- ItemThe role of the biological soil-root interface in seasonal variation of N and P recycling for Aspalathus linearis(Stellenbosch : Stellenbosch University, 2023-03) Du Toit, Elbe; Valentine, A. J. ; Kleinert, Aleysia ; Lotter, Daleen; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: Aspalathus linearis is found in the diverse Cape Floristic Region, known for its nutrient poor soil with a low pH and weather conditions that consists of dry, hot summers and cold, wet winters. Aspalathus linearis is well adapted to the soil in this region by producing cluster roots, forming symbiotic relationships with nitrogen fixing bacteria and by increasing the soil pH through OH- secretion in the roots. Few studies have investigated the seasonal variation in A. linearis physiology with regards to N and P acquisition. This study aims to determine the seasonal variation in N and P acquisition for A. linearis grown in two different soils and determine the biological activities in the soil associated with A. linearis. It is expected that an increase in N and P enzyme activity will be seen for soil with higher nutrient levels. Two soils were identified, one with higher P and C concentrations and a higher clay percentage. Nitrogen and P acquisition was determining using enzyme assays, including GS, NR, GDH-D, GDH- A, Phytase, APase, RNAse and Pi. Soil chemical and physical properties were investigated to determine the differences in the soil and microbial community diversity were investigated in the study. The differences in soil showed a great impact on soil enzyme activity on enzymes such as NR and APase, while having little effect on other soil enzymes. Seasonal growth caused a significant increase in bacterial activity and diversity for both soils identified and an increase in wetter season was found in microbial diversity. Soil conditions such as higher C and P did not show a consistent impact on plant enzyme activity. Enzymes such as GS, GDH-A, GDH-D and Pi were affected by soil conditions, however, seasons affected activity more than soil. Amino acid concentrations increased in winter, with soil in higher C and P having significantly lower concentrations than amino acid concentrations from all winter material measured. The study found that an increase in nutrients including C and P does not lead to an overall increase in N and P acquisition and that enzyme activity varied in dependency on soil conditions. Further studies should investigate the effect of different soil conditions on drought for A. linearis.
- ItemSeasonal patterns of evapotranspiration, soil moisture, rainfall, and normalized difference vegetation index in a Nama-Karoo ecosystem(Stellenbosch : Stellenbosch University, 2023-03) Gqaleni, Nosipho Onwabile; Midgley, Guy; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH SUMMARY: This study mainly focused on the South African semi-arid region, the Eastern Karoo, which is located on an ecotone between the Nama Karoo and the Grassland Biomes. The Eastern Karoo is classified as a summer rainfall area, and is known for its highly variable rainfall patterns, which vary in time, space, amount and duration. Seasonal changes in rainfall patterns can affect the hydrological cycle, environmental processes, vegetation, and other biological processes of this region. Dryland ecosystems such as in the Eastern Karoo seem to be one of the most responsive ecosystems to climate change and rising atmospheric CO2, for example, leading to globally widespread desert “greening”. Despite these important changes underway globally and locally, and the value of semi-arid systems in southern Africa, much remains to be learned about their functioning and resilience in response to biophysical drivers. The main aim of this study was to determine if the soil moisture regime, and the related vegetation evapotranspiration and production response to rainfall and other climatic drivers in the Eastern Karoo are predictably linked, and if the ecosystem shows predominantly “pulsed” (eventdriven) or seasonal functioning. The inter-biome ecotonal location was specifically selected to enhance the likelihood of gathering data representing a range of climatic conditions over a succession of years. Results suggest that while soil moisture in this system is somewhat more strongly event-driven rather than seasonally driven, particularly for shallow soil depths, system evapotranspiration appears to be under strong plant physiological control, especially via a reliance of evapotranspiration on deeper soil moisture. These findings do not support the notion that the Nama-Karoo is strongly event-driven with respect to vegetation physiological activity. They also do not support the inference that ongoing climate change will necessarily increase evapotranspiration and cause system aridification. Results for vegetation productivity showed that average normalized difference vegetation index (NDVI) is driven by evapotranspiration and air temperature, not soil moisture, while short term shifts in NDVI (DNDVI) are driven by rainfall and soil moisture conditions. Finally, in-field daily average NDVI measurements on a dominant grass species strongly indicated that shallow soil moisture is a significant driver of the C4 grass productivity response. Taken together, these results show the value of using a range of approaches as lenses in answering questions of how climate conditions affect vegetation water use and production as a whole, and that in-field sampling adds value to the mechanistic understanding gained by studying the individual components of biodiversity. Future research could usefully build on this platform to develop a fuller picture of the interplay between these components of plant biodiversity, and how they contribute to the resilience of overall productivity under climatic variability and ongoing change.
