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The process of thicket encroachment in semi-arid savanna: community patterns and biotic interactions

  • Authors: Nell, Rhys
  • Date: 2022-10-14
  • Subjects: Scrub encroachment , Savanna ecology , Biotic interaction , Plant nutrients , Plant-water relationships
  • Language: English
  • Type: Academic theses , Master's theses , text
  • Identifier: http://hdl.handle.net/10962/364966 , vital:65666
  • Description: Bush encroachment in savannas is widespread in South Africa and is concerning, from both socio-economic and conservation viewpoints, as it affects ecosystem services, functioning and productivity. This phenomenon depends on multiple factors such as history, vegetation, management and environmental conditions, and their interplay. Encroachment into savannas has been relatively well-documented, however understanding of the different roles of tree-tree interactions between species that occur during this process is still limited. This includes the interactions causing spatial patterning, or how interactions and outcomes change over time in terms of encroachment succession from open savanna to closed-canopy thicket. The main objectives of this research are to document thicket establishment in a savanna ecosystem and consider the ecological roles of the key woody species and the abiotic properties of their micro-sites. Determining interactive effects of species co-occurrence is critical to understanding or predicting patterns and changes in biodiversity, nutrient distribution and available water resources. It is also imperative in determining correct and effective land management practices, particularly for reducing bush encroachment and its negative effect on rangelands. All data were collected on Endwell farm, located in the Smaldeel region of the Eastern Cape, South Africa. Endwell farm is a semi-arid savanna with a mean annual rainfall of 730 mm. First, I examine and describe the thicket encroachment process by exploring the associations between species and their size classes in the field. This was done by using plot-based belt transects and looking at changes in species size-class compositions from early to late successional stages. Association rules (market basket) analysis was used to identify the most common species size-class association patterns. The association between the savanna tree Vachellia karroo and the thicket pioneer Scutia myrtina was the most prevalent at all stages, with V. karroo being central to all associations in the first stage of encroachment; during later stages of encroachment, associations shift to incorporating other thicket pioneer species. The demography and clump formation of S. myrtina was strongly linked to associations with V. karroo to initiate bush clump formation. Results suggest that mature V. karroo facilitate the establishment and growth of S. myrtina. These two species were the focus of more detailed investigations to explore the nature and magnitude of their interspecific interactions. I then examined the effects of pairwise tree interactions between V. karroo and S. myrtina on soil and leaf nutrient content. I measureddifferences betweeninter-canopy and sub-canopy soil nutrient content, and the effect of associations on plant leaf nutrients, between pair-size combinations and individual controls. Results confirmed that pair-size tree interactions affected both soil nutrient and leaf nutrient content. All individuals increased soil K, N and organic C in the sub-canopy, while association with V. karrooincreased S. myrtinafoliar N, Pand K. In contrast, association with S. myrtinaloweredV. karroofoliar N, P and K. Small S. myrtina individuals werefound to benefit most from establishing and growing next to a large V. karroo individual, through mechanisms affecting soil and foliar nutrients. Scutia myrtina individuals establishing in association with smaller size classes of V. karroo showed no significant effects. I tested for positive and negative effects of pairwise tree interactions between Vachellia karroo and Scutia myrtina on available soil water and plant water potential (Ψ). This was done by looking at differences betweeninter-canopy and sub-canopy soil moisture and bulk density and associations on plant water stress (pre-dawn and mid-day leaf Ψ), between pair-size combinations and individual controls. I also selectively removed large V. karroo individuals from pairs to confirm the effects of competition andfacilitation. Similar to other studies, results confirmed positive and negative effects of pairwise tree interactions. Small S. myrtina individuals weremost facilitated by establishing and growing up next to a large V. karroo individual, through mechanisms affecting soil water content, bulk density and leaf Ψ. Scutia myrtina establishing in association with other size classes of V. karroo were much less facilitated, showing no significant effects. In contrast, large S. myrtina showed competitive interactions with V. karroo. , Thesis (MSc) -- Faculty of Science, Botany, 2022
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  • Date Issued: 2022-10-14

Modelling plant water use of the grassland and thicket biomes in the Eastern Cape, South Africa: towards an improved understanding of the impact of invasive alien plants on soil chemistry, biomass production and evapotranspiration

