Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Plant ecology and degradation of basalt-derived dambos in semi-arid southeastern Zimbabwe

Auteur : Clegg BW

Université de soutenance : University of KwaZulu-Natal

Grade : MScAgric (Grassland Sciences) 1999

Résumé
The relationship between vegetation change, soil-water processes and degradation, and methods of re-vegetating sites with sparse herbaceous cover were investigated within dambos on basalt-derived soils in semi-arid southeastern Zimbabwe. The dambos were divided into two groups, namely vertic and non-vertic, according to soil type. Between 1977 and 1997, tall grassland communities in the dambos with vertic soil experienced a marked decline in tall, mesic Ischaemum afrum and Setaria incrassata with a corresponding increase in short, more xeric Urochloa mosambicensis. The compositional changes were associated with a marked decline in herbaceous basal cover. Woody cover increased in both dambo types between 1963 and 1991. In the dambos with vertic soil, marked increases in woody cover were confined to small, locaKsed patches (approximately 30 ha out of a total dambo area of 1294 ha) where thickets of Acacia borleae formed. In contrast, a large proportion of the dambo with non-vertic soil was invaded by woody plants (principally Colophospermum mopane). In both dambo types, vegetation changes appeared to be related to desiccation, but the mechanisms of desiccation differed. Woody invasion into the dambos with non-vertic soil was related to a disruption of water retention, while vegetation change within the dambos with vertic soil appeared to have occurred in response to a lack of water supply rather than a reduced ability to retain water. Portions of both dambo types appeared to have become degraded. Degradation was manifested in a reduced ability to retain water and garner replacement resources. Planting with tillers as opposed to seeds appeared to be a more reliable method of establishing perennial herbaceous cover on degraded sites with vertic soils. Where abiotic conditions were satisfactory for tiller survival protection from animal impact was advantageous.

Présentation (NRF)