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Vegetation ecology of the pans (playas) of Soetdoring Nature Reserve, Free State Province
Copyright © NISC Pty Ltd
South African Journal of Botany 2003, 69(3): 401–409 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) inside Soetdoring Nature Reserve, a freshwater and a brackish pan, is presented. These ephemeral pan communities are also compared to the vegetation of the hot spring and two permanently inundated earth dams inside the brackish pan’s basin. The vegetation of the freshwater communities differs significantly from that of the brackish pan itself. The freshwater pan was classified as a Mixed Grass Pan and the brackish pan as an Open Diplachne Pan. The Mixed Grass Pan is characterised by a Cynodon transvaalensis–Gnaphalium declinatum community, associated with the following subcommunities: Selago dinteri–Cynodon transvaalensis, Panicum schinzii–Cynodon transvaalensis and
Portulaca oleracea–Cynodon dactylon subcommunity. Cynodon transvaalensis is the dominant species in this pan. The Open Diplachne Pan is characterised by a Diplachne fusca–Eragrostis bicolor community with a Diplachne fusca subcommunity and an Eragrostis bicolor–Diplachne fusca subcommunity associated with it. Diplachne fusca is dominant in the pan basin. A TWINSPAN classification, refined by Braun-Blanquet procedures, revealed five distinct plant communities for the pan basins and freshwater areas inside the basin collectively. This information is important since very few published analyses are available for pans in the Free State Province, as well as in South Africa as a whole.
Introduction The word wetland is a generic term used to group those features of the landscape, the formation of which has been dominated by water, in which processes and characteristics are largely controlled by water (Maltby 1986), and which are commonly referred to as pans (playas), vleis, floodplains, marshes, swamps and bogs, as a single type of ecosystem (Breen and Begg 1991). Endorheic pans (pans with an inlet but no outlet) are relatively easily defined ecosystems. ‘Pan’ is a South African vernacular term for any large, flat, sediment-filled depression that collects water after rain (Mepham and Mepham 1987, Shaw 1988, Seaman et al. 1991, Davies and Day 1998). Pans usually dry up seasonally, mainly due to evaporation (Geldenhuys 1982). Their shape is circular to oval (Allan 1987b). They are relatively shallow, even when fully inundated, and usually less than three metres deep (Allan et al. 1995). ‘Playa’ is the most common name for a pan used in world geomorphological literature (Neal 1975, Davies and Day 1998), which is Spanish for shore (Waisel 1972). Pans are physically unstable, changing in a season or even in a single rainstorm which can cause them to fill with water. Plant and animal species in the pan change according to the depth, duration and volume of flooding, surrounding topography and soil types. Thus, pans provide classic examples of ecological succession, where a plant communi-
ty alters environmental conditions in a way that makes the habitat less favourable for its own survival but more favourable for the development of a different community (Maltby 1986). Pans are widespread in South Africa, but not ubiquitous. Most of them occur on the arid side of both the 500mm mean annual isohyet and the 1 000mm free surface evaporation loss isoline (Le Roux 1978, Goudie and Thomas 1985). Pans are distributed throughout various biomes, being especially common in the Grassland, Nama Karoo and Kalahari Biomes (Allan et al. 1995). Within the arid area there are some major concentrations of pans. Pans are concentrated in, but not restricted to, a pan ‘belt’ that stretches from the North West Province through the Northern Cape Province, and the western Free State to south of the Orange River. In the western Free State the belt runs southwards from Kroonstad, through Wesselsbron, Boshof and Dealesville to south of Kimberley in the Northern Cape Province (Goudie and Thomas 1985, Seaman 1987, Shaw 1988). The pan density is particularly high in the Dealesville–Bultfontein area, according to Goudie and Thomas (1985), and this is the area in which the Soetdoring Nature Reserve falls. Geldenhuys (1982) determined the numbers and densities of pans per sixteenth degree square in the western Free State, and found that in the 2826 CC
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square, in which a part of Soetdoring Nature Reserve falls, a total of 69 pans can be found, with a density of 10.2 pans per 100km–2. Several classification schemes of pans exist in South Africa (Allan et al. 1995), those according to vegetation structure being the most commonly used (Noble and Hemens 1978, Geldenhuys 1982, Allan 1987a). These schemes have been loosely based on the vegetation associated with pans, but very few published analyses of the vegetation exist for South Africa. A comprehensive, although unpublished, survey of the vegetation of a pan is that of Zimbatis (1975) covering Barberspan, near Delareyville in the North West Province. These results, however, refer to the entire Barberspan reserve and include both aquatic and terrestrial plant species. An additional, but superficial, description of the Barberspan flora is provided by Milstein (1975). More recently, Fuls (1993) and Malan (1998) included the vegetation of some pans encountered in the northern and southern Free State, respectively, in their study, while Meintjies (1992) gave a broad description of the vegetation of the Bainsvlei pans, near Bloemfontein. These three mentioned surveys are, however, unpublished. Parris (1984) gave a broad description of the vegetation associated with pans in the Kalahari (South Africa and Botswana). Allan et al. (1995) provide the only other published data that could be found for pans in South Africa, in the form of a list of ninetyseven plant species recorded in the former Transvaal Highveld, (North West Province, Gauteng and Mpumalanga). The aims of this study were to identify, classify and describe the plant communities of the pans in Soetdoring Nature Reserve and to compare them to the vegetation of the hot spring and earth dams inside one of the pan basins. This study is important in providing information on the vegetation associated with pans, since very little information is available for the Free State at present. Study Area Soetdoring Nature Reserve is situated about 34km northwest of Bloemfontein, Free State Province, between latitudes 28°48’–28°53’S and longitudes 25°56’–26°07’E. The Bloemfontein–Dealesville road in the west and the Bloemfontein–Bultfontein road in the east bound the reserve. The average altitude is 1 450m. The reserve covers a total surface area of 6 173ha and is situated around the Modder River and Krugersdrift Dam (Figure 1) (Watson 1993). The reserve is underlain by the Tierberg Formation of the Ecca Group (Karoo Supergroup) (Van Riet et al. 1997). The Tierberg Formation consists of shale, siltstone and sandstone. An extensive layer of sand, of aeolian origin, obscures the Ecca Group over a part of the reserve. Alluvium and scree can be found next to the river (Nolte 1995). Soetdoring Nature Reserve falls in the Grassland Biome (Rutherford and Westfall 1994). The vegetation type in the reserve is Dry Sandy Highveld Grassland, according to Low and Rebelo (1996), and Dry Cymbopogon–Themeda Veld (A50), according to Acocks (1988). There are two pans present, one in the northern part and the other in the southern part of the reserve, on each side of
Janecke, Du Preez and Venter
the Modder River (Figure 1). A shallow branchlet, that contains water most of the time, splits off from the river close to each of the pans. When the river is in flood, the water pushes up into these branchlets and eventually overflows into the two pans. The southern pan is also part of a palaeo-drainage line of the river (Grobler et al. 1988). This pan is further characterised by having two man-made earth dams and a hot spring in its basin. The lay-out of the southern pan is presented in Janecke (2002). Methods The classification of the vegetation was done on the basis of the floristic-sociological (Zurich-Montpellier) approach or Braun-Blanquet method. The essential viewpoint of this method is that plant communities are units of classification, based primarily on species composition (Kent and Coker 1996). In order to accommodate the growing season, sampling was conducted in the spring and summer of 2000–2001. Plot sizes were fixed on 16m2 and stratified random plots were used in order to accommodate the different zones in the pan. In each sample plot all species present were recorded and the cover abundance of each species, according to the Braun-Blanquet scale (Mueller-Dombois and Ellenberg 1974), was noted. Species with less than five occurrences, in the total data set of 55 relevés, were considered to be of too low frequency to make a significant contribution to the communities and have subsequently been omitted. A complete list of these species are given in Janecke (2002). Taxon names conform to the PRECIS species list of the NBI (updated in August 2001), as incorporated in the TURBOVEG database (Hennekens 1996a) of southern African flora. The classification algorithm TWINSPAN (Hill 1979) was applied to the floristic data set and then refined by applying Braun-Blanquet procedures in the MEGATAB table editing program (Hennekens 1996b), in order to compile a phytosociological table (Table 1). Amongst others, these procedures were successfully used by Van Wyk et al. (2000) and Malan et al. (1994) for wetland areas. Results Five plant communities were recognised (Table 1) between the two pans, including the freshwater areas inside the basin. The hierarchical classification is as follows: 1. Cynodon transvaalensis–Gnaphalium declinatum community 1.1 Hemarthria altissima–Cyperus denudatus subcommunity 1.2 Selago dinteri–Cynodon transvaalensis subcommunity 1.3 Panicum schinzii–Cynodon transvaalensis subcommunity 1.4 Portulaca oleracea–Cynodon dactylon subcommunity 1.5 Eleusine coracana–Cynodon dactylon subcommunity 2. Diplachne fusca–Eragrostis bicolor community 2.1 Diplachne fusca subcommunity 2.2 Eragrostis bicolor–Diplachne fusca subcommunity 2.2.1 Polygonum aviculare–Eragrostis bicolor variant 2.2.2 Sporobolus ioclados–Eragrostis bicolor variant 3. Helictotrichon turgidulum–Phyla nodiflora community 4. Juncus rigidus community
Species Group A Hemarthria altissima Xanthium strumarium Cyperus denudatus Species Group B Panicum schinzii Echinochloa holubii Verbena brasiliensis Frankenia pulverulenta Species Group C Portulaca oleracea Cotula microglossa Chenopodium murale Limosella longiflora Species Group D Eleusine coracana Bulbostylis burchellii Species Group E Cynodon transvaalensis Gnaphalium declinatum Alternanthera sessilis Agrostis lachnantha Species Group F Sporobolus ioclados Salsola glabrescens Species Group G Eragrostis bicolor Lycium horridum
Community number Subcommunity number Variant number
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Database nr.
