Browsing by Author "Tedder, Michelle Jennifer."

Now showing 1 - 12 of 12

The causes and consequences of Seriphium plumosum L. encroachment in semi-arid grassland communities of Gauteng province, South Africa.
(2021) Pule, Hosia Turupa.; Tedder, Michelle Jennifer.; Tjelele, Julius Tlou.
Abstract available in PDF.
The competitive effect of vetiver grass on native grasses and implications for grassland rehabilitation in South Africa.
(2018) Dlamini, Lindokuhle Xolani.; Kirkman, Kevin Peter.; Tedder, Michelle Jennifer.
Astract available in PDF file.
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.
Effects of drought on grassland forb reproduction.
(2024) Mbambo, Sibonokuhle Thandwayo.; Tedder, Michelle Jennifer.; Steenhuisen, Sandy-Lynn.
No abstract available.
Effects of increased temperature on growth and nutritional value of mesic grasslands, with or without woody legume seedling competition.
(2020) Gili, Nikilita.; Tedder, Michelle Jennifer.; Scogings, Peter Frank.; Mkhize, Nthuthuko Raphael.
Mesic grasslands are complex ecosystems covered in grasses and other graminoid vegetation. The species composition varies due to variation in rainfall and temperature; these grasslands are climatically supported. They also vary in nutritive value and grasslands with high species richness have low nutritive value because grasses differ genetically. Grasses’ response to high temperature and competition is species dependent. Grasslands are ideal for ecological experiments because grasses grow fast and their response to environmental changes is noticeable. Therefore, an experiment of induced warming with legume seedlings interaction was conducted at the University of KwaZulu-Natal, using open top chambers and Vachellia sieberiana var. woodii. This was done to determine the effect of increased temperature on the biomass, growth, morphology and nutritive value of Themeda triandra and Aristida junciformis. These species were chosen because they occur naturally and dominate in the Ukulinga farm, where the experiment was conducted. They also have contrasting palatability, T. triandra is highly palatable and A. junciformis is less palatable. Vachellia sieberiana seedlings were grown from seed and transplanted after two months to the field to interact with grasses for four months. The results suggest that the interaction of warming and woody seedlings reduces the biomass of the investigated species. This implies that warming reduces grass biomass. Plant traits such as grass height, leaf area, tiller width and tuft diameter responded differently to the treatments. The fibre (neutral detergent fibre) of T. triandra was increased by warming. The interaction of warming and woody seedlings had no effect on the regrowth fibre content. The interaction of warming and woody seedlings increased the protein content of A. junciformis. Warming and woody seedlings independently increased the protein content of T. triandra. The response of plants to increased warming will help ecologists understand the effects of global warming. To provide more insight into these findings, further research on specific species with longer experimental duration and high woody seedling neighbour density is of importance.
The impact of soil water and nitrogen variability on the fitness and performance of Neolema abbreviata Larcordaire (Chrysomelidae) a biological control agent for Tradescantia fluminensis.
(2018) Mbande, Abongile.; Chidawanyika, Frank.; Tedder, Michelle Jennifer.
