Vegetation of the rock habitats of the Sekhukhuneland Centre of Plant Endemism , South Africa

A hierarchical classification, description, and ecological and floristic interpretations are presented on the vegetation types o f the ultramafic rock habitats of the Sekhukhuneland Centre o f Plant Endemism. Relevés were compiled in 100 strati­ fied random plots. A TWINSPAN classification, refined by Braun-Blanquet procedures, revealed 17 plant communities, which are classified into 13 associations belonging to four proposed alliances. Many new syntaxa are ecologically inter­ preted and described. For each syntaxon, the species richness, endemism and conserv ation status was determined. Much of the plant community distribution can be ascribed to specific habitat preference. The floristic and habitat information, pro­ posed classification, general description and vegetation key are provided to aid future identification of conservation areas, land use planning and research. An ordination (DECORANA) based on floristic data confirmed potential relationships that could exist between the plant communities and associated habitats and environmental gradients.


INTRODUCTION
Although several phytosociological studies have been conducted on vegetation types o f the north east ern D rakensberg Escarpm ent and adjacent areas of Lim popo [Northern Province] and M pum alanga (Deall 1985;Bloem 1988;M atthews 1991;Burgoyne 1995), the vegetation o f the Sekhukhuneland Centre o f Plant Endem ism (SCPE) (Van Wyk & Smith 2001;Siebert & Van Wyk 2001) has never been studied in detail.Recently much work has been done to docum ent the plant diversity and vegetation o f this poorly known m icro-regional Centre of Plant Endemism (Siebert et al. 2002a-e;Siebert et al. 2003).The vegetation of rock habitats is the last m ajor vegetation type of Sekhukhuneland to be described.Its extent o f occur rence is ± 4 000 k n r and is characterized by co n sider able diversity in geology (Kent 1980) and p h ysiogra phy (Land Type Survey Staff 1987).In South A frica rock outcrop com m unities have received very little attention from botanists and environm entalists in the past (B redenkam p & D eutschlander 1995).probably due to their low agricultural potential.However, ultram afic rock habitats are floristically notew orthy and have high conservation significance, because many endem ics with distributions correlated with this geological substrate occur here (M adulid & Agoo 1995;M eirelles et al. 1999;Reddy et al. 2001;Siebert et al. 2001).
Various vegetation types have been identified for the rock habitats o f the northeastern Drakensberg Escarp ment (M atthews et al. 1991;Matthews et al. 1992) and western Rustenburg Layered Suite (Coetzee 1975;Van der Meulen 1979).areas that have a strong floristic rela tionship with the SCPE (Siebert 1998).The areas where the rocky outcrops o f the SCPE occur were mapped as three major Veld Types by Acocks (1988), namely Mixed Bushveld (18), Sourish Mixed Bushveld (19) and .A more generalized classification of the same region's vegetation is given by Low & Rebelo (1996), who recognize one broad Vegetation Type for the study area, namely Mixed Bush veld (18).
The vegetation of the SCPE can be broadly described as mountain bushveld that forms a mosaic with moist grassland in the south and semi-arid bushveld in the north.In this region of undulating hills and mountains, a predominant and characteristic feature is the scattered rock habitats.The vegetation of rock habitats described here only includes those plant communities identified as the Hippobromus pauciflorus-Rhoicissus tridentata Rock Outcrop Vegetation by Siebert et al. (2002a).This vegetation forms a mosaic distribution with the other major vegetation types of the SCPE.Forty-one of the Sekhukhuneland endemics/near-endemics and threat ened taxa are known to occur in rock habitats (Siebert 1998).
The present paper forms part of a comprehensive veg etation and floristic survey of the SCPE (Siebert 2001).It is envisaged that the identification, classification and description of the various vegetation units will contribute to the knowledge of the plant diversity and biological intricacies of the region.This paper provides ecological and floristic data of the region's rocky habitat communi ties on ultramafic outcrops and associated habitats, by characterizing and interpreting the vegetation units.Classification of the vegetation is basic to the formula tion of management policies to co-ordinate and imple ment proper land use in bioregional planning activities.An assessment of the plant diversity, endemism and Red Data List taxa of the plant communities of the study area is supplied to aid future conservation actions and ecosys tem management strategies.Such information is needed to build an adequate database of natural features and other land uses related to sustainable development (Bedward et al. 1992).

