(Playas) of Soetdoring Nature Reserve, Free State Province
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South African Journal of Botany 2003, 69(3): 401–409 Copyright © NISC Pty Ltd Printed in South Africa — All rights reserved SOUTH AFRICAN JOURNAL OF BOTANY ISSN 0254–6299 Vegetation ecology of the pans (playas) of Soetdoring Nature Reserve, Free State Province BB Janecke*, PJ du Preez and HJT Venter Department of Plant Sciences, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa * Corresponding author, e-mail: [email protected] Received 3 December 2002, accepted in revised form 22 April 2003 A comparison of the vegetation of two pans (playas) Portulaca oleracea–Cynodon dactylon subcommunity. inside Soetdoring Nature Reserve, a freshwater and a Cynodon transvaalensis is the dominant species in this brackish pan, is presented. These ephemeral pan com- pan. The Open Diplachne Pan is characterised by a munities are also compared to the vegetation of the hot Diplachne fusca–Eragrostis bicolor community with a spring and two permanently inundated earth dams Diplachne fusca subcommunity and an Eragrostis inside the brackish pan’s basin. The vegetation of the bicolor–Diplachne fusca subcommunity associated freshwater communities differs significantly from that with it. Diplachne fusca is dominant in the pan basin. A of the brackish pan itself. The freshwater pan was clas- TWINSPAN classification, refined by Braun-Blanquet sified as a Mixed Grass Pan and the brackish pan as an procedures, revealed five distinct plant communities for Open Diplachne Pan. The Mixed Grass Pan is charac- the pan basins and freshwater areas inside the basin terised by a Cynodon transvaalensis–Gnaphalium decli- collectively. This information is important since very natum community, associated with the following sub- few published analyses are available for pans in the communities: Selago dinteri–Cynodon transvaalensis, Free State Province, as well as in South Africa as a Panicum schinzii–Cynodon transvaalensis and whole. Introduction The word wetland is a generic term used to group those fea- ty alters environmental conditions in a way that makes the tures of the landscape, the formation of which has been habitat less favourable for its own survival but more dominated by water, in which processes and characteristics favourable for the development of a different community are largely controlled by water (Maltby 1986), and which are (Maltby 1986). commonly referred to as pans (playas), vleis, floodplains, Pans are widespread in South Africa, but not ubiquitous. marshes, swamps and bogs, as a single type of ecosystem Most of them occur on the arid side of both the 500mm mean (Breen and Begg 1991). annual isohyet and the 1 000mm free surface evaporation Endorheic pans (pans with an inlet but no outlet) are rela- loss isoline (Le Roux 1978, Goudie and Thomas 1985). Pans tively easily defined ecosystems. ‘Pan’ is a South African are distributed throughout various biomes, being especially vernacular term for any large, flat, sediment-filled depres- common in the Grassland, Nama Karoo and Kalahari Biomes sion that collects water after rain (Mepham and Mepham (Allan et al. 1995). Within the arid area there are some major 1987, Shaw 1988, Seaman et al. 1991, Davies and Day concentrations of pans. Pans are concentrated in, but not 1998). Pans usually dry up seasonally, mainly due to evap- restricted to, a pan ‘belt’ that stretches from the North West oration (Geldenhuys 1982). Their shape is circular to oval Province through the Northern Cape Province, and the west- (Allan 1987b). They are relatively shallow, even when fully ern Free State to south of the Orange River. In the western inundated, and usually less than three metres deep (Allan et Free State the belt runs southwards from Kroonstad, through al. 1995). ‘Playa’ is the most common name for a pan used Wesselsbron, Boshof and Dealesville to south of Kimberley in world geomorphological literature (Neal 1975, Davies and in the Northern Cape Province (Goudie and Thomas 1985, Day 1998), which is Spanish for shore (Waisel 1972). Seaman 1987, Shaw 1988). Pans are physically unstable, changing in a season or The pan density is particularly high in the even in a single rainstorm which can cause them to fill with Dealesville–Bultfontein area, according to Goudie and water. Plant and animal species in the pan change accord- Thomas (1985), and this is the area in which the Soetdoring ing to the depth, duration and volume of flooding, surround- Nature Reserve falls. Geldenhuys (1982) determined the ing topography and soil types. Thus, pans provide classic numbers and densities of pans per sixteenth degree square examples of ecological succession, where a plant communi- in the western Free State, and found that in the 2826 CC 402 Janecke, Du Preez and Venter square, in which a part of Soetdoring Nature Reserve falls, the Modder River (Figure 1). A shallow branchlet, that con- a total of 69 pans can be found, with a density