A study of the interactions between C₃ and C₄ grasses on Signal Hill, Cape Town

Master Thesis


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University of Cape Town

Current concerns with reference to global climate change can help us to focus on the competitive interaction of grasses with different photosynthetic pathways. The dominance of eight different common grass species occurring on Signal Hill was investigated, and three species which occurred in localized dense stands were suitable for detailed investigation. Distribution patterns of the three selected grass species, Hyparrhenia hirta (L.) Stapf (C₄), Merxmuellera disticha (Nees) Conert (C₃) and Themeda triandra Forsskal (C₄) were interpreted using a dispersion index (R). Regularity in dispersion of individual grasses was quite common, whilst aggregated and random dispersion was not. Potential competitive interactions between those grasses (perennial) on the east- and west-facing slopes of Signal Hill ·were then described. Using nearest-neighbour analysis it was possible to show that both inter- and intra-specific competition was occurring between the grass species investigated. Pattern analysis was supported by removal experiments and it was possible to infer that the three grass species are competing for water and space. The measurements of carbon dioxide fixation rates under greenhouse conditions demonstrated that C₄ grasses were more competitive, relative to C₃ grass and responded more strongly to changes in soil moisture status. Species with the C₄ pathway showed substantial increase in photosynthetic rate in response to an increase in soil moisture content whilst a C₃ grass' carbon dioxide assimilation rate at those moisture content was relatively lower. Seedling transplant experiments showed that monospecificity of patches could chiefly be attributed to familial clumping, limited seed dispersal, and vegetative reproduction. Phenological aspects like growth, production, and reproduction in terms of flowering tillers of neighboured and non-neighboured individuals, and of individuals one year and two years after fire, were studied to assess the importance of competitive interactions and fire. The results of this study posed the following question: In the light of the global warming effect, which grass species (representing C₃ and C₄ pathways) are likely to prevail in different parts of the world?

Bibliography: pages 116-141.