versión On-line ISSN 2071-0771
versión impresa ISSN 0075-6458
Koedoe vol.52 no.1 Pretoria ene. 2010
Robert F. BrandI; Leslie R. BrownI; Pieter J. du PreezII
IApplied Behavioural Ecology and Ecosystem Research Unit, Department of Environmental Sciences, University of South Africa, Florida campus, South Africa
IIDepartment of Plant Sciences, University of the Free State, South Africa
A checklist of vascular plants of Platberg was compiled to determine species richness, rarity and endemism. The floristic analysis is part of the Department of Economic, Tourism and Environmental Affairs Free State biodiversity assessment programme and conservation management plan for Platberg.
The analysis identified a total of 669 species belonging to 304 genera and 95 families, with 214 species belonging to the Monocotyledoneae and 438 species to the Dicotyledoneae. The largest family is Asteraceae with 126 species, followed by Poaceae with 73 species, Cyperaceae with 39 species, Fabaceae with 33 species, and Scrophulariaceae with 27 species. Various fynbos species were found, as well as 26 endemic/near-endemic species belonging to the Drakensberg Alpine Centre or Eastern Mountain Region.
The results of this study revealed that Platberg shares inselberg floral richness and endemism that can be tracked via the Afromontane archipelago-like string of inselbergs and mountains, which stretch north through the Chimanimani Mountains, into Malawi, the Eastern Arc Mountains via Tanzania and north through Ethiopia, into Eurasia.
Conservation implications: Platberg, as an inselberg, is a site of significant biological diversity, with high species richness, vegetation selection and ecosystem complexity. It shares floral richness and endemism via inselbergs and mountains throughout Africa. The high species richness, Red Data species and ecosystems make this area an important conservation site that should be legislated and protected.
Keywords: Drakensberg Alpine Centre; endemism; Platberg; Red Data species; species richness
With the human population steadily on the rise, the natural environment is under greater pressure than ever before (Huntley 1991). Natural areas act as reservoirs for plant and animal populations (IUCN 1980). Platberg, overlooking the town of Harrismith in the Free State, is an inselberg that presents a refuge for indigenous plants and animals (Burke 2001; Gröger et al. 1996; Mutke et al. 2001; Porembski et al. 1997, 1998; Porembski & Brown 1995). Vegetation surveys provide information on the different plant communities and plant species present and form the basis of any management plan for a specific area (Brown et al. 1996).
Little is known about the different taxa of Platberg and hence a detailed floristic and ecological survey was undertaken to quantify threats to the native flora and to establish whether links exist with higher-altitude Afro-alpine flora occurring on the Drakensberg. No extensive vegetation surveys had been undertaken on Platberg prior to this study; only limited opportunistic floristic collections were done in the mid-1960s by Mrs. Jacobs (these vouchers were mounted and authenticated in 2006 and are now housed at the Geo Potts Herbarium, Botany Department, University of the Free State). Similarly, 50 relevés were sampled between 1975 and 1976 by Professor H.J.T. Venter, Department of Genetics and Plant Sciences, University of the Free State.
The town of Harrismith is located at the foot of Platberg (29º10' S and 28º16' E), along the N3 highway between Johannesburg and Durban (Figure 1).
Platberg's altitude ranges from 1 900 m to 2 394 m. The surface area covers approximately 3 000 ha. The slopes are steep with numerous vegetated gullies and boulder scree slopes below vertical cliffs that are 20 m to 45 m high. Waterfalls cascade down the southern cliffs after rain. A permanent stream arising from the Gibson Dam on the undulating plateau flows off the escarpment and cascades as a waterfall (Figure 2).
From a distance, Platberg appears to have a distinct flat top. However, once on the summit the plateau is found to be undulating, with rolling grass-covered slopes. The vegetation of the plateau is dominated by grassland, with a few rocky ridges, sheet rock and rubble patches, as well as numerous seasonal wetlands and a permanent open playa (pan) on its far western side.
Woody patches of the genera Leucosidea, Buddleja, Kiggelaria, Polygala, Heteromorpha and Rhus shrubs, as well as the indigenous Mountain bamboo Thamnocalamus tessellatus, grow along the base of the cliffs. The shrubland vegetation is concentrated on the cool side of Platberg, on sandstone of the Clarens Formation, in gullies, on scree slopes, mobile boulder beds, and on rocky ridges. Shrubs and trees also occur in a riparian habitat in the south-facing cleft, in which the only road ascends steeply to the summit.
