Wetland plant communities in the Potchefstroom Municipal Area , North-West , South Africa

Wetlands in natural areas in South Africa have been described before, but no literature exists concerning the phyto­ sociology of urban wetlands. The objective of this study was to conduct a complete vegetation analysis of the wetlands in the Potchefstroom Municipal Area. Using a numerical classification technique (TWINSPAN) as a first approximation, the classification was refined by using Braun-Blanquet procedures. The result is a phytosociological table from which a number of unique plant communities are recognised. These urban wetlands are characterised by a high species diversity, which is unusual for wetlands. Reasons for the high species diversity could be the different types of disturbances occurring in this area. Results of this study can be used to construct more sensible management practises for these wetlands.


INTRODUCTION
According to the Ramsar Convention, which is also known as The Convention on Wetlands of International Importance, wetlands can be described as areas of marsh, fen, peatland or water, whether natural or artificial, per manent or temporary (Cowan 1995).The water of wet lands can be static or flowing, fresh, brackish or salty, including areas of marine water the depth of which at low tide docs not exceed six metres.Wetlands may include adjacent riparian and coastal areas (Cowan 1995).Wetlands also refer to a mosaic of ecosystems that typically form transition zones between uplands and aquatic environments, although some do have discrete boundaries thus not providing gradual transitions (Catallo 1993).
Wetlands have many beneficial functions which are of chemical, biological, socio-economical and physical or hydrological nature (Williams 1990) and they play an important role as water reservoirs, stream flow regula tors, flood attenuators, water purifiers and controllers of soil erosion (Walmsley 1988).Additionally, wetlands form specialised habitats for a great variety of plant and animal species.
Many natural wetlands have been destroyed in the course of agricultural, industrial and urban development (Archibald & Batchelor 1992), and are regarded as one of South Africa's most endangered ecosystem types (Walmsley 1988).The increasing use of fertilisers and insecticides in catchment areas of wetlands and the fur ther threat of pollution by industries and the almost indis criminate use of some areas for grazing, contribute great ly to the degradation of wetlands (Fuls et al. 1992).As early as 1951, Louw made the observation that the mod ification of natural vegetation and the destructive conse quences of man's interference in the Potchefstroom area, is nowhere as evident as along the numerous vleis.wet lands and spruits.
Conservation and management of wetlands need to be addressed to ensure the natural functioning of these areas as well as the maintenance of species diversity (Eckhardt et al. 1993a).The need for the conservation of urban wetlands resulted in the development of programmes such as the Metropolitan Open Space Systems (MOSS) programme of which the Durban MOSS (D'MOSS) is a prime example.This programme proposed a new holistic approach to city planning, one whereby indigenous plants form an integral part of the urban landscape (Roberts 1993).Because rivers, streams, pans, marshes, estuaries, and lagoons are critically important to both man and wildlife (Cooper & Duthie 1992), wetlands within urban areas should form the essential core areas of any MOSS network.The MOSS approach is presently also being implemented in Durban.Pietermaritzburg.East London, Port Elizabeth.Bloemfontein, the East Rand.Port Alfred and Empangeni.
One of the first priorities, before a system such as MOSS can be implemented, is an extensive phytosocio logical survey of the plant communities within the municipal borders (Roberts 1993).Before conservation strategies can be successfully implemented, detailed knowledge and information of individual biotopes, their ecological characteristics, location, and distribution within the municipal area should be obtained, as well as knowledge of the faunal and floral composition of these biotopes (Starfinger & Sukopp 1994).The presence or absence of some species in these biotopes may often serve as a bio-indication of the disturbance or pollution in these biotopes (Starfinger & Sukopp 1994).
Although phytosociological studies of wetlands in nat ural areas in the Grassland Biome have been done by, among others, Kooij et al. (1991), Fuls et al. (1992), Bloem et al. (1993), Eckhardt et al. (1993a).Myburgh et al. (1995) and Smit et al. (1995).no literature concerning the phytosociology of urban wetlands exists.Bezuidenhout (1993) did not study wetlands in his comprehensive syntaxonomical and synecological study of the western Transvaal Grasslands, which include the natural areas around Potchefstroom.The only study on the wetlands of the Potchefstroom area was that of Louw (1951).
The objective of this study was, therefore, to conduct a complete vegetation analysis of wetlands within the Potchefstroom Municipal Area.Through this study it should be possible to ascertain the extent of disturbance due to urbanisation.This study forms part of an extensive study of urban open spaces in a number of cities in the North-West, South Africa, with the ultimate aim of a phytosociological and syntaxonomical synthesis of all urban open spaces.

