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University of the Witwatersrand (1987)

Response of three semi-arid savannas on contrasting soils to the removal of the woody component

Scholes, Robert John

Titre : Response of three semi-arid savannas on contrasting soils to the removal of the woody component

Auteur : Scholes, Robert John

Université de soutenance : University of the Witwatersrand

Grade : Doctor of Philosophy (1987)

Résumé
A three-year study was undertaken in the semi-arid (500mm p.a.) north-eastern lowland area of South Africa. All the woody plants were removed from one-hectare savanna plots on three different soil types, and key hydrological and biological changes were monitored relative to adjacent controls. Runoff increased initially, but decreased once the grass cover increased. Deep drainage and lateral subsurface flow increased on the sandiest site. Evaporation from the soil surface increased on the heavier textured soils. The duration of plant-available water in the soil increased on all cleared plots. The pre-clearing woody-plant aboveground biomass was in the range of 5.6 to 11.2 t.ha-1, of which 0.66 to 0.80 t.ha-1 was leaf biomass. The annual herbaceous production was strongly rainfall dependent, averaging 1 to 1.5 t. ha -1, and increased by 0.4-0.6 t. ha -1 except on the most fertile site (0.6-2.0 t. ha -1). Total available forage increased with clearing, but so did its variability. The observed changes in herbaceous layer palatability could not be attributed to clearing. Woody plants and grasses were shown to have water-use niche separation in both rooting depth and time of water use. Simulation over forty years of wetting patterns indicated 75 to 85 % niche overlap, with separation on the depth axis more important in sandy sites, and on the time axis in clayey sites. Competition between woody plants and grasses was strongly asymmetrical in favour of woody plants. Grasslands derived from savannas by clearing are inherently unstable. Maintenance of a treeless grassland by the use of fire is feasible only if herbivory is controlled. Unchecked regrowth will result in reversion to savanna in about 15 years. Clearing is not recommended on sandy upland soils or highly erodible sites

Mots clés : Savanna ecology—South Africa ; Deforestation ; Vegetation dynamics ; Soils—Analysis

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Page publiée le 28 juin 2017, mise à jour le 3 janvier 2019