The Effects of Different Forms of Land Use on the Ecology of a Semi-Arid Region in South-Eastern Rhodesia

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Abstract
A study was made in 1970-72 of the effects of 4 different utilization regimes on savanna vegetation (lowveld) on shallow basalt soils in Rhodesia. These were nil utilization, light utilization by wild herbivores, moderate utilization by cattle and intensive utilization by cattle and goats. Total above-ground standing crop of shrubs and trees varied between 31 t [tons] ha-1 (nil utilization) and 9 t ha-1 (intensive utilization). The seasonal production of new leaves and twigs varied from 2 t ha-1 (nil utilization) to 0.6 t ha-1 (intensive utilization). Most of the differences in the herbaceous vegetation attributable to utilization were a result of the intensity of use rather than the different species of herbivores involved. It was dominated by annual grasses in intensively utilized areas and by perennial grasses in areas of light and moderate utilization. The areas dominated by annual grasses were characterized by large fluctuations in aboveground herbaceous production between seasons (up to 400% change) in response to rainfall, while much smaller changes occurred in the swards dominated by perennials (about 100%). Differences in litter cover (23, 45 and 68%, respectively, in areas of intensive, moderate and light utilization) had large effects on infiltration of rainfall since water was found to infiltrate 9 times faster into soil covered by litter than into bare soil. In a dry year (about 300 mm rainfall) seasonal aboveground herbaceous production varied from about 0.25 t ha-1 in areas of intensive use to about 1 t ha-1 in areas of light use. In a wet season (about 600 mm rainfall) production for the same areas was about 1.2 t ha-1 and 2 t ha-1, respectively. Protein content of swards dominated by annual grasses remained high throughout the season, never dropping below about 9%, while that of communities dominated by perennial grasses dropped from about 12% in the early season to below 5% in the late season. However, total quantities of protein and other foodstuffs available were, at all times, much greater where utilization was moderate or light than where intensive. Continuous intensive use was resulting in all grasses in these areas being completely consumed during the growing season, leaving little or no herbage for the 7- or 8-mo. dry season. In areas dominated by perennial grasses, the large standing crop of forage, although not of very high nutritive quality, resulted in considerable quantities of herbage remaining for dry season consumption. The energy content of the herbaceous vegetation was 17,522 J g-1 (4185 kcal g-1) and the efficiency of the herbaceous component of the vegetation in trapping radiant energy over the 5-mo. growing season ranged 0.01-0.06%. Swards exhibiting the most desirable characteristics were those in areas of light and moderate use. In areas of non-use sward vigor was declining and the proportion of bare soil surface and scrub was increasing. Intensive utilization resulted in a change in dominance from perennial to annual grasses, with increasing year to year fluctuations in seasonal production, a large proportion of bare soil and increasing runoff of rainwater.