STUDY AREA
Potchefstroom is situated in the Dry Sandy Highveld Grassland (Bredenkamp & Van Rooyen 1996) of the Grassland Biome (Rutherford & Westfall 1994).
The present study covered the wetlands within the Potchefstroom Municipal Area (between 27° 04 ' and 27° 07' longitude and 26° 40' and 26° 44' latitude) in the North-West province (Figure 1).Potchefstroom is locat ed around the Mooi River, near the confluence of the Mooi River and Loopspruit.Geologically this area is underlain by Pretoria Group strata, namely shale and quart/.itcwith major diabase intrusives.The diabase sills and the Pretoria Group shale are major sources of clay, which may be supplemented by Karoo age clays weath ered from palaeokarst in Malmani Dolomite along the upper reaches of the Mooi River (Nel et al. 1939).
The rainfall in Potchefstroom is erratic, hut the mean annual rainfall exceeds 600 mm.Summer temperatures are high, the mean monthly maximum temperatures exceed 32° C during October to January, whereas the mean monthly minimum temperatures are below -1° C during the months of June to August (Weather Bureau 1988).
The entire Potchefstroom area is drained by the Vaal River and its tributaries, of which the Mooi River and Loopspruit are the most important.The latter has its ori gin near Losberg.The Mooi River is a perennial stream, led by a number of strong springs in the dolomite forma tion, within and beyond the northern limits of the area.
The study area includes three large marshland areas which will be referred to as the Arboretum.Van der Hoff Park and Prozesky marshlands (Figure 1) and a number of very small marshland areas which will not specifical ly be referred to.The Arboretum is a conservation area on the city margin in the northern part of Potchefstroom.northwest of the Potchefstroom Dam and is bordered by natural grasslands.This area is relatively undisturbed in comparison with the other two marshlands, although it is invaded by large reedswamps.The Van der Hoff Park marshland is situated between two residential areas close to the city centre, but is heavily disturbed due to grazing especially by horses.It is a very flat bottomland area without any deep depressions or drainage canals.The marshlands referred to as the Prozesky marshlands are situated in the O.P.M. Prozesky Bird Sanctuary.This conservation area, which stretches from relatively close to the city centre to the southern municipal border, is characterised by deep depressions and drainage canals with higher-lying areas in between, which are heavily grazed by cattle.This wetland is interrupted by a road before it links to a number of dams of the municipal sewage plant.The remaining parts of the banks of the Mooi River houses recreational facilities, sports grounds, including a trim park and a golf course, as well as a num ber of residential gardens (Figure 1).The Wasgoedspruit which drains the western parts of the city, including industrial areas, as well as natural areas, into the Mooi River, is also part of the system.The Wasgoedspruit is disturbed and part of it is layed out with concrete.

MATERIALS AND METHODS
Relevés were compiled in 102 sample plots during the period January to March 1996.Plot sizes were fixed at 16 nr for herbaceous communities and 100 m: for woody communities (Bredenkamp & Theron 1978).For each species present in the sample plots, the cover-abundance values according to Braun-Blanquet scales were used (Mueller-Dombois & Ellcnberg 1974) Habitat parameters recorded, included topography, aspect, slope, soil type and various soil properties, including physical and chemical analyses.The soil prop erties in the A and B horizons included percentage grav el.sand, silt and clay; exchangeable K*.Na2+ .Mg* and Ca+ ; soil conductivity, soil pH (H;0), soil depth and the in situ soil compaction (together with gravimetric water content) in accordance to the Soil Classification Work Group (1991).Different aspects of direct or indirect anthropogenic influences (human impact) such as mow ing, weeding, trampling, grazing, the use of chemicals and erosion, were also recorded as thoroughly as possi ble.It was impossible, though, to quantify the intensities of these anthropogenic influences, but terms such as heavy trampling, heavy grazing and frequent mowing are used in the text to express extreme levels of anthro pogenic inlluences.
The TWINSPAN classification algorithm (Hill 1979a) and the BBPC suite (Bezuidenhout et al. 1996) were used for analysing the floristic data, as first approx imation, and subsequently Braun-Blanquet procedures were used to refine these results.A phytosociological table (Table 1) was obtained through these procedures.
This approach proved to produce ecologically reliable results in many phytosociological studies in natural areas  (Behr & Bredenkamp 1988;Bredenkamp et al. 1989).An additional software package (TURBOVEG) was used for capture, processing, and presentation of phytosociological data (Hennekens 1996a) as well as a visu al editor (MEGATAB) for phytosociological tables (Hennekens 1996b).Introduced species are clearly marked in Table 1.The occurrence of each species in a specific stratum is indicated with a symbol, namely, tl for a high tree layer (> 10 m), t2 for an intermediate tree layer (5-10 m), t3 for a low tree layer (< 5 m), si for a high shrub layer (> 2 m), s2 for a low shrub layer (< 2 m) and hi for a herbaceous layer, in Table 1.Differentiation between trees and shrubs was based on definitions pro posed by Edwards (1983).Two different numbering sys tems were used in Table 1, namely a relevé number which starts at 1 and indicates the relevé numbers which were used in this particular study and a TURBOVEG number which indicates the unique relevé number in the South African phytosociological data base.Species which were only encountered once or twice during the study or which are not diagnostic for a specif ic community or group of communities are not included in Table 1.These species are, however, included in Table 2 and are also mentioned in the results.An ordination algorithm, DECORANA (Hill 1979b) was applied to the floristic data to determine floristic relationships between communities and to detect possible habitat and/or distur bance gradients associated with vegetation gradients.
Taxa names conform to those of Arnold & De Wet (1993), but are updated to November 1996 according to the PRECIS floristic data base of South Africa, managed by the National Botanical Institute in Pretoria.Soil nomenclature follows the classification of the Soil Classification Working Group (1991).No attempt was made to formally fix syntaxa names because this is nor mally avoided in detailed local studies.Formal syntaxonomy will follow after the analyses of all the different land use types in the urban area of Potchefstroom.Formal syntaxa which arc referred to in this study were already described by other authors.The names of these formal syntaxa arc used as they were published without any attempt to validate invalid names or to correct any typographical errors.