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1 | 1.2 | 1.3 | | | | | | 4 | . . . . . . | . . . . . . . + | . . . . . . | . . . . . . . 3 | . . . . . . | . . . 1 . . .
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Table 1: Phytosociological table of the vegetation in the pans (playas) of Soetdoring Nature Reserve
South African Journal of Botany 2003, 69(3): 401–409 403
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| . . . . | . . . . . . | . . . . . . . | . . . | . . . | . . . . | . . . | b . | . . | . . | . . . . . . | 4 3 4 | 3 . . . . . | . . . | . . . . | . . 1 + 1 b | . . . . . . . | . . . | . . . | . . . . | . . . | b . | . . | . . | . . . . . . | b b b | . . . . . . | . . .
| . . . . | . . . . . . | . . . . . . . | . . . | . . . | . . . . | . . . | . . | . . | + 3 | . . . . . . | . . . | . . . . . . | . . . | . . . . | . . . . . . | . . . . . . . | . . . | . . . | . . . . | . . . | . . | . b | b b | . . . . . . | . . . | . . . . . . | . . .
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Species with an occurrence of less than five have been omitted from the table
Species Group H Diplachne fusca Species Group I Helictotrichon turgidulum Panicum coloratum Eragrostis plana Eragrostis obtusa Species Group J Cyperus bellus Eragrostis biflora Species Group K Digitaria eriantha Selago dinteri Species Group L Phragmites australis Species Group M Juncus rigidus Species Group N Sporobolus virginicus Species Group O Cynodon dactylon Phyla nodiflora Polygonum aviculare
Table 1 cont.
404 Janecke, Du Preez and Venter
South African Journal of Botany 2003, 69(3): 401–409
405
Dealesville
N
Bultfontein
Krugersdrift Dam Bloemfontein
Johannesburg
10km Bloemfontein
NORTHERN PROVINCE BOTSWANA
MPUMALANGA Pretoria Nelspruit Mafkeng Johannesburg NORTH WEST GAUTENG Klerksdorp
NAMIBIA
Upington Kimberley
Bloemfontein FREE STATE LESOTHO
NORTHERN CAPE
Umtata EASTERN CAPE
ATLANTIC OCEAN
Cape Town
Bloemhof 2
Pietersburg
WESTERN CAPE
East London Mossel Bay
KWAZULUNATAL
3 Kimberley
Bethlehem Harrismith
Winburg
1
4
BLOEMFONTEIN
Pietermaritzburg Durban
INDIAN OCEAN
Cape Town
Port Elizabeth
Pans Entrance Gate Picnic Spots Lion and Wild Dog Camps
Welkom
LEGEND
Bethulie 5
1 - Soetdoring Nature Reserve 2 - Sandveld Nature Reserve 3 - Willem Pretorius Nature Reserve 4 - Sterkfontein Dam Nature Reserve 5 - Tussen die Riviere Nature Reserve
Figure 1: Map of Soetdoring Nature Reserve and its location in the Free State Province, Republic of South Africa, in relation to other nature reserves. The dotted lines indicate public and maintenance roads
4.1 Cyperus bellus–Eragrostis biflora subcommunity 4.2 Cynodon transvaalensis–Juncus rigidus subcommunity 4.3 Digitaria eriantha–Selago dinteri subcommunity 4.4 Phragmites australis–Juncus rigidus subcommunity 5. Sporobolus virginicus community. Description According to the classification schemes of pans (Noble and Hemens 1978, Geldenhuys 1982, Allan 1987a), the pan in the northern part of the reserve, with its thick growth of hygrophilous grasses, is classified as a Mixed Grass Pan, while the southern pan, with its brackish conditions and dominant Diplachne fusca, is classified as an Open Diplachne Pan. One of the characteristic features of the vegetation of pans is the concentric zonation of the vegetation around a pan, caused by physical and chemical gradients in the soil (Leistner 1967, Parris and Child 1973, Parris 1984). This
characteristic feature is evident around the Mixed Grass Pan. On the outer edge a gray coloured concentric zone occurs, consisting mostly of Gnaphalium declinatum and Phyla nodiflora. Inside this ring, a zone dominated by Cynodon transvaalensis is found, while the pan basin is covered by a mixture of grasses, such as Panicum schinzii and Echinochloa holubii. In contrast, no concentric zonation occurs in the Open Diplachne Pan, except that the vegetation of the pan dune differs from that of the basin in having a mixture of dwarf karroid shrubs, like Chrysocoma ciliata, Salsola glabrescens and Rosenia humilis; grasses like Themeda triandra, Eragrostis trichophora, Digitaria eriantha, Cynodon dactylon, Chloris virgata and Aristida adscensionis; as well as some species from the pan basin, namely Diplachne fusca, Sporobolus ioclados, Eragrostis bicolor and Cynodon dactylon; and others (Janecke 2002). These relevés were not included in the table, because of the ecotonal nature of the pan dune. Pan ecosystems can pass through many successional
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stages, emphasising their dynamic yet ephemeral nature (Maltby 1986). It is therefore important to note that the communities described here may not be representative of all the successional stages, but only of the final stage present in the dry phase of the pan, before it is inundated again. The vegetation associated with the Mixed Grass Pan is represented by community 1, excluding the Eleusine coracana– Cynodon dactylon subcommunity (1.5), and the vegetation associated with the Open Diplachne Pan by communities 2 to 5 and subcommunity 1.5 (Table 1). 1. Cynodon transvaalensis–Gnaphalium declinatum community This community (Species Group E) characterises the vegetation of the freshwater Mixed Grass Pan. Cynodon transvaalensis is dominant and covers large areas of the pan basin and also occurs in the inner concentric zone on the edge of the pan. This zone is characterised by the grayish forb Gnaphalium declinatum, together with Alternanthera sessilis and Agrostis lachnantha (Species Group E). The Cynodon transvaalensis–Gnaphalium declinatum community subdivides into the following five subcommunities. 1.1 Hemarthria altissima–Cyperus denudatus subcommunity This hygrophilous subcommunity is not part of the vegetation of the Mixed Grass Pan basin itself, but occurs on the edge of the pan on damp soil, between the pan and the Modder River. The diagnostic species of this subcommunity are given in Species Group A. Hemarthria altissima is the dominant species and is a highly palatable, creeping, matforming grass (Van Oudtshoorn 1999). Hemarthria altissima prefers wet areas such as vleis (or swamps) or river banks and is always found growing in or near water (Van Oudtshoorn 1999). Cyperus denudatus, a sedge, is usually also associated with water and occurs in dense stands between Hemarthria altissima. Xanthium strumarium, or cocklebur, occurs in high densities but is a Category 1 invader weed. This means that Xanthium strumarium is an undesirable plant and must be eradicated (Bromilow 2001). 1.2 Selago dinteri–Cynodon transvaalensis subcommunity This subcommunity was mostly present on the edge of the Mixed Grass Pan, between the pan and Acacia karroo thornveld. No diagnostic species occur, but the subcommunity differs from other subcommunities with respect to the absence of Species Groups A–D and the presence of Selago dinteri (Species Group K). Cynodon transvaalensis (Species Group E) has high cover abundance values. Eragrostis bicolor (Species Group G), Gnaphalium declinatum (Species Group E), as well as Phyla nodiflora and Polygonum aviculare (Species Group O) are scattered throughout the subcommunity. 1.3 Panicum schinzii–Cynodon transvaalensis subcommunity The vegetation of this subcommunity covers the basin of the Mixed Grass Pan, excluding the circular zones on the outside. No salt precipitate is visible on this pan’s basin. The presence of this subcommunity contributes largely to the classification of the pan as a Mixed Grass Pan. The species of Species Group B is diagnostic. The grasses Panicum
