Masters Degrees (Botany and Zoology)

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Browsing Masters Degrees (Botany and Zoology) by Subject "Acacia -- Effect of climatic changes on"

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    Familiar mutualist interactions during biological invasions: Consequences for invaders and impacts on natives.
    (Stellenbosch : Stellenbosch University, 2020-12) Warrington, Staci; Le Roux, Johannes Jacobus; Ellis, Allan G.; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Mutualisms are important for plant survival but are easily disrupted when plants are introduced into new environments. This acts as a strong barrier to establishment success. However, non-native plants can form novel mutualisms with resident species or, when co-introduced, can maintain familiar associations. Plants that co-invade ecosystems with their mutualists usually impact native species more severely than invasive plants that form novel associations. Invasive Australian acacias (genus Acacia Mill.) make use of both mutualist reassembly pathways to facilitate their invasion success in nutrient poor environments. These acacias frequently alter (a)biotic soil conditions, e.g. via soil nutrient enrichment, leading to positive feedbacks. The first aim of this thesis was to determine the relative contributions of novel vs familiar rhizobial associations to the establishment success of Acacia saligna across different soils in South Africa’s Core Cape Subregion. As a second aim, I also investigated whether leaf litter of Acacia saligna benefits its seedlings’ establishment under competition with a native legume, and how this may act synergistically with familiar rhizobial associations to improve the competitive ability of the species. For the first aim, I grew A. saligna and the native legume, Psoralea pinnata, in a glasshouse experiment in five different CCR soils under two inoculum addition treatments. Australian bradyrhizobia isolated from acacias were used as inocula. Various performance measures were recorded and next-generation sequencing (NGS) barcoding methods used to identify rhizobia associating with the two legumes across treatments. For both legumes, few significant inoculum effects were found for any performance measures. Plant performance responded more strongly to soil type. Barcoding revealed that A. saligna and P. pinnata were predominantly associating with Australian Bradyrhizobium and native Mesorhizobium, respectively, irrespective of treatment x soil combination. For the second aim, I grew A. saligna and P. pinnata together in pots containing Psoralea-conditioned soils and exposed them to Australian inoculum and acacia topsoil (which represented acacia leaf litter) treatments in a fully factorial design. I incorporated data for seedlings grown in the same soil from the glasshouse experiment discussed under aim one to compare performances when grown alone vs in mixture so as to determine how Australian bradyrhizobia may facilitate acacia performance. I also compared the performances of each legume grown together in mixture between the four inoculum and topsoil treatment combinations. Overall, I found no significant inoculum or topsoil effects on the performance of either legume. NGS revealed similar rhizobial associations as in the first experiment. Overall, this thesis revealed that both legume species formed familiar associations regardless of Acacia-Bradyrhizobium cointroductions or acacia-mediated positive feedbacks. This suggests that P. pinnata may be valuable for restoration projects after acacia clearing. The presence of Australian bradyrhizobia in all soils (including uninoculated soils) also suggests that these strains are already present and proliferating within the CCR, and can thereby facilitate future Australian acacia invasions as mutualist absence may no longer be a barrier to acacia establishment success.