Floristic composition and species diversity of urban vegetation in Bloemfontein , Free State , South Africa

Bloemfontein is a medium-sized city in the Free State province, and is situated in a region characterised by intensive commercial farming. The farming, coupled with increased urbanisation, resulted in degradation and fragmentation of the natural vegetation. An ecological approach to urban open space planning has been suggested (Florgård 2000; Poynton & Roberts 1985; Thompson 2002), which would ensure that open space areas centrally placed in cities are linked with open spaces towards the periphery of cities by dispersal corridors such as rail and roadside vegetation, including ruderal and disturbed vegetation (Poynton & Roberts 1985). Roadsides and railway tracks can have high species richness, especially in terms of rare and endangered plant species that can be harboured in such habitats (Forman & Alexander 1998; Galera et al. 2014).


Introduction
Bloemfontein is a medium-sized city in the Free State province, and is situated in a region characterised by intensive commercial farming. The farming, coupled with increased urbanisation, resulted in degradation and fragmentation of the natural vegetation. An ecological approach to urban open space planning has been suggested (Florgård 2000;Poynton & Roberts 1985;Thompson 2002), which would ensure that open space areas centrally placed in cities are linked with open spaces towards the periphery of cities by dispersal corridors such as rail and roadside vegetation, including ruderal and disturbed vegetation (Poynton & Roberts 1985). Roadsides and railway tracks can have high species richness, especially in terms of rare and endangered plant species that can be harboured in such habitats (Forman & Alexander 1998;Galera et al. 2014).
The ecological approach to urban open space planning and management is a sensible and achievable objective, but it is constrained in part by lack of ecological expertise from the relevant government authorities, lack of infrastructure and financial support and also by public opinion (Cilliers, Müller & Drewes 2004). Public opinion is especially important because, for example, even though urban dwellers show a general desire for contact with nature, there is a consistently negative public perception when it comes to ruderal and spontaneous vegetation on derelict sites (Millard 2004). ecological processes (Bolund & Hunhammar 1999;Federer 1976;Goddard, Dougill & Benton 2009;Godefroid & Koedam 2007). Therefore, cities with relatively large or many conserved open spaces may, for example, have higher species diversity, less water run-off, reduced noise and air pollution (Bolund & Hunhammar 1999;Litschke & Kuttler 2008;Tratalos et al. 2007;Whitford, Ennos & Handley 2001).
The proper management and conservation of urban open spaces requires in-depth knowledge of the spatial distribution, floristic, structural and functional compositions of the major vegetation types (VTs) within the urban environment. The present vegetation study was initiated to identify the main VTs of the open spaces within the Bloemfontein metropolitan area, and to determine the composition and diversity of plant species found in the area. Such urban vegetation studies are relatively few in South Africa, limited to those conducted by among others Roberts (1993), Cilliers, Van Wyk and Bredenkamp (1999) and Grobler, Bredenkamp and Brown (2006).

Research method and design Study area
Bloemfontein extends from approximately 29°00' to 29°15' south and 26°07' to 26°21' east, with altitude ranging from 1350 m to 1450 m above sea level. According to the climate statistics from the South African Weather Service, the annual mean maximum and minimum temperatures are 24.6 °C and 7.6 °C, respectively. Rainfall mainly occurs in summer in the form of thunderstorms, and it averages 550 mm annually. The main geologic feature of the study area is the Karoo Supergroup, represented by the Tierberg Formation of the Ecca Group and the Adelaide Sub-Group of the Beaufort Group; there are also dolerite intrusions of the post-Karoo age (Johnson et al. 2006). Prominent soil groups are oxidic (Hutton form), plinthic (Bainsvlei form), duplex (Valsrivier, Swartland and Sterkspruit forms), cumulic (Oakleaf form), vertic (Arcadia form) and melanic (Milkwood form) (Fey 2010;Soil Classification Working Group 1991). Bloemfontein is situated in the Grassland Biome (Rutherford & Westfall 1994), and is part of the Central Variation of the Dry Cymbopogon-Themeda Veld (Acocks 1988). Other classifications describe Bloemfontein's vegetation as Dry Sandy Highveld Grassland (Bredenkamp & Van Rooyen 1996) and Dry Highveld Grassland (Mucina et al. 2006).

Vegetation survey
The first step of the survey entailed the stratification of vegetation prior to sampling. Stratification of the area was done on 1:50 000 scale maps and 1:30 000 aerial photographs, based on the topography and relative homogeneity of physiognomic units. The topographic units recognised were the watercourses, flat plains as well as the hills and ridges. A total of 248 relevés were compiled; 160 were compiled for the first time, while 88 were from existing data (Muller 1970;Rossouw 1983). Sample plots ranging in size between 16 m 2 for the grassland vegetation and 100 m 2 for the woody vegetation were surveyed across the study area. All plant species present in each sample plot were recorded and each was given a cover-abundance value according to the Braun-Blanquet scale (Kent & Coker 1996;Mueller-Dombois & Ellenberg 1974). Plant taxonomy generally follows Germishuizen and Meyer (2003). For each relevé, habitat attributes were also noted, including rock type (geology), terrain type (topographical position) and an estimation of the percentage of rockiness of the soil surface. Soil characteristics such as soil depth, pH, organic matter and texture were used for the study. Other noted observations included the extent of soil erosion and forms of biotic influence such as utilisation by herbivores and management practices.

