Browsing by Subject "vegetation pattern"

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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 phenological pattern of vegetation in Namaqualand, South Africa and its climatic correlates using NOAA-AVHRR NDVI Data
    (Taylor & Francis, 2005) Fox, S C; Hoffman, M T; Hoare, D
    Remote sensing techniques were used to reveal the vegetation patterns in the greater Namaqualand area and to relate them to climatic variables. We related the normalised difference vegetation index (NDVI) to a range of different rainfall and temperature indices. Mean annual NDVI is significantly related to precipitation and potential evapotranspiration (PET). A multiple regression model explains 52% of the variance when Mean Annual NDVI is related to climatic variables. Mean NDVI in August (the month of maximum NDVI in most of Namaqualand) is significantly related to PET and the current plus two previous months of precipitation. A multiple regression model for mean NDVI in August and climatic variables explains 57% of the variance. We also assessed the annual and seasonal NDVI pattern exhibited within seven different vegetation types. The climatic variables influencing the summer rainfall and winter rainfall vegetation types differ significantly from each other. All vegetation types have a peak NDVI signal in August although Mountain Fynbos and North Western Mountain Renosterveld values are significantly higher than the rest. Upland, Lowland and Strandveld Succulent Karoo exhibit similar NDVI responses while the two summer rainfall vegetation types (Bushmanland and Orange River Nama Karoo) have a very low NDVI signal throughout the year. The results suggest that NDVI can be used successfully to detect growth and phenology patterns within different vegetation types in Namaqualand.