  • Authors: Gwate, Onalenna
  • Date: 2018
  • Subjects: Grasslands -- South Africa -- Eastern Cape , Invasive plants -- South Africa -- Eastern Cape , Rangelands -- South Africa -- Eastern Cape , Range ecology-- South Africa -- Eastern Cape , Rangelands -- Water-supply , Rangelands -- Weed control , Evapotranspiration , Plant-water relationships
  • Language: English
  • Type: text , Thesis , Doctoral , PhD
  • Identifier: http://hdl.handle.net/10962/54800 , vital:26617
  • Description: It is imperative to understand the strong coupling between the carbon capture process and water use to sustainably manage rangelands. Woody encroachment is undermining rangelands grass production. Evapotranspiration (ET) highlights the links between ecosystem carbon capture process and water use. It forms the biggest flux of the hydrological cycle after precipitation yet it is not well understood. The Grassland and the Albany Thicket (AT) biomes in the Eastern Cape, South Africa, provide an interesting space to study the dynamics in rangelands biomass production and the associated water use. Therefore, the main purpose of this study was to contribute towards management of rangelands by understanding the dynamics in rangeland grass production and water use. To achieve this aim, the impact of Acacia mearnsii, an invasive alien plant, on soil chemical properties and rangelands grass production was investigated. This was achieved by analysing the biophysical attributes of A. mearnsii as they related to grass production. Secondly, selected soil variables that could be used as a prognosis for landscape recovery or deterioration were evaluated. In addition, aboveground grass biomass was measured in areas cleared of A. mearnsii and regression equations were prepared to help model aboveground grass biomass in areas cleared of A. mearnsi. The thesis also explored dynamics in water vapour and energy fluxes in these two biomes using an eddy covariance system. Consequently, water vapour and energy fluxes were evaluated in order to understand landscape water use and energy partitioning in the landscape. The study also tested the application of Penman-Monteith equation based algorithms for estimating ET with micrometeorological techniques used for validation. Pursuant to this, the Penman- Monteith-Leuning (PML) and Penman-Monteith-Palmer (PMP) equations were applied. In addition, some effort was devoted to improving the estimates of ET from the PMP by incorporating a direct soil evaporation component. Finally, the influence of local changes in catchment characteristics on ET was explored through the application of a variant of the Budyko framework and investigating dynamics in the evaporative index as well as applying tests for trends and shifts on ET and rainfall data to detect changes in mean quaternary catchment rainfall and ET. Results revealed that A. mearnsii affected soil chemical properties and impaired grass production in rangelands. Hence, thinning of canopies provided an optimal solution for enhanced landscape water use to sequestrate carbon, provide shade, grazing, and also wood fuel. It was also shown that across sites, ET was water limited since differences between reference ET and actual ET were large. ET was largely sensitive to vapour pressure deficit and surface conductance than to net radiation, indicating that the canopies were strongly coupled with the boundary layer. Rangeland ET was successfully simulated and evaporation from the soil was the dominant flux, hence there is scope for reducing the so-called ‘unproductive’ water use. Further, it was shown that the PML was better able to simulate ET compared to the PMP model as revealed by different model evaluation metrics such as the root mean square error, absolute mean square error and the root mean square observations standard deviation ratio. The incorporation of a soil evaporation component in the PMP model improved estimates of ET as revealed by the root mean square error. The results also indicated that both the catchment parameter (w) and the evaporative index were important in highlighting the impacts of land cover change on ET. It was also shown that, despite changes in the local environment such as catchment characteristics, global forces also affected ET at a local scale. Overall, the study demonstrated that combining remote sensing and ground based observations was important to better understand rangeland grass production and water use dynamics.
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  • Date Issued: 2018

Effects of removing Acacia Mearnsii on the water table, soil and vegetation properties in the Tsomo Valley of the Eastern Cape Province, South Africa