Platberg falls within the Grassland Biome, generally containing short to tall sour grasses (Acocks 1988; Low & Rebelo 1996). The vegetation map of Mucina and Rutherford (2006) shows Platberg as a prominent isolated vegetation 'island' with affinities to the Drakensberg Grassland Bioregion, embedded in a lower lying matrix of Eastern Free State Sandy Grassland (Gm 4). Platberg also has elements of Fynbos (Brand et al. 2008; Mucina & Rutherford 2006), False Karoo (Acocks 1998) and Succulent Karoo (Low & Rebelo 1996), as well as elements of Temperate and Transitional Forest, specifically Highland Sourveld (44a, Acocks 1988) veld types.
Platberg consists of layers of the Karoo Supergroup, which stratagraphically lie immediately below the volcanic rocks of the Drakensberg Formation, the youngest unit of the Stormberg Group. The 1 900 m contour marks the initiation of the footslopes of Platberg and is constituted by the aeolian and waterborne sediments of the Clarens Formation (Du Toit 1954; Loock et al. 1991; Norman & Whitfield 1998; Truswell 1970). Several dolerite dykes intersect Platberg, with a close-set sequence of parallel dykes forming the woody/shrub community on the northern and southern slopes. A dolerite dyke is evident just below the Gibson Dam and is exposed by a borrow pit used for quarrying. Numerous other dykes are visible on aerial photographs. These dyke swarms occur mostly on the flat terrain at lower altitudes, with only a few intersecting Platberg itself. These dykes are also visible in plantation road-cuts on the southern side of Platberg.
Topography and soils
The steep slopes are composed of shale and mostly sandstone of the Clarens Formation, intersected by dolerite dykes, resulting in benches, deep ravines and terraces. Numerous sandstone and basalt boulders form scree and boulder slopes, resulting in miscellaneous soils forms, characteristic of the Ib land type, specifically Mispah (Ms) and Glenrosa (Gs) soil forms (du Preez 1991; MacVicar et al. 1991; Mucina & Rutherford 2006). There are affinities with Fa land types' undulating rocky landscape dominated by shallow Ms and Gs soil forms (Land Type Survey Staff 1991; Smit et al. 1993).
The summit plateau is composed of igneous rock, specifically basalt and dolerite. These soils are generally shallow (20 mm - 200 mm), with the deepest being < 500 mm. Soils are derived from basalt and are shown to have even proportions of course sand, fine sand, clay and silt, with moderate to high organic matter content. The main land type is Ea (clay vertic, melanic or red diagnostic horizons) (MacVicar 1991; Mucina & Rutherford 2006; Smit et al. 1993) with a clay content of 8% to 60%, depending on the soil series (Land Type Survey Staff 1991).
The mean annual precipitation for the study area ranges between 800 mm and 1 200 mm, with the mean daily maximum temperature fluctuating between 22 ºC and 24 ºC for the hottest month of January. During the coldest months (June and July) the daily mean temperature ranges from -2 ºC to 0 ºC. Rainfall occurs throughout the year; the highest rainfall is recorded in the summer months (October to March; Figure 3). The daily mean relative humidity for the most humid month, March, varies between 68% and 72%, and the daily minimum relative humidity for July and August varies between 32% and 38% (Mucina & Rutherford 2006; Schulze 1997; South African Weather Service 2007; Van Zinderen Bakker 1973).
Snow occurs on the top of Platberg during most winters and can remain on the ground for up to two weeks, while water in south facing gullies may be frozen for several weeks at a time (Mucina & Rutherford 2006; Van Zinderen Bakker 1973). Possible heavy soils are present on Mtabazwe, the highest point on Platberg (Hilliard & Burtt 1987) and may be a significant factor affecting vegetation at high altitudes (Hilliard & Burtt 1987; Mucina & Rutherford 2006).
The climate of Platberg and the Drakensberg is part of a continental, north-south trending Austro-Afro-alpine climate, with noticeably higher precipitation than the surrounding lowland. Precipitation along the Drakensberg Escarpment is over 1 400 mm in some areas, with a gradual decrease both to the north and south, as well as westwards (560 mm) to the drier sub-humid continental climate towards Bloemfontein, some 400 km away (Scott 1988; Van Zinderen Bakker 1973). Precipitation is orographic, not only from violent thunderstorms in summer, but also from dense mist and fog (Smit et al. 1993) as well as snow. According to Cowling et al. (1992), climatic heterogeneity associated with montane landscape is a more important determinant of plant diversity than geological heterogeneity.