Tradescantia fluminensis Vell. (Commelinaceae) is a plant of Neotropical origin native to the southern parts of Brazil bordering Argentina. In South Africa, it is classified as a category 1B invader species in the National Environmental Management Biodiversity Act (NEMBA) owing to its incipient phase of invasion. The occurrence of naturalised populations of T. fluminensis has so far been confirmed in all provinces except the Free State, Northern Cape and North West. In cognisance of the devastating effects of invasive alien plants on native biodiversity, ecosystem health and ultimately provision of ecosystem services, several control methods have been employed with varying degrees of success. Classical biological control, which involves the release of exotic natural enemies (pathogens and herbivorous insects), is one such method widely-used because of its relatively low costs and minimal non-target effects. For T. fluminensis, Neolema abbreviata (Larcodaire) Coleoptera: Chrysomelidae) is one agent that is earmarked for release in South Africa following a successful introduction in New Zealand. However, little is known how novel environments presented by soil water and nutrient gradients may indirectly influence its herbivore performance and life-history through alterations in host-plant quality. In this era of global climate change where anthropogenic activities have led to changes in rainfall patterns and biogeochemical cycles of major elements such as nitrogen, investigation of species responses to such is important. Results from my study show that both water and nitrogen (N) variability influenced plant biomass accumulation, foliar N content and subsequent herbivore performance, and life-history traits of both adult and larval N. abbreviata. The longest vines were on plants that had optimal irrigation under excess fertiliser whilst severely water stressed plants that had excess fertiliser had the shortest vines. Foliar N content was highest in plants that had excess fertiliser under both pulsed and optimal irrigation whilst lowest foliar N content was in plants under optimal irrigation without any fertiliser. Optimally irrigated plants that received moderate fertiliser had their highest rate of egg deposition in both no-choice and multi-choice conditions suggesting quality-based host ranking behaviour in N. abbreviata. The consequent larval performance traits which included weight gain and time to pupation were superior in this treatment thereby providing support for the preference-performance hypothesis (PPH). Feeding patterns between larvae and adults among plant treatments were largely similar suggesting uniform nutritional requirements across the life-stages. There were limited parental effects of plant quality on the life-history traits in both larvae and adults across F1 and F2 generations. In reciprocal diet transplant experiments, there were no significant responses to parental diet effects on larval weight, mortality, feeding damage, pupal weight and days to pupation. However, there were significant parental diet x test diet interactions with offspring from parents fed on high N plants generally performing better on low N test plants in traits such as larval weight gain and final pupal weights. Oviposition selection, feeding weight and longevity did not respond to the effects of parental diet nor its interaction with test diet, unlike the case with larval traits. There were significant correlations between pupal weight and number of days to pupation, pupal weight and eclosion success. I conducted a 3 x 3 full factorial experiment to determine the impact of water and fertiliser variability on the performance of Neolema abbreviata (and its host plant Tradescantia fluminensis. My results show differential responses to parental diet between larvae and adults of the same generation among an insect species with both actively feeding larval and adult life-stages. However, there was no correlation between adult weight and longevity. Overall, my thesis contributes to the growing body of literature on the impacts of anthropogenic global change on plant-insect interactions. It will also assist land managers when applying biological control of T. fluminensis. Furthermore, my results show the implications on the successful biological control (mass-rearing and field release) of T. fluminensis resulting from variable nitrogen and water conditions.
Interactive effects of fire history and elevated 2 temperature on aboveground productivity in a high 3 altitude mesic grassland in South Africa.
(2021) Mvelase, Thembeka Ayanda.; Tedder, Michelle Jennifer.; te Beest, Mariska.
Abstract available in PDF.
Local and global controllers of grassland ecosystem stability during global change.
(2019) Demmer, Stuart.; Kirkman, Kevin Peter.; Tedder, Michelle Jennifer.
Human impacts on grasslands dramatically affect grassland biodiversity which impacts the ability of ecosystems to sustainably provide ecosystem services. As the extents of these anthropogenic impacts increase (due to agricultural intensification, for instance) solutions to this problem are becoming increasingly important. The ecosystem stability concept provides a framework to investigate how biological systems such as grasslands respond to disturbances. However, there is uncertainty relating to the ecosystem components which influence the various facets of ecosystem stability. Therefore, the aim of this dissertation is to 1) outline the current academic consensus pertaining to the drivers of grassland ecosystem stability, 2) contribute to underrepresented research areas identified in the literature review, and 3) investigate whether there are general environmental conditions which predispose to grassland destabilisations following anthropogenic disturbance. Academic consensus was assessed using a systematic map of review articles discussing grassland ecosystem stability concepts. This review highlighted the many complex interactions that exist in grassland ecosystems. There was also a strong consensus that diversity mediates ecosystem functioning and stability. Other ecosystem processes such as fire, herbivory, woody encroachment, and plant invasions were also well represented and discussed in these review publications, however, climatic impacts on grasslands were identified as an important knowledge gap. To address this, nutrient enriched grassland stability responses to temperature variability were studied using a long-term nutrient addition experiment. Surprisingly, nutrient enriched grassland productivity was more stable than control grasslands in response to temperature variability. Finally, environmental drivers of grassland stability changes following nutrient addition were assessed using a globally replicated experiment. This investigation showed that grasslands with a history of intensive anthropogenic management are positively affected by nutrient addition whilst stability in more naturally assembled grasslands is greatly reduced following nutrient addition. Stability changes were also associated with changes in nutrient availability and soil macronutrient (specifically Ca and K, but not micronutrient) status. Sward structure changes (such as increased compositional dissimilarity, greater dominance, and reduced asynchrony) were associated with stability reductions following nutrient addition. The findings of these three investigations highlight the serious impacts that human activities which result in increased nutrient deposition in grasslands are having on grassland ecosystems. In relation to the prevailing consensus identified in the review literature concerning the positive effects of grassland diversity on ecosystem stability and functioning, this dissertation advocates for the increased preservation of intact grasslands.
