Vegetation, soil and grazing relationships in the Middelburg District of the Eastern Cape

[摘要] ENGLISH ABSTRACT:Arid and semi-arid rangelands of the Nama-Karoo Biome are believed to have changedconsiderably since the arrival of domestic livestock in the veld. Severe grazing pressureis considered to be one of the prime factors responsible for the perceived degradation ofvegetation and concurrent soil degradation. To understand the process of degradationand to make further recommendations for future veld restoration, a large-scale projectwas undertaken in the Eastern Cape. This project focused on the role that landscapeheterogeneity plays in providing refuges for plant species. The key question asked inthis project was: What role do these hypothetically less grazed mesas play in theconservation of rangelands in the Nama-Karoo of the Eastern Cape?This study, a component of the larger project, investigated grazing and soil landscapegradients on- and off- the three mesas (Tafelberg, Folminkskop and Buffelskop) in theMiddelburg District of the Eastern Cape, South Africa and the possible influences thatthey might have on the veld. The flats surrounding the mesas were mostly used asgrazing camps in contrast to the plateaux of the mesas, which had varied levels ofaccessibility.In the absence of direct observations, the primary objective of the study was to test thedifferential dung pellet abundance and impact of animals on different parts of thelandscape. As such, dung pellet counts in this study were correlated with surrogates ofsoil physical properties including bare ground, trampling and litter cover. Variation indung pellet density was found at the different habitats (flats, slopes and plateaux) of allmesas. The flats to the north-eastern and south-western of Tafelberg mesa were foundto be more heavily utilized by livestock and herbivores, while the plateaux and southeasternslopes of Folminkskop and Buffelskop were also utilized by grazers. TheTafelberg mesa was the only study site that was consistent with the hypothesis whichstated that grazers would be less concentrated on the plateau compared to thesurrounding flats due to its inaccessibility, whilst the high mean dung pellet density onthe plateaux of the smaller Folminkskop and Buffelskop mesas due to easier accesscontradicted the original hypothesis. It appeared that dung pellet density did not clearlyturn out to be an indicator of habitat use in this study, but showed where slopes andplateaux were accessible to herbivores, as in the case for the Buffelskop mesa, a higherabundance of dung pellets were found suggesting that higher intensities of habitat usetook place.During this study a strong pattern was observed of bare soil patches on the flatssurrounding the mesas. There was a decrease in percentage of bare soil along thegradient of the three mesas with a high percentage of bare soil on the plateaux of themesas. A significant correlation was found between bare soil and dung pellet density.However, the plateaux of Folminkskop and Buffelskop had a high percentage of bare soilcompared to the plateau of Tafelberg mesa. Farmers mainly used these smaller mesasas grazing camps for their livestock and herbivores. A positive correlation between baresoil and litter cover of the different habitats was evident in this study. A lowerpercentage of litter cover at these sites was associated with a high percentage of baresoil. Litter is very important in a healthy vegetation community in terms of nutrientcycling and fertile patches.A detailed assessment of soil chemical and physical properties would reveal, firstly, ifvegetation change is better explained by soil or grazing effects and, secondly, if changesin soil have resulted from land use. Differences in macro- and micro-site variationsbetween open-canopy (between shrubs) and closed-canopy (under shrubs) sites foreach habitat were determined to differentiate between local scales due to land use andlandscape scales due to geomorphology. The results suggested that carbon,phosphorus, calcium, magnesium, copper and manganese levels in soils at landscapescale better explain vegetation changes between habitats. At a local scale (open- andclosed-canopy sites) land use was responsible for little changes in soils. Changes inonly soil potassium, zinc and boron elements were actually a consequence of localscales due to land use. The soil nutrient content on the slopes appeared to beintermediate between the flats and plateaux of all three mesas. It appeared that doleritecapped Tafelberg and Folminkskop mesas had high silt and clay content, whileBuffelskop (sandstone) mesa had a lower silt and clay content. Consequently, thetexture and parent material of the soils contributed to the variations in soil nutrientcomposition between these mesas. High infiltration rate together with low nutrientcontent on the flats clearly showed that these flats, surrounding the mesas weredegraded. The high infiltration rates were caused by high activity by livestock and otherindigenous animals on the flats which breaks the surface crusting of bare soil and improve infiltration. It was concluded that high levels of grazing at these sites have alsoaltered the textural and soil properties.Endozoochory dispersal and the deposition of dung pellets in areas of small patchdisturbances play an important role in veld regeneration in degraded areas. Dungpellets collected from permanent study sites on the southeastern and northwestern flatsand slopes, and all the study sites on the plateau of Tafelberg mesa, was sown inseedling trays, watered and monitored for seedling germination. Species list were thencompared to below-ground soil seed bank data and above-ground vegetation datacollected by other researchers at the same permanent study sites. Higher seedlingpercentages were recorded from dung pellets collected on the flats than on the plateau.A total of sixteen species were found to germinate in dung pellets collected on the flatscompared to ten species germinating in dung pellets collected on the slopes and twospecies on the plateau of Tafelberg mesa. The seeds that germinated represent avariety of palatable grasses and shrub species. Aristida sp., Eragrostis bicolor,Eragrostis ch/orome/as and Eragrostis obtusa were palatable grasses recorded for dungpellets collected on the flats. Of the species recorded, Aristida sp., Chenopodium sp.and Pentzia sp. were found in dung pellets but were not recorded in parallel soil seedbankand vegetation studies.Successful restoration of veld conditions requires strict grazing management practices.Germination of seed in dung pellets might be considered to be a valuable means ofindicating restoration potential and rangeland conditions for the identification of bothdegraded and conservation worthy areas. With appropriate land management skills andrestoration measures, these challenges can be constructively and creatively faced.

[发布机构] Stellenbosch University