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schinzii, Echinochloa holubii (Species Group B) and Cynodon transvaalensis (Species Group E) are dominant and jointly cover the entire basin. These are hygrophilous grasses, typical of pans or vleis (Van Oudtshoorn 1999). Panicum schinzii is a highly palatable grass, Cynodon transvaalensis has relatively high grazing value, while Echinochloa holubii has an average grazing value (Van Oudtshoorn 1999). Verbena brasiliensis (Species Group B) also occurs frequently between the grasses, while Frankenia pulverulenta (Species Group B), Polygonum aviculare and Phyla nodiflora (Species Group O) are occasionally found in the pan basin. 1.4 Portulaca oleracea–Cynodon dactylon subcommunity This subcommunity occurs on damp soil in the area between the Mixed Grass Pan and the short, shallow branchlet from the river, filled with stagnant water. Mostly pioneer species, such as Portulaca oleracea, Cotula microglossa, Chenopodium murale and Limosella longiflora (Species Group C), characterise this subcommunity. The high cover values of Alternanthera sessilis (Species Group E) and the high constancy of Cynodon dactylon (Species Group O) are also diagnostic. Cynodon dactylon is a pioneer grass often found in damp places, but not restricted to them (Van Oudtshoorn 1999). Cynodon transvaalensis, Gnaphalium declinatum (Species Group E), Phyla nodiflora and Polygonum aviculare (Species Group O) were also encountered in varying densities. 1.5 Eleusine coracana–Cynodon dactylon subcommunity This subcommunity occurs in the Open Diplachne Pan on the banks of the one earth dam. The earth dam is permanently inundated, thus its vegetation differs from that of the surrounding ephemeral pan, as well as from the vegetation of the shallow branchlet from the river that fills and retreats. Eleusine coracana (Species Group D) is dominant. The diagnostic species of this subcommunity are those of Species Group D, while prominent species include Cynodon transvaalensis, Gnaphalium declinatum, Alternanthera sessilis (Species Group E), Cynodon dactylon and Phyla nodiflora (Species Group O). 2. Diplachne fusca–Eragrostis bicolor community This community characterises the vegetation of the Open Diplachne Pan’s basin. It does not, however, cover the whole basin. There are bare patches where high salt concentrations, visible as crystals on the unvegetated soil surface, are present. Diplachne fusca (Species Group H) is the dominant and diagnostic species in the pan and occurs in varying densities. Other species that frequent the pan basin, are Eragrostis bicolor, Lycium horridum (Species Group G) and Polygonum aviculare (Species Group O). This community also covers the lunette dune on the north-western side of the pan that merges into the grassland. The following two distinct subcommunities are included in the Diplachne fusca –Eragrostis bicolor community. 2.1 Diplachne fusca subcommunity This subcommunity is typical of the pan basin. The habitat is that of extremes with dry, hot, brackish conditions when the
South African Journal of Botany 2003, 69(3): 401–409
pan is dry, in contrast to the conditions when it is inundated. These conditions may even be present at the same time in different parts of the pan. These extreme conditions eliminate almost all plants, except Diplachne fusca, from growing in the habitat. It is one of very few grass species that grow successfully in habitats with brackish soils that are periodically inundated (Van Oudtshoorn 1999). The high cover abundance values of Diplachne fusca (Species Group H) and the absence of all other species, except Sporobolus ioclados (Species Group F), Eragrostis bicolor (Species Group G) and Polygonum aviculare (Species Group O), are diagnostic in this subcommunity. Diplachne fusca is a highly palatable, rhizomatous swamp grass, with different growth forms (Van Oudtshoorn 1999). The growth form in Soetdoring Nature Reserve is of a flat creeping character — the culms are about 0.1m tall and the inflorescences are 50mm long. However, Van Oudtshoorn (1999) described it as having culms 0.3–1.5m tall and inflorescences 0.2–0.35m long. 2.2 Eragrostis bicolor–Diplachne fusca subcommunity This subcommunity (Species Groups G and H) can be found in patches on the basin, as well as on the edge of the Open Diplachne Pan. Diplachne fusca (Species Group H) is dominant in the pan basin. The high cover values of Eragrostis bicolor (Species Group G) is diagnostic. Eragrostis bicolor is known to grow in pans, mostly in moist, brackish soils (Van Oudtshoorn 1999). Lycium horridum (Species Group G) occurs at low density throughout the pan basin. This subcommunity can be subdivided into the following two variants. 2.2.1 Polygonum aviculare–Eragrostis bicolor variant This variant represents the vegetation of the basin of the Open Diplachne Pan where water remains for longer periods. Eragrostis bicolor (Species Group G) and Diplachne fusca (Species group H) are dominant, while Lycium horridum (Species Group G) also has a high frequency. The presence of Polygonum aviculare, Cynodon dactylon and Phyla nodiflora (Species Group O), as well as the absence of all other species groups are characteristic of this variant. 2.2.2 Sporobolus ioclados–Eragrostis bicolor variant This variant occurs on the edge of the pan and contains a mixture of vegetation from the pan basin and from that of the adjacent grassland. Species Groups F and K contain the diagnostic species. Sporobolus ioclados (Species Group F) is the dominant species, along with Salsola glabrescens (Species Group F) and Eragrostis bicolor (Species Group G). Sporobolus ioclados normally grows in and around seasonal pans, since it is well adapted to brackish and other saline soils (Van Oudtshoorn 1999). Diplachne fusca (Species Group H), Digitaria eriantha and Selago dinteri (Species Group K) have equally high cover values in the variant. 3. Helictotrichon turgidulum–Phyla nodiflora community This community occurs in a disturbed area of the pan basin, where water is retained for longer periods than in the rest of the basin. The site is next to a dirt road traversing the pan, at a storm-water drainage pipe. The diagnostic species are those of Species Group I. Helictotrichon turgidulum (Species
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Group I) is dominant, along with Cynodon dactylon and Phyla nodiflora (Species Group O). Panicum coloratum, Eragrostis plana (Species Group I), Eragrostis biflora (Species Group J), Verbena brasiliensis (Species Group B), Sporobolus ioclados (Species Group F) and Eragrostis obtusa (Species Group I) also occur in the community with varying cover abundance values. Except for the forb Verbena brasiliensis, the others are all grass species of moist grassland (Van Oudtshoorn 1999). 4. Juncus rigidus community This community, characterised by Juncus rigidus (Species Group M), surrounds and connects the hot spring and earth dam in the Open Diplachne Pan’s basin. The rush, Juncus rigidus (Species Group M), forms an almost homogeneous stand, where the few open spaces between the Juncus tussocks are occupied by species like Cynodon transvaalensis (Species Group E), Polygonum aviculare (Species Group O) and Frankenia pulverulenta (Species Group B). According to Waisel (1972), the various species of Juncus play an important part in the succession of saline habitats, and under certain conditions form a long prevailing community (sere climax), which will remain as long as the water table and salt levels are constant. Most species are inhabitants of brackish water, but tend to avoid permanently inundated habitats with stagnant water, and concentrate on the fringes of salines (Waisel 1972). Chapman (1974) described the principal association of thermal saline areas in central Africa as a Juncus rigidus–Sporobolus virginicus community, which shows some affinity with this community. The Juncus rigidus community consists of four subcommunities. The first two subcommunities occur at the earth dam and the others are associated with the hot spring of the Open Diplachne Pan. 4.1 Cyperus bellus–Eragrostis biflora subcommunity The earth dam inside the basin of the pan is a permanent water body, the vegetation of which completely differs from that of the pan (note the absence of the Diplachne fusca community (2). This subcommunity occurs on the disturbed soils of the dam wall and is characterised by the species of Species Group J. Cynodon dactylon (Species Group O) is dominant, while Cyperus bellus, Eragrostis biflora (Species Group J), Phyla nodiflora, Polygonum aviculare (Species Group O) and Cynodon transvaalensis (Species Group E) are rather prominent as well. 4.2 Cynodon transvaalensis–Juncus rigidus subcommunity This subcommunity occurs in the seepage area of the earth dam and is represented by Species Groups E and M. No diagnostic species occur, but the subcommunity is characterised by the absence of all species groups, except for Species Groups B, E, G, M and O. Juncus rigidus (Species Group M) is the dominant species, while Cynodon transvaalensis (Species Group E), Cynodon dactylon, Phyla nodiflora and Polygonum aviculare (Species Group O) also form a prominent part of the vegetation. The species of Species Group O have a high constancy in this subcommunity. Phyla nodiflora mostly occupies the zone next to the water’s edge, while the other species are mostly present on the dam walls.
