Fire and the dynamics of semi-arid grassland: influence on plant survival, productivity and water-use efficiency

Abstract

There is a lack of knowledge concerning the effect of fire behaviour on the vegetation dynamics of semi-arid climates. Arising from this, an investigation was initiated to characterise the behaviour of fires burning with the wind (head fire) and against the wind (back fire). The behaviour of these fires was related to the response of the vegetation by monitoring the recovery and productivity of the grass sward after only one burn over a two year period (2000/01 to 2001/02 seasons). The results showed that head fires had significantly greater flame height and rate of spread than back fires. However, at ground level back fires were more intense and had a negative effect on recovery of the grasses, resulting in lower litter, yield and water-use efficiency (aboveground phytomass or crude protein produced per unit of evapotranspiration). Fire had a significant effect on plant density and cover, with basal cover 68% lower than that of unburnt grassland two months after the fire. Cymbopogon plurinodis with large tufts had the most die-back of all the species. With the exception of plant cover and density, which takes longer to recover, it takes approximately two complete growing seasons to recover from most of the detrimental effects of fire. The initial increased crude protein content of the plant material caused by fire did not off set the approximately 50% reduction in plant production the first growing season after the fire. Fire significantly decreased WUE (water-use efficiency), with an average over the two growing seasons of 3.06kg ha-1 mm-1 and 2.45kg ha-1 mm-1 for unburnt and burnt grassland respectively. In various semi-arid grassland areas, the use of fire as a management tool is questionable if there is no specific purpose for it.