Natural woodland vegetation and plant species richness of the urban open spaces in Gauteng , South Africa

It is estimated that approximately 60 % of the world’s population will be living in urban areas by 2025. In Gauteng, the most densely populated province in South Africa, the natural open spaces are continually under threat from development. Vegetation is the most physical representation of the environment on which all animals are ultimately dependent. In order to evaluate an areas potential for development or conservation it is necessary to make a thorough inventory of the plant communities and their associated habitats. A survey of the natural woodlands was undertaken as part of a project describing the vegetation of the natural open spaces within the Gauteng region. Relevés were compiled in 73 stratified random sample plots in selected open spaces within the study area. A TWINSPAN classification, refined by Braun-Blanquet procedures, indicated six woodland communities that can be grouped into two major woodland communities. A classification and description of these communities as well as their species richness are presented. The results indicate that there are still patches of well-preserved natural vegetation within the study area and contribute to the limited knowledge that presently exists for the vegetation of the area.


Introduction
Although the smallest of the nine provinces of South Africa, covering an area of only 16 191 km², Gauteng has a population of approximately 7.3 million people making it the most densely populated province in South Africa (Gauteng 2000).It is therefore understandable that urbanisation and its associated impacts are the biggest threats to the natural areas that still persist in the Gauteng urban areas.Kowarik (1990) confirms this by stating that human impact has been recognised as one of the most important influences on the composition of vegetation in urban environments.These impacts often include the loss of complete habitats due to the construction of residential, industrial or other developments.Natural areas adjacent to urban areas are equally affected by humanrelated activities such as trampling, footpaths and exotic plant invasions, including stray products from gardens.Management practices such as mowing of natural grasslands and changing the natural veld fire frequency also have an effect on these natural ecosystems.
In the past, little attention was paid to the natural environment when making decisions on development in Gauteng.By overlooking the natural environment in urban development planning, wasteful exploitation of the natural resources occurred (Bredenkamp & Brown 1998a, 1998b).In European countries, landscape ecological mapping and evaluation have become an essential part of the planning process (Spellerberg 1992).Such planning assists in restricting disturbance in areas with sensitive habitats or species and prevent undue fragmentation of wildlife habitat (Helliwell 1973).Until recently, vegetation surveys in urban areas in South Africa were unknown.After a pioneer survey in the Durban Municipal Area (Roberts 1993), surveys were undertaken in the urban areas of Potchefstroom (Cilliers & Bredenkamp 1998, 1999a, 1999b, 1999c, 2000;Cilliers et. al. 1998), Klerksdorp (Van Wyk et al. 1997) in the North-West Province, and Bloemfontein (Dingaan 1999) in the Free State.
The present study reports on a wider assessment of the vegetation of the urban areas of Gauteng, attempting to include areas of high species richness in urban areas and to consolidate the studies previously done in the study area.The purpose of this study was therefore to identify the plant communities of different types of habitat present in the urban areas of Gauteng.This information will help to motivate conservation actions, ensuring that nature conservation strategies are incorporated into land-use planning initiatives within the urban environment.