Data analysis
Phytosociological data were first captured and processed in the TURBOVEG database (Hennekens 1996a), and then exported to the MEGATAB computer program (Hennekens 1996b) for classification using TWINSPAN (Hill 1979a). The result was a synoptic table that shows a hierarchical classification of the syntaxa, with each synrelevé representing a plant community. The principle of synoptic tables is based on rating the presence of each species within a community on a constancy scale (Kent & Coker 1996;Mueller-Dombois & Ellenberg 1974). An ordination technique, Detrended Correspondence Analysis (DECORANA) (Hill 1979b), was applied to the data set to illustrate floristic relationships between the plant communities and to detect possible relationships between the communities and the environment. Canonical Correspondence Analysis (CANOCO) (Ter Braak & Šmilauer 2009), an extension of DECORANA, was also carried out to further illustrate the correlations between the vegetation data and the environmental variables.
Patterns of species diversity were analysed using two types of diversity, that is, α-diversity and β-diversity, and also evenness. Two aspects of α-diversity were analysed, the first being species richness (S) that is defined as the number of species per sample plot. Because S can be exaggerated by the presence of rare species, α-diversity was also measured with the Shannon-Wiener diversity index (H′). It is a weighted expression of species richness and the proportion in which each species is represented in a sample plot, which is calculated as: where S total is the total number of species present in each VT (γ-diversity) and S ave is the average species richness (α-diversity) for each sample plot in a community.

Vegetation classification and ordination
A synoptic classification of the vegetation is presented, showing only the major VTs and not the lower ranked syntaxa constituting each VT. The following five major vegetation units and four subdivisions were recognised from the study area, as summarised in Table 3: VT

Soil characteristics of vegetation types
The deepest soils were recorded in the R. lanceolatus-C. longus Streambed vegetation (433 mm ± 78 mm) and V. karroo-A. laricinus Streambank vegetation (475 mm ± 62 mm); these soils also have the highest pH of 7.2 ± 1.1 and 7.4 ± 0.9, respectively ( Table 4). The O. europaea-B. saligna Shrubland has the highest clay content (30% ± 5%) and organic matter content (4.65% ± 2.02%). The soils of the A. congesta-T. triandra Grassland and the F. muricata-T. triandra Grassland have the highest sand content at 76% ± 12% and 66% ± 8%, respectively. These communities also occur on relatively shallower soils with the average depth of 285 mm ± 105 mm and 205 mm ± 68 mm, respectively. No soil samples were collected for the D. pottsii-C. orbiculata Grassland and the O. capense-E. nindensis Grassland. The collecting of samples was mainly restricted by the shallow nature of the soil, compounded by the extensive dolerite rocks and boulders in habitats where these communities occur. The CANOCO biplot ( Figure 3) reveals community correlations with soil depth, texture (clay, sand and silt contents), pH and organic matter. Axis 1 (eigenvalue 0.618) shows correlations with soil depth and clay content. Axis 2 (eigenvalue 0.553), on the other hand, is correlated with silt, organic matter, pH and sand.

Patterns of species diversity
The D.    Cilliers, Schoeman and Bredenkamp (1998) reported similar species richness patterns, characterised by low species richness in waterlogged soils compared to the drier river banks.
The R. lanceolatus-C. longus Streambed vegetation has high β w , as there are few common species within the vegetation unit. This high species turnover can mainly be ascribed to the habitat-specific nature of hydrophytic species. The F. muricata-T. triandra Grassland also has high β w , and according to Lennon et al. (2001), inflated β w could result from large differences in species richness between sample plots. There is a high variation of S in the F. muricata-T. triandra Grassland, ranging from 1 to 19. This variation is possibly because of the disturbed and unstable nature of some habitats where parts of this vegetation unit are found, such as on roadsides and along railway tracks. For example, situations where only one species was encountered in a sample plot were along roadsides where Enneapogon cenchroides was found dominating.
A high H′ was recorded for the D.  Africa can also be species rich, and should be properly managed and conserved.

Conclusion
We identified five major VTs and four sub-units in the Bloemfontein area and found the wetlands and rocky outcrops to be most threatened habitats. The O. rosea-B. catharticus wetlands (VT 1) possess a large number of highly palatable species and as a result are subjected to frequent overgrazing and trampling. As a conservation measure, access to these wetland areas could be restricted and this can be achieved by fencing off the most vulnerable areas. The A. diffusa subsp. burkei-C. nudicaulis grassland of the rocky outcrops (VT 3) is threatened by the expansion of Bloemfontein city to the north. This is a botanically diverse VT that occurs exclusively in the Seven Dams Conservancy, and represents an isolated type of vegetation not found in any other parts of Bloemfontein. The area should therefore be regarded as a conservation priority because of its uniqueness and high botanic diversity.