STUDY AREA
The study area is located in the SCPE and is defined in Siebert et al. (2002a) as an area more or less restrict ed to the Rustenburg Layered Suite, one of the stratigraphic units of the Bushveld Complex (Figure 1).The Rustenburg Layered Suite is known for its concentric belts of norite and pyroxenite (Visser et al. 1989).In addition, large quantities of heavy metals such as iron, chromium and platinum are present in the different zones o f the suite (Schurm ann et al. 1998;Viljoen & Schurmann 1998).
The study area lies in the sum m er rainfall region and the mean annual rainfall is 578 mm (South African W eather Bureau 1998), of which nearly half (48%) is received between Decem ber and February (summer); a mean of 283 mm for these three months (Erasmus 1985).However, the rainfall pattern is strongly influ enced by the local topography and varies from as little as 400 mm in the Steelpoort and Olifants River valleys in the north, to an estimated 700 mm on the Leolo M ountains and the Dwarsrivier M ountains in the south  (Siebert 1998).Perhaps the most outstanding climatic feature of the drier central and northern parts of the SCPE is that it lies in the rainshadow of the northeastern Drakensberg Escarpment.
Annual tem peratures o f the study area range from -4.5°C to 38°C, with a daily mean of 18.5°C (South Afri can Weather Bureau 1998).The northern and western parts of the study area are on average warmer than the southern and eastern parts (Siebert 1998).The northern parts of the region exhibit mean daily temperatures of 28.3°C maximum and 7.2°C minimum.Temperatures vary at different localities within the area, also correlat ing strongly with physiographic features, being higher in the low-lying valleys and lower on the high-lying plateau (Buckle 1996).

METHODS
A first approximation of a vegetation classification, based on the total floristic data set of 415 stratified ran dom sample plots (Siebert et al. 2002a) was obtained by the application of Two-Way Indicator Species Analysis (TWINSPAN) (Hill 1979a).This first step of an objec tive multivariate classification identified six major vege tation types for the Sekhukhuneland Centre (Siebert et al. 2002a).These results were used to subdivide the data set into phytosociological tables.One of these tables rep resented floristic and habitat data of rock habitats (100 relevés) and was again subjected to TWINSPAN.The resultant classification w'as further refined by Braun-Blanquet procedures in the MEGATAB computer pro gramme (Hennekens 1996b).
For sampling purposes, stratification was based on terrain type, aspect and vegetation structure.To stan dardize the plot size and to counter the bias of different scale (Jonsson & Moen 1998), plot size was fixed at 400 m 2. Within each sample plot, all species were recorded and a cover-abundance value was assigned to each species according to the Braun-Blanquet scale (M ueller-Dombois & Ellenberg 1974).Plant species names conform to those of R etief & Herman (1997), and practice followed in the H.G.W.J. Schweickerdt Herbarium (PRU), University of Pretoria.Terminology to describe vegetation structure follow s Edw ards (1983).Environmental data recorded for each sample plot include terrain type (Land Type Survey Staff 1987, 1988. 1989).aspect, slope, geology (Visser et al. 1989), soil type (M acVicar et al. 1991) and rockiness of soil surface.Longitude and latitude readings were also recorded for each sam ple plot using a G lobal Positioning System (GPS).All relevé data are stored in the TURBOVEG database (Hennekens 1996a), m an aged by the D epartm ent o f Botany, University of Pretoria (M ucina et al. 2000).Syntaxa are named in accordance with the Code of Phytosociological N om en clature (Weber et al. 2000).
The ordination algorithm Detrended Correspondence Analysis (DECORANA) (Hill 1979b) (Figure 2) was applied to highlight potential gradients in the vegetation, and the relationship between these plant communities and the physical environment.
To facilitate the identification of areas of high conser vation potential, the alpha diversities of the different plant communities were calculated.The alpha diversity (plant species richness) is defined as the number of species per unit area within a homogeneous community or the total number of species per community (Whittaker 1977).A 400 n r sample plot was taken as the unit area.
The geographical distribution of all the taxa was ver ified at the National Herbarium (PRE), Pretoria, to iden tify any taxa endem ic/near-endem ic to the region (Siebert 1998).All taxa were also checked against the Red Data List of southern African plants (Hilton-Taylor 1996) to determine their conserv ation status.