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4.3 Digitaria eriantha–Selago dinteri subcommunity This subcommunity is associated with the vegetation of the hot spring in the Open Diplachne Pan. It occurs on elevated ‘islands’, isolated from the rest of the pan basin. The soil of these ‘island’ areas has a more sandy texture in comparison to the clayey texture of the pan basin. No salt precipitate is visible in these elevated mounts. The subcommunity can also be found between the dense Phragmites australis stand, that occurs around the eye of the spring, and the dominant stand of Juncus rigidus further away from the eye (Subcommunity 4.4). Digitaria eriantha is the dominant species, with Selago dinteri also present in high densities (Species Group K). Cynodon dactylon (Species Group O) occurs in small patches on the islands. All the pan species, except Juncus rigidus (Species Group M), are absent (Species Groups A–J, L and N). These islands have a grassland character, therefore the mixture of grass species associated with Juncus rigidus. 4.4 Phragmites australis–Juncus rigidus subcommunity This subcommunity represents the vegetation of the hot spring. The spring at Vlakkraal 23 (a farm now incorporated into the reserve) is one of three mineral springs within a relatively small area near the Modder River. The others are at Florisbad and Lombards Drift 832, respectively — immediately east of the bridge on the Soutpan tarred road (Grobler and Loock 1988). According to Kent (1949), all three springs are classified as warm springs. Phragmites australis forms a dense stand around the eye of the spring in the reserve. There is no standing water visible, but the vegetation differs completely from that of the ephemeral pan. Species Group L characterises this subcommunity, along with Species Group M. Phragmites australis (Species Group L) is the dominant species, with Juncus rigidus (Species Group M) the only other frequent species. Digitaria eriantha (Species Group K) and Cynodon dactylon (Species Group O) are found scattered between these two dominant species. 5. Sporobolus virginicus community This unique community occurs only in a small patch inside the Open Diplachne Pan, located in the area between the hot spring and the lunette dune. The rhizomatous grass, Sporobolus virginicus (Species Group N), characterises this community, while the absence of almost all the other species listed on Table 1 is also notable. Diplachne fusca (Species Group H) and Polygonum aviculare (Species Group O) are the only other species noted. Sporobolus virginicus is a halophytic, mat-forming, perennial grass with salt glands on its leaves (Naidoo and Naidoo 1998). Salt secretion in halophytes is regarded as an adaptive mechanism to cope with high substrate salinity (Waisel 1972, Naidoo and Naidoo 1998). Although this grass is common along the South African coastal areas, the species may also occur inland around saline water bodies, according to Naidoo and Naidoo (1998).
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etation zones are, however, not a feature of the brackish Open Diplachne Pan on the southern side of the Modder River. The species composition of the two pans differs to a large extent. The Mixed Grass Pan is characterised by a Cynodon transvaalensis–Gnaphalium declinatum community, associated with the following subcommunities: Selago dinteri– Cynodon transvaalensis, Panicum schinzii–Cynodon transvaalensis and Portulaca oleracea–Cynodon dactylon subcommunity. The Hemarthria altissima–Cyperus denudatus subcommunity is not really part of the pan vegetation, but rather represents riparian vegetation, as it occurs between the pan and the river. The Open Diplachne Pan itself is characterised by a Diplachne fusca–Eragrostis bicolor community with a Diplachne fusca subcommunity and an Eragrostis bicolor– Diplachne fusca subcommunity associated with it. The last mentioned subcommunity is subdivided into two variants, namely the Polygonum aviculare–Eragrostis bicolor variant and the variant that represents the lunette dune vegetation — the Sporobolus ioclados–Eragrostis bicolor variant. The Helictotrichon turgidulum–Phyla nodiflora community indicates a disturbed area inside the Open Diplachne Pan, while the Sporobolus virginicus community represents only a small area between the hot spring and the lunette dune, but is absent from the rest of the pan basin. The two earth dams inside the Open Diplachne Pan are respectively characterised by the Eleusine coracana– Cynodon dactylon subcommunity of the Cynodon transvaalensis–Gnaphalium declinatum community, and the Juncus rigidus community along with its Cyperus bellus– Eragrostis biflora and Cynodon transvaalensis–Juncus rigidus subcommunities. This differs from the normal pan vegetation and rather represents vegetation of more permanent water bodies. The hot spring is also characterised by the Juncus rigidus community, but along with the Digitaria eriantha–Selago dinteri and Phragmites australis–Juncus rigidus subcommunities. The pans of Soetdoring Nature Reserve justify conservation, not only as habitat for animals and birds (Janecke 2002), but also by possibly helping to reduce floods further downstream of the Modder River. Water from the overflowing river fills the pans, by means of branchlets and hence they serve as storage areas for water. Consequently, the pans inside the reserve are subjected to impacts from the catchment of the pans that lies outside the reserve. Drainage from ploughed fields situated in the pans’ catchments can introduce pesticides and fertilisers resulting in contamination and eutrophication, which is exacerbated by the endorheic nature of these wetlands (Geldenhuys 1982, Allan 1987b, Seaman 1987). Large areas of the Open Diplachne Pan are not even in the natural state any more, due to man’s previous actions, i.e. the creation of the permanent earth dams in the pan basin (Janecke 2002). Care should be taken that the pans are properly conserved inside the nature reserve.
Discussion A characteristic feature of freshwater pans seems to be the concentric zones of vegetation. The concentric zones may indicate a sere of hygrophytic communities. Concentric veg-
Acknowledgements — Special thanks to Dr PWJ van Wyk, Department of Plant Sciences, for time and assistance in creating the figure. The management of Soetdoring Nature Reserve is thanked for assistance and kindness.
