Shrub Layer Dynamics Under Different Herbivore Densities in an Arid Savanna, Botswana

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Applied Ecology. SUMMARY(1) A grazing experiment in an arid savanna in Botswana examined heavy and moderate grazing by cattle. The dynamics of the shrub layer were followed for 5 years.(2) In the areas with no and moderate grazing, shrub densities fluctuated but showed no consistent change, while density increased with heavy grazing. At the end of the experiment, density, cover, total mass, shoot and leaf mass and leaf area of shrubs were higher in the area under heavy grazing than in those under no or moderate grazing. There were no differences in shrub abundance between ungrazed and moderately grazed areas.(3) The increase in shrub abundance with heavy grazing was accounted for by two of the five recorded species, Acacia mellifera and Grewia flava. Both are shallow rooted suggesting that they were favoured by an increase in water availability in the surface soil following overgrazing of the grass layer.

Woody Encroachment as a Social-Ecological Regime Shift

“…Sustained heavy grazing by livestock ranching promotes woody seedling regeneration through reduced grass competition, provided that seedling mortality is not increased through consumption and trampling, and there has been above average rainfall in more arid savannas [1,12,52]. Heavy grazing in mesic savannas reduces fuel load through consumption and trampling, thereby reducing fire frequency and more significantly fire intensity [1,31,36,44,53].…”

“…Overgrazing has also been reported to reduce the effect of grass competition on tree seedlings and saplings, as a dense grass layer can negatively affect tree growth and survival [54][55][56]. Long-term grazing trials in both mesic and arid systems have consistently reported increases in the density of woody plants over 5-40 year periods of observation [52].…”

Woody Plant-Herbivore Interactions in Semi-Arid Savanna Ecosystems

“…Noumi et al (2010) reported browsing as improving the regeneration of Acacia tortilis trees through the recruitment of new individuals. Skarpe (1990) argued that browsing accelerates tree growth in semi-arid savanna through reducing competition for moisture from herbaceous plants. …”

Understanding the causes of bush encroachment in Africa: The key to effective management of savanna grasslands

“…In communal grazing lands of Botswana, the bush encroachment zone has been observed between 0 and 300 m from foci (boreholes), where there is high concentration of grazers (Moleele et al 2002). The possible explanation is that overgrazing suppresses the dominance of grass species and favors the growth and multiplication of woody species, because they then have increased access to available soil moisture (Skarpe 1990). Grazing also indirectly contributes towards bush encroachment through dispersal of encroacher plants' seeds.…”