Studies on herbaceous layer production in Burkea africana savanna

Abstract

The effects of fire, woody vegetation and defoliation on components of the herbaceous layer in an area of Burkea africana savanna were studied for one year. Biomass (live material) was significantly greater, over the year, in the open subhabitat than under the dominant Burkea africana trees and Ochna oulchra shrub clumps, in the case of the sward as a whole. Biomass of the palatable 'forage and intermediate' species group was, however, of the same order in the open as under ~- africana, while that under Q. pulchra remained less than in the open. Biomass of the non-forage species group was significantly greater in the open than in the two canopied subhabitats. The amount of in-vitro digestible organic matter showed the same trends as the biomass, but there was more crude protein in forage and intermediate species under B. africana trees than in the open or under o. pulchra clumps. An intense backfire applied in.September 1978 resulted in a significant 29,4% less biomass over the following year, when compared with an unburnt area. This was largely due to the significant reduction in biomass and basal cover of the somewhat moribund non-forage species, mainly Eragrostis pallens, on the burnt plots. Basal cover and biomass of the leafier forage and intermediate species were not significantly affected by the backfire. An eight-weekly clipping regime imposed on burnt plots resulted in a slower rate of biomass accumulation than was measured on burnt plots allowed unchecked regrowth. The slowest biomass accumulation rate was recorded, in the case of the sward as a whole, on unburnt, unclipped plots whilst the fastest was on burnt, unclipped plots. Burning followed by eight-weekly clipping reduced the biomass accumulation rate of the non-forage species relative to unburnt non-forage species, but stimulated the rate of accumulation of forage and intermediate species' biomass. Exclusion of herbivores from burnt areas for differing periods resulted in the greatest peak standing crop occurdng in those sites protected for the longest period (101 days) after the fire, in both Burkea africana and Acacia spp areas. A six-weekly clipping interval resulted in relatively good dry matter yields of Digitaria eriantha ssp pentzii, both in a controlled and a natural environment, compared with more and less frequent clipping intervals. A similar clipping frequency also resulted in relatively good yields obtained from Brachiaria nigropedata, without adversely affecting carbohydrate reserves. A two cm clipping height resulted in greatest mortality of both species, whereas an eight cm height resulted in 100% survival. An estimate of 71,87 gm-2 yr-1 net grazeable annual above-ground production was obtained for the sward (on an oven-dry basis).