Study area
The study area is located between 27º40'E-28º25'E and 25º40'S-26º20'S approximately in the center of the Gauteng province (Fig. 1) and includes Pretoria, Midrand, Johannesburg and parts of the West Rand.Altitudes in Gauteng vary from 1081 m to 1899 m above sea level, with a mean altitude of 1512 m (Gauteng 2000).
Mean monthly temperature in the study area is 16.8 ºC with a mean maximum of 22.6 ºC and a mean minimum of 10.8 ºC.The mean winter temperature in the study area is 13.8 ºC and mean summer temperature, 25.6 ºC (Weather Bureau 2000).Mean annual rainfall in Gauteng is 670 mm per year (Gauteng 1997).The geology of the area includes the rock types dolomite, chert, quartzite, granite, diabase, shale and andesitic lava.The most important land type units in this study area are Ab, Bb, Ba that are mainly associated with with flat or slightly undulating landscapes and the Ib land type which is often associated with ridge areas.The main soil forms found in the study area are Mispah and Glenrosa. (Land Type Survey Staff 1985, 1987a, 1987b).Acocks (1988) described the vegetation of the study area as "False" Grassland of the Central variation of Bankenveld (Veld Type 61b).Patches of woodland vegetation are found at sheltered sites on hillslopes and rocky outcrops within this veld type.Bredenkamp & Van Rooyen (1996a) described this vegetation as the Rocky Highveld Grassland vegetation type, which covers the largest part of the study area.A small patch of Moist Cool Highveld Grassland (Bredenkamp & Van Rooyen 1996b) occurs in the southeastern corner of the study area.The area, therefore, predominantly represents the Grassland Biome (Rutherford & Westfall 1986).The open and closed woodland patches in the grassland biome resemble the vegetation of the Waterberg Moist Mountain Bushveld (Bredenkamp & Van Rooyen 1996b) also described by Acocks (1988) as Sour Bushveld (20).The woodland layer of the area is further represented by the Mixed Bushveld vegetation type (Bredenkamp & Van Rooyen 1996a) representing the savanna biome.

Methods
The open spaces within the study area were identified and delineated using 1:50 000 aerial photographs and were stratified into relatively homogeneous areas within which sample plots were randomly allocated.A total of 73 sample plots were randomly located within the woodland vegetation to ensure that all variations were considered and sampled.Plot sizes of 200 m² were used in accordance with Bredenkamp & Theron (1978).
Total floristic composition was noted for every sample plot.Additionally, the average height and percentage cover of the tree, shrub and herbaceous layers were estimated and recorded.The percentage cover of bare soil was also estimated and recorded.The coverage of the tree layer was based on crown cover where the canopies are vertically projected onto the ground (Werger 1974) The same technique was used for the shrub and herbaceous layers.The dominant tree, shrub, and herbaceous species were noted and the cover/abundance for each species was estimated according to the Braun Blanquet coverabundance scale (Mueller-Dombois & Ellenberg 1974).
Environmental data included slope inclination measured in degrees, topography including hills, slopes and crests, aspect (north, south, west, east, and aspects in between) and disturbance factors such as exotic vegetation, trampling and erosion.Information on geology, land type, soil forms and clay contents were obtained from Land Type Survey Staff (1985Staff ( , 1987)).Soil classification is according to Macvicar (1991).
The computer programme TURBOVEG (Hennekens 1996a) and the TWINSPAN classification algorithm (Hill 1979) were used for capture, processing and presentation of phytosociological data.Further refinement was achieved with Braun Blanquet procedures by using MEGATAB (Hennekens 1996b), a visual editor for phytosociological tables.
Names and authors of taxa are in accordance with Arnold & De Wet (1993).For the purposes of this study, Aloe greatheadii var.davyana and Aloe transvaalensis were lumped as one species and referred to as Aloe greatheadii var.davyana.The classification of species growth forms were done according to Van Wyk & Malan (1998).