The relative tolerance of mesic grassland species to defoliation and competition.
(2018) Zama, Naledi Zola.; Tedder, Michelle Jennifer.; Morris, Craig Duncan.; Mkhize, Ntuthuko Raphael.
Effective grazing management is dependent on understanding grass species responses to herbivory. These responses to herbivory can be broadly grouped into 3 categories, namely decreaser (plants that decline in abundance) and increasers (plants that increase in abundance). Tolerance is defined as the capacity of a plant to withstand herbivory, while suffering little loss in growth or its ability to reproduce and it can be equated to the ability of the plant to compensate. The relevance of this strategy to rangeland management has become increasingly apparent and has allowed researchers to investigate more questions and test long-standing ideas within the Grassland Science discipline. Therefore, the general aim of this research was to determine how Increaser and Decreaser grass species common in mesic grasslands tolerate defoliation and competition. Two experiments were conducted as controlled pot trial experiments under shade cloth at the NM Tainton Arboretum. Simulated herbivory in the form of clipping was used for both experiments. Categorising species into four grazing response groups (Increaser I, II,III and Decreaser) has led to generalisations made across and between species in terms of responses to herbivory. To determine if these generalisations are appropriate, the growth response of two Decreaser (Themeda triandra and Tristachya leucothrix) and two Increaser (Eragrostis curvula and Eragrostis plana) grass species was investigated. Results indicate that defoliation tolerance is not necessarily explained by response groups and differences can be observed between species, within response groups. Grouping species into response groups may be an over simplification. This implies that species identity may be more important in understanding species composition changes within natural communities than originally thought. Plant traits, such as shoot biomass, roots biomass , tuft height and root to shoot biomass ratios also responded differently across species highlighting the importance of further research on specific species as generalisations may not be entirely useful. To provide more insight into this, the response of T. triandra to defoliation and competition with E. curvula was investigated. Results show that veld dominated by T. triandra and few E. curvula tufts should be leniently grazed every other year with rest applied following a growing season, to allow T. triandra tufts to regrow. A non-selective grazing system should be adopted by veld managers during the growing season to lower the competitive pressure exerted on T. triandra tufts by other species and to enhance growth. These results need to be considered as a basis to understand how T. triandra swards respond on a small scale and further investigations are necessary to validate impact on natural communities. Overall, defoliation tolerance is species specific and depends on the combined effects of defoliation and competition as these affect the cumulative and morphological 2 responses of important mesic grassland species.
Responses of a South African mesic grassland to long-term nutrient enrichment and cessation of nutrient enrichment.
(2023) Zama, Naledi Zola.; Kirkman, Kevin Peter.; Magadlela, Anathi.; Mkhize, Ntuthuko Raphael.; Tedder, Michelle Jennifer.