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Malan PW (1998) Vegetation Ecology of the Southern Free State. PhD Thesis, University of the Free State, Bloemfontein Malan PW, Venter HJT, Du Preez PJ (1994) Phytosociology of the Bloemfontein-West district: Vegetation of the Ae land type. South African Journal of Botany 60: 245–250 Maltby E (1986) Waterlogged Wealth — Why Waste the World’s Wet Places? International Institute for Environment and Development, London. ISBN 0–905347–63–3 Meintjies S (1992) A Contribution to the Biology of Triops granarius (Lucas). PhD Thesis, University of the Free State, Bloemfontein Mepham RH, Mepham JS (1987) Seasonal and semi-permanent pans. In: Burgis MJ, Symoens JJ (eds) African Wetlands and Shallow Water Bodies. Editions de l’ORSTOM, Paris, pp 476–486. ISBN 2709908816 Milstein P le S (1975) The biology of Barberspan, with special reference to the avifauna. Ostrich Supplement 10: 6–15 Mueller-Dombois D, Ellenberg H (1974) Aims and Methods of Vegetation Ecology. Wiley, New York. ISBN 0–471–62290–7 Naidoo Y, Naidoo G (1998) Sporobolus virginicus leaf salt glands: morphology and ultrastructure. South African Journal of Botany 64: 198–204 Neal JT (1975) Playas and Dried Lakes: Occurrence and Development. Dowden, Hutchinson & Ross, Stroudsburg. ISBN 0–470–63084–1 Noble RG, Hemens J (1978) Inland water ecosystems in South Africa — a review of research needs. South African National Scientific Program Report No. 34: 1–150 Nolte CC (1995) The Geology of the Winburg Area. Explanation of Sheet 2826, Scale 1:250,000. Council for Geoscience, Geological Survey of South Africa. ISBN 1–875061–15–0 Parris R (1984) Pans, rivers and artificial waterholes in the protected areas of the south-western Kalahari. Supplement to Koedoe, pp 63–82 Parris R, Child G (1973) The importance of pans to wildlife in the Kalahari and the effect of human settlement on these areas. Journal of South African Wildlife Management Association 3: 1–8 Rutherford MC, Westfall RH (1994) Biomes of Southern Africa: An Objective Categorization. National Botanical Institute, Pretoria. ISBN 1–874907–24–2 Seaman M (1987) Pans of the Orange Free State. African Wildlife 41: 237–238 Seaman MT, Ashton PJ, Williams WD (1991) Inland salt waters of southern Africa. Hydrobiologia 210: 75–91 Shaw PA (1988) Lakes and pans. In: Moon BP, Dardis GF (eds) The Geomorphology of Southern Africa. Southern Book Publishers, Johannesburg, pp 120–140. ISBN 1–86812–072–4 Van Oudtshoorn F (1999) Gids tot Grasse van Suider Afrika. Briza Publications, Pretoria. ISBN 187509307–9 Van Riet W, Claasen P, Van Rensburg J, Van Viegen T, Du Plessis L (1997) Environmental Potential Atlas for South Africa. JL van Schaik Publishers, Pretoria. ISBN 0–627–02315–0 Van Wyk E, Cilliers SS, Bredenkamp GJ (2000) Vegetation analysis of wetlands in the Klerksdorp Municipal Area, North West Province, South Africa. South African Journal of Botany 66: 52–62 Waisel Y (1972) Biology of Halophytes. Academic Press, New York. ISBN 0–12–730850–4 Watson JP (1993) Bestuursplan van Soetdoring Natuurreservaat. Unpublished Report for the Department of Environmental Affairs and Tourism, Free State Province Zimbatis N (1975) A floristic survey of provincial nature reserves, Transvaal. Barberspan Nature Reserve. Unpublished Report on project No TN 6.3.3. Transvaal Division of Nature Conservation, pp 1–8
South African Journal of Botany 2003, 69(3): 401–409 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) inside Soetdoring Nature Reserve, a freshwater and a brackish pan, is presented. These ephemeral pan communities are also compared to the vegetation of the hot spring and two permanently inundated earth dams inside the brackish pan’s basin. The vegetation of the freshwater communities differs significantly from that of the brackish pan itself. The freshwater pan was classified as a Mixed Grass Pan and the brackish pan as an Open Diplachne Pan. The Mixed Grass Pan is characterised by a Cynodon transvaalensis–Gnaphalium declinatum community, associated with the following subcommunities: Selago dinteri–Cynodon transvaalensis, Panicum schinzii–Cynodon transvaalensis and
Portulaca oleracea–Cynodon dactylon subcommunity. Cynodon transvaalensis is the dominant species in this pan. The Open Diplachne Pan is characterised by a Diplachne fusca–Eragrostis bicolor community with a Diplachne fusca subcommunity and an Eragrostis bicolor–Diplachne fusca subcommunity associated with it. Diplachne fusca is dominant in the pan basin. A TWINSPAN classification, refined by Braun-Blanquet procedures, revealed five distinct plant communities for the pan basins and freshwater areas inside the basin collectively. This information is important since very few published analyses are available for pans in the Free State Province, as well as in South Africa as a whole.
Introduction The word wetland is a generic term used to group those features of the landscape, the formation of which has been dominated by water, in which processes and characteristics are largely controlled by water (Maltby 1986), and which are commonly referred to as pans (playas), vleis, floodplains, marshes, swamps and bogs, as a single type of ecosystem (Breen and Begg 1991). Endorheic pans (pans with an inlet but no outlet) are relatively easily defined ecosystems. ‘Pan’ is a South African vernacular term for any large, flat, sediment-filled depression that collects water after rain (Mepham and Mepham 1987, Shaw 1988, Seaman et al. 1991, Davies and Day 1998). Pans usually dry up seasonally, mainly due to evaporation (Geldenhuys 1982). Their shape is circular to oval (Allan 1987b). They are relatively shallow, even when fully inundated, and usually less than three metres deep (Allan et al. 1995). ‘Playa’ is the most common name for a pan used in world geomorphological literature (Neal 1975, Davies and Day 1998), which is Spanish for shore (Waisel 1972). Pans are physically unstable, changing in a season or even in a single rainstorm which can cause them to fill with water. Plant and animal species in the pan change according to the depth, duration and volume of flooding, surrounding topography and soil types. Thus, pans provide classic examples of ecological succession, where a plant communi-
ty alters environmental conditions in a way that makes the habitat less favourable for its own survival but more favourable for the development of a different community (Maltby 1986). Pans are widespread in South Africa, but not ubiquitous. Most of them occur on the arid side of both the 500mm mean annual isohyet and the 1 000mm free surface evaporation loss isoline (Le Roux 1978, Goudie and Thomas 1985). Pans are distributed throughout various biomes, being especially common in the Grassland, Nama Karoo and Kalahari Biomes (Allan et al. 1995). Within the arid area there are some major concentrations of pans. Pans are concentrated in, but not restricted to, a pan ‘belt’ that stretches from the North West Province through the Northern Cape Province, and the western Free State to south of the Orange River. In the western Free State the belt runs southwards from Kroonstad, through Wesselsbron, Boshof and Dealesville to south of Kimberley in the Northern Cape Province (Goudie and Thomas 1985, Seaman 1987, Shaw 1988). The pan density is particularly high in the Dealesville–Bultfontein area, according to Goudie and Thomas (1985), and this is the area in which the Soetdoring Nature Reserve falls. Geldenhuys (1982) determined the numbers and densities of pans per sixteenth degree square in the western Free State, and found that in the 2826 CC
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square, in which a part of Soetdoring Nature Reserve falls, a total of 69 pans can be found, with a density of 10.2 pans per 100km–2. Several classification schemes of pans exist in South Africa (Allan et al. 1995), those according to vegetation structure being the most commonly used (Noble and Hemens 1978, Geldenhuys 1982, Allan 1987a). These schemes have been loosely based on the vegetation associated with pans, but very few published analyses of the vegetation exist for South Africa. A comprehensive, although unpublished, survey of the vegetation of a pan is that of Zimbatis (1975) covering Barberspan, near Delareyville in the North West Province. These results, however, refer to the entire Barberspan reserve and include both aquatic and terrestrial plant species. An additional, but superficial, description of the Barberspan flora is provided by Milstein (1975). More recently, Fuls (1993) and Malan (1998) included the vegetation of some pans encountered in the northern and southern Free State, respectively, in their study, while Meintjies (1992) gave a broad description of the vegetation of the Bainsvlei pans, near Bloemfontein. These three mentioned surveys are, however, unpublished. Parris (1984) gave a broad description of the vegetation associated with pans in the Kalahari (South Africa and Botswana). Allan et al. (1995) provide the only other published data that could be found for pans in South Africa, in the form of a list of ninetyseven plant species recorded in the former Transvaal Highveld, (North West Province, Gauteng and Mpumalanga). The aims