Ad hoc plant collections and reconnaissance surveys commenced in 2003 and 2004. The main collections associated with the Braun-Blanquet survey on Platberg were made from October 2005 through to January 2007. These collections were of plants growing exclusively in the 393 sample plots, which comprised the entire survey for this study. All specimens were pressed, dried and labelled according to standard field and herbaria practices. Identification was done at the Geo Potts Herbarium, UFS. Problematic material was identified at the National Herbarium of the South African National Biodiversity Institute (SANBI) in Pretoria. All floristic material currently resides at the Geo Potts Herbarium.
The final species list was compiled from material collected at Platberg for this study. This species list was compiled from all collections of Platberg housed at the UFS and SANBI-Pretoria Herbaria, supplemented by information contained in numerous reference books.For trees and wildflowers the following books were consulted: Hilliard (1990, 1992), Hilliard and Burtt (1987), Killick (1990), Pooley (1997, 1998, 2003), and Van Wyk and Smith (2001); for grasses: Gibbs-Russell et al. (1991), Moffett (1997) and Van Oudtshoorn (1999); and for the sedges: Gordon-Grey (1995).
Information compiled by Germishuizen et al. (2006) was used for nomenclature, the latest taxonomic changes and arrangement, as well as an author update. Alien species (introduced exotics) are marked with an asterisk (*). The conservation status of plants was checked against the National Red List of South African Plants (2009), as well as Golding (2002), Hilton-Taylor (1996) and Scott-Shaw (1999). Endemic or near-endemic classification was ascertained from Carbutt & Edwards (2004, 2006), Pooley (2003) and Scott-Shaw (1999).
The complete species list for Platberg (Tables 2-4 in online Appendix) was sorted according to the Englerian system (SANBI-Pretoria) that conforms to the system used by Arnold and De Wet (1993). Red Data status was included, where applicable. Throughout this discussion the term Drakensberg Alpine Centre (DAC) will be used in line with the terminology and updated nomenclature as followed by Carbutt and Edwards (2006).
An assumption and basic hypothesis has been that Platberg is linked, phytosociologically to, and is a floristic extension of, the DAC. The phytosociological analysis of Platberg as described by Brand (2007) and Brand et al. (2008) shows strong links with the DAC and supports this assumption. The DAC is regarded as the only true Alpine region in southern Africa (Carbutt & Edwards 2004; Killick 1963, 1978a) and, together with the Cape Floristic Region (CFR), contributes significantly to the flora of Africa (Hilliard & Burtt 1987), via east and west coast mountain tracks (Mutke et al. 2001). The flora of the DAC and, by extension, Platberg is an amalgamation of numerous floras from both temperate and subtropical as well as Afromontane sources (Hilliard & Burtt 1987) and contains significant taxonomic influences from the CFR (Goldblatt & Manning 2000). The DAC and CFR link is reported in studies of the Eastern Cape Drakensberg region by Hoare and Bredenkamp (2001), in studies of the Stormberg and Drakensberg and the grasslands of southern KwaZulu-Natal by Bester (1998), and in studies of the adjacent Drakensberg regions by Perkins (1997) and Perkins et al. (1999a, 1999b).
According to Van Wyk and Smith (2001) the total number of species for the DAC is about 2 200, with no endemic/near-endemic families, four endemic/near-endemic genera, and > 400 endemic/near-endemic species/taxa (18.2%), with 5% succulents amongst the endemics. Other surveys have covered smaller regions. There exists no complete survey for the entire Drakensberg that provides comprehensive and accurate figures. Table 1 presents a summary of regional floras of the DAC and includes the most recent figures for a total of 2 520 species, with no endemic families and six near-endemic genera (Carbutt & Edwards 2004).
Floristic analysis of Platberg
The Platberg vascular plant species list comprised a total of 669 species, represented by 304 genera and 95 families (Table 1 in online Appendix). The flowering plants are represented by Monocotyledoneae with 214 species from 23 families (23% of total number of families) and Dicotyledoneae with 438 species from 63 families (67%). Pteridophytes are represented by 16 species and eight families (8.4%), and Gymnosperms by one family and one species (1%). The complete species list, separated into the different divisions and sorted by family, appears in the online Appendix.
