Mean annual rainfall and the population growth (1993-2002) and potential distribution of Themeda triandra in the shrubby vegetation of the southern Nama-karoo

We investigated the relationship between mean annual rainfall (MAR) and the population growth (1993-2002) of Themeda triandra, to test the hypothesis that annual rainfall is a factor controlling the potential distribution of this palatable, perennial bunchgrass on the shrub-dominated plains of the southern Nama-karoo. Seedlings of T. triandra were raised from seed collected in the northeastern portion of the Eastern Mixed Nama Karoo where the long-term MAR is 448mm. One hundred seedlings were transplanted in a shrubland in the southwestern portion of the Eastern Mixed Nama Karoo approximately 250km from the site of seed collection. The long-term MAR at this transplant site is 322mm and the site was open to grazers. Another 150 seedlings were transplanted in a shrubland of the Great Nama Karoo located a further 250km to the southwest. The long-term MAR at this site is 156mm and two-thirds of the seedlings were transplanted in a plot closed to grazers while the remaining transplants were exposed to grazers. Births and deaths of the grasses were monitored for nine years, and nine years of annual rainfall were obtained from climatic records. The T. triandra population in the southwestern portion of the Eastern Mixed Nama Karoo grew exponentially when MAR was 316mm and grazing was permitted. In contrast, population growth in both grazed and ungrazed treatments in the Great Nama Karoo site was zero when MAR was 230mm. These results show a close relationship between MAR and the population growth of T. triandra and suggest that the potential distribution of the grass in the Eastern Mixed Nama Karoo is larger than its present distribution.