Nitrogen and phosphorus are two of the most important limiting nutrients required for plant growth and production within grasslands. South African ecosystems are generally considered to be nutrient poor with acidic soils. Increased soil acidity exacerbated by nutrient enrichment are expected to transform South African grasslands over a long period. This PhD research project investigated the cumulative effect of 70 years of nitrogen (N) – in the form of limestone ammonium nitrate (LAN) and ammonium sulphate (ASU), phosphorus (P) – in the form of superphosphate and dolomitic lime application on the Ukulinga Grassland Nutrient Experiment (UGNE). The UGNE has been in operation since 1951 and is located on the plateau at the Ukulinga Research Farm, KwaZulu-Natal, South Africa. Here, the purpose was to evaluate potential changes in above-ground net primary productivity (ANPP), species composition, soil variables and species diversity. In the first experiment (Chapter 2), nutrient-enriched plots were abundantly dominated by taller grass species like Megathyrsus maximus rather than shorter species like Themeda triandra and Tristachya leucothrix. Limestone ammonium nitrate and P did not affect species richness as strongly as ASU, suggesting nutrient identity to be an important factor to consider. Ammonium sulphate enrichment was also associated with low soil pH, enhancing these plots' elevated aluminium (Al) concentration. Considering this key result, careful monitoring of soil pH and Al concentration is required moving forward as Al toxicity may cause a threat to sensitive plant species. To further emphasize the negative effects of soil acidification induced by N on the UGNE, the greenhouse pot-trial experiment (Chapter 5) revealed that the N-enriched soils are extremely acidic and P- deficient, further stunting nodulation development in Vachellia sieberiana saplings. Vachellia sieberiana is a common nitrogen-fixing species that encroaches mesic environments in South Africa, but its competitive ability may be reduced within grasslands under extremely acidic conditions. Phosphorus enrichment can replenish important soil nutrients and further improve soil fertility. The effects of long-term P enrichment (70 years) and short-term cessation of P enrichment (3 years) has received little to no attention in South African grasslands, therefore this was investigated in the second experiment (Chapter 3). Phosphorus is an important macro-nutrient and essential for biological nitrogen fixation (BNF). Therefore, it is expected that long-term P enrichment would modify soil properties and indirectly influence plant cover percentage and composition. What was unknown was if the cessation of the P enrichment would revert plant cover and composition towards untreated/control conditions. In chapter 3, the results revealed that changes in plant cover among the three treatments of control, P enrichment and cessation of P enrichment were consistent. Furthermore, no difference was observed among the treatments for the ANPP, species diversity indices and richness. However, the Indicator Species Analysis proved that Setaria nigrirostris and Bidens pilosa were strongly associated with the cessation of P enrichment treatment. The third experiment (Chapter 4) provided the opportunity to assess the best models that predict species compositional changes and species richness declines. Here, it was clear that the enrichment of more nutrients shifted both grass and forb species composition. For grass species composition, the shift in cover-abundance was from the shorter Tristachya leucothrix (no nutrients added) species to Aristida junciformis (only LAN added) and Megathyrsus maximus (LAN + P added). For forb species composition, Cephalaria pungens occurred in a high cover-abundance in control plots and Bidens pilosa occurred in LAN + P enriched plots. Interestingly, LAN-only and LAN + P enriched plots showed the greatest shift in both grass and forbs when compared to ASU-only, ASU + P and P-only enriched plots. In terms of overall species declines, the models presented identified the number of nutrients added and ANPP as the best predictors. Light was not identified as a significant predictor. The results provided partial evidence for the nitrogen detriment hypothesis and biomass-driven hypothesis over the niche dimension hypothesis. The key findings from all the experiments highlight that the following factors: 1) nutrient-poor soils, 2) type of nitrogen used for enrichment, 3) soil acidification and 4) high soil aluminium concentrations are important in the observed changes in the soil chemical properties, species composition, species diversity, species richness and species growth dynamics on the UGNE. The results of this research project also emphasize the importance of long-term studies in assessing if the cessation of nutrient enrichment is a strategy for ecosystem rehabilitation. It was identified here that long-term nutrient enrichment heavily modifies a mesic grassland community and alternative rehabilitation methods may need to be implemented.
Selective impacts on the vigour and mortality of Aristida junciformis (subsp. junciformis)
(2019) Scharlach, Anke.; Kirkman, Kevin Peter.; Tedder, Michelle Jennifer.; Morris, Craig Duncan.
