The structure of a grass community in Burkea africana savanna during recovery from fire

Three adjacent sites, differing in their fire histories, in the Eragrostis pallens–Digitaria eriantha association of the Burkea africana savanna at Nylsvley, northern Transvaal were compared. One site has been burned annually while the second and third sites had last been burned 5 and at least 12years previously. Grass species richness was highest on the annual burn and decreased with an increasing period of protection from fire. Total mean basal areas of the grasses increased by 78% from the annual burn to the protected site. The dominant grasses in this association, Eragrostis pallens Hackel and Digitaria eriantha Steudel, increased in abundance with protection from fire until they comprised approximately 97% of the total number of individuals and total mean basal area found at the protected site. Annual burning reduced the density and average diameter of individuals of E. pallens, allowing opportunistic species to colonize this association. Significantly more individuals of E. pallens were found in the 5-year burn. Its numbers declined in the protected site, presumably as a result of self-thinning. The number of tillers produced, the number that flowered and flowering phenology of E. pallens was similar in populations on the annual burn and in the protected site. On the burnt area after 5years of protection, plants of this species produced significantly fewer tillers and the time of flowering was delayed compared with other treatments. Significantly more tillers of E. pallens were grazed upon on the sites protected for 5 and 12years. In contrast, D. eriantha plants showed no increase in size from the annual to the protected site but their density did increase

Competitive interactions between the perennial shrub Leipoldtia constricta and an annual forb, Gorteria diffusa

Competitive interactions were studied between individuals of the perennial shrub, Leipoldtia constricta L. Bol., and an annual forb, Gorteria diffusa Thunb. in the winter rainfall region of the Karoo using nearest-neighbour analysis. Strong competitive interactions were measured between intraspecific and interspecific nearest-neighbour pairs. In particular, the size and reproductive potential of the annual forb was reduced whenever it established next to the perennial shrub. This reduction was greater than that occurring intraspecifically among individuals of the annual forb. Despite this effect, more annuals established close to the perennial shrub, presumably as the result of the latter’s multiple-stemmed growth habit which traps soil at its base providing a ‘safe site’ for germination and establishment