Doctoral Degrees (Botany and Zoology)
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Browsing Doctoral Degrees (Botany and Zoology) by browse.metadata.advisor "Boucher, C."
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- ItemThe riparian vegetation of the Hottentots Holland Mountains, Western Cape, South Africa(Stellenbosch : University of Stellenbosch, 2003-12) Sieben, E. J. J.; Boucher, C.; University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: Riparian vegetation has received a lot of attention in South Africa recently, mainly because of its importance in bank stabilization and its influence on flood regimes and water conservation. The upper reaches have thus far received the least of this attention because of their inaccessibility. This study mainly focuses on these reaches where riparian vegetation is still mostly in a pristine state. The study area chosen for this purpose is the Hottentots Holland Mountains in the Southwestern Cape, the area with the highest rainfall in the Cape Floristic Region, which is very rich in species. Five rivers originate in this area and the vegetation described around them covers a large range of habitats, from high to low altitude, with different geological substrates and different rainfall regimes. All of these rivers are heavily disturbed in their lower reaches but are still relatively pristine in their upper reaches. All of them are dammed in at least one place, except for the Lourens River. An Interbasin Transfer Scheme connects the Eerste-, Berg- and Riviersonderend Rivers. The water of this scheme is stored mainly in Theewaterskloof Dam. Another big dam for water storage, Skuifraam Dam, will be built on the Berg River near Franschhoek in the nearby future. In order to study the vegetation around a river, a zonation pattern on the river bank is described and several physical habitats are recognized. A primary distinction is made between a Wet Bank (flooding at least once a year) and a Dry Bank (flooding less than once a year). The Dry Bank is further subdivided into a Lower Dynamic, a Shrub/Tree and a Back Dynamic Zone. In the lower reaches these zones are very distinct, but in the upper reaches of a river they tend to blend into each other and some zones can be absent or very narrow. Vegetation has been sampled in transects across the riverbed, following the Braun-Blanquet method. Additional vegetation samples have been recorded in the bogs and mires at the sources of the rivers. Vegetation structure and physical habitat has been described to contribute to the description of the vegetation types. In order to understand the environmental processes that determine the vegetation, environmental parameters were recorded in every vegetation sample, such as, slope, aspect, rockiness and soil variables. The classification of the vegetation samples resulted in the identification and subsequent description of 26 riverine and 11 mire communities. The riverinecommunities have been subdivided into ten Community Groups, including a group of Aquatic communities and three groups of Wet Bank communities. The main distinction within the Wet Bank Zone is the importance of erosion or deposition as a driving force of the ecosystem. Three groups of Fynbos communities are identified in the Back Dynamic Zone, with Asteraceous Fynbos occurring on shales and granites, Ericaceous Fynbos occurring on Table Mountain Group sandstones and Transitional Fynbos on a variety of substrates. One community group is characterized by the dominance of Cliffortia odorata, which shows affinity with some renosterveld communities known from literature. The two final groups contain the Afromontane Forests and Riparian Scrub communities, respectively. Discharges are calculated from data recorded at existing gauging weirs. The recurrence intervals, inundation levels and stream power of several flood events are derived from these data and are extrapolated to upstream sites. It appears that most vegetation types in the zonation pattern on the riverbank can be explained by these flood events, except for the Afromontane Forests, which are dependent on other sitespecific factors including protection from fire. Constrained and unconstrained ordinations are used to relate vegetation patterns to the environment. The vegetation is determined by three environmental gradients, operating at different scales. The lateral gradient across the riverbed is mainly determined by inundation frequency and stream power, which are difficult to measure in rocky mountain situations, although variables like distance from the water’s edge, elevation above the water level and rockiness are correlated to them. The longitudinal gradient is the gradient along the length of the river, from high to low altitude. This gradient has the least influence on the riparian vegetation. The geographical gradient reflects the large-scale climatic processes across the mountain range. This gradient accounts for the biggest part of the total explained variation. Important variables are especially the ratio between the summer and winter rainfall and the geological substrate. In the Fynbos Biome, where gamma diversity is extremely high, large-scale environmental processes are important in azonal vegetation as well. The most species-rich vegetation associated with the rivers is found furthest from the water’s edge at intermediate altitudes. Knowledge about the vegetation types and environmental processes in Western Cape rivers is essential for monitoring and maintaining these special ecosystems. Specific threats are related to possible abstraction of water from theTable Mountain Group aquifer and from climate change, which might result in an overall drying of the ecosystem.