- ItemLeaf-level physiological response and carbon allocation in two eucalypt hybrids with contrasting water use strategies under drought(Stellenbosch : Stellenbosch University, 2023-03) Tonkin, Christopher; Midgley, Guy F.; Drew, David; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: Eucalypts are among the fastest growing trees on earth, with high production rates. However, the high species diversity of eucalypts has allowed this taxon to be successful across a broad climatic range, and allowed the forestry industry to match species with diverse sites around the world that may demand resilience to adverse conditions in addition to productivity. The creation of hybrids has further facilitated the selection of traits which can optimize productivity and resilience under adverse conditions. Stomatal regulation is thought to underpin this optimization, but a coherent predictive theory remains elusive, with a gain:risk framing now shaping new advances. Solutions to this challenge could help position eucalypts as a vital element of nature-based solutions to anthropogenic climatic change, addressing mitigation needs while considering optimal resource use, especially of water. This study explored drought responses of two eucalypt hybrids (Eucalyptus grandis x longirostrata (E.gl) and Corymbia henryi x torelliana (C.ht)) with contrasting water use strategies, to identify how key leaf-level functional traits (stomatal and light harvesting behaviour) influence the trade-off between growth rate and drought tolerance traits. Experimental plants were grown from rooted cuttings of each clone, and subjected to a 12-week drought period. Repeated harvesting quantified growth effect using biomass, stem height and stem diameter measures. Photosynthetic response analysis determined the differential effect of drought on the photosynthetic apparatus of the two varietals. Stomatal conductance and soil water potential were measured repeatedly while varietals were subjected to soil dry-down, and used to parameterize an optimization model to estimate hydraulic parameters for both varietals. The data showed distinct water-use strategies in the two varietals, resulting in a differential fit between responses predicted by a gain:risk approach in the two varietals. Model predictions more closely matched measurements in E.gl, which showed a more isohydric response to drought (i.e. more rapid stomatal closure) and less closely in C.ht which showed a more anisohydric response (i.e. lower but less drought- responsive stomatal conductance). Both varietals showed no damage to photosynthetic apparatus under drought conditions, revealing the resilience of light harvesting in both water use strategies. This suggests that stomatal optimization modelling approaches need to better incorporate longer term adaptive responses that affect the gain:risk trade-off. Varietal differences in growth measures reflected these leaf level functional traits, with C.ht showing a significantly higher relative increase in biomass than E.gl by the end of the experiment, regardless of water treatment. Prioritisation of stem diameter growth in C.ht may support xylem development that is more tolerant of cavitation, while the E.gl varietal prioritised apical stem growth which would likely support greater productivity under sustained ideal water supply conditions, but this accumulative effect would require more time to emerge than permitted by this experiment.
- ItemFly pollination of generalist daisies in the Greater Cape Floristic Region(Stellenbosch : Stellenbosch University, 2023-03) Vlotman, Carly Bianca; Ellis, Allan G. ; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: The majority of flowering plant species rely on insect pollen vectors for reproduction. A key factor in determining the dependence of plants on certain pollinators is understanding the specialisation of plant-pollinator interactions. Generalist pollinated plants are visited by a wide diversity of pollinators, and so do not necessarily exclude visitation from insects which are not offering effective pollination services. This ultimately results in effectiveness hierarchies where some effective pollinators are most important for ecological persistence and individual fitness of plants, while many other visitors may contribute little to reproduction. Daisies (Asteraceae) are a classic example of generalist pollinated plants due to their radially symmetrical and open access inflorescence morphology which allows visitation from a variety of insect visitors. However, in contrast to this expectation of generalism, recent work suggests that the dominant annual daisy species of the Namaqualand mass flowering displays may be strongly reliant on few fly pollinators (particularly Bombyliidae: Mariobezziinae, and Tabanidae: Rhigioglossa) for reproduction. To test whether daisies in the Western Cape are also specialised, I investigate the extent to which dominant annual daisies in the Western Cape spring flowering displays are dependent on flies as pollinators. In Chapter 2, I examine the prevalence of flies in daisy pollination across the Greater Cape Floristic Region. Based on recent studies conducted in Namaqualand, I expect daisy visitor communities to be diverse, but dominated by flies. In this chapter, I ask what the diversity and composition of flower visiting insect communities associated with spring flowering annual GCFR daisies is. By conducting surveys of pollinator communities across the Western Cape, I find that the dominant, showy annual daisy species of the spring flowering displays are visited by an average of eight visitor species, making them relatively generalized compared to the daisies of Namaqualand. Secondly, I