Short-term response of burnt grassland to defoliation in a semi-arid climate of South Africa

Abstract

The impact of defoliation on the productivity and soil temperature of burnt grassland (head and back fires) was investigated over the short term (one and two growing seasons after burning). Head fires had greater flame height and rate of spread than back fires. However, back fires were more intense and exceeded 100°C at ground level. Botanical composition was little influenced by both head and back fires, though fire had a drastic impact on the basal cover and tuft die-back. Basal cover was still 28% and 11% respectively lower than that of unburnt grassland, one and two growing seasons after burning. Those grass species normally forming large tufts, such as Cymbopogon plurinodis and Themeda triandra, had the greatest percentage die-back per tuft. Defoliation up to and during October, after the grassland had rested for a full growing season after burning, still significantly (P ≤ 0.01) lowered production for the subsequent full growing seasons. For sustainable production in this semi-arid area, it seems grassland should not be defoliated before December of the second growing season following fire. When defoliation took place later in the same season following burning, it had a smaller negative impact on the water-use efficiency. Defoliation significantly (P ≤ 0.01) increased the soil temperature up to a depth of 50mm over the first half of the growing season right after the fire. The importance of the correct time of utilisation of burnt semi-arid grassland, with respect to sustained high production, has been emphasised.

Keywords: aboveground phytomass, defoliation, head and back fires, soil temperature, tuft mortality, water-use efficiency

African Journal of Range & Forage Science 2006, 23(1): 1–11