The effect of fire on the soil seed bank of a semi-arid grassland in South Africa

During the dormant winter period the semi-arid grassland areas of South Africa are characterised by planned and unplanned fires, either by lightning or negligence by man. The influence of burning (head and back fire) on the germinability of grass species in the soil seed bank was quantified in the greenhouse over the 2000/01 to 2001/02 growing seasons for a semi-arid grassland. The behaviour of the head and back fires was also monitored. Soil seed bank samples were collected immediately after burning and then every third month over a two-year period from 0.25m(2) blocks 50mm deep. The same was done on unburnt grassland. At ground level, the back fire exceeded 100 degrees C, while the head fire had temperatures of less than 100 degrees C. The fire caused a flush of seedlings from the seed bank over the first season after the fire. The head fire stimulated seedling density significantly (P <= 0.01) more than the back fire. The mean (head and back fire) seed bank density the first season following the fire, for burnt and unburnt grassland, was respectively 84.6 and 52.1 grass seedlings m(-2) for September, 50.5 and 21.9 grass seedlings m(-2) for December and 347 and 58 grass seedlings m(-2) for March. The pioneer species Aristida congesta and Tragus koelerioides only occurred in the soil seed bank of the burnt grassland, while the climax species Cymbopogon plurinodes, Digitaria eriantha and Panicum stapfianum only germinated in the unburnt grassland seed bank. The Eragrostis species showed the highest germination due to fire. The burning of semi-arid grassland did have a quantified impact on the soil seed bank over the short-term.