Classification
The classification resulted in the recognition of one major community.14 communities, nine subcommuni ties and three variants.The specific position of some of the communities in the three marshland areas are shown in Figure 2.
From the phytosociological table ( This woodland community occurs on the banks of the entire Mooi River, with the exception of parts of the Van der Hoff Park marshland (Figure 2).The soils are deep (> 120 cm) but shallower and with lower clay content (< 50%) than that of the marshland communities.The dominant species in this community is Salix babxlonica (species group D. Table 1), an introduced tree which is naturalised along watercourses in southern Africa (Henderson 1991).These trees are planted at dams and along riverbanks, but its extensive occurrence along the Mooi River is most likely due to self (vegetative) propa gation and dispersal by floodwaters.
Although these trees are aesthetically pleasing and beneficial to man and the environment under certain cir cumstances.they also have disadvantages such as being a potential threat to the conservation of indigenous ripar ian and wetland vegetation (Henderson 1991).The pro lific lateral root system of Salix babylonica is very effec tive in arresting soil erosion of the river banks, but it may also lead to the damming up of small streams and possi bly even flooding during high rainfall seasons.
The diagnostic species of this community are those of species group D (Table 1).and include introduced trees such as Salix babylonica, Celtis sinensis, Morus alba and Ligustrum lucidum, as well as the declared invader Melia azedarach and the indigenous tree.Celtis africana.Other diagnostic spccies are the shrubs Solatium nigrum and Pyracantha coccinea, the climbers Araujia sericifera and Merremia tridentata and the highly invasive rhizomatous fern-like plant, Equisetum ramosissimum (species group D. Table 1).Other species in this commu-    1), the indigenous but invasive shrub.Asparagus laricinus (species group J. Table 1) and intro duced annuals such as Tagetes minuta and Bidens bipinnata (species group R, Table 1).
Although Eckhardt et al. (1993a) mentioned the pres ence of huge specimens of Salix babylonica along the Klip River and in the Seekoeivlei area, northeastern Free State, no Salix babylonica Community was described.
Four subcommunities can be distinguished in the Salix babylonica Woodland Community, based on spe cies composition:

Rhus pyroides Subcommunity
This subcommunity is situated mainly on the city mar gin in relatively dry areas on the river banks, but occurs on lower-lying areas which are seasonally waterlogged, as well.It is commonly associated with vertic soils, for exam ple the Arcadia soil form.It is further characterised by the presence of a dense shrub stratum (canopy cover of 70% and up to 3 m tall) underneath the Salix babylonica trees.The diagnostic shrubs are the indigenous species Rhus pyroides and Maytenus heterophylla and the introduced species Pyracantha angustifolia (species group A, Table 1).Another shrub with relatively high cover in this subcom munity is Asparagus laricinus (species group J, Table 1) which is an important component of bush encroachment in disturbed areas in the Grassland Biome (Friedel 1987;Cilliers & Bredenkamp in press).The tree stratum of 70% canopy cover and height of 8 m is equally well developed and consists of the trees mentioned in species group D (Table 1).One other tree.Robinia pseudoacacia and a number of herbaceous species which occurred only once in this subcommunity are mentioned in Table 2.The herba ceous stratum is poorly developed with a canopy cover of 5% and a height of 90 cm.An average number of 17 species was recorded per sample plot of which 73% were introduced and only 23% were therophytes.

Populus x canescens Subcommunity
On wetter areas on the river banks and on small islands in the Mooi River, the prolific invader Populus x canescens (species group B.  1), and the shrub Asparagus laricinus (species group J, Table 1).An average number of nine species was recorded per sample plot of which 81% were introduced and only 12% were therophytes.

Populus wislizenii Subcommunity
This subcommunity is characterised by the presence of the diagnostic tree Populus wislizenii (species group C, Table 1), a very large and conspicuous shade tree which is planted in adjacent parks.These trees propagate by means of cuttings and do not show any great tenden cy to sucker and therefore, it is not sure how they are spreading into the Salix babylonica Community from the parks.This subcommunity, established on the higher river banks, which are built up by man or which is the result of deep streambed incission, rarely occurs at water level.It is situated mainly on the man-made or anthro pogenous soil type, Witbank, but still with a clay per centage of close to 40%.The very well-developed tree stratum with a canopy cover of nearly 100% and a max imum height of 12 m, makes this subcommunity very conspicuous.The shrub and herbaceous strata are poorly developed.An average of 14 species per sample plot was recorded for this community of which 76% were intro duced and only 7% were therophytes.

Salix babylonica-Pennisetum clandestinum Sub community
The Salix babylonica-Pennisetum clandestinum Sub community occurs mainly along the river banks in parks, sports grounds and other recreational areas, but was occasionally encountered in some of the marshy areas as well.The main soil type on which this subcommunity occurs is the vertic Arcadia soil form, but it is also asso ciated with the anthropogenous soil form, Witbank.The soils are relatively deep, with a high clay content and is sometimes heavily compacted (up to 4 kg/m2).Although no diagnostic species occur in this subcommunity it is characterised by the absence of the species of species groups A, B and C (Table 1) and a relatively high cover of the turf grass, Pennisetum clandestinum (species group G, Table 1), which invades from adjacent lawns.The dominant tree is Salix babylonica in the well-devel oped tree stratum (canopy cover of 65% and up to 8 m high).The total lack of young trees establishing in this subcommunity is worth mentioning.The shrub stratum which consists mainly of Asparagus laricinus close to the stems of the trees, has a canopy cover of 20% and an average height of 1.5 m.Apart from Pennisetum clan destinum, the well-developed herbaceous stratum (70% canopy cover) consists mainly of introduced forbs such as Taraxacum officinale (Table 2) and the annuals, Ipomoea purpurea, Bidens pilosa (species group F, Table 1), Tagetes minuta and Bidens bipinnata (species group R, Table 1).Parts of this subcommunity are mown every 4-6 weeks together with adjacent lawns, according to Mr P. Labuschagne, Department of Parks and Recrea tion, Potchefstroom Municipality.An average number of 15 species per sample plot was recorded of which 64% were introduced and 13% were therophytes.

