Browsing by Author "Tedder, Michelle Jennifer."

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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.
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 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 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.
The influence of drought and abiotic drivers on a mesic grassland.
(2024) Maziko, Yonela.; Tedder, Michelle Jennifer.; Kirkman, Kevin Peter.; Everson, Theresa Mary.
A significant percentage of southern African grasslands have undergone deteriorative transformation due to climate-related changes. This PhD research study investigated the impacts of drought on species composition dynamics in a South African mesic grassland with data collected over a period of 3 years. Chapter 3 investigated the effects of a short-term drought on plant species composition under different burning management regimes on the long-term veld burning experiment at Ukulinga Research Farm, University of KwaZulu Natal, South Africa. The results showed that annual burning produced higher species diversity and richness while no burning promoted the development of woody plants such as Vachellia nilotica and forbs. The short-term drought did not have a significant impact on the species composition of the grassland. Chapter 4 assessed the influence of drought on grass-on-grass interactions in pot trials at the Neil Tainton Arboretum greenhouse and the study found that the presence of competition reduced phytometer biomass, but at varying levels with different species. Broad-leaved species had a stronger competitive effect over narrow-leaved species as they reduced the number of tillers, biomass production and tuft size of narrow-leaved species. The competitive strength within broad-leaf species was species-specific, with Megathrysus maximus being more competitive than Cymbopogon excavatus. Palatable species had a dominant competitive effect over unpalatable species without exposure to defoliation and Tristachya leucothrix had a stronger competitive effect over Themeda triandra. Drought did not have any significant impact on final dry biomass, but significantly reduced the tiller numbers for palatable species as well as tiller numbers and tuft circumference for broad-leaved species. Chapter 5 focused on how the grass sward composition, biomass production as well as root biomass and length were affected by different moisture levels, including drought at 50% rainfall reduction on the Drought Net Experiment at Ukulinga Research Farm. Species diversity, richness and aboveground biomass were significantly affected by varying weather patterns, while all were not changed by moisture manipulations. Species diversity and richness increased in 2021/2022 while the biomass was greater in 2020/2021. On the contrary, high moisture levels reduced root biomass and this can be attributed to resources being allocated to greater above-ground biomass during moisture availability. Greater root biomass was found in the top 16cm of soil. Chapter 6 assessed the influence of nutrient addition (nitrogen, phosphorus, potassium) on species composition, diversity and biomass production under different moisture levels including drought (50% rainfall reduction) over 3 years on the Drought Net Experiment at Ukulinga Research Farm. Interannual climate variability significantly increased species richness and diversity by 2021/2022. An interaction between fertilization and years shows a significant difference in response to interannual climate variability across fertilizer levels. Annual weather patterns refer to the differences in each 12-month cycle, centred around the growing season The unfertilized plots were unaffected by annual climate fluctuations and the fertilized plots showed significantly greater diversity in 2021/2022, the growing season with the highest rainfall. Species evenness fluctuated annually in the high and ambient rainfall treatments while it remained unchanged in the drought treatment. Tristachya leucothrix, Themeda triandra and Cymbopogon excavatus were abundant in both unfertilized and fertilized sites but most species increased in abundance in the fertilized areas. High moisture levels significantly increased plant biomass production while drought had the opposite effect. The findings from this study have shown that interannual climate variability and fertilization had a significant impact on species composition and productivity. The length of drought is a key factor that determines if a drought causes significant impacts on a plant community and long-term experimental applications such as fire, have more pronounced implications on a grassland community. Lastly, the competitive effect of plants is species-specific, and influenced by plant morphological traits. The results of this research study emphasize the importance of long-term experimental research for varying levels of drought and fire on mesic grasslands.
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.