Largest plant families on Platberg
The 13 largest families comprised 411 species from 158 genera, which represented 50.7% of the Angiosperms and 61.4% of the total flora thus far recorded for Platberg, while the remaining 82 families reflected the other 38.6%.
The two largest families were Asteraceae with 126 species from 40 genera, constituting 18.8% of the total flora, and Poaceae with 73 species from 39 genera, constituting 10.9 % of the total flora. With a significantly lower number of species, the next 11 largest families (in descending order) were the following:
Cyperaceae with 39 species from 18 genera (5.8%)
Fabaceae with 33 species from 13 genera (4.9%)
Scrophulariaceae with 27 species from 13 genera (4.0%)
Hyacinthaceae with 21 species from 10 genera (3.1%)
Orchidaceae with 16 species from eight genera (2.4%)
Iridaceae with 17 species from six genera (2.5%)
Crassulaceae with 13 species from three genera (1.9%)
Geraniaceae with 12 species from three genera (1.8%)
Thymelaeaceae with 12 species from three genera (1.8%)
Lobeliaceae with 12 species from three genera (1.8%)
Campanulaceae with 10 species from two genera (1.5%).
All other families had fewer than 10 species.
The Pteridophytes comprised 8.5% of the total number of families for Platberg and Gymnosperms comprised 1%. Angiosperms represented 90.5%, with a breakdown into Monocotyledons constituting 26.7% and Dicotyledons constituting 73.3% of vascular plants.
The largest 17 genera each had five species or more, with the genera Helichrysum and Senecio each having a considerably larger number of species (31 and 30 species, respectively) than any of the other genera (Table 2).
Twenty-nine Red Data, endemic or near-endemic species were found to represent 4.3% of the total flora for Platberg, all of which were Angiosperm (Table 3). Introduced species (see Tables 2-4 in online Appendix) amounted to 22, which represented 3.3% of the total plant species.
Three of the six near-endemic genera for the DAC were found on Platberg, two of which were Monocotyledoneae: the mountain bamboo Thamnocalamus (Poaceae) and Rhodohypoxis (Hypoxidaceae), and one the Dicotyledoneae Craterocapsa (Campanulaceae). The other remaining three near-endemic genera, not recorded on Platberg, were Guthriea (Achariaceae), Huttonaea (Orchidaceae) and Glekia, which made a total of 11 endemic or near-endemic genera for the DAC (Carbutt & Edwards 2004).
An analysis of the flora for Platberg (online Appendix 1) shows that the largest family is Asteraceae, with 126 species (18.8% of the total species). This is not unusual for the Grassland Biome or the DAC (Carbutt & Edwards 2004), or even for South Africa, where in semi-arid to arid areas Asteraceae is the largest family (Goldblatt & Manning 2000). The Poaceae is the next largest family (10.9%), with the majority of grasses using C4 metabolism, which is characteristic of adaptations to high temperatures and low rainfall. This is somewhat analogous for Platberg, with its higher altitude and rainfall as well as its close proximity to the Drakensberg massif. However, a more detailed analysis shows the composition of grasses to be a mix of C3 and C4, with about a quarter using a combination of the metabolic pathways of both (Brand 2007). The position of Fabaceae (fourth largest, 4.9%) and Scrophulariaceae (fifth largest, 4%) is also a reflection of their position globally, as well as across drier parts of Africa (Goldblatt & Manning 2000). The high value of Cyperaceae is not unexpected; it shows similar patterns for South Africa and related areas. A comparison of Cyperaceae for inselbergs (Brand 2007) confirms these high values for Cyperaceae, which may be a result of the harsh conditions found on inselbergs that favour Cyperaceae development and radiation (Gröger & Barthlott 1996).
A comparison of the ratio of Monocotyledoneae to Dicotyledoneae for Platberg (Table 2) is 1:2.15, which is almost identical to that for Korannaberg of 1:2.16 (Du Preez 1992). However, the ratio for Mountain Zebra National Park, Eastern Cape, is 1:8 (Pond et al. 2002), which is higher than the average for the CFR of 1:3 (Goldblatt & Manning 2000). The latter is the average for floras worldwide (Goldblatt & Manning 2000). A comparison of the five largest Angiosperm families of Platberg, the DAC and other relevant areas shows striking similarities.
Twenty-nine Red Data, endemic and near-endemic species (all of which are Angiosperms) comprise 4.3% of the total flora of Platberg (Table 3), which is lower than for the DAC at 7.2% (Carbutt & Edwards 2004).