Classification of vegetation
The floristic data analysis resulted in the identifica tion of 17 plant communities of the Hippobromus pauciflorus-Rhoicissus tridentata Rock Outcrop Vegetation (Table 1).These were subsequently hierarchically classi fied into 13 associations.Four major syntaxa are recog nized on the grounds of the physical environment and are proposed as potential alliances, with all 13 associations and eight subassociations classified under them.No macro-climatic or geological variation plays a role in            .+ ad a! TS?

S i -?
x -r   Bothalia 33,2 (2003) local differentiation of the plant communities.Plant com munities relate to soil type, rockiness and terrain type, with aspect and slope also of importance.The four major communities (potential alliances) are distinctive and eas ily distinguishable in the field.This might be attributed to the uniformity of the environmental factors for each of the major communities, causing a distinct distribution pattern of habitats and associated vegetation.
The hierarchical classification of the vegetation is reinforced by the correlation between habitat and plant communities (Figure 2).The distribution of Sekhukhu neland Centre endemic/near-endemic and Red Data List taxa among various plant communities is listed in Table 2.A summary of selected community attributes is sup plied in Table 3

Plant community descriptions
The Hippobromus pauciflorus-Rhoicissus tridentata Rock Outcrop Vegetation is largely restricted to the slopes and plateaus of undulating ultramafic hills.Surface rocks are predominant and abundant in these habitats, with rock percentage varying from 25% on the rocky flats to more than 50% in the rocky refugia.The vegetation can structurally be classified into forest/woodland (rocky refugia).woodland/thicket (rocky outcrops and ridges) and herbland (rocky flats) (Edwards 1983).The rock habitats of Sekhukhuneland, like those in other parts of the world (Madulid & Agoo 1995), constitute an impor tant feature which occurs in islands differing significant ly from surrounding areas.
Environmental data: the vegetation of this alliance of rocky outcrops is a thicket or woodland.It is found on all aspects of gently sloped (1-5°) rock intrusions on mid slopes, scarps and occasionally in valleys (Table 3).Soil forms are shallow and rocky.The soil surface is covered by 60-90% of rock with a large average diameter of 2.5-8 m (Table 3).
Notes on floristic diversity: floristic links with the rest of the data set are visible in species groups K. Z and AG (Table I ).These few' and weak links support the propos al of this major group as an alliance.The mean number of species encountered per sample plot in this group is 29, with a total number of 107 plant taxa (13 relevés) (Table 3).There are 16 plant taxa of conservation value, ten are SCPE endemics and six are SCPE near-endemics.One of the endemics.Euphorbia sekukuniensis, is a Red Data List taxon assessed as Rare (Table 2).Eight ende mics of conservation value are restricted to this commu nity of the SCPE.Bothalia 33,2 (2003) 1. Vepro reflexae-Mimusopetum zeyheri ass.nova hoc loco Nomenclatural type: relevé 190 (holotypus).
Environmental data: the vegetation is a sparse, tall thicket of rock outcrops on the banks of seasonal streams and rivers.It is mostly found along watercourses that flow from the mountains towards the valley of the Steelpoort River.The habitat has gentle, sloped sides (1-3°) (Table 3).The dominant soil type is the Bonheim Form, a melanic A-horizon underlain with a pedocutanic B. Mean rock size varies from 5.5 to 7 m in diameter and covers 70-80% of the soil surface.Notes on floristic diversity: this community is rare in the SCPE.and exhibits a slight floristic affinity with other rock habitats of the SCPE in species groups K and Z (Table 1).The mean number of species per relevé is 31.and the total number of species recorded for the association is 46 (three relevés) (Table 3).Five plant taxa of conservation value occur (Table 2), namely one SCPE endemic, Rhoicissus sekhukhuniensis, and four SCPE near-endemics.
Environmental data: the vegetation is a sparse, short thicket on rocky outcrops in the Steelpoort River Valley.The plant community is associated with exposed norite or pyroxenite outcrops on level to gently sloped ( 0 -1 ) midslopes and scarps of hills.Surface rock cover per centage is 60-90% .with the stacked exposed boulders reaching diameters between 2.5 and 8 m (Table 3).The dominant soil type is the Mispah Form, indicating very shallow soils over rock.Notes on floristic diversity: this plant community ex hibits a slight floristic link with the rocky ridges of the Centre in species groups K. T and Z (Table 1).The mean number of species encountered per sample plot is 27, with the total number for this association being 91 (10 relevés) (Table 3).A high number of fourteen taxa with conservation status are recorded for the association (Table 2).Ten are SCPE endemics, which is the highest number recorded for any plant community of rock habi tats in the SCPE.Four are SCPE near-endemics.Three Red Data List taxa are recorded.O f all the rock habitats, this association has the highest number of plant taxa with conservation value restricted to it (seven).