Amaranthus hybridus-Pennisetum clandestinum Ruderal Community
This community is situated on alluvial deposits in the beds of parts of the Wasgoedspruit as well as on its gen tly sloping inner banks.It is also developing on debris which is the result of an attempt to cover the beds of some areas with concrete.The Wasgoedspruit drains some of the natural areas on the west side of Potchef stroom, as well as industrial areas where a number of chemical plants are situated.Sporadic chemical pollution of this drainage canal was, therefore, encountered in the past.The pollution is probably one of the reasons why no true wetland or reedswamp communities developed in this area.The Wasgoedspruit is mostly dry, but is heavi ly disturbed during floods, making this a true ruderal community.The Amaranthus hybridus-Pennisetum clandestinum Ruderal Community is associated with the Witbank (anthropogenic) soil form with a clay content of < 40%, and which are at places rather heavily compact ed (4.5 kg/m2).
No tree or shrub stratum exists, but the herbaceous stratum is relatively well developed with a canopy cover of about 50% and an average height of 1 m.The turf grass Pennisetum clandestinum (species group G, Table 1) which invades from adjacent parks, is the dominant species.The diagnostic species include introduced forbs and grasses, such as the annuals Amaranthus hybridus and Datura stramonium, which is also a declared weed (Wells et al. 1986) and the perennials Modiola caroliniana and Sorghum halepense, as well as the indigenous forbs, Solarium panduriforme and Commelina benghalensis (species group E, Table 1).Other species which were only recorded once in this study are indigenous grasses such as Chloris virgata, Urochloa mosambicensis, Brachiaria eruciformis, Echinocloa colona and Aristida congesta (Table 2).Other species in this com munity are the highly invasive pioneer grass Cynodon dactylon and the conspicuous tall-growing annuals, Tagetes minuta and Bidens bipinnata (species group R, Table 1).An average number of 14 species was recorded per sample plot, of which 46% were introduced and 35% were therophytes.

Berkheya radula-Thetneda triandra Grassland Com munity
The Berkheya radula-Themeda triandra Grassland Community forms a transitional area between adjacent higher-lying grasslands and the lower-lying wetland communities in the study area (Figure 2).Although situ ated in bottomlands, it is still much higher than the marshlands.This community occurs on clay loam to sandy clay loam soil types, as well as on duplex soils with a high clay content (> 50%), representing the Valsrivier and Rensburg soil forms, and occasionally on rocky soils of the Glenrosa soil form.
The diagnostic species of this community are those of species group H (Table 1).It includes the two dominant species, the indigenous grass Themeda triandra, which is widespread in the Grassland Biome.and the indigenous forb Berkheya radula which is characteristic of soils with a high clay content.Other diagnostic species are all indigenous grasses such as Eragrostis chloromelas, Digitaria eriantha, Panicum stapfianum and Cymbopogon plurinodis.Species which are also abundant in this com munity are the indigenous grass, Setaria sphacelata (species group U, Table 1) and the introduced forb, Physalis viscosa (species group K, Table 1).Forbs which are typically found in the adjacent grasslands, such as Crabbea angustifolia, Rhynchosia totta, Corchorus asplenifolius and Monsonia angustifolia (Table 2) were also encountered in this community.This community is fur ther characterised by the encroachment of the shrub.Asparagus laricinus (species group J. Table 1) forming a canopy cover of up to 12% in some areas.No tree stra tum exists in this community.An average number of 13 species per sample plot was recorded of which 25% were introduced and 11% were therophytes.This community resembles, to a certain extent, the Themeda triandra Variant of the Eragrostidetum planae, an association described for the seasonally wet bottom lands of the Be land type in which Potchefstroom is sit uated (Bezuidenhout & Bredenkamp 1991).A similar grassland community which represents a transitional zone between relatively dry and wet grasslands, the Themeda triandra-Eragrostis plana dry/wet grassland was described by Eckhardt et al. (1993b) for the north eastern Free State.

Hyparrhenia hirta Grassland Community
The Hyparrhenia hirta Grassland Community is also situated on the seasonally Hooded bottomlands adjacent to marshlands, but usually higher than them (Figure 2).It is situated next to the Berkheya radula-Themeda triandra Community in some areas but on the lower-lying wetter parts on soils of the Valsrivier and Rensburg soil forms.
The dominant and also one of the diagnostic species is the indigenous grass Hyparrhenia hirta (species group I, Table 1).Other species in this community are those of species groups J, K, Q, R and U (Table 1).The herba ceous stratum is well developed with a canopy cover of 70% and a height of 1.5 m.No tree stratum exists in this community, but the shrub stratum with Asparagus larici nus and Rhus pyroides is relatively well developed (canopy cover of up to 30% and 1.8 m tall).The abun dance of the invasive grass, Cynodon dactylon and the introduced annuals.Tagetes minuta and Bidens bipinna ta (species group R. Table 1) is an indication of the grad ual degradation of this community.An average number of 12 species was recorded per sample plot of which 47% were introduced and 23% were therophytes.

