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University of Stellenbosch (2003)

The regeneration potential of Themeda triandra in the Middelburg district of the Eastern Cape

Hendricks, Noel Colin

Titre : The regeneration potential of Themeda triandra in the Middelburg district of the Eastern Cape

Auteur : Hendricks, Noel Colin

Université de soutenance : University of Stellenbosch

Grade : Master of Science (MSc) 2003

Résumé partiel
common topographical feature of the Nama-Karoo are mesas, commonly known as "platkoppies" , that provide an interesting source of landscape heterogeneity to an otherwise flat landscape. Although these isolated mesas are geologically and edaphically distinct from the surrounding flats, many species are shared between these habitats. These include palatable species such as Themeda triandra. A question asked by the broad umbrella project under which this project falls was : to what extent do mesas provide refuges for palatable species that are under pressure from heavy overstocking on the surrounding flats ? A study on the regeneration potential of T triandra on and off the Tafelberg Mesa in the Middelburg district of the Eastern Cape, South Africa, was undertaken within the context of a broader umbrella project "Restoration of degraded Nama-Karoo : role of conservation islands". The Nama-Karoo has had centuries of heavy commercial livestock production and it is considered to have been transformed from a relatively (by arid ecosystems) productive system to one dominated by shrubs and somewhat less productive species. Themeda triandra is one of the preferred grass species for livestock production. Although T. triandra is a preferred grass species, relatively, little is known about its ability to produce viable seed and the establishment of seedlings particularly in restoration and rehabilitation programmes. This study examines the pattern of seed production and seedling survival, seed dispersal, seed germination, and the morphological and ecophysiological variability of T. triandra, a species with great potential for restoration of degraded Nama-Karoo sites, particularly those in the Eastern Cape. Themeda triandra was found to be one of the dominant species on summit of the Tafelberg Mesa. In comparison, it occurred in small isolated populations on the flats surrounding the mesa. The flats and slopes are grazed more intensely by domestic livestock than the summit of the mesa. This is due to the inaccessibility to livestock due to a steeper topography and the lack of water at the higher altitudes. Annual seed production of T. triandra per plant and per m2 was highest for the populations on the flats despite these populations being grazed most intensively. Rainfall had an effect on annual seed production, which was monitored over two years. Rainfall increased from less than 20mm in November 1999 to 125, 110, 50 and 135mm in December 1999, January 2000, February 2000 and March 2000 respectively. With the increased rainfall prior to the May 2000 sampling period, more seeds were produced per plant and per m2 for the flats and slopes habitats of the Tafelberg Mesa. The opposite trend occurred on the summit of the mesa, where seed production actually decreased. This could be attributed to increased competition or to lower grazing intensities. Increase in rainfall also had a positive effect on the cover of other grasses (excluding T. triandra) and T. triandra itself. Despite higher levels of seed production in populations of T. triandra on the flats, seedling survival was clearly low whereas seedlings on the slopes and summit had significantly higher seedling survivorship. This negative impact could be explained due to the trampling effect of domestic herbivores. The results of a seed dispersal experiment clearly suggest that the seed dispersal distance of T. triandra to "safe" microsites is short distance (majority of seeds disperse up to 60cm) and that the dispersal agent is wind. Microsites for re-establishment was found to be open or rocky sites. In a controlled experiment, seed emergence of T. triandra indicated that optimal sowing depths varied with soil type.

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