Environmental data: in the SCPE this alliance of rocky ridges is characterized as open to closed moist bush clumps, with predominantly shallow, black and red clay soil forms.This vegetation occurs on midslopes and scarps of undulating ultramafic hills, on varying slopes of 1-15° on all aspects.Rocks cover 45-80% of the soil surface and vary in diameter from 1 to 5 m (Table 3).Notes on floristic diversity: this major group is domi nant and floristic relationships exist with all the other plant communities of rock habitats, indicating that it forms the core of this vegetation type in the SCPE (Table 1).Associations 4 to 9 represent bush clumps in the Brachiario serratae-M elhanietum randii Rocky Grassland of the Roossenekal Subcentre (Siebert et al. 2002d) and Associations 3, 5 and 10 represent bush clumps in the Loudetio simplicis-Eucleion linearis and Setario sphacelatae-Acacietum caffrae of the Steelpoort Subcentre (Siebert et al. 2002b).The mean number of species encountered per sample plot is ± 35. and the total number of plant species is 150 taxa (62 relevés) (Table 3).Thirty-one taxa of conserv ation value are part of the proposed alliance, and 13 are restricted to it (Table 2).Thirteen SCPE endemics and 15 SCPE near-endemics were recorded.Nine taxa are listed on the Red Data List.
Environmental data: this association represents bush clumps on rocky ridges on warm north and northeast aspects of norite and pyroxenite hills in the Steelpoort Subcentre.It occurs on midslopes and scarps on red loam soils of the Glenrosa and Mispah Forms.It covers gentle to moderate sloped areas (3-7°).Rock cover on the sur face is 70-90% , with rocks reaching 2-4.5 m in diameter (Table 3).Notes on floristic diversity: this association is not strongly linked with the other associations of its group and is probably more related to the Englerophytum magalismontanum-Acacia caffra Mountain Bushveld (Winterbach et al. 2000).The mean number of species encountered per sample plot is 30, with a total number of 89 plant taxa (nine relevés) (Table 3).Four SCPE en demics.seven SCPE near-endemics and one Red Data List taxon are present in this association (Table 2).None of the 11 taxa of conservation value are restricted to it.
Species group J (Table I ).
Environmental data: this association represents bush clumps on moderate to steep-sloped (1-15°) midslopes and scarps of norite (sometimes pyroxenite) hills.It occurs on deeper red and black clay soils of the Mayo and Milkwood Forms, which are interspersed with shal low gravel soils of the Glenrosa Form.Rock cover is 45-80% o f the soil surface with sizes between 0.5 and 7 m in diameter (Table 3).Notes on floristic diversity: the association has a species combination typical for rocky outcrops in species group K. shared with Associations 1, 2 and 3 (Table 1).In species group P it shares a species combination typi cal of rocky ridges with Association 5 to 8 (Table I ).This community is also unique in that it shares forest species with Associations 12 and 13 in species group AF (Table 1).The mean number of species encountered per sample plot is ± 44. the highest mean number recorded per relevé for any of the major vegetation units of rock habitats in the SCPE.A total number of 122 plant taxa were record ed (22 relevés) (Table 3) of which 20 have conservation status (the highest number for any association of SCPE rocky habitats).Nine SCPE endem ics, eight near endemics (the highest number for rocky habitats) and six Red Data List taxa (the highest number for rocky habi tats) are recorded for this association (Table 2).A high number of six taxa is restricted to it.Species group F (Table 1).
Environmental data: this sub-association represents dry.warm bush clumps on northern aspects of hills.It occurs on deeper red and black clay soils of the Mayo and Milkwood Forms and lies on moderately sloped (5-15°) midslopes and scarps.Rock cover on the surface is 45-50% , with rock sizes between 0.5 and 1 m in diame ter (Table 3).Notes on floristic diversity: the subassociation has a grassland-savanna species combination in species group H which it shares with Subassociation 4.2 (Table 1).The mean number of species encountered per sample plot is 46, together with Subassociation 4.2, the highest number recorded per relevé for any of the vegetation units of rock habitats.A total number of 105 plant taxa were recorded (eight relevés) (Table 3).Five SCPE endemics, of which two are Red Data List taxa namely Ele phantorrhiza praetennissa and Zantedeschia pentlandii, and five near-endemics are found in this subassociation (Table 2).
Environmental data: see description of Subassocia tion 4.1.This subassociation represents bush clumps of rocky ridges, which is restricted to grassland on moist, cool southern aspects (Table 3).Notes on floristic diversity: Species group H (Table 1) shows a strong floristic connection w ith Subassociation 4.1 due to the similar geographical distribution, just on different aspects of the same hills.The mean number of species encountered per sample plot is 46. the highest mean number recorded per relevé in the data set.The total number of plant species for this subassociation is 110 (seven relevés).the rich est diversity of species recorded for any rock habitat in the SCPE (Table 3).Of the 10 taxa of conservation value in this subassociation, four are SCPE endemics and five are SCPE near-endemics.Of these, three are Red Data List taxa (Table 2).Three taxa are restricted to the subassociation, namely Berkheya insignis (endemic form).Eucomis montana (Rare in the Red Data List) and Gnidia caffra (endemic form).
Environmental data: a subassociation dominated by a dense herbaceous cover on level ridges situated on mid slopes and scarps of grass-covered norite hills.It occurs on red clay soils of the Mispah Form, with the soil sur face covered by 70-80% rock of a relatively large size of 4 to 7 m in diameter (Table 3).Notes on floristic diversity: a noteworthy floristic relationship exists w ith Association 2 in species group C (Table 1), which can be ascribed to occurrence on the scarps of hills.The mean number of species encountered per sample plot is 39 and the total number for the sub association is 103 (seven relevés) (Table 3).Six SCPE endemics, six near-endemics and five Red Data List taxa are recorded (Table 2).O f these 14 taxa of conserv ation value, three near-endemics are restricted to it, namely Aloe reitzii var.reitzii (Indeterminate in Red Data List), Chlorophytum cyperaceum and Plectranthus xerophilus.