Results
A mixture of tree, shrub and herbaceous species characterises the woodland areas of the entire study area.General species present in the woodlands are listed under Species Group K (Table 1).
The vegetation of all the woodland communities is characterised by the presence of the small trees Diospyros lycioides, Canthium gilfillanii, Zanthoxylum capensis, Rhus leptodictya, Maytenus heterophylla and Rothmannia capensis together with the exotics Lantana camara and Solanum mauritianum.
The small shrub Asparagus suaveolens, the forbs Commelina africana, Helichrysum rugulosum, the succulent Aloe greatheadii var.davyana, the slender twining herb Pentarrhinum insipidum, the exotic weeds Tagetes minuta, Bidens pilosa, the noxious parasite Cuscuta campestris and the fern Pellaea calomelanos are often encountered.et al. (1995), and also exhibits an affinity to the Afro-montane vegetation of the Drakensberg.The plant species composition of the Combretum molle-Setaria lindenbergiana Open to Closed Woodland relates to the Sub-humid Warm Temperate Bushveld as described by Coetzee et al. (1993b), and shows an affinity to the Bushveld vegetation of the Savanna Biome (Rutherford & Westfall 1986).Bredenkamp & Brown (1998a, 1998b) also described vegetation to the west and north of Johannesburg indicating floristic resemblance to this vegetation unit.The vegetation of the Ruimsig Entomological Reserve in Roodepoort, also shows a floristical resemblance to the woodland species identified in this study (Deutschländer & Bredenkamp 1999).
The common species associated with the Rhus pyroides Forest and Canthium gilfillanii-Aristida transvaalensis-Cymbopogon marginatus Savanna communities identified by Bredenkamp & Theron (1978) An average of 37 species per 200 m² was recorded in this plant community.
In general, the species composition of this community compares well with that of the Acacia caffra -Euclea crispa woodland within the Sub-humid Cool Temperate Mountain Bushveld described by Coetzee et al. (1994Coetzee et al. ( , 1995)), found on north-facing slopes, crests and upper south-facing steep slopes of the rocky outcrops mainly within the Ib Land Type in the Pretoria area.
This community also shows floristic resemblance with the Acacia caffra -Setaria spacelata Closed Woodland described by Coetzee et al. (1995), the Acacia caffra -Setaria sphacelata Closed Woodland in the Roodeplaatdam Nature Reserve (Van Rooyen 1984), the vegetation on the Bronberg in Pretoria (Bredenkamp 1997), the Olea europaea -Cymbopogon excavatus savanna and Olea europeae -Schistotephium heptalobium bush clumps vegetation of the Rietfontein Nature Reserve (Bredenkamp 1991), the Acacia robusta woodland (Ellery 1994) and the Acacia caffra vegetation of the  Bredenkamp & Theron (1978) also show resemblance to this community.Bredenkamp & Brown (1998a, 1998b) described similar vegetation to the west and north of Johannesburg.
Three communities were recognised under this major woodland community: 1 The average number of species recorded in this plant community per 200 m² is 32.
Showing resemblance to this community is the Combretum erythrophyllum -Celtis africana Major Community described by Behr & Bredenkamp (1988) from the Witwatersrand Botanic Garden in Roodepoort and the Celtis africana -Ziziphus mucronata community (Bredenkamp 1997b) from the Fort Klapperkop area in Pretoria and the Lonehill Nature Reserve (Bredenkamp 1992).Bredenkamp (1992) conducted a more detailed assessment on the small Lonehill Nature Reserve and described more communities.Bredenkamp & Brown (1998a, 1998b)

Vegetation
The species group C (Table 1) is diagnostic for this plant community.The diagnostic species are the grass Setaria lindenbergiana and the climber shrub Rhoicissus tridentata.
The most prominent species in this plant community include the trees Acacia caffra, Celtis africana and Euclea crispa, the shrub Diospyros lycioides and the bulbous geophyte Ledebouria revoluta, the noxious weed Tagetes minuta and the succulent Aloe greatheadii var.davyana.Disturbance to this plant community was recorded at most of the sites, with specific reference to the low herbaceous cover under the trees.
An average of 31 species per 200 m² was recorded for this plant community.
The average number of species recorded in this plant community per 200 m² is 41.
In parts of the plant community stands of Protea caffra (Species Group F) and the scarcer Protea roupelliae (Species Group F) are prominent, indicating an affinity with the Drakensberg Highveld Sourveld (Sourveld) vegetation.
The Olea europaea -Cymbopogon excavatus savanna and Olea europeae -Schistotephium heptalobium bush clumps vegetation of the Rietfontein Nature Reserve (Bredenkamp 1991) also show resemblance to this community.

Habitat
This sub-community is found on gradual north and south-facing slopes predominantly in the Pretoria area, but also in Clayville, Fourways and Alberton.This plant community is associated with closed woodland in association with rocky outcrops.This plant community was recorded from the Ib1b land types.