Drought responses of C3 and C4 (NADP-ME) Panicoid grasses

  • Authors: Frole, Kristen Marie
  • Date: 2008
  • Subjects: Botany -- Research , Grasses -- Physiology -- South Africa , Grasses -- Effect of drought on , Grasses -- Drought tolerance , Plant-water relationships
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:4193 , http://hdl.handle.net/10962/d1003762 , Botany -- Research , Grasses -- Physiology -- South Africa , Grasses -- Effect of drought on , Grasses -- Drought tolerance , Plant-water relationships
  • Description: The success of C₄ plants lies in their ability to concentrate CO₂ at the site of Rubisco thereby conferring greater efficiencies of light, water and nitrogen. Such characteristics should advantage C₄ plants in arid, hot environments. However, not all C₄ subtypes are drought tolerant. The relative abundance of NADP-ME species declines with increasing aridity. Furthermore, selected species have been demonstrated as being susceptible to severe drought showing metabolic limitations of photosynthesis. However there is a lack of phylogenetic control with many of these studies. The aims of this study were to determine whether the NADP-ME subtype was inherently susceptible to drought by comparing six closely related C₃ and C₄ (NADP-ME) Panicoid grasses. Gas exchange measurements were made during a natural rainless period and a controlled drought / rewatering event. Prior to water stress, the C₄ species had higher assimilation rates (A), and water use efficiencies (WUE[subscript leaf]) than the C₃ species, while transpiration rates (E) and stomatal conductances (g[subscript s]) were similar. At low soil water content, the C₃ species reduced gs by a greater extent than the C₄ species, which maintained higher E during the driest periods. The C₄ species showed proportionally greater reductions in A than the C₃ species and hence lost their WUE[subscript leaf] and photosynthetic advantage. CO₂ response curves showed that metabolic limitation was responsible for a greater decrease in A in the C₄ type than the C₃ type during progressive drought. Upon re-watering, photosynthetic recovery was quicker in the C species than the C₄ species. Results from whole plant measurements showed that the C₄ type had a significant whole plant water use efficiency advantage over the C₃ type under well-watered conditions that was lost during severe drought due to a greater loss of leaf area through leaf mortality rather than reductions in plant level transpiration rates. The C₃ type had xylem characteristics that enhanced water-conducting efficiency, but made them vulnerable to drought. This is in contrast to the safer xylem qualities of the C₄ type, which permitted the endurance of more negative leaf water potentials than the C₃ type during low soil water content. Thus, the vulnerability of photosynthesis to severe drought in NADP-ME species potentially explains why NADP-ME species abundance around the world decreases with decreasing rainfall.
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  • Date Issued: 2008

Water requirements and distribution of Ammophila arenaria and Scaevola plumieri on South African coastal dunes

  • Authors: Peter, Craig Ingram
  • Date: 2000
  • Subjects: Scaevola plumieri , Sand dune plants , Sand dune planting , Plants -- Transpiration , Sandworts , Plant-water relationships , Evapotranspiration , Plants, Effect of heat on
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:4219 , http://hdl.handle.net/10962/d1003788 , Scaevola plumieri , Sand dune plants , Sand dune planting , Plants -- Transpiration , Sandworts , Plant-water relationships , Evapotranspiration , Plants, Effect of heat on
  • Description: Phenomenological models are presented which predicts transpiration rates (E) of individual leaves of Scaevola plumieri, an indigenous dune pioneer, and Ammophila arenaria, an exotic grass species introduced to stabilise mobile sand. In both cases E is predictably related to atmospheric vapour pressure deficit (VPD). VPD is calculated from measurements of ambient temperature and humidity, hence, where these two environmental variables are known, E can be calculated. Possible physiological reasons for the relationships of E to VPD in both species are discussed. Scaling from measurements of E at the leaf level to the canopy level is achieved by summing the leaf area of the canopy in question. E is predicted for the entire canopy leaf area by extrapolation to this larger leaf area. Predicted transpiration rates of individual shoot within the canopy were tested gravimetrically and shown to be accurate in the case of S. plumieri, but less so in the case of A. arenaria. Using this model, the amount of water used by a known area of sand dune is shown to be less than the rainfall input in the case of S. plumieri in wet and dry years. The water use of A. arenaria exceeds rainfall in the low-rainfall year of 1995, while in 1998 rainfall input is slightly higher than water extraction by the plants. Using a geographic information system (GIS), regional maps (surfaces) of transpiration were calculated from surfaces of mean monthly temperature and mean monthly relative humidity. Monthly surfaces of transpiration were subtracted from the monthly median rainfall to produce a surface of mean monthly water deficit. Areas of water surpluses along the coast correspond with the recorded distribution of both species in the seasons that the plants are most actively growing and reproducing. This suggests that unfavourable water availability during these two species growth periods limit their distributions along the coast. In addition to unfavourable water deficits, additional climatic variables that may be important in limiting the distribution of these two species were investigated using a discriminant function analysis.
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  • Date Issued: 2000
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