Ecological significance of variation in Themeda triandra Forsk : a case of intra-specific divergence in life history strategies?

Master Thesis


Permanent link to this Item
Journal Title
Link to Journal
Journal ISSN
Volume Title

University of Cape Town

Rangelands are a important resource for commercial, subsistence and game enterprises. However, their variabiligy poses a challenge for their effective management. This thesis was motivated by the need to contribute to a functional classification system for rangelands to guide managers in a way that adequately, yet simply, addresses ecological variation in South African rangelands. The approach was to investigate life history characteristics of rangelands at four sites representing climate extremes. These differed in the amount, and predictability, of rainfall and the incidence of frost. The approach was novel in that a single species, Themeda triandra, was used in an attempt to test predictions on sets of key traits associated with the different rainfall regimes. The underlying hypothesis was that density-dependent processes would be most important in mesic grasslands, requiring frequent defoliation by fire or grazing to maintain a productive Themeda sward. Climate variabiligy would be the most important determinant of grass growth in semi-arid grasslands. Vegetive and reproductive traits of Themeda would be expected to have diverged in response to these different selection pressures in the different populations. Observations were made on reproductive and vegetative traits in field populations in the four study areas and on plants grown from clonal material and seeds in a glasshouse to determine key axes of variation. Population responses to varying light and moisture levels were compared in a glasshouse experiment simulating the effects of different moisture regimes and competitive environments of source populations. Fecundity, seedbank characteristics, seedling numbers and population size structure were compared in field and glasshouse experiments. Production characteristics were compared in a clipping experiment conducted in the field. Results showed that different populations of Themeda exhibit divergent life history characteristics associated with different rainfall and temperature regimes. These differences are evident in the morphology, allocation patterns, leaf traits, germination biology and phenology of populations. The ecological significance and implications of trait divergence were clearly evident in demographic characteristics of the populations, in experimental responses to watering and shading, and in divergent growth responses to clipping experiments. The results were consistent with classic life history theory. In low, unpredictable rainfall regions, populations have evolved r-selected strategies whereas populations from high and predictable rainfall are characterized by K-selected traits. However the incidence of frost is also a factor influencing growth form divergence.

Includes bibliographical references (leaves 132-143).