Vegetation
Species that dominate the plant community are the trees Acacia caffra, Celtis africana and Ehretia rigida, the shrublet Asparagus suaveolens, the grasses Themeda triandra, Setaria sphacelata, the succulent Aloe greatheadii var.davyana and the asteraceous Helichrysum rugulosum.Rhus pyroides is locally dominant and Panicum maximum is sometimes prominent.
Species group E (Table 1) is diagnostic for this community.Diagnostic tree species include Acacia karroo, the shrub Asparagus laricinus that forms impenetrable thickets, the slender shrub Sida dregei, the erect shrublets Hibiscus lunarifolius and Teucrium trifidum, the herbaceous shrublet Pavonia burchellii, the twining forb Clematis brachiata, the forb Conyza podocephala and the exotic weeds Oxalis corniculata and Conyza albida.
The average number of species recorded in this plant community per 200 m² is 36.

Habitat
This sub-community is found throughout the study area on a variety of aspects and on gradual to steep slopes of ridges and hills and rocky outcrops.This plant community is prominent on the higher altitude slopes and crests.

Vegetation
Species group F ( The average number of species recorded in this plant community is 36 per 200m². Floristically the Bequartiodendron magalismontanum shrub land of Ellery (1994) also compares well with this community.

Discussion
The results of this study indicate that patches of well-preserved vegetation do occur in urban Gauteng.The sites chosen were relatively undisturbed and many could be important to conserve.Some sites are presently utilised for agricultural purposes.In spite of this, weedy species, eg., Tagetes minuta and Bidens pilosa occur on a large number of plots indicating disturbances of the herbaceous layer.
Species group J (Table 1) indicates a floristic relationship between the Acacia caffra-Themeda triandra-Hyparrhenia hirta Open to Closed Woodland and the Combretum molle-Setaria lindenbergiana Open to Closed Woodland.Both these communities occur on slopes of fairly high altitude.
The classification procedures followed, resulted in clearly defined vegetation units that can be related to the environmental factors observed.This classification contributes to the limited knowledge that presently exists for the vegetation of urban open spaces in Gauteng.The results of this study can be used as a baseline study and future studies can be measured against it.The species richness determined per 200 m² per vegetation type could also be used to determine environmental and ecological degradation and actions could be taken to manage these changes.
The results of this study could further be used to determine how the surrounding landuses have influenced the vegetation of urban open spaces.The results could assist in guiding development in future open spaces in a constantly expanding urban environment.

Fig. 1 .
Fig. 1.Location of the study area within the Gauteng Province.
The species that dominate this plant community are the trees Rhus leptodictya, Celtis africana, Acacia caffra, Canthium gilfillanii and Zanthoxylum capense, the shrub Diospyros lycioides and small shrub Asparagus suaveolens, the succulent Aloe greatheadii var.davyana, the common weed Tagetes minuta, the asteraceous forb Helichrysum rugulosum, and the grasses Themeda triandra, Hyparrhenia hirta and Setaria sphacelata.The high occurrence of the weed Tagetes minuta as well as the other exotic weedy species Verbena bonariensis, Conyza albida, Lantana camara, Zinnia peruviana and the succulent Opuntia ficus-indica in the herbaceous layer together with Melia azedarach in the shrub and tree layer, indicates that this plant community is susceptible to disturbance and generally not in a good condition in the study area.

Table 1
A phytosociological table of the woodland vegetation of the urban areas of Gauteng

Table 1
davyana, the fern Pellaea calomelanos and the grass Melinis nerviglumis.Table 1).Characteristic species associated with this plant community are the trees Pittosporum viridiflorum, Croton gratissimus, Osyris lanceolata and Pouzolzia mixta and the shrubs Ancylobotrys capensis and Rhus magalismontana and the sbrublet Waltheria indica.Other species include forbs like the succulent Crassula setulosa, Vernonia sutherlandii, Cyperus sp. and Sutera caerulea and the grasses Aristida transvaalensis and Enneapogon scoparius.Species that dominate the plant community are the trees Combretum molle, Vangueria infausta, Canthium gilfillanii and Diospyros lycioides the grasses Cymbopogon validus, Melinis repens and the forb Commelina africana.
The most prominent species in this plant community are the trees Combretum molle and Vangueria infuasta, the shrubs Xerophyta retinervis, the succulent Aloe greatheadii var.