of 10.2 pans tains water most of the time, splits off from the river close to per 100km–2. each of the pans. When the river is in flood, the water push- Several classification schemes of pans exist in South es up into these branchlets and eventually overflows into the Africa (Allan et al. 1995), those according to vegetation two pans. The southern pan is also part of a palaeo-drainage structure being the most commonly used (Noble and line of the river (Grobler et al. 1988). This pan is further char- Hemens 1978, Geldenhuys 1982, Allan 1987a). These acterised by having two man-made earth dams and a hot schemes have been loosely based on the vegetation asso- spring in its basin. The lay-out of the southern pan is pre- ciated with pans, but very few published analyses of the veg- sented in Janecke (2002). etation exist for South Africa. A comprehensive, although unpublished, survey of the vegetation of a pan is that of Methods Zimbatis (1975) covering Barberspan, near Delareyville in the North West Province. These results, however, refer to The classification of the vegetation was done on the basis of the entire Barberspan reserve and include both aquatic and the floristic-sociological (Zurich-Montpellier) approach or terrestrial plant species. An additional, but superficial, Braun-Blanquet method. The essential viewpoint of this description of the Barberspan flora is provided by Milstein method is that plant communities are units of classification, (1975). More recently, Fuls (1993) and Malan (1998) includ- based primarily on species composition (Kent and Coker ed the vegetation of some pans encountered in the northern 1996). In order to accommodate the growing season, sam- and southern Free State, respectively, in their study, while pling was conducted in the spring and summer of Meintjies (1992) gave a broad description of the vegetation 2000–2001. Plot sizes were fixed on 16m2 and stratified ran- of the Bainsvlei pans, near Bloemfontein. These three men- dom plots were used in order to accommodate the different tioned surveys are, however, unpublished. Parris (1984) zones in the pan. In each sample plot all species present gave a broad description of the vegetation associated with were recorded and the cover abundance of each species, pans in the Kalahari (South Africa and Botswana). Allan et according to the Braun-Blanquet scale (Mueller-Dombois al. (1995) provide the only other published data that could be and Ellenberg 1974), was noted. Species with less than five found for pans in South Africa, in the form of a list of ninety- occurrences, in the total data set of 55 relevés, were con- seven plant species recorded in the former Transvaal sidered to be of too low frequency to make a significant con- Highveld, (North West Province, Gauteng and tribution to the communities and have subsequently been Mpumalanga). omitted. A complete list of these species are given in The aims of this study were to identify, classify and Janecke (2002). Taxon names conform to the PRECIS describe the plant communities of the pans in Soetdoring species list of the NBI (updated in August 2001), as incor- Nature Reserve and to compare them to the vegetation of porated in the TURBOVEG database (Hennekens 1996a) of the hot spring and earth dams inside one of the pan basins. southern African flora. This study is important in providing information on the vege- The classification algorithm TWINSPAN (Hill 1979) was tation associated with pans, since very little information is applied to the floristic data set and then refined by applying available for the Free State at present. Braun-Blanquet procedures in the MEGATAB table editing program (Hennekens 1996b), in order to compile a phyto- Study Area sociological table (Table 1). Amongst others, these proce- dures were successfully used by Van Wyk et al. (2000) and Soetdoring Nature Reserve is situated about 34km north- Malan et al. (1994) for wetland areas. west of Bloemfontein, Free State Province, between lati- tudes 28°48’–28°53’S and longitudes 25°56’–26°07’E. The Results Bloemfontein–Dealesville road in the west and the Bloemfontein–Bultfontein road in the east bound the Five plant communities were recognised (Table 1) between reserve. The average altitude is 1 450m. The reserve covers the two pans, including the freshwater areas inside the a total surface area of 6 173ha and is situated around the basin. The hierarchical classification is as follows: Modder River and Krugersdrift Dam (Figure 1) (Watson 1. Cynodon transvaalensis–Gnaphalium declinatum community 1993). 1.1 Hemarthria altissima–Cyperus denudatus subcom- The reserve is underlain by the Tierberg Formation of the munity Ecca Group (Karoo Supergroup) (Van Riet et al. 1997). The 1.2 Selago dinteri–Cynodon transvaalensis subcommunity Tierberg Formation consists of shale, siltstone and sand- 1.3 Panicum schinzii–Cynodon transvaalensis subcom- stone. An extensive layer of sand, of aeolian origin, obscures munity the Ecca Group over a part of the reserve.