of this study were to identify, classify and describe the plant communities of the pans in Soetdoring Nature Reserve and to compare them to the vegetation of the hot spring and earth dams inside one of the pan basins. This study is important in providing information on the vegetation associated with pans, since very little information is available for the Free State at present. Study Area Soetdoring Nature Reserve is situated about 34km northwest of Bloemfontein, Free State Province, between latitudes 28°48’–28°53’S and longitudes 25°56’–26°07’E. The Bloemfontein–Dealesville road in the west and the Bloemfontein–Bultfontein road in the east bound the reserve. The average altitude is 1 450m. The reserve covers a total surface area of 6 173ha and is situated around the Modder River and Krugersdrift Dam (Figure 1) (Watson 1993). The reserve is underlain by the Tierberg Formation of the Ecca Group (Karoo Supergroup) (Van Riet et al. 1997). The Tierberg Formation consists of shale, siltstone and sandstone. An extensive layer of sand, of aeolian origin, obscures the Ecca Group over a part of the reserve. Alluvium and scree can be found next to the river (Nolte 1995). Soetdoring Nature Reserve falls in the Grassland Biome (Rutherford and Westfall 1994). The vegetation type in the reserve is Dry Sandy Highveld Grassland, according to Low and Rebelo (1996), and Dry Cymbopogon–Themeda Veld (A50), according to Acocks (1988). There are two pans present, one in the northern part and the other in the southern part of the reserve, on each side of
Janecke, Du Preez and Venter
the Modder River (Figure 1). A shallow branchlet, that contains water most of the time, splits off from the river close to each of the pans. When the river is in flood, the water pushes up into these branchlets and eventually overflows into the two pans. The southern pan is also part of a palaeo-drainage line of the river (Grobler et al. 1988). This pan is further characterised by having two man-made earth dams and a hot spring in its basin. The lay-out of the southern pan is presented in Janecke (2002). Methods The classification of the vegetation was done on the basis of the floristic-sociological (Zurich-Montpellier) approach or Braun-Blanquet method. The essential viewpoint of this method is that plant communities are units of classification, based primarily on species composition (Kent and Coker 1996). In order to accommodate the growing season, sampling was conducted in the spring and summer of 2000–2001. Plot sizes were fixed on 16m2 and stratified random plots were used in order to accommodate the different zones in the pan. In each sample plot all species present were recorded and the cover abundance of each species, according to the Braun-Blanquet scale (Mueller-Dombois and Ellenberg 1974), was noted. Species with less than five occurrences, in the total data set of 55 relevés, were considered to be of too low frequency to make a significant contribution to the communities and have subsequently been omitted. A complete list of these species are given in Janecke (2002). Taxon names conform to the PRECIS species list of the NBI (updated in August 2001), as incorporated in the TURBOVEG database (Hennekens 1996a) of southern African flora. The classification algorithm TWINSPAN (Hill 1979) was applied to the floristic data set and then refined by applying Braun-Blanquet procedures in the MEGATAB table editing program (Hennekens 1996b), in order to compile a phytosociological table (Table 1). Amongst others, these procedures were successfully used by Van Wyk et al. (2000) and Malan et al. (1994) for wetland areas. Results Five plant communities were recognised (Table 1) between the two pans, including the freshwater areas inside the basin. The hierarchical classification is as follows: 1. Cynodon transvaalensis–Gnaphalium declinatum community 1.1 Hemarthria altissima–Cyperus denudatus subcommunity 1.2 Selago dinteri–Cynodon transvaalensis subcommunity 1.3 Panicum schinzii–Cynodon transvaalensis subcommunity 1.4 Portulaca oleracea–Cynodon dactylon subcommunity 1.5 Eleusine coracana–Cynodon dactylon subcommunity 2. Diplachne fusca–Eragrostis bicolor community 2.1 Diplachne fusca subcommunity 2.2 Eragrostis bicolor–Diplachne fusca subcommunity 2.2.1 Polygonum aviculare–Eragrostis bicolor variant 2.2.2 Sporobolus ioclados–Eragrostis bicolor variant 3. Helictotrichon turgidulum–Phyla nodiflora community 4. Juncus rigidus community
Species Group A Hemarthria altissima Xanthium strumarium Cyperus denudatus Species Group B Panicum schinzii Echinochloa holubii Verbena brasiliensis Frankenia pulverulenta Species Group C Portulaca oleracea Cotula microglossa Chenopodium murale Limosella longiflora Species Group D Eleusine coracana Bulbostylis burchellii Species Group E Cynodon transvaalensis Gnaphalium declinatum Alternanthera sessilis Agrostis lachnantha Species Group F Sporobolus ioclados Salsola glabrescens Species Group G Eragrostis bicolor Lycium horridum
Community number Subcommunity number Variant number
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Table 1: Phytosociological table of the vegetation in the pans (playas) of Soetdoring Nature Reserve
South African Journal of Botany 2003, 69(3): 401–409 403
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Species with an occurrence of less than five have been omitted from the table
Species Group H Diplachne fusca Species Group I Helictotrichon turgidulum Panicum coloratum Eragrostis plana Eragrostis obtusa Species Group J Cyperus bellus Eragrostis biflora Species Group K Digitaria eriantha Selago dinteri Species Group L Phragmites australis Species Group M Juncus rigidus Species Group N Sporobolus virginicus Species Group O Cynodon dactylon Phyla nodiflora Polygonum aviculare
Table 1 cont.
404 Janecke, Du Preez and Venter
South African Journal of Botany 2003, 69(3): 401–409
405
Dealesville
N
Bultfontein
Krugersdrift Dam Bloemfontein
Johannesburg
10km Bloemfontein
NORTHERN PROVINCE BOTSWANA
MPUMALANGA Pretoria Nelspruit Mafkeng Johannesburg NORTH WEST GAUTENG Klerksdorp
NAMIBIA
Upington Kimberley
Bloemfontein FREE STATE LESOTHO
NORTHERN CAPE
Umtata EASTERN CAPE
ATLANTIC OCEAN
Cape Town
Bloemhof 2
Pietersburg
WESTERN CAPE
East London Mossel Bay
KWAZULUNATAL
3 Kimberley
Bethlehem Harrismith
Winburg
1
4
BLOEMFONTEIN
Pietermaritzburg Durban
INDIAN OCEAN
Cape Town
Port Elizabeth
Pans Entrance Gate Picnic Spots Lion and Wild Dog Camps
Welkom
LEGEND
Bethulie 5
1 - Soetdoring Nature Reserve 2 - Sandveld Nature Reserve 3 - Willem Pretorius Nature Reserve 4 - Sterkfontein Dam Nature Reserve 5 - Tussen die Riviere Nature Reserve
Figure 1: Map of Soetdoring Nature Reserve and its location in the Free State Province, Republic of South Africa, in relation to other nature reserves. The dotted lines indicate public and maintenance roads
4.1 Cyperus bellus–Eragrostis biflora subcommunity 4.2 Cynodon transvaalensis–Juncus rigidus subcommunity 4.3 Digitaria eriantha–Selago dinteri subcommunity 4.4 Phragmites australis–Juncus rigidus subcommunity 5. Sporobolus virginicus community. Description According to the classification schemes of pans (Noble and Hemens 1978, Geldenhuys 1982, Allan 1987a), the pan in the northern part of the reserve, with its thick growth of hygrophilous grasses, is classified as a Mixed Grass Pan, while the southern pan, with its brackish conditions and dominant Diplachne fusca, is classified as an Open Diplachne Pan. One of the characteristic features of the vegetation of pans is the concentric zonation of the vegetation around a pan, caused by physical and chemical gradients in the soil (Leistner 1967, Parris and Child 1973, Parris 1984). This
characteristic feature is evident around the Mixed Grass Pan. On the outer edge a gray coloured concentric zone occurs, consisting mostly of Gnaphalium declinatum and Phyla nodiflora. Inside this ring, a zone dominated by Cynodon transvaalensis is found, while the pan basin is covered by a mixture of grasses, such as Panicum schinzii and Echinochloa holubii. In contrast, no concentric zonation occurs in the Open Diplachne Pan, except that the vegetation of the pan dune differs from that of the basin in having a mixture of dwarf karroid shrubs, like Chrysocoma ciliata, Salsola glabrescens and Rosenia humilis; grasses like Themeda triandra, Eragrostis trichophora, Digitaria eriantha, Cynodon dactylon, Chloris virgata and Aristida adscensionis; as well as some species from the pan basin, namely Diplachne fusca, Sporobolus ioclados, Eragrostis bicolor and Cynodon dactylon; and others (Janecke 2002). These relevés were not included in the table, because of the ecotonal nature of the pan dune. Pan ecosystems can pass through many successional