Platberg: Environmental conditions on inselbergs and floristic comparisons
Inselbergs represent sites with extreme environmental conditions (Gröger & Barthlott 1996), which include high levels of daily temperature change, namely 8 ºC to 28 ºC for December and -2 ºC to 18 ºC for July (Schulze 1997). Frost is another important environmental factor for inselbergs; the average number of days with frost is between 61 and 120 days (Schulze 1997). Shallow soils, characteristic of large sections of inselbergs, have limited water retention, which results in harsh conditions for the vegetation; intermittent water is a major limiting factor, resulting in edaphic and micro-climatic xeric islands (Gröger & Barthlott 1996; Körner 2003).
A comparison of the inselberg vegetation of Platberg and Korannaberg is shown in Table 4, with a close correlation of taxa and their numbers. There is a clear topographic diversity pattern associated with inselbergs. Climate and topography are only part of the explanation for high biodiversity; it is rather the diversity of plant communities that provides a measure of habitat diversity (Cowling & Lombard 2002) and is a reflection of the response to climate change (Mucina & Rutherford 2006; Scott et al. 1997).
There is a high degree of similarity between the Angiosperm flora of Platberg and Korannaberg (Table 4), with the higher numbers for Korannaberg possibly the result of its much larger area (Cowling & Lombard 2002; MacArthur & Wilson 2001) - as the surface area increases more species can be supported (Gröger & Barthlott 1996; Linder 2003). Additionally, it was noted that the higher species numbers for Korannaberg could be due to its position, which has strong Afromontane, Nama Karoo and Savanna floristic influences (Du Preez 1991, 1992; Du Preez et al. 1991; Du Preez & Bredenkamp 1991).
Compared to other regional floras, Platberg has one of the highest species-to-area ratios (Figure 4). For the Bourke's Luck site, the figure of 5.244 is converted from 524.4 hectares, selected from the total Reserve size of 26 818 hectares, with similar rainfall (700 mm - 1 400 mm per annum), slightly higher temperatures, and situated on the Escarpment at 1 640 m to 1 900 m (Brown et al. 2005; Mucina & Rutherford 2006).
Endemism, species richness and floristic links with the DAC
Phytosociologically, geologically, topographically, climatically and biogeographically, Platberg may be seen as an extension of the DAC. It would then be expected that strong floristic affinities would be seen between the two sites (Table 5). The DAC is seen as a transition zone, a migratory pathway and repository for taxa of diverse regions and biomes (Carbutt & Edwards 2004; Hilliard & Burtt 1987; Killick 1963, 1978a, 1978b; Mucina & Rutherford 2006). The DAC has close affinities with the CFR. This is particularly seen in the fynbos vegetation unit elements found on Platberg and the DAC, and described as the Gd 9 Drakensberg-Amathole Afromontane Fynbos, as well as the Northern Escarpment Afromontane Fynbos (Gm 24; Mucina & Rutherford 2006). Other floristic affinities are seen with the Magaliesberg, with the Gm 29 Waterberg-Magaliesberg Summit Sourveld (Mucina & Rutherford 2006), and the surrounding Bankenveld, Witwatersrand areas (Behr & Bredenkamp 1988; Bredenkamp & Brown 2003; Grobler et al. 2002, 2006). Floristic links between the DAC, KwaZulu-Natal and the CFR are elucidated in studies by Bester (1998), Hill (1996), Hoare and Bredenkamp (2001), Pond et al. (2002), Perkins (1997), Perkins et al. (1999a), Smit et al. (1993, 1995), and described as the Gs 1 Northern Zululand Mistbelt Grassland (Mucina & Rutherford 2006).
The ecological survey undertaken by Kay et al. (1993) of the Golden Gate National Park does not give a floristic analysis with the breakdown into the numbers of families, genera and species. The phytosociological table given lists a total of 114 species and makes no reference to a total species count. The paper by Roberts (1969) on the vegetation of the Golden Gate Park lists a total of 331 vascular plants and is 'a list of the more abundant species present' (Roberts 1969). It is thus not possible to make comparisons with the flora of the only other site that has high-altitude grasslands in the proximity of Platberg.