ask how these communities are structured across daisy genera/species and across space. I find strong variation in the pollinator communities across space, which is characteristic of generalized pollination systems, and substantial overlap in visitor communities across daisy genera. Lastly, I ask if flies are consistently the most abundant visitors to daisies across the GCFR. As expected, flies were found to be one of the dominant daisy visitor groups. However, beetles (particularly Nitidulidae and Melyridae) were far more abundant visitors than flies. Surprisingly bees, that are dominant daisy pollinators globally, were virtually absent as visitors to the annual mass flowering daisies of the GCFR. In Chapter 3, I build on my findings in chapter 2, and determine the relative importance of flies and beetles for the reproduction of Dimorphotheca pluvialis, a dominant daisy species across the Greater Cape Floristic Region, by quantifying their pollination effectiveness. While previous studies have used pollinator visitation rates and/or frequencies as a measure of a pollinator’s importance, in this chapter I experimentally examine both the quantity (visitation rates) and quality (single visit pollen deposition and seed set) components to fully understand the role that these visitors play in female plant fitness. Overall, I find that Nitidulidae beetles and Mariobezziinae flies are offering similar levels of pollination effectiveness. This is certainly a surprising result since nitidulids have previously been disregarded as effective pollinators due to their low mobility, small body size and low overall hairiness. However, this study finds that nitidulids are not only active visitors, but they also carry substantial pollen loads which are ultimately deposited on receptive stigmas. Taken together, this study provides the first baseline data on the pollination systems of the foundational daisy species in the Western Cape spring flowering displays. This thesis confirms the importance of Mariobezziinae bee flies as daisy pollinators, and further highlights the importance of nitidulid beetles, which have previously been assumed to be ineffective pollinators, in the persistence of the spring-mass flowering, annual daisies of the GCFR. Further investigation into the relative importance of flies and beetles as selective agents on daisy floral traits in the GCFR is required.
- ItemThe impacts of invasive fish on ghost frog tadpoles(Stellenbosch : Stellenbosch University, 2023-03) Van Blerk, Daniel; Measey, John; Pegg, Josephine; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: The global impacts of invasive fish on native amphibians are diverse and ongoing, with documented negative impacts. The severity of impacts is expected to differ based on shared evolutionary history of amphibians with fish. Lotic systems are characterised by fast flowing water, which historically have limited upstream fish dispersal and ensured the lack of co- evolution between amphibians and fish. Water bodies associated with little to no flow are lentic systems and are often inhabited by both fish and amphibians. To date, there is a lack of comparison between the impacts of invasive fish on native amphibians between lotic and lentic systems. A comparison of the literature using a meta-analysis is conducted followed by a field-based study. A global meta-analysis was conducted to answer the question: Are the impacts of invasive fish on native amphibians are more severe in lotic than in lentic systems? Due to a paucity of data on other amphibian response variables, only amphibian behaviour could be compared between these systems. Behavioural responses show identical directions of effect between the two systems, with larger effect sizes for lotic amphibians. This does not support the expected more severe impacts on lotic systems, instead indicating a potential enhanced response to invasive fish by native lotic amphibians. The results identify a paucity of impact data in Africa and South America, with a general lack of suitable data for lotic systems. This is partly attributed due to the design of studies in most lotic systems. I recommend the use of control treatments for future lotic impact studies. I conducted a field-based study to quantify the impacts of invasive fish on ghost frog tadpoles (Family Heleophrynidae) in streams of South Africa. I used extensive field sampling of tadpole abundance, invasive fish, environmental and habitat variables to achieve the aim of investigating whether the preliminary negative effects of invasive fish on ghost frog tadpoles are reflected across a broader scale. Generalised linear-mixed models highlight a significant negative effect of invasive fish on ghost frog tadpoles, with further water quality parameters and physical habitat features playing significant roles impacting tadpole abundance. This thesis is the first to compare lentic and lotic amphibian responses to invasive fish and answers the question of whether there is a differential effect on amphibian behaviour in response to invasive fish, whilst elucidating the global lack of adequate lotic data. The second chapter not only represents the type of study needed to contribute towards lotic amphibian data but is also the largest scale at which the impacts of invasive fish on ghost frog tadpoles have been quantified. The results reported in this thesis contribute towards the motivation for global studies on the impact of invasive fish on native amphibians; and will aid in future conservation decisions regarding South Africa’s endemic lotic amphibians. The results of this thesis will likely have implication towards lotic amphibian conservation in South Africa and the world.