Browsing by Author "Buwa, Ziphokazi Siyasanga"

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    The interaction between a keystone plant species and its dominant epiphyte on Marion Island : climate change implications
    (Stellenbosch : Stellenbosch University, 2007-12) Buwa, Ziphokazi Siyasanga; McGeoch, M. A.; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Climate has shown some remarkable changes over the past century, especially at the polar and sub-polar regions. Southern Ocean Islands provide good models for studies related to climate change effects, since effects may be evident in the short term and may also be clearer. Marion Island is an example of such a system with a harsh abiotic environment, and low species richness often vulnerable to change. Climate change is predicted, and also reported, to have biological consequences on plant communities, affecting the phenology, morphology, and the interaction between individuals and species. This study examines the association between the keystone plant species, Azorella selago Hook. (Apiaceace), and its dominant epiphyte Agrostis magellanica Lam. (Poaceae). Two complimentary approaches were used, one observational and the other experimental. The main objective for the observational study was to quantify bioticallyrelevant microclimate temperature, as well as the morphology, epiphyte load and phenology of A. selago at three different altitude sites on Marion Island. This provided information on baseline variation for understanding specific variability in plant response to the experimental part of this study, against which future patterns arising from biological monitoring can be compared. Studying plants at different altitudes provides a possible analogue for temperature-related climate change consequences for the ecology of A. selago, and its interaction with A. magellanica. The microclimate temperature associated with A. selago differed between the three sites examined. This difference was related to local topographic conditions and altitude differences. Cushion size differed distinctively between the three altitude sites, with this difference related to environmental heterogeneity such as differences in age and substrate structure. Azorella selago annual growth rate was estimated through stem length and the number of leaves on both exposed and shaded stems. Within-site variability, as well as epiphyte cover were found to be the contributing factors on A. selago annual growth rate. This highlights the importance of site-specificity when estimating growth rate within and between different altitude sites. Leaf characteristics differed between the sites, with this attributed to local habitat conditions, such as topography, as well as epiphyte cover. As expected, the difference in leaf size between exposed and shaded leaves demonstrated a larger specific leaf area on leaves shaded by A. magellanica. Agrostis magellanica abundance and density were altitudinally related, with A. selago demonstrating facilitation effects on A. magellanica. The trend shown in this study suggests that in spite of general facilitative effect of A. selago on A. magellanica towards higher altitudes, the abiotic environmental threshold for A. magellanica occurs at lower altitudes than it does for A. selago. Phenological differences were also apparent between the three sites. The objectives of the experimental part of this study were to quantify the effect of the dominant epiphyte, A. magellanica on biotically-relevant microclimatic temperatures, as well as on the phenology and physical condition of A. selago. Different treatments were applied to cushions at the three altitude sites to examine the shading effect of epiphytic A. magellanica on cushion plants, as well as the effect of treatment-related disturbance. Azorella selago microclimate temperature showed no significant difference between treatments, suggesting that on average epiphytic A. magellanica cover has no effect on cushion microclimate temperature. The percentage of flower budding and flowering of A. selago was negatively related to epiphyte cover. Cushion vitality was also responsive to epiphyte cover, with higher vitality scores on low grass covered cushions than on high grass cushions. This shows that A. magellanica competes with A. selago, while A. selago facilitates A. magellanica. Heavy epiphyte numbers impose negative effects on A. selago vegetative and reproductive performance, as well as cushion vitality. Therefore, the results of this research show that the vegetative and reproductive performance of A. selago and cushion vitality are likely to be negatively affected under ongoing climate change on Marion Island if this brings about heavier epiphyte loads on this keystone cushion plant species.