Species group L (Table 1).
Environmental data: this association is a dry bush clump of any aspect, situated on exposed iron-rich norite and magnetite ridges, on midslopes and scarps o f hills.It occurs on red and black clay soils of the Mispah Form (ortic A-horizon) and Milkwood Form (melanic A-horizon) underlain by hard rock.The soil surface is covered by 50-60% rock, o f an average size of 2.5-3 m in diameter (Table 3).Slope of the habitat is usually moderate (3-7°).Notes on floristic diversity : the community has a marked floristic grassland affinity in species group X with Asso ciation 11 (Table 1).The mean number of species encoun tered per sample plot is 40.with the total number for this association being relatively high at 109 species (seven relevés) (Table 3).Six SCPE endemics and six SCPE near endemics, of which three are Red Data List taxa, were recorded (Table 2).
Environmental data: this association represents bush clumps in grassland on all aspects of hills with norite and ferrogabbro rocky ridges.It usually occurs at higher alti tudes than the other associations, and is found on mid slopes and scarps on red clay soils of the Mayo and Mispah Forms.It lies on gently sloped areas (1-5°).Rock cover on the surface is 60-80% .with rocks reach ing an average size of 3-5 m in diameter (Table 2).
Notes on floristic diversity: no noteworthy floristic links are evident with associations outside the major group (alliance).The mean number of species encoun tered per sample plot is 34.w ith a high total number of 109 plant taxa (seven relevés) (Table 3).Five SCPE endemics and five near-endemics, of w hich three are Red Data List taxa.are found in this association (Table 2).

Related to the Zantedeschio pentlandi-Aloetum castaneae (Siebert et al. 2()02d).
Environmental data: this association is an open, sparse bush clump of norite and pyroxenite ridges, on midslopes and scarps of hills.It occurs on black and red clay soils of the Glenrosa and Mispah Forms on cool south and southeast aspects.The soil surface is covered by 60-80% rock of an average size of 2.5-3.5 m in diam eter (Table 3).Slope of the habitat is usually 1-5°.Notes on floristic diversity: the association has sever al floristic relationships with the rest of the data set.The mean number of species encountered per sample plot is 33, with 89 species the total number for this association (six relevés) (Table 3).O f the 12 taxa of conservation value, no taxa are restricted to it.Six SCPE endemics and five SCPE near-endemics were recorded, of which four are Red Data List taxa (Table 2).

