Abstract
Riparian vegetation, which normally differs structurally and compositionally from surrounding vegetation, has been degraded in many parts of the fynbos biome by several species of invasive Australian Acacia . Systematic clearing of dense stands of these alien trees was initiated in 1995, and information is urgently needed to guide the restoration of riparian habitats. A problem is that degradation of these communities is so advanced and widespread that in many cases managers do not know what species to use in restoration, or what kinds of target communities to aim for. This study's aim was to provide baseline information on riparian plant community structure and composition from non-transformed habitats. Species and environmental data were recorded from 76 sites located along the headwater systems of six rivers in the southwestern part of the Western Cape province. Analysis of the data, applying multivariate classification (TWINSPAN) and ordination (Detrended Correspondence Analysis) techniques, identified four prospective plant communities: 1) a Nivenia corymbosa - Brachylaena neriifolia Community; 2) a Leucadendron salicifolium - Berzelia lanuginosa Community; 3) a Cliffortia ruscifolia - Metrosideros angustifolia Community; and 4) a Kiggelaria africana - Brabejum stellatifolium Community. These formed a continuum with only the Leucadendron and Kiggelaria communities separating in ordination space. Soil pH differed between the latter two communities, reflecting different geology. It was found that many riparian specialist species are relatively widespread. For the study area, it is concluded that where information on the historical composition of riparian communities is lacking, target communities for restoration can be defined from pristine communities with similar geology, and secondly, altitude. In all cases the target community will comprise a large proportion of widespread, predominantly resprouting, riparian species.
Highlights
There exist no formal published descriptions or classification of riparian vegetation in the Western Cape
No distinctive communities emerged from the initial TWINSPAN classification, which comprised a high proportion of common indigenous taxa (Table 2)
Riparian vegetation in the Western Cape mountain and foothill zones comprises a mixture of terrestrial fynbos elements as well as non-fynbos plants that are adapted to the specific ecological conditions present close to the river (Sieben 2003)
Summary
There exist no formal published descriptions or classification of riparian vegetation in the Western Cape. Boucher 1978a, Macdonald 1988, Taylor 1996), but riparian zones have mostly received less attention owing to the narrow band they form within the terrestrial matrix. The geomorphology of most Western Cape rivers is characterised by the Cape Fold Belt mountains that dominate the area, rising to an elevation of about 2 000m. These mountains comprise rocks of the Table Mountain Group series (mainly sandstones) that yield predominantly nutrient-poor substrata. From the steep mountainous terrain, the rivers flow through a foothill zone, often on Cape Granite Suite soils, onto the coastal plains, the latter comprising the Malmesbury or Bokkeveld Group shales. The dominant geomorphological process here is deposition, in contrast to the mountain stream zone where it is erosion (Davies and Day 1998)
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