Browsing by Author "Khomo, Lesego"

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    Biogeochemical niche construction in the forest-fynbos mosaic of Jonkershoek Nature Reserve, South Africa
    (2014) Dalwai, Raheem; Khomo, Lesego
    The south-western Cape of South Africa is dominated by fynbos vegetation with patches of forest restricted to rock screes and stream banks owing to the more favourable moisture status of these microhabitats. A detailed analysis of soil underlying forest and fynbos vegetation in Jonkershoek Nature Reserve was investigated. A total of nine soil pits were dug in forest and on fynbos slopes with high and low gradients. Pits were analysed by depth examining texture while soil was also sent for x-ray fluorescent analysis and carbon and nitrogen analysis. Evidence for dust deposition varied spatially, although this could be a result of the rapid erosion experienced during winter. Soil properties, mainly texture and nutrient concentration differed distinctly between forest and fynbos. I argue that these differences are caused by topographical diversity and maintained by vegetation which influences nutrient enrichment via dust accumulation and plant litter decomposition. Thus I concluded that soil texture favours nutrient retention in forest soil more than in fynbos soil enhancing the disparity between nutrients in the respective environments. By influencing litter decomposition and aeolian inputs of dust, vegetation is responsible for modifying its niche increasing the difference between fynbos and forest patches.
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    Evidence for aeolian origins of heuweltjies from buried gravel layers
    (2016) CRAMER, MICHAEL D; von Holdt, Johanna; Khomo, Lesego; Midgley, Jeremy J
    Abstract Although heuweltjies (19–32 m diameter) dominate the surface of much of the southwestern Cape of South Africa, their origins, distribution and age remain controversial. Current hypotheses are that the heuweltjies are (1) constructed by the excavation and mounding habits of burrowing animals; (2) the result of erosion by water of areas between patches protected from fluvial action by denser vegetation or (3) the product of localised aeolian sediment accumulation beneath denser vegetation associated with termitaria. At a site where quartz-containing gravels occur on the soil surface in areas between heuweltjies, these gravels were found to extend as a relatively intact layer of uniform concentration from the inter-mound area into the mound at the same plane as the surrounding soil surface. This buried layer suggests that heuweltjies were either built-up by deposition on a previous soil surface layer or eroded from sediment accumulated above the buried gravel layer. Mounds contain a relatively large proportion of silt consistent with sediment deposition. Mound sediment elemental composition was strongly correlated with that of local shale, indicating a local source of sediment. Pedogenesis was considerably more advanced off- than on-mound. There was no evidence of extensive regional aeolian sediment mantling over the vast area in which the heuweltjies occur. These findings and observations support the aeolian deposition hypothesis of heuweltjie origins combined with a degree of erosion, rather than a termite bioturbation hypothesis or a predominantly erosion-based hypothesis.
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    The Influence of dust deposition, carbonates and erosion on the formation of Clanwilliam heuweltjies
    (2013) Adams, Ismat; Khomo, Lesego
    Heuweltjies (Afrikaans for "little hills") are non-anthropogenic, regularly dispersed earth mounds up to 32 meters in diameter and approximately 1.4 meters in height, that dot about 25% of the land surface of south-western southern Africa. The zoogenic "termite" hypothesis has been widely accepted as a hypothesis of heuweltjie origin. However, the recent "vegetation-patch-erosion" (VPE) hypothesis suggests an equally likely explanation for heuweltjie formation. The aim of this investigation was firstly to determine the influence of dust deposition and carbonates on heuweltjie formation in order to gain insight into the formative process of heuweltjies. The second part of this investigation sought to further test the VPE hypothesis by examining whether erosion alone could give rise to heuweltjies within a reasonable time frame. Soil surveys were conducted on 8 heuweltjies and at 2 inter-heuweltjie areas at depths of up to 0.9m. Textural analysis of soil samples collected from the 8 heuweltjies as well as an additional 11 heuweltjies was conducted. Bulk density was also evaluated for each horizon of each heuweltjie and inter-heuweltjie. Carbonates in all soil samples were determined by acid digestion of soils. A digital elevation model of the Clanwilliam area was constructed and used to obtain heuweltjie area and elevation. There was no difference in average soil texture moving from the centre of a heuweltjie to its periphery (indicating that they formed by layered stratification rather than radially from the centre). There was an apparent difference in texture and carbonates between heuweltjie and inter-heuweltjie zones. Mass of fines (silt and clay) and carbonates were both positively correlated with heuweltjie mass, but were also collinear. Non-parametric regression of heuweltjie mass against both fines and carbonates revealed a clear linear trend. However, the trend between fines and carbonates suggested that fines played a central role in both heuweltjie and carbonate formation and that carbonate took on only a secondary role in heuweltjie formation. Erosion alone would have taken 11000 years to form the heuweltjies. However, some calcretes within heuweltjies have been found to be older than 11000 years, suggesting that erosion has not had a major influence on heuweltjie formation. These findings support the vegetation-patch-erosion hypothesis. However they support dust deposition rather than erosion as a primary contributor to heuweltjie formation.