data collected on the Pretoriuskop EBPs reveals that there have been significant changes in the woody vegetation in Pretoriuskop between 1954 and 1996. The density of the woody vegetation increased between 1954 and 1996 by almost 200%. The number of species and the species diversity of the woody vegetation also increased between 1954 and 1996. In 1954, there were approximately equal numbers of single-stem and multi-stem individuals, while in 1996 there were more multi-stem individuals than single-stem individuals. The increase in atmospheric CO2 levels between 1954 and 1996 is believed to have been a factor that has driven the changes in the woody vegetation of Pretoriuskop between 1954 and 1996.Item Spatial and temporal variation of the fire regime in Mkuzi Game Reserve(2006-11-16T09:06:32Z) Mulqueeny, CraigFire is a key determinant of savanna dynamics, and would thus have a major influence on the vegetation dynamics of Mkuzi Game Reserve. Given this logic, it is an important and commonly used management tool in this reserve. Its main uses in the reserve are for either removing moribund material or for reducing woody plant encroachment, both of which normally entail dry season burns. As a consequence, fire often results in a green flush of vegetation that is highly favoured by grazing herbivores. A further management goal is maintaining or improving biological diversity by promoting vegetation heterogeneity. Current policy prescribes this should be achieved through point-source ignitions rather than by block-burning, which was the earlier practice. This study explores spatial and temporal fire patterns at a landscape scale in Mkuzi Game Reserve using Geographic Information Systems (GIS). Much of our understanding of the dynamics of fire has previously been determined at a plot scale and scaling up of these insights to a landscape scale is problematic, hence this project aimed to contribute to our understanding of the dynamics of fire at a landscape scale. The study also specifically examined how the fire regime in the reserve has changed with a change in the burning philosophy and strategy, namely from block burning to the point source ignition (PSI) strategy, which began to be implemented in the mid-1980's. Fire frequency was related to both geological type and vegetation type. The fact that geology was related to fire frequency was not surprising because the relationship between geology and vegetation in the reserve has previously been established. The varying amount of herbaceous material per vegetation type apparently influenced fire frequency. Spatial variation in fire frequency was also positively related to rainfall variation over the reserve, while the total area burnt per annum was positively related to the preceding wet season rainfall, but not for years with a high dry season rainfall. The influence of rainfall on grass production and thus fuel load explained these relationships. In addition, there was some evidence of a carry over effect of rainfall where the previous wet season rainfall together with the preceding wet season rainfall influenced total annual area burnt, but this was only significant for years when dry season rainfall was low. Contrary to an expected negative influence, dry season rainfall had no effect on the total annual area burnt. Grazer biomass had a significant limiting effect on fire frequency over the reserve (spatially), most likely due to consumption of herbaceous ii material, but there was no relationship between grazer biomass and total annual area burnt (temporally). Dry season burns were significantly larger than wet season burns and can be attributed to the more favourable fuel condition during the dry season. Intense burns were also generally larger than the cooler burns, namely those rated as patchy/very patchy and clean. This was mainly attributed to a high fuel load which is critical for intense fires but also positively influences the spread of fire. The comparison of the block burning strategy and the point source ignition (PSI) strategy showed that fire frequency was greater during the PSI burning period than during the block burning period. The total area burnt per annum was greater during the PSI burning period than during the block burning period, but individual burn sizes were not significantly different between the two strategies. Evidence showed that individual burns that occurred during the PSI period had boundaries that were more irregular than those of block burns. Fires were most common during the dry season for both burning strategies, but the proportion of the burns that occurred during the dry season was greater for the PSI burning period than for the block burning period. Evidence also showed that a much greater emphasis was put on applying dry season prescribed burns during the PSI period than during the block burning period. A greater effort was also made during the PSI period to burn firebreaks, which were only implemented during the dry season. Arson fires (started deliberately or accidentally by neighbours) were more common during the block burning period than during the PSI period, while under both burning strategies, they were more common during the dry season than the wet season. There was no distinguishable difference in the burn intensity patterns between block and PSI burning, that is, the proportions of burns in the different burn intensity classes were not significantly different between the two burning strategies. Although the contribution of the individual fire barrier types showed some change with a change from block burning to a PSI strategy, the combined contribution of natural barriers did not increase, and that of management barriers did not decrease, as would have been expected. In addition, natural and management barriers were apparently of equivalent importance during both burning strategies.