The floristic composition for Platberg and the related DAC shows Asteraceae as the largest family (Table 5). This is also the case for the Cape flora, with significant correlation within the top 12 to 20 families (Goldblatt & Manning 2000). Fabaceae is the second largest family, followed by Aizoaceae, Ericaceae and Iridaceae, which is a unique aspect of the Cape flora (Goldblatt & Manning 2000). These families are thus represented in the top 10 for Platberg as well as the DAC. The second most speciose family, Poaceae, followed by Cyperaceae, reflects Platberg's position in the Grassland Biome with similar floral composition to the DAC (Carbutt & Edwards 2004, 2006; Mucina & Rutherford 2006) and the more grassy regions to the west and north (Du Preez 1991; Du Preez & Bredenkamp 1991; Eckhardt et al. 1993a, 1993b, 1995; Fuls 1993; Kooij et al. 1990a, 1990b, 1990c; Malan 1998). This is in contrast to the Cape flora, where Poaceae and Cyperaceae are poorly represented, with Restionaceae filling the environmental and floristic position of the Poaceae (Table 5; Goldblatt & Manning 2000).
Platberg and affinities with Fynbos and other Cape floral elements in high-altitude floras (DAC)
South Africa has about 20 500 vascular plant taxa (Germishuizen et al. 2006; Goldblatt & Manning 2000; National Red List of South African Plants 2009). The CFR has the richest flora for its area, with 9 000 species (Goldblatt & Manning 2000), as it is an important centre of endemic species and radiation for genetic material (Linder 2003). As the CFR is confined to the southern tip of the African continent and thus at a 'dead end', species are forced to migrate to the north (Linder 2003; Mucina & Rutherford 2006). A floristic comparison with the CFR would elucidate links with high-altitude taxa that may have migrated out of the CFR, through the DAC, and further north.
The CFR is also the largest contributor to DAC flora in the KwaZulu-Natal Drakensberg, with 67 genera (Carbutt & Edwards 2004). The number of CFR elements decrease further northwards from the DAC (Carbutt & Edwards 2004). However, unlike the CFR (Table 6), the largest families for the DAC show similar trends with Mount Mulanje and the Nyika Plateau, with the exception of Orchidaceae (which is the most plentiful family), represented by 122 and 200 species respectively for these centres (Burrows & Willis 2005). The next three highest species-rich families are Asteraceae, Poaceae and Fabaceae, with Rubiaceae the fifth largest family (82 and 90 species respectively). A further floristic comparison of endemics and near-endemics with the DAC and the Afromontane archipelago, complied by Burrows & Willis (2005), shows the following:
Nyika Plateau (Malawi) comprises 1 891 total species, of which 46 are endemic or near-endemic.
Mount Mulanje (Malawi) comprises 1 303 total species, of which 94 are endemic or near-endemic.
Chimanimani Mounatins (Mozambique) comprises 859 total species, of which 70 are endemic or near-endemic.
Simien Highlands (Ethiopia) comprises 550 total species, of which 12 (plus) are endemic or near-endemic.
Mount Cameroon (Cameroon) comprises 2 435 total species, of which 49 are endemic or near-endemic.
The largest family for all three regions is Asteraceae, with 18.9% for Platberg, 17.1% for the DAC, and 11.5% for the Cape. Comparing the density of Asteraceae within South Africa (11%) to other regions of the world, we find that Hawaii comprises 15.9%, the Sonora Desert 15%, Texas 13.4%, eastern North America 12.7%, New Zealand 12.5% and Europe 12% (Goldblatt & Manning 2000; Hilliard & Burtt 1987). The DAC has a significantly higher percentage than the Cape. The percentage for Platberg is higher than for the DAC, which is significantly higher than for Hawaii (which has the largest percentage globally). Asteraceae show significant spread in the DAC and more so on Platberg. These areas have been geologically stable for a considerable time (McCarthy & Rubidge 2005), and have many different abiotic parameters that are an important for high speciation (Mutke et al. 2001). Terrain heterogeneity and a relatively stable quaternary climate (Scott et al. 1997) may have enabled this genus to spread and establish so successfully at altitude (Linder 2003). Alternatively, this may be a recent phenomenon, where new taxa respond rapidly to a new environment and fill available niches (Linder 2003), or perhaps it is a response to climate change over longer periods, possibly from the start of the Holocene epoch 30 million years ago.
The low species numbers for Poaceae in the CFR is explained by it being a replacement genus for Restionaceae that fills its niche and thus is more plentiful (Carbutt & Edwards 2006; Haaksma & Linder 2001; Linder 2003). In the CFR, Restoniaceae also displaces Cyperaceae (Goldblatt & Manning 2000), which may partially explain the low number of Cyperaceae species and its ranking at twelfth for the CFR, third for Platberg and sixth for the DAC (Table 6) as the environmental parameters for all regions are analogous (Stock et al. 2004).
