The riparian vegetation of the Hottentots Holland Mountains, Western Cape, South Africa

[摘要] ENGLISH ABSTRACT: Riparian vegetation has received a lot of attention in South Africa recently, mainlybecause of its importance in bank stabilization and its influence on flood regimes andwater conservation. The upper reaches have thus far received the least of this attentionbecause of their inaccessibility. This study mainly focuses on these reaches whereriparian vegetation is still mostly in a pristine state. The study area chosen for thispurpose is the Hottentots Holland Mountains in the Southwestern Cape, the area withthe highest rainfall in the Cape Floristic Region, which is very rich in species. Fiverivers originate in this area and the vegetation described around them covers a largerange of habitats, from high to low altitude, with different geological substrates anddifferent rainfall regimes.All of these rivers are heavily disturbed in their lower reaches but are stillrelatively 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 inTheewaterskloof Dam. Another big dam for water storage, Skuifraam Dam, will bebuilt on the Berg River near Franschhoek in the nearby future.In order to study the vegetation around a river, a zonation pattern on the riverbank is described and several physical habitats are recognized. A primary distinctionis made between a Wet Bank (flooding at least once a year) and a Dry Bank (floodingless than once a year). The Dry Bank is further subdivided into a Lower Dynamic, aShrub/Tree and a Back Dynamic Zone. In the lower reaches these zones are verydistinct, but in the upper reaches of a river they tend to blend into each other and somezones can be absent or very narrow.Vegetation has been sampled in transects across the riverbed, following theBraun-Blanquet method. Additional vegetation samples have been recorded in thebogs and mires at the sources of the rivers. Vegetation structure and physical habitathas been described to contribute to the description of the vegetation types. In order tounderstand the environmental processes that determine the vegetation, environmentalparameters were recorded in every vegetation sample, such as, slope, aspect,rockiness and soil variables.The classification of the vegetation samples resulted in the identification andsubsequent description of 26 riverine and 11 mire communities. The riverinecommunities have been subdivided into ten Community Groups, including a group ofAquatic communities and three groups of Wet Bank communities. The maindistinction within the Wet Bank Zone is the importance of erosion or deposition as adriving force of the ecosystem. Three groups of Fynbos communities are identified inthe Back Dynamic Zone, with Asteraceous Fynbos occurring on shales and granites,Ericaceous Fynbos occurring on Table Mountain Group sandstones and TransitionalFynbos on a variety of substrates. One community group is characterized by thedominance of Cliffortia odorata, which shows affinity with some renosterveldcommunities known from literature. The two final groups contain the AfromontaneForests and Riparian Scrub communities, respectively.Discharges are calculated from data recorded at existing gauging weirs. Therecurrence intervals, inundation levels and stream power of several flood events arederived from these data and are extrapolated to upstream sites. It appears that mostvegetation types in the zonation pattern on the riverbank can be explained by theseflood events, except for the Afromontane Forests, which are dependent on other sitespecificfactors including protection from fire.Constrained and unconstrained ordinations are used to relate vegetationpatterns to the environment. The vegetation is determined by three environmentalgradients, operating at different scales. The lateral gradient across the riverbed ismainly determined by inundation frequency and stream power, which are difficult tomeasure in rocky mountain situations, although variables like distance from thewater'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 tolow altitude. This gradient has the least influence on the riparian vegetation. Thegeographical gradient reflects the large-scale climatic processes across the mountainrange. This gradient accounts for the biggest part of the total explained variation.Important variables are especially the ratio between the summer and winter rainfalland the geological substrate. In the Fynbos Biome, where gamma diversity isextremely high, large-scale environmental processes are important in azonalvegetation as well. The most species-rich vegetation associated with the rivers isfound furthest from the water's edge at intermediate altitudes.Knowledge about the vegetation types and environmental processes inWestern Cape rivers is essential for monitoring and maintaining these specialecosystems. Specific threats are related to possible abstraction of water from theTable Mountain Group aquifer and from climate change, which might result in anoverall drying of the ecosystem.

[发布机构] Stellenbosch University