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stages, emphasising their dynamic yet ephemeral nature (Maltby 1986). It is therefore important to note that the communities described here may not be representative of all the successional stages, but only of the final stage present in the dry phase of the pan, before it is inundated again. The vegetation associated with the Mixed Grass Pan is represented by community 1, excluding the Eleusine coracana– Cynodon dactylon subcommunity (1.5), and the vegetation associated with the Open Diplachne Pan by communities 2 to 5 and subcommunity 1.5 (Table 1). 1. Cynodon transvaalensis–Gnaphalium declinatum community This community (Species Group E) characterises the vegetation of the freshwater Mixed Grass Pan. Cynodon transvaalensis is dominant and covers large areas of the pan basin and also occurs in the inner concentric zone on the edge of the pan. This zone is characterised by the grayish forb Gnaphalium declinatum, together with Alternanthera sessilis and Agrostis lachnantha (Species Group E). The Cynodon transvaalensis–Gnaphalium declinatum community subdivides into the following five subcommunities. 1.1 Hemarthria altissima–Cyperus denudatus subcommunity This hygrophilous subcommunity is not part of the vegetation of the Mixed Grass Pan basin itself, but occurs on the edge of the pan on damp soil, between the pan and the Modder River. The diagnostic species of this subcommunity are given in Species Group A. Hemarthria altissima is the dominant species and is a highly palatable, creeping, matforming grass (Van Oudtshoorn 1999). Hemarthria altissima prefers wet areas such as vleis (or swamps) or river banks and is always found growing in or near water (Van Oudtshoorn 1999). Cyperus denudatus, a sedge, is usually also associated with water and occurs in dense stands between Hemarthria altissima. Xanthium strumarium, or cocklebur, occurs in high densities but is a Category 1 invader weed. This means that Xanthium strumarium is an undesirable plant and must be eradicated (Bromilow 2001). 1.2 Selago dinteri–Cynodon transvaalensis subcommunity This subcommunity was mostly present on the edge of the Mixed Grass Pan, between the pan and Acacia karroo thornveld. No diagnostic species occur, but the subcommunity differs from other subcommunities with respect to the absence of Species Groups A–D and the presence of Selago dinteri (Species Group K). Cynodon transvaalensis (Species Group E) has high cover abundance values. Eragrostis bicolor (Species Group G), Gnaphalium declinatum (Species Group E), as well as Phyla nodiflora and Polygonum aviculare (Species Group O) are scattered throughout the subcommunity. 1.3 Panicum schinzii–Cynodon transvaalensis subcommunity The vegetation of this subcommunity covers the basin of the Mixed Grass Pan, excluding the circular zones on the outside. No salt precipitate is visible on this pan’s basin. The presence of this subcommunity contributes largely to the classification of the pan as a Mixed Grass Pan. The species of Species Group B is diagnostic. The grasses Panicum
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schinzii, Echinochloa holubii (Species Group B) and Cynodon transvaalensis (Species Group E) are dominant and jointly cover the entire basin. These are hygrophilous grasses, typical of pans or vleis (Van Oudtshoorn 1999). Panicum schinzii is a highly palatable grass, Cynodon transvaalensis has relatively high grazing value, while Echinochloa holubii has an average grazing value (Van Oudtshoorn 1999). Verbena brasiliensis (Species Group B) also occurs frequently between the grasses, while Frankenia pulverulenta (Species Group B), Polygonum aviculare and Phyla nodiflora (Species Group O) are occasionally found in the pan basin. 1.4 Portulaca oleracea–Cynodon dactylon subcommunity This subcommunity occurs on damp soil in the area between the Mixed Grass Pan and the short, shallow branchlet from the river, filled with stagnant water. Mostly pioneer species, such as Portulaca oleracea, Cotula microglossa, Chenopodium murale and Limosella longiflora (Species Group C), characterise this subcommunity. The high cover values of Alternanthera sessilis (Species Group E) and the high constancy of Cynodon dactylon (Species Group O) are also diagnostic. Cynodon dactylon is a pioneer grass often found in damp places, but not restricted to them (Van Oudtshoorn 1999). Cynodon transvaalensis, Gnaphalium declinatum (Species Group E), Phyla nodiflora and Polygonum aviculare (Species Group O) were also encountered in varying densities. 1.5 Eleusine coracana–Cynodon dactylon subcommunity This subcommunity occurs in the Open Diplachne Pan on the banks of the one earth dam. The earth dam is permanently inundated, thus its vegetation differs from that of the surrounding ephemeral pan, as well as from the vegetation of the shallow branchlet from the river that fills and retreats. Eleusine coracana (Species Group D) is dominant. The diagnostic species of this subcommunity are those of Species Group D, while prominent species include Cynodon transvaalensis, Gnaphalium declinatum, Alternanthera sessilis (Species Group E), Cynodon dactylon and Phyla nodiflora (Species Group O). 2. Diplachne fusca–Eragrostis bicolor community This community characterises the vegetation of the Open Diplachne Pan’s basin. It does not, however, cover the whole basin. There are bare patches where high salt concentrations, visible as crystals on the unvegetated soil surface, are present. Diplachne fusca (Species Group H) is the dominant and diagnostic species in the pan and occurs in varying densities. Other species that frequent the pan basin, are Eragrostis bicolor, Lycium horridum (Species Group G) and Polygonum aviculare (Species Group O). This community also covers the lunette dune on the north-western side of the pan that merges into the grassland. The following two distinct subcommunities are included in the Diplachne fusca –Eragrostis bicolor community. 2.1 Diplachne fusca subcommunity This subcommunity is typical of the pan basin. The habitat is that of extremes with dry, hot, brackish conditions when the
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pan is dry, in contrast to the conditions when it is inundated. These conditions may even be present at the same time in different parts of the pan. These extreme conditions eliminate almost all plants, except Diplachne fusca, from growing in the habitat. It is one of very few grass species that grow successfully in habitats with brackish soils that are periodically inundated (Van Oudtshoorn 1999). The high cover abundance values of Diplachne fusca (Species Group H) and the absence of all other species, except Sporobolus ioclados (Species Group F), Eragrostis bicolor (Species Group G) and Polygonum aviculare (Species Group O), are diagnostic in this subcommunity. Diplachne fusca is a highly palatable, rhizomatous swamp grass, with different growth forms (Van Oudtshoorn 1999). The growth form in Soetdoring Nature Reserve is of a flat creeping character — the culms are about 0.1m tall and the inflorescences are 50mm long. However, Van Oudtshoorn (1999) described it as having culms 0.3–1.5m tall and inflorescences 0.2–0.35m long. 2.2 Eragrostis bicolor–Diplachne fusca subcommunity This subcommunity (Species Groups G and H) can be found in patches on the basin, as well as on the edge of the Open Diplachne Pan. Diplachne fusca (Species Group H) is dominant in the pan basin. The high cover values of Eragrostis bicolor (Species Group G) is diagnostic. Eragrostis bicolor is known to grow in pans, mostly in moist, brackish soils (Van Oudtshoorn 1999). Lycium horridum (Species Group G) occurs at low density throughout the pan basin. This subcommunity can be subdivided into the following two variants. 2.2.1 Polygonum aviculare–Eragrostis bicolor variant This variant represents the vegetation of the basin of the Open Diplachne Pan where water remains for longer periods. Eragrostis bicolor (Species Group G) and Diplachne fusca (Species group H) are dominant, while Lycium horridum (Species Group G) also has a high frequency. The presence of Polygonum aviculare, Cynodon dactylon and Phyla nodiflora (Species Group O), as well as the absence of all other species groups are characteristic of this variant. 2.2.2 Sporobolus ioclados–Eragrostis bicolor variant This variant occurs on the edge of the pan and contains a mixture of vegetation from the pan basin and from that of the adjacent grassland. Species Groups F and K contain the diagnostic species. Sporobolus ioclados (Species Group F) is the dominant species, along with Salsola glabrescens (Species Group F) and Eragrostis bicolor (Species Group G). Sporobolus ioclados normally grows in and around seasonal pans, since it is well adapted to brackish and other saline soils (Van Oudtshoorn 1999). Diplachne fusca (Species Group H), Digitaria eriantha and Selago dinteri (Species Group K) have equally high cover values in the variant. 3. Helictotrichon turgidulum–Phyla nodiflora community This community occurs in a disturbed area of the pan basin, where water is retained for longer periods than in the rest of the basin. The site is next to a dirt road traversing the pan, at a storm-water drainage pipe. The diagnostic species are those of Species Group I. Helictotrichon turgidulum (Species
