Seed Production of Species in the Highveld Secondary Succession

Abstract

After cessation of cropping in the Trachypogon-other species grassland of the South African Highveld, the usual secondary succession was described as follows by Rose-Innes (1939). (1) The ruderal or weed stage, dominated by species such as Erigeron canadensis.t Grasses such as Eleusine indica can dominate with high soil fertility (Davidson 1962). (2) The primary grass stage, usually dominant 2-3 years after abandonment. Characteristic species are the tussock grass Eragrostis curvula and the stoloniferous Cynodon dactylon. (3) The secondary grass stage, usually dominant some 10 years after abandonment. The characteristic species is the tussock grass Hyparrhenia hirta. (4) The sub-climax grassland. Rose-Innes suggested that after 30 or so years the seral areas would have a botanical composition similar to undisturbed Trachypogon-other species grassland. The undisturbed grassland will be referred to as sub-climax grassland, a term used by previous workers (e.g. Davidson 1962). Characteristic species are T. spicatus and Tristachya hispida. In almost all seral communities there are some species from each of the first three stages, although one particular stage may be dominant. Seral grasses are also found scattered throughout sub-climax grassland. Studies by Rose-Innes (1939) showed that seed viability of species in the Highveld succession declined as succession advanced, but no studies have been made of the seed output of different seral stages of the Highveld succession. Salisbury (1942) showed that in general seed output of dicotyledons decreased as species came from later seral stages, but no reports have been encountered which have followed seed output through succession in one particular locality. Investigations of seed production were made during the 1961-62 growing season on the Frankenwald Research Station of the University of the Witwatersrand. The station is on latitude 26?S and longitude 28?E. The soils are acid and light textured. The annual rainfall is approximately 30 in. (760 mm) per year and is virtually confined to the summer months.