Cyperus longus Major Wetland Community
This major wetland community is the largest of all the communities, fringing the reedswamps and is mainly sit uated in two of the three marshlands in the study area, namely Van der Hoff Park and Prozesky (Figures 1 and  2).It occurs in a very wide spectrum of habitats, namely from dry to seasonally wet to waterlogged areas, as well as in different stages of habitat degradation.All these variations are covered by the different communities, sub communities and variants.
With the exception of isolated and scattered occur rence of trees and shrubs, no real tree and shrub strata is present in this community.The herbaceous stratum is well developed.The diagnostic species of this major wetland community are those of species group X (Table 1).The most conspicuous of these species are the tallgrowing perennial sedge, Cyperus longus, which shows a great degree of plasticity as it is encountered in aquat ic, helophytic and mesophytic habitats.Because of the effects of grazing and trampling, accompanied by the development of habitats better suited for other species, Cyperus longus is disappearing from some areas where it was dominant some years ago.The diagnostic species include true wetland species which are also diminishing in certain areas, such as Ranunculus multifidus, Falckia oblonga, Stenotaphrum sec undatum and Crinum bulbispermum, and invasive species such as Paspalum dilatatum and Senecio inomatus (species group X.Table 1).
A similar broad major wetland community, the Echinochloa holubii-Cyperus longus Wetland was described by Kooij et al. (1991) for watercourses, riverbanks, valley flats, flood plains and stream channels in the Kroonstad area.Free State.Although the Cyperus longus Major Wetland Community, currently described, is situated on the same soil types it differs from the com munity described by Kooij et al. (1991) with respect to species composition.The difference in species composi tion is probably due to the type and intensity of distur bances in urban areas.
The Cyperus longus Major Wetland Community is divided into the following communities based on habitat and type and intensity of the disturbances:

Cichorium intybus-Xanthium strumarium Invasive Community
This community is highly disturbed and is situated in the heavily grazed areas in the Van der Hoff Park and Prozesky marshland areas (Figure 2).It is characterised by the extensive invasion of the dominant species, the grass Cynodon dactylon (species group R. Table 1), which could suggest an enriched nitrogen status of the soil (Kooij et al., 1991), probably due to biotic effects, e.g. from animal excretions.This community is further characterised by the decrease of the sedge, Cyperus longus (species group X, Table 1), which probably was the dominant species in this area.The diagnostic species of this community are the introduced forbs, Cichorium intybus and the declared weed Xanthium strumarium (Wells et al. 1986) as well as the indigenous, perennial grasses Helictotrichon turgidulum and Eragrostis plana (species group P. Table 1).Other species which occur in this community are those of species groups Q and U (Table 1).
Three subcommunities can be distinguished in this community based on water content of the soil and degree of disturbance.

Paspalum dilatatum Subcommunity
This subcommunity is situated on the higher-lying, somewhat drier areas occupied by the Cichorium intybus-Xanthium strumarium Invasive Community (Figure 2).No diagnostic species occur but it is characterised by the abundance of the conspicuous introduced grass, Paspalum dilatatum (species group X, Table 1).
Three variants, which are forming interconnected mosaics or patches with each other, could be distin guished in the Paspalum dilatatum Subcommunity:

Cynodon dactylon Variant
The Cynodon dactylon Variant occurs on the Hatter areas in footpaths (in the O.P.M. Prozesky Bird Sanctuary) and near entrance gates which are heavily trampled by man and grazing cattle and horses.The foot paths are sometimes mown as well.This variant is asso ciated with the Witbank (anthropogenic) and Rensburg soil forms with deeply cracking, vertic clays, and is also heavily compacted (> 4.5 kg/m2).With the exception of a number of herbaceous species which were recorded once or twice (Table 2), no diagnostic species were encountered in this variant.It is characterised, however, by the absence of species of species groups L and M (Table 1), the total dominance of Cynodon dactylon and a low species diversity in comparison with the other vari ants.Other species which occur are typically ruderal species which can probably withstand the effect of tram pling and grazing such as Physalis viscosa (species group K, Table 1), Plantago lanceolata and the prostrate indigenous forb, Conyza podocephala (species group U, Table 1).An average of 12 species was recorded per sample plot of which 44% were introduced and 20% were therophytes.

Ambrosia psilostachya Variant
The Ambrosia psilostachya Variant of the Paspalum dilatatum Subcommunity is mainly associated with the drier, overgrazed areas in the wetlands of Prozesky, on the same soil types as the Cynodon dactylon Variant.The diagnostic species are the forbs Ambrosia psilostachya and Oxalis pes-caprae and the indigenous grasses Eragrostis micrantha and Andropogon schirensis (spe cies group L, Table 1).Although no tree or shrub stratums exists in this variant, the indigenous shrub Asclepias decipiens and the introduced tree Gleditsia triacanthos (Table 2) have a scattered distribution.Due to the high levels of disturbance an unusually high average number for wetland areas of 22 species per sample plot was recorded, of which 42% were introduced and only 12% were therophytes.