The aim of grazing management is to maximise livestock production by maintaining high sward quality. Many southern African grasslands have become degraded allowing grass species unfavourable for livestock production, such as Aristida junciformis subsp. junciformis, to become dominant thereby reducing the available sward quality. Aristida junciformis persists once established and is remarkably understudied. Three studies were conducted to investigate the dynamics of this grass and to find focused management techniques to control and manage A. junciformis. The studies compared the impact of a high density graze (HDG), targeted herbicide application and a control on the survivorship and productivity of A. junciformis tufts, on the species and cover composition and on the post-treatment seedbank. Tufts exposed to herbicide had a lower probability of survival (p = 0.887) than those subjected to a HDG (p = 1.000) or control (p = 1.000). After treatment implementation, grazed tufts were significantly (p = 0.0018) shorter than control tufts. The tufts displayed a linear growth rate under the control (F1,8 = 456.84; P < 0.001), increasing steadily over time, and a quadratic growth rate under the HDG (F2,7 = 125.35; P < 0.001), initially growing rapidly then declining towards the end of the growing season. There was no significant difference in the height (p = 0.9481) and the aboveground net primary productivity (ANPP) (p = 0.7053) between the tufts in the control and HDG paddocks. The plant species composition (p = 0.4169) and cover composition (p = 0.4169) did not differ among treatments, however there were significant shifts in species composition (p = 0.0002) and cover composition (p = 0.0005) over time (p = 0.0002). The directional shift in species and cover composition were similar in all paddocks. Most of the grazing resistant perennial grasses, or ‘mtshiki’ species (Eragrostis curvula, E. plana, Sporobolus pyramidalis and S. africanus) and A. junciformis increased and Themeda triandra decreased over time. Total vegetation cover increased across all paddocks for all grass and forb species such that the bare soil cover was reduced from 53% to 34%. No A. junciformis seedlings emerged from the seedbank study. Most of the seedlings emerging from the seedbank (92%) and field studies (40%) were forbs. In the field study A. junciformis (30%) was second most dominant, followed by T. triandra (13%). The plant species composition of emerged seedlings did not differ among treatments (p = 0.8134). Aristida junciformis is and remains a persistent, indigenous weed that is difficult to eradicate. More research is required to prevent its establishment in areas not yet dominated but prone to its invasion and to eradicate it in veld where it is already dominant.
Vachellia sieberiana var. woodii a high-altitude encroacher: the effect of fire, frost, simulated grazing and altitude.
(2018) Russel, Jennifer Mary.; Tedder, Michelle Jennifer.
ABSTRACT There is increasing evidence that savannas and grasslands throughout the world are experiencing bush encroachment. The replacement of grassy biomes with woody biomes has serious consequences for net primary productivity. The grasslands of South Africa are not exempt from this phenomenon. Despite this, the drivers of the tree:grass dynamics are still robustly debated. In mesic and moist savannas and grasslands, the tree:grass balance appears to be maintained mainly through disturbance such as fire, frost and herbivory or a combination of disturbances. Other factors such as competition for resources may play a modifying role. High altitude grasslands are frequently within a climatic zone that would support trees, yet trees are absent. The answer as to what mechanism excludes trees from these grassy biomes continues to elude researchers. Very often low temperature is cited as a possible mechanism. Vachellia sieberiana var. woodii is a typical savanna tree which is absent from high altitudes. However, it has been encroaching into the grasslands along the escarpment of the Drakensberg, KwaZuluNatal, South Africa, over several decades, although is still excluded from the top of the escarpment. I acquired aerial photographs and satellite images covering the Van Reenen’s Pass area, north-western KwaZulu-Natal, dating from 1955 to 2015. These images confirmed that V. sieberiana was increasing in density along the escarpment, but that no V. sieberiana was present on top of the escarpment, despite the successful establishment of other tree species. The photographs and images also suggested that V. sieberiana was extending its range into higher altitudes. Because fire, frost and herbivory are generally thought to be the determinants of the structure of grasslands and savannas, I conducted field trials along the altitudinal gradient on Van Reenen’s Pass, investigating the effect of these determinants on the establishment of transplanted V. sieberiana saplings at three different altitudes. Competition for resources were briefly taken into consideration, although they were not the main thrust of the project: soil nutrients and root gaps. Soil moisture was not a concern as the area is what is defined as mesic. The transplanted saplings were smallest at the high-altitude site and largest at the low-altitude site after two growing seasons in the field. The response of the saplings to the various treatments was not consistent at the three sites. There was no response to the treatments at the highaltitude site; a significant response to fire, frost and simulated grazing at the mid-altitude sites; and a significant response to frost and simulated grazing at the low-altitude site.