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Group I) is dominant, along with Cynodon dactylon and Phyla nodiflora (Species Group O). Panicum coloratum, Eragrostis plana (Species Group I), Eragrostis biflora (Species Group J), Verbena brasiliensis (Species Group B), Sporobolus ioclados (Species Group F) and Eragrostis obtusa (Species Group I) also occur in the community with varying cover abundance values. Except for the forb Verbena brasiliensis, the others are all grass species of moist grassland (Van Oudtshoorn 1999). 4. Juncus rigidus community This community, characterised by Juncus rigidus (Species Group M), surrounds and connects the hot spring and earth dam in the Open Diplachne Pan’s basin. The rush, Juncus rigidus (Species Group M), forms an almost homogeneous stand, where the few open spaces between the Juncus tussocks are occupied by species like Cynodon transvaalensis (Species Group E), Polygonum aviculare (Species Group O) and Frankenia pulverulenta (Species Group B). According to Waisel (1972), the various species of Juncus play an important part in the succession of saline habitats, and under certain conditions form a long prevailing community (sere climax), which will remain as long as the water table and salt levels are constant. Most species are inhabitants of brackish water, but tend to avoid permanently inundated habitats with stagnant water, and concentrate on the fringes of salines (Waisel 1972). Chapman (1974) described the principal association of thermal saline areas in central Africa as a Juncus rigidus–Sporobolus virginicus community, which shows some affinity with this community. The Juncus rigidus community consists of four subcommunities. The first two subcommunities occur at the earth dam and the others are associated with the hot spring of the Open Diplachne Pan. 4.1 Cyperus bellus–Eragrostis biflora subcommunity The earth dam inside the basin of the pan is a permanent water body, the vegetation of which completely differs from that of the pan (note the absence of the Diplachne fusca community (2). This subcommunity occurs on the disturbed soils of the dam wall and is characterised by the species of Species Group J. Cynodon dactylon (Species Group O) is dominant, while Cyperus bellus, Eragrostis biflora (Species Group J), Phyla nodiflora, Polygonum aviculare (Species Group O) and Cynodon transvaalensis (Species Group E) are rather prominent as well. 4.2 Cynodon transvaalensis–Juncus rigidus subcommunity This subcommunity occurs in the seepage area of the earth dam and is represented by Species Groups E and M. No diagnostic species occur, but the subcommunity is characterised by the absence of all species groups, except for Species Groups B, E, G, M and O. Juncus rigidus (Species Group M) is the dominant species, while Cynodon transvaalensis (Species Group E), Cynodon dactylon, Phyla nodiflora and Polygonum aviculare (Species Group O) also form a prominent part of the vegetation. The species of Species Group O have a high constancy in this subcommunity. Phyla nodiflora mostly occupies the zone next to the water’s edge, while the other species are mostly present on the dam walls.
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4.3 Digitaria eriantha–Selago dinteri subcommunity This subcommunity is associated with the vegetation of the hot spring in the Open Diplachne Pan. It occurs on elevated ‘islands’, isolated from the rest of the pan basin. The soil of these ‘island’ areas has a more sandy texture in comparison to the clayey texture of the pan basin. No salt precipitate is visible in these elevated mounts. The subcommunity can also be found between the dense Phragmites australis stand, that occurs around the eye of the spring, and the dominant stand of Juncus rigidus further away from the eye (Subcommunity 4.4). Digitaria eriantha is the dominant species, with Selago dinteri also present in high densities (Species Group K). Cynodon dactylon (Species Group O) occurs in small patches on the islands. All the pan species, except Juncus rigidus (Species Group M), are absent (Species Groups A–J, L and N). These islands have a grassland character, therefore the mixture of grass species associated with Juncus rigidus. 4.4 Phragmites australis–Juncus rigidus subcommunity This subcommunity represents the vegetation of the hot spring. The spring at Vlakkraal 23 (a farm now incorporated into the reserve) is one of three mineral springs within a relatively small area near the Modder River. The others are at Florisbad and Lombards Drift 832, respectively — immediately east of the bridge on the Soutpan tarred road (Grobler and Loock 1988). According to Kent (1949), all three springs are classified as warm springs. Phragmites australis forms a dense stand around the eye of the spring in the reserve. There is no standing water visible, but the vegetation differs completely from that of the ephemeral pan. Species Group L characterises this subcommunity, along with Species Group M. Phragmites australis (Species Group L) is the dominant species, with Juncus rigidus (Species Group M) the only other frequent species. Digitaria eriantha (Species Group K) and Cynodon dactylon (Species Group O) are found scattered between these two dominant species. 5. Sporobolus virginicus community This unique community occurs only in a small patch inside the Open Diplachne Pan, located in the area between the hot spring and the lunette dune. The rhizomatous grass, Sporobolus virginicus (Species Group N), characterises this community, while the absence of almost all the other species listed on Table 1 is also notable. Diplachne fusca (Species Group H) and Polygonum aviculare (Species Group O) are the only other species noted. Sporobolus virginicus is a halophytic, mat-forming, perennial grass with salt glands on its leaves (Naidoo and Naidoo 1998). Salt secretion in halophytes is regarded as an adaptive mechanism to cope with high substrate salinity (Waisel 1972, Naidoo and Naidoo 1998). Although this grass is common along the South African coastal areas, the species may also occur inland around saline water bodies, according to Naidoo and Naidoo (1998).
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etation zones are, however, not a feature of the brackish Open Diplachne Pan on the southern side of the Modder River. The species composition of the two pans differs to a large extent. The Mixed Grass Pan is characterised by a Cynodon transvaalensis–Gnaphalium declinatum community, associated with the following subcommunities: Selago dinteri– Cynodon transvaalensis, Panicum schinzii–Cynodon transvaalensis and Portulaca oleracea–Cynodon dactylon subcommunity. The Hemarthria altissima–Cyperus denudatus subcommunity is not really part of the pan vegetation, but rather represents riparian vegetation, as it occurs between the pan and the river. The Open Diplachne Pan itself is characterised by a Diplachne fusca–Eragrostis bicolor community with a Diplachne fusca subcommunity and an Eragrostis bicolor– Diplachne fusca subcommunity associated with it. The last mentioned subcommunity is subdivided into two variants, namely the Polygonum aviculare–Eragrostis bicolor variant and the variant that represents the lunette dune vegetation — the Sporobolus ioclados–Eragrostis bicolor variant. The Helictotrichon turgidulum–Phyla nodiflora community indicates a disturbed area inside the Open Diplachne Pan, while the Sporobolus virginicus community represents only a small area between the hot spring and the lunette dune, but is absent from the rest of the pan basin. The two earth dams inside the Open Diplachne Pan are respectively characterised by the Eleusine coracana– Cynodon dactylon subcommunity of the Cynodon transvaalensis–Gnaphalium declinatum community, and the Juncus rigidus community along with its Cyperus bellus– Eragrostis biflora and Cynodon transvaalensis–Juncus rigidus subcommunities. This differs from the normal pan vegetation and rather represents vegetation of more permanent water bodies. The hot spring is also characterised by the Juncus rigidus community, but along with the Digitaria eriantha–Selago dinteri and Phragmites australis–Juncus rigidus subcommunities. The pans of Soetdoring Nature Reserve justify conservation, not only as habitat for animals and birds (Janecke 2002), but also by possibly helping to reduce floods further downstream of the Modder River. Water from the overflowing river fills the pans, by means of branchlets and hence they serve as storage areas for water. Consequently, the pans inside the reserve are subjected to impacts from the catchment of the pans that lies outside the reserve. Drainage from ploughed fields situated in the pans’ catchments can introduce pesticides and fertilisers resulting in contamination and eutrophication, which is exacerbated by the endorheic nature of these wetlands (Geldenhuys 1982, Allan 1987b, Seaman 1987). Large areas of the Open Diplachne Pan are not even in the natural state any more, due to man’s previous actions, i.e. the creation of the permanent earth dams in the pan basin (Janecke 2002). Care should be taken that the pans are properly conserved inside the nature reserve.
Discussion A characteristic feature of freshwater pans seems to be the concentric zones of vegetation. The concentric zones may indicate a sere of hygrophytic communities. Concentric veg-
Acknowledgements — Special thanks to Dr PWJ van Wyk, Department of Plant Sciences, for time and assistance in creating the figure. The management of Soetdoring Nature Reserve is thanked for assistance and kindness.
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