Conservation implications for Platberg
The floristic analysis of Platberg shows its value as both a site of diversity and endemism. This value is also linked to the DAC flora and shows its position as the main inselberg in the chain of such prominent landscape features. The high plant diversity of Platberg and its main genetic source, the DAC, is because it includes elements of Mesic Karoo-Nama vegetation, the Afromontane Podocarpus elements, mesic KwaZulu-Natal escarpment, and drier Eastern Cape Stormberg and Lesotho floras.
Platberg has high terrain heterogeneity, with steep contours, deeply incised gullies and numerous slope orientations. According to Cowling et al. (1992), Low & Rebelo (1996), Mucina and Rutherford (2006), Perkins (1997) and van Wyk and Smith (2001) this provides more habitats for species and is more important than other factors in determining high biodiversity. In addition, Platberg is the single largest and best preserved high-altitude grassland in the Free State (Mucina & Rutherford 2006). Immediate threats are from the invasion of Pinus patula on the footslopes and plateau area of Platberg to the south and east. This poses a considerable conservation problem because Pinus patula is in the process of rapidly out-competing the local indigenous vegetation, particularly on the cool south slope and in the stream flowing down the gully on the south side of Platberg.
The threats to genetic diversity (Anderson 2001) also influenced by global warming are key considerations for conservationists and highlight the importance of mountains and the associated inselbergs as areas of refuge to plant species. This, therefore, stresses the urgent need for their protection. Additionally, high-altitude vegetation, specifically alpine flora as defined by White (1983) and Körner (2003), is the only biogeographic unit on land with a global distribution, accounting for nearly 3% of the land surface and including about 10 000 species, or about 4% of higher order plants (Körner 2003).
The threat posed by global warming to the world's alpine regions, including Africa, was predicted by Peters (1992), who indicated that as little as a 3 ºC increase in temperature over 100 years would be equivalent to a 500 m upward shift in altitudinal zones. In the Afromontane and Afro-alpine biota, this would cause a significant reduction in the distribution of plants, changing their structure and composition, and forcing some taxa to higher altitudes (Taylor 1996). Furthermore, in some cases (e.g. Platberg), a significant reduction of the taxa could possibly occur where the altitude is under 2 500 m.
Platberg is centred in the Grassland Biome, with a lesser mix of Fynbos, Nama-Karoo and Forest Biomes. Platberg consists of 3 000 ha of high-altitude grassland composed of a mix of C3 and C4 grasses and other plant families that use this metabolic pathway. Between 30% and 55% of the Grassland Biome has been transformed, only 5.5% of which is under protection. Numerous abiotic environmental factors (soil, altitude, slope, aspect, moisture availability, temperature range, fire, modern and palaeoclimatic factors) and biotic interactions between plants and plants, plants and pollinators, and herbivores and plants have resulted in a relatively high species richness and diversity. The Drakensberg Alpine Centre is the main floristic influence on Platberg, with strong affinities with the Cape Floral Region.
The result of this study has shown that Platberg is a centre of significant biological diversity, with high species richness, vegetation structure and ecosystem complexity, and a centre of genetic diversity and variation. It occurs as an island in the Grassland 'sea' and shares inselberg floral richness and endemism that can be tracked via the Afromontane archipelago-like string of inselbergs and mountains, which stretches north through the Chimanimani Mountains, into Malawi, the Eastern Arc Mountains via Tanzania and north through Ethiopia, into Eurasia. It also shows a western tract via the Congo, Ivory Coast and Cameroon inselbergs and mountains.
The current vegetation patterns on Platberg reflect changes in palaeo-environmental cycles of cooling and warming, which, since Miocene times, have had the greatest influence on the texture and composition of plants and speciation on Platberg. Current trends towards climate change will alter the composition and species numbers of grasses as well as other plant families on Platberg.
As an important high-altitude grassland, it is imperative that Platberg be provided with protection legislated on at least a provincial level.
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Leslie R. Brown
Department of Environmental Sciences, UNISA
Florida campus, Private Bag X6
1710, Florida, South Africa
Received: 18 June 2009
Accepted: 11 Oct. 2009
Published: 29 Mar. 2010
This article is available at: http://www.koedoe.co.za