Senecio inornatus Variant
The Senecio inornatus Variant is encountered in the slightly lower and wetter parts of the higher-lying areas in the wetlands of the O.P.M. Prozesky Bird Sanctuary.Although the diagnostic species is the indigenous shrub Gomphocarpus fruticosus (species group M. Table 1).the variant is better characterised by its dominant species, namely the introduced, prostrate grass Cynodon dactylon and the indigenous tall-growing forb, Senecio inornatus.Although Senecio inornatus is also present in other wet land communities, it develops here into relatively dense patches of conspicuous plants due to their height (2 m and higher) and their corymbs of brightly yellow coloured capitula.The sedge, Cyperus longus, which was the dom inant species in this area about two to three years ago, is totally absent in this variant.An average number of 15 species was recorded per sample plot of which 52% were introduced and 16% were therophytes.

Cyperus marginatus Subcommunity
This subcommunity is associated with lower-lying, seasonally waterlogged areas (Figure 2) on the Rensburg soil form with soil compaction of up to 5 kg/m2 or on the Bloemdal soil form with soil compaction of less than 2 kg/m2.Although the annual, introduced forb Physalis angulata (species group N. Table 1) is the only diagnostic species, this subcommunity is better characterised by the presence of the tall-growing sedge, Cyperus marginatus (species group W, Table 1).The invasive grass.Cynodon dactylon and the other tall-growing sedge, Cyperus longus are still the dominant species.Louw (1951) described Cyperus marginatus as a robust species which plays a major role in succession in the Vaal River, but which is rel atively unimportant elsewhere.Although this species was found in the Potchefstroom urban wetlands, the subcom munity in which it exists seems to diminish.This is prob ably due to the effects of trampling and grazing, because it is only situated in relatively small patches in lower-lying, wetter areas.The abundance of introduced annuals such as Physalis angulata (species group N, Table 1), Tagetes minuta and fíidens bipinnata and the declared weed Xanthium strumarium (species group R. Table 1) (Wells et al. 1986) is a further indication of the degraded state of the Cyperus marginatus Subcommunity.An average number of 15 species was recorded per sample plot of which 5 1 % were introduced and 17% were therophytes.

Sesbania bispinosa Subcommunity
This subcommunity represents a more degraded form ot the Cyperus marginatus Subcommunity.The diagnos tic species are the introduced, annual to biannual inva-sive shrub.Sesbania bispinosa and the introduced forb, Flaveria biclentis (species group O. Table 1).An average number of 11 species was recorded per sample plot of which 53% were introduced and 30% were therophytes.

Benda erecta Community
The Berula erecta Community is found in seasonally wet bottomland areas which were at the time of the study partly under standing water of up to 10 cm (Figure 2).The soils are quite deep, undrained vertic soils with a high clay content (> 60%).The community is associated with areas close to or adjacent to the river, in the absence of a tree stratum.
The diagnostic species of this community are the lowgrowing, indigenous forbs, Berula erecta and Mentha aquatica (species group T, Table 1).Other species are the tall-growing sedge, Cyperus longus, the low-growing, mat-forming forb, Falckia oblonga and the indigenous vlei grass, Stenotaphrum secundatum (species group X, Table 1).The Berula erecta Community is further char acterised by the absence of the species of species groups P, Q and R (Table 1), although Cynodon dactylon invades into the margins of this community.The vegetation seems to be relatively undisturbed, but trampling by the grazing animals keeps the wet soil in a permanently pud dled condition, which favours the establishment of Phragmites australis and Typha capensis from the adja cent reedswamps (Communities 10 and 11).Although not recorded during the study, blooms of the very inva sive water fern, Azolla filiculoides can sometimes be found in these puddles, as well.The indigenous geophyte, Crinum bulbispermum (species group X.Table 1), although not abundant, has a scattered distribution in this community.
Two subcommunities could be distinguished in this community.

Rumex conglomeratus Subcommunity
On slightly lower-lying areas of the Berula erecta Community, where water seems to stagnate when the rainfall is high (Figure 2), small clumps of the perennial forb, Rumex conglomeratus (species group S, Table 1) establish.An average number of only eight species per sample plot was recorded of which none were introduced and only 8% were therophytes.

Falckia oblonga Subcommunity
Although no diagnostic species were recorded for this subcommunity, it is characterised by the formation of small but dense mats of the small, rhizomatous forb.Falckia oblonga (species group X, Table 1) in areas where water drainage is faster (Figure 2), and where Rumex conglomeratus is absent.Although Falckia oblon ga was also found on bare areas on the higher slopes of dry vleis, as was suggested by Louw (1951), they are always dominated by the invasive grass, Cynodon dacty lon, in these areas.It is possible that Falckia oblonga finds refuge in the wetter areas, where Cynodon dactylon has not established so extensively.An average of 10 species per sample plot was recorded of w hich 34% were introduced and only 4% were therophytes.

Leersia hexandra Wetland Community
This community was encountered in shallow depres sions or pools which are seasonally waterlogged in the O.P.M. Prozesky Bird Sanctuary and occasionally in water on the river's edge in the Arboretum, but never in deep water (Figure 2).Although no diagnostic species were recorded, this community is characterised by the high cover of rhizomatous grass species such as Leersia hexandra (species group Z. Table 1) and Stenotaphrum secundatum (species group X.Table 1).Other species include the tall-growing sedge, Cyperus longus and the forb Ranunculus multifidus (species group X.Table 1), as well as the low-growing sedge, Care.x schlechteri (species group V. Table 1).An average number of 10 species was recorded per sample plot of which 33% were introduced and only 13% were therophytes.
A similar community, the Leersia hexandra-Schoenoplectus paludicola Wetland was described by Fuls et al. (1992) for the northern Free State.

