Dissertation (PhD)--University of Stellenbosch, 2003. / 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. / AFRIKAANSE OPSOMMING: Riviere se oewerplantegroei kry die laaste tyd baie aandag in Suid-Afrika,
hoofsaaklik vanweë die belang vir die beheer van vloede, stabilisasie van die oewers
en die bewaring van drinkwater. Die hoë-liggende dele van die riviere het tot dusver
die minste aandag geniet omdat hulle tot ’n groot mate ontoeganklik is weens die
onherbergsame terrein waarin hulle geleë is. In hierdie studie is daar veral na
bergstrome gekyk waar die plantegroei nog taamlik natuurlik en onversteur is. Die
studiegebied wat vir hierdie doel gekies is, is die Hottentots-Holland berge in die
Wes-Kaap. Die gebied het die hoogste reënval in die Kaapse Floristiese Ryk en is ook
baie ryk aan spesies. Vyf riviere het in hierdie gebied hulle oorsprong. Die plantegroei
wat hier voorkom sluit ‘n wye reeks habitatte in: van hoog tot laag in hoogte bo
seespieël, verskeie geologiese substrate asook verskillende reënval patrone.
Al die vyf riviere wat ondersoek is, is baie versteur in hul onderlope, maar is
nog grotendeels natuurlik in hul hoë-liggende dele. Almal is reeds opgedam deur een
of meer damme, behalwe die Lourensrivier. ’n Tussenopvanggebied-oordragskema
verbind tans die Eerste-, Berg- en Riviersonderendriviere met mekaar. Die water uit
hierdie riviere word tans hoofsaaklik in die Theewaterskloofdam opgegaar. ’n
Verdere groot opgaardam, die sogenaamde Skuifraamdam, word binnekort in die
Bergrivier te Franschhoek gebou.
Al die riviere se onderlope is tot ’n mindere of meerdere mate vervuil met
landbou- en rioolafvoerprodukte. Uitheemse indringerplante, wat die natuurlike
oewerplantegroei verdring, skep veral probleme stroomaf van plantasies en dorpe.
Om die plantegroei van die rivieroewers na te vors, te klassifiseer en te
beskryf, is variasies in die fisiese omgewing bepaal en korrelasies gesoek om die
verspreiding van die plantegroei te verklaar. Die belangrikste verdeling in die
oewerplantegroei wat gevind is, is tussen die Nat-oewersone (dit word meer as een
keer per jaar oorstroom) en die Droë-oewersone (dit word minder as een keer per jaar
oorstroom). Die Droë-oewersone word verder onderverdeel in die Laerdinamiesesone,
die Boom/Struiksone en die Agter-dinamiesesone. In die laer dele van
die rivier is hierdie soneringspatrone baie duidelik, maar in die boonste dele van die
rivier kan die onderverdelings dikwels nie van mekaar onderskei word nie omdat
hulle gemeng is, of kan die sones baie smal wees of selfs heeltemal afwesig wees.Die plantegroei is gemonster in transekte wat dwarsoor die rivierloop uitgelê
is. Die Braun-Blanquet monstertegniek is gevolg. Bykomende monsterpersele is
opgemeet in die moerasse in die boonste dele van die berg-opvanggebiede. Om die
omgewingsprosesse wat die plantegroei bepaal te verstaan, is ’n aantal
omgewingsfaktore in elke monsterperseel aangeteken, wat, onder andere, helling,
aspek en bedekking van rotse ingesluit het, terwyl die variasie in samestelling van die
bodem ook aangeteken is.
Die klassifikasie van die plantegroei het tot die beskrywing van 26
plantgemeenskappe in die riviere en 11 gemeenskappe in die moerasse gelei. Die
struktuur van die plantegroei asook kenmerke van die fisiese habitat is in die
beskrywing van die plantegroei-eenhede ingesluit. Die gemeenskappe in die riviere is
onderverdeel in tien gemeenskapsgroepe. Daar is een gemeenskapsgroep wat die
akwatiese gemeenskappe en drie wat die Nat-oewersone gemeenskappe insluit. Die
belangrikste verskille tussen die verskillende Nat-oewersone gemeenskappe word
bepaal deur die mate waartoe erosie of deposisie voorkom. Daar is ook drie
gemeenskapsgroepe van Fynbos onderskei wat in die Agter-dinamiesesone voorkom.
Dit sluit in die Aster-fynbos op die skalies en graniete, die Erica-fynbos op die
sandstene en die Oorgangs-fynbos op gemengde substrate. Een gemeenskapsgroep is
deur die dominansie van Cliffortia odorata gekenmerk. Dit toon verwantskap met
renosterveld gemeenskappe wat reeds in die literatuur beskryf is. Die laaste twee
groepe sluit die Afromontane woude en Oewerstruikbosse in.
Die waterafloop is bereken deur middel van data verkry vanaf bestaande
keerwal meetstasies. Die herhalings-intervalle, oorstromingsdiepte en vloei-sterkte
van verskillende vloedtipes word vanaf hierdie data afgelei en stroomop
geekstrapoleer. Die meeste plantegroeivariasie op die oewers kan deur die vloede
verklaar word, behalwe in die geval van die Afromontane woude, wat deur ander
omgewingsfaktore bepaal is.
Beperkte en onbeperkte ordinasie is gebruik om die verband tussen die
plantegroeipatrone en die omgewing te bepaal. Die plantegroei se verspreiding is
bepaal deur drie omgewingsgradiënte, wat op verskillende skale ‘n uitwerking het.
Die laterale gradiënt oor die rivierbedding is hoofsaaklik bepaal deur
oorstromingsfrekwensie en stroomvloeisterkte. Hierdie veranderlikes is moeilik
bepaalbaar, alhoewel ander soos, afstand vanaf die rivier, hoogte bo watervlak en
bedekking van rotse, wat hieraan gekorreleer is, wel meetbaar is. Die lengte gradiënt,dit is die gradiënt wat van oorsprong na einde langs die lengte van die rivier
teenwoordig is, het die minste invloed op die plantegroei. Die geografiese gradiënt
weerspieel die grootskaalse klimaatsveranderinge oor die bergreeks. Deur hierdie
gradiënt word die grootste deel van die totale variasie tussen die monsters verklaar.
Die belangrikste veranderlikes is die verhouding van somer- teenoor winter-reënval
en die geologiese substraat.
Soortgelyk aan die fynbos in die Fynbosbioom, waar gammadiversiteit
buitegewoon hoog is, is die grootskaalse omgewingsprosesse, ook vir asonale
oewerplantegroei, baie belangrik. Die spesierykste plantegroei rondom die rivier word
die verste van die oewer op gemiddelde hoogtes bo seespieël gevind.
Kennis oor die plantegroei en die omgewingsprosesse in die riviere in die
Wes-Kaap is belangrik vir die monitering en effektiewe beheer van hierdie besondere
ekosisteem. Spesifieke bedreigings is gekoppel aan die potensiële ontginning van
water uit die akwifer in die Tafelberggroep-sedimente asook deur grootskaalse
klimaatsveranderinge waartydens die hoeveelheid water, volgens voorspellings,
waarskynlik sal afneem in hierdie ekosisteem.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16069 |
| Date | 12 1900 |
| Creators | Sieben, E. J. J. |
| Contributors | Boucher, C., University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xvi, 262 leaves : ill. |
| Rights | University of Stellenbosch |
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