Patterns of plant diversity and their management across South African Rangelands

Doctoral Thesis


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University of Cape Town

Forbs dominate southern Africa's rangeland plant diversity, but have been ignored in favour of grasses that are important for livestock production. To facilitate better conservation management I examined the interaction between patterns of diversity, forb recruitment and disturbance effects across the mesic to semi-arid rangelands east of the escarpment in southern Africa. Results were compared to a well studied North American tallgrass prairie, Konza. Sampled between 1 m2 and 1000 m2 across 25 sites, species and family richness increased with mean annual precipitation up to â2000 m a.s.l., above which cooler temperatures accounted for a decline. Forb functional types ranged from persistent (long-lived, resprouters) in the fire-prone mesic rangelands to more recruiting (short-lived, reseeders) taxa in the drought-prone semi-arid areas. Mesic rangelands should therefore tolerate frequent, low intensity defoliation, and semiarid floras be better suited to infrequent intense disturbances. Seedling recruitment matched richness patterns, primarily increasing with precipitation. Recruitment was highest in the fire treatment at the three mesic sites, but proportionately higher in the animal digging and drought treatments at the three semi-arid sites. Seedling composition differed from the adult sward, suggesting that some species may not recolonise transformed lands. Despite predictions of the Milchunas et al. (1988) and Olff and Ritchie (1998) grazing models, exclusion of large mammalian grazers from 40 m x 40 m plots did not appear to impact forb diversity. Instead, grazing effects sampled in 100 x 1m' quadrats per treatment, across ten sites, were overridden by the mesic to semi-arid gradient along which the sites were orientated. Species richness declined under heavy communal cattle grazing along fence-line contrasts at two mesic sites, with soft-leaved monocot (orbs being replaced by short-lived divot weeds not found inside the adjacent lightly grazed nature reserves. Marked differences in the diversity and disturbance responses of tallgrass prairie and southern African rangelands suggest that management strategies cannot be extrapolated from similar ecosystems around the world.