Abstract:
Natural and anthropogenic environmental variables pressures exert fitness pressures on our flora and vegetation and necessitate an understanding of the mechanisms of plant responses to those pressures. Overgrazing and desertification are examples of the stressors that affect plant community composition, on both large and small scales. In order to investigate the plant community response to these stressors it has been shown that it is more practical and informative to group plant species according to their functional characteristics into functional types than attempting to predict plant response solely based on previous range distribution data. The present study involved a survey of the dunefield vegetation of several privately owned farms as well as a communal farm in the southern Kalahari. The farms were subjectively chosen to represent a gradient from underutilized to overutilized. Plant community surveys were conducted on the farms and the dominant plant species were sampled and researched for selected functional traits to explore possible relationships between traits and grazing pressure. Ordination techniques were used to identify plant functional grazing response types. The identification of plant functional types through easy measurable traits will be a great aid in modelling and longterm environmental monitoring approaches. Of the three dunefield habitats, the dune street habitat was identified as the most vulnerable to overgrazing. A reduction in cumulative cover and an increase in bare surface cover indicated increased grazing pressure in the dune crest habitat. In the dune streets shrub cover and annual grass cover increased when veld was overgrazed. Functional analysis of 112 Kalahari plant species for a set of 47 plant traits showed that species clustering were strongly related to vegetative characteristics. The emerging clusters were strongly associated with major life forms, which comprised phanerophytes of different size classes, geophytes and herbaceous perennials, as well as herbaceous annuals. These clusters were identifiable even with the inclusion of reproductive traits. Selected vegetative traits are good indicators of plant response to land-use intensity since these traits lend competitive advantages to plant persistence under utilisation pressure. Plant height, spinescence, specific leaf area, leaf dry matter content, leaf sclerophylly and stem density were found to be examples of such traits. Phanerophyte and chamaephyte dominated functional groups were mostly associated with lower range condition and hemicryptophyte dominated groups with higher range condition, whereas herbaceous therophyte dominated groups did not show any specific association.