Cyperus fastigiatus-Paspalum distichum Wetland Community
This community sometimes occurs on the river's edge, but is mainly associated with shallow-to deep drainage depressions or pools which are seasonally waterlogged (Figure 2).In pools where water is stagnant for long periods, this community is mainly situated at the edges.The soils on which this community can be found are vertic soils of the Rensburg soil form, but occasion ally also on the Champagne soil form.
The diagnostic species are the tall-growing sedge.Cyperus fastigiatus (species group Y, Table 1).The dom inant species is the rhizomatous grass Paspalum dis tichum (species group BB.Table 1) which form very dense mats, and tends to grow in deeper water as well.Louw (1951) made the observation in the Vaal River that when this grass migrates to deeper water, its robustness increases and it behaves like a floating aquatic species.This community shows an affinity with the Leersia hexa ndra Community of the Cyperus longus Major Wetland Community, because of the presence of the grass, Leer sia hexandra and the forb Rumex lanceolatus (species group Z. Table 1).The Cyperus fastigiatus-Paspalum distichum Wetland Community differs, however, from the Leersia hexandra Community with respect to the habitat and the fact that the tall-growing sedge Cyperus longus, is replaced by another tall-growing sedge, Cyperus fastigiatus.
Other species occurring in this community which is worth mentioning arc the water fern Marsilea capensis and the tall-growing forb Persicaria lapathifolia (Table 2: under the heading, no specific communities).An aver age number of only six species was recorded per sample plot of which 26*$ were introduced and none were thero phytes.
A similar community with an equally low species diversity, the Paspalum distichum Community associat ed with marshes, was described in the Grootvlei area in the eastern Transvaal (Myburgh et al. 1995).

Schoenoplectus corymbosus Community
This community is situated in deeper water in the Mooi River as well as in deep depressions or pools with stagnant water on the Champagne soil form (Figure 2).The diagnostic species is the tall-growing, robust sedge Schoenoplectus corymbosus (species group AA.Table 1).Occasionally the rhizomatous grass, Paspalum dis tichum (species group BB, Table 1) invades from the river banks into this community.No other species were recorded in this community.

Eleocharis palustris Wetland Community
This community is relatively undisturbed and occurs only on the city margin, in the Arboretum.It is situated on vertic soils which are seasonally waterlogged in bot tomland areas fringing the reedswamps (Figure 2).
The diagnostic species of this community are the sedge, Eleocharis palustris and the forbs, Chironia palustris, Samolus valerandi and Lobelia thermalis (species group CC, Table 1).The forb Chironia palustris, when flowering in summer, transforms this specific communi ty into a conspicuous and aesthetically pleasing pink mass, which was encountered nowhere else in the study area.Although the Eleocharis palustris Community is situated in the same type of habitat in which the Berula erecta Community occurs in other wetlands, there is no real affinity between these two communities regarding species composition.Eleocharis palustris, does, however, have an isolated distribution in the Berula erecta Com munity.An average number of 14 species was recorded per sample plot of which 17% were introduced and 5% were therophytes.

Carex acutiformis Wetland Community
This particularly species-poor community occurs in two of the three marshland areas (Figure 2), namely the Arboretum and Van der Hoff Park, as well as in other areas along the Mooi River, where it is associated with the Typha capensis Reedswamp Community.It either forms a narrow fringe along the reedswamps in water logged areas on the Champagne soil form, or it estab lishes in dense clumps on seasonally waterlogged, high er-lying areas on the Rensburg soil form.This communi ty seems to invade the Berula erecta Community and the Eleocharis palustris Community.
The diagnostic and dominant, and most of the time the only species occurring in this community is the sedge, Carex acutiformis (species group DD, Table 1).All other species which were recorded are species from adjacent wetland communities.

Typha capensis Reedswamp Community
This community which occupies large areas on sea sonally or permanently waterlogged soils (Figure 2) was regarded by Louw (1951) as one of the important con stituents of the reedswamps, together with Phragmites australis.The Typha capensis Community, when occur ring together with the Phragmites australis Community, is always situated on its outside in shallower water, act ing as a pioneer in reedswamp succession in some cases.The Typha capensis Community seems to expand rapid ly into other communities such as the Berula erecta Community.It occurs on the Champagne soil form but also on vertic soils such as the Rensburg and Arcadia soil forms.It was observed in wetlands all over the world that small-seeded anemochorous plants such as Typha species, colonise mudflats during periods of drawdown, as well as areas that have been recently disturbed (Ellison & Bedford 1995).
The diagnostic species of this community is the indige nous, rhizomatous bulrush Typha capensis and to a lesser extent, the forb Hydrocotyle verticillata (species group EE, Table 1).Other species which only occurred once in this study includes the indigenous species Kniphofia ensifolia (Table 2: under the heading, no specific communi ties), which is situated at the edge of the Typha capensis Reedswamp Community.An average number of only six species was recorded per sample plot of which 27% were introduced and 17% were therophytes.

Phragmites australis Reedswamp Community
The Phragmites australis Reedswamp Community is mainly situated in permanently waterlogged areas, some times in water as deep as 2 m, in the river bed and in all three marshland areas (Figure 2).It colonises, together with the Typha capensis Community, also seasonally waterlogged and even higher-lying bare areas along the river.These bare areas are formed due to the destabilisa tion of substrates in the inner city after heavy rains, when water collects in larger volumes and passes through more quickly than usual.
The dominant and only diagnostic species of this community is the tall-growing reed Phragmites australis (species group FF, Table 1).Other species occurring on the edge of this community are ruderal species or species of adjacent wetland communities.This community can easily be distinguished from the other community in the reedswamp, the Typha capensis Commnunity, as it grows much higher (> 3 m) and is much denser in certain areas.The very dense growth of Phragmites australis may block waterways which lead to water stagnation and pro vide breeding places for mosquitoes and bilharzia snails (Marks et al. 1994;Bromilow 1995).An average number of five species was recorded per sample plot of which 55% were introduced and 44% were therophytes.
A similar community, the Phragmites australis Vlei was described by Bloem et al. (1993) in the North-east ern Sandy Highveld, Transvaal..

Azolla filiculoides Floating Community
The Azolla fdiculoides Floating Community invades stagnant water in depressions, pools and drainage canals in some of the wetland areas in Potchefstroom (Figure 2).The only species in this community is the perennial, mat-forming, free-floating, aquatic fern Azolla fdicu-loides (species group GG, Table 1).This species is a declared invader and is regarded as one of the most dan gerous invaders of water bodies in temperate regions in southern Africa (Henderson 1995).This species was not mentioned by Louw (1951)  With the exception of the subcommunities of the Salix babylonica Community (Community 1) and the subcom munities and variants of the Cichorium intybus-Xanthium strumarium Invasive Community (Community 5.1), the different plant units are restricted to specific spatial areas as shown in the scatter diagram (Figure 3).The diagram also illustrates a gradient along ordination axis 1 which could be related to depth of the soil water table (position in the whole wetland system, and ability to establish in deep water).Depth of the water table could have a major influence on the distribution pattern of vegetation (Eckhardt et al. 1993b).The communities found under extreme conditions, namely away from water or in water, occur on the periphery of the diagram (Figure 3).The grassland communities (Communities 3 and 4) which occur on the higher-lying areas, fringing the marshlands and the Salix babylonica Community (Community 1) which occurs on the river banks are situ ated at the left of the scatter diagram.Communities which typically establish in shallow to deep water (Commmunitics 6. 7, 10 and 11) are situated to the right  (1993a);Myburgh et al. (1995) andSmit et al. (1995), in natural wetland areas.The range and intensity of distur bances in certain areas of the Potchefstroom urban wet land, as well as the scale on which the current study was done, probably permitted the recognition of more com munities which may also form small mosaics with each other.In none of the mentioned studies on natural wet lands, was the abundance of the invasive grass, Cynodon dactylon so profound, as in the current study.
Variations in those plant communities which could be related to specific environmental conditions are mainly ascribed to river bank wetness, period of wetness, depth of soil water table, soil type and drainage tempo of soil.With the exception of the anthropogenous soil form, Witbank, all the other soils are deep, poorly drained soils with high clay contents.No correlation between the dif ferent plant communities and data from either physical or chemical soil analyses, were found.
The present study should be used as the basis for future management and conservation of urban wetlands in Potchefstroom.and it should also lead to the develop ment of a Metropolitan Open Space System (MOSS) net work for Potchefstroom.The development of such a net work.together with the involvement of the public in the conservation of the wetlands may prove to be the salva tion of the Potchefstroom wetlands.One important aspect that should immediately be addressed in the man agement of these wetlands is the termination of all mow ing and grazing practises, and the careful monitoring of any successional changes in the vegetation, to be able to assess the exact influence these grazing and mowing practises have on the vegetation.
The classification and description of the wetland communities in the Potchefstroom Municipal Area also serve as a basis for further studies in urban wetlands.Future research must concentrate more specifically on the conservation status and ecological importance of cer tain species and plant communities, the vegetation dynamics of wetland ecosystems under specific human influences and habitat changes, and the effect of certain toxic agents (ecotoxicology) on wetlands.All this infor mation can eventually be used in the development of a model, such as the one of Ellison & Bedford (1995), which can predict the consequences of anthropogenic disturbances in wetlands.
FIGURE 1.-Location o f the wetlands and adjacent areas with different land use in the Potchefstroom Municipal Area, North-West, South Africa.
T A B LE 2.-List of species which occurred only a few times and have relatively low cover-abundance values, as well as species not restricted to specific communities in wetlands of Potchefstroom Municipal Area, North-West, Figure 3 represents the distribution of 98 of the 102 relevés along the first and second axes of a DECORANA ordination.Although the rclevés of the Carex acutiformis Wetland Community and the Azolla filiculoides Floating Community are excluded from Figure3, they show a very clear discontinuity with each other and with all the other communities.This discontinuity was verified by the ordination of the total data set which is not presented in this paper.

FIGURE 3 .
FIGURE 3.-Relative positions of plant communities along first two axes of ordination of wet land plant communities in Pot chefstroom Municipal Area, North-West, South Africa.

TABLE 1 .
-Phytosociological table of wetlands in the Potchefstroom Municipal Area, North-W est.South Africa

TABLE 1 .
-Phytosociological table of wetlands i n the Potchefstroom Municipal Area, North-West, South Africa (cont.)

Table 1
Although Populus x canescens can also arrest soil erosion, it has the same disadvantages as Salix babylonica, namely a threat to indigenous wetland communities and it may also cause flooding.The shrub and herbaceous strata are poorly developed, in compari son with the tree stratum (canopy cover of 80% and 8 m tall).Not many other species grow in this subcommuni ty, apart from climbers such as Araujia sericifera and Merremia tridentata (species group D, Table B. Van der Hoff Park C. O P M Prozesky Bird Sanc tuary All other symbols ex plained in text due to root suckering.