Environmental data: this vegetation type is a typical ecotone between Brachiario serratae-Melhanietum randii
Rocky Grassland and Themedo triandrae-Combretion mollis of Rocky Ridges in the southern region of the SCPE.It is open shrubland in moist grassland on mid slopes and scarps of undulating norite or pyroxenite hills.
The habitat has a level slope of 1-3°, restricted to south and west aspects.Soils are typical red and black clays of the Mayo and M ilkwood Forms.Rock size is 1-3.5 m in diameter and cover 50-70% of the soil surface (Table 3).Notes on floristic diversity: this association's floristic relationships are typical for the alliance (Table 1).The mean num ber o f species encountered per sam ple plot is 34 and the total num ber o f plant species for this associ ation is 75 (three relevés) (Table 3).A high num ber of taxa o f conservation value are found in this association (13), and o f these seven are SCPE endem ics and six are SCPE near-endem ics.Four of these are Red Data List taxa (Table 2).Two taxa with conservation status are restricted to the association, nam ely the near-endemic Helichrysum albilanatum and the Rare (R) endemic Rhus sekhukhuniensis.
Environmental data: this is a distinct association of moist riverbank thicket on raised banks o f mountain streams in the valleys between undulating grass-covered norite and pyroxenite hills.It lies on a gentle slope of 1-3°.Soils are characteristically a moist 'humus-rich' sandy loam on a rocky substrate.Approximately 60-70% of the soil surface is covered by rocks, with a size of 3.5-5.5 m in diameter (Table 3).Notes on floristic diversity: the association follows the floristic affinities of the alliance, but is characterized by the absence of the taxa in species group P and Y due to the presence of moist, lowland soils (Table 1).The mean number of species encountered per sample plot in this association is 34, with the total number of 64 plant species (five relevés) (Table 3).Four taxa of interest occur in this association, namely two SCPE endemics and two near-endemics (Table 2).Species group S (Table 1).
Environmental data: this vegetation type represents transformed savanna on cool, south and east aspects of norite hills.It covers moderate midslopes and scarps (3-9°).The community is restricted to sandy loam soils in the northern parts of the SCPE.Approximately 20-40% of the soil surface is covered by rocks, with a diameter of >500 mm (Table 2).Notes on floristic diversity: no clear-cut floristic links exist with other plant communities of rocky ridges and it is characterized by the absence of the taxa of rocky habi tats listed in species group Y (Table 1).Intense harvesting of wood and overgrazing of the veld is indicated by the low frequency of taxa in species groups T. Z and AG (Table 1).The mean number of species encountered per sample plot is 20 and the total number is 35 (three relevés) (Table 3).These numbers are the lowest recorded for the rock habitats of the SCPE.None of the five plant taxa with conservation value are restricted to it and comprise three SCPE endemics and two near-endemics, of which one is a locally common Red Data List taxon (Table 2).
Environmental data: alliance of rocky flats on footslopes, midslopes and scarps of predominantly ultramafic hills and to a lesser extent also hills of the Transvaal Sequence.The habitat occurs on all aspects and is gently to moderately sloped (1-9°).Approximately 60-90% of the soil surface is covered by flat rocks exposed at ground level with a relatively large diameter of >10 m (Table 3).Soils are sandy and "humus-rich' in hollows and fissures in the bedrock.

Dominant/prominent taxa: see description of Associa tion 11.
Notes on floristic diversity: a strong floristic affinity exists with the alliance of rocky ridges, which is con firmed by species groups Y and Z (Table 1).The mean number of species encountered per sample plot is ± 29. and the total number of plant species is 83 taxa (14 relevés) (Table 3).This alliance has 12 plant taxa of conservation value, of which three are SCPE endemics and seven near endemics.Six Red Data List taxa occur.Together with Association 4. this is the highest number of Red Data List taxa recorded for rock habitats in the SCPE.None of these taxa are restricted to the proposed alliance.
Environmental data: the vegetation structure is shrub by and grassy and widespread throughout hillsides of the study area and variations of this association are probably widespread along the north-eastern escarpment.It occurs on all aspects o f footslopes.midslopes and scarps.It lies on gentle to moderate slopes (1-9°) and is found pre dominantly on moist, 'humus-rich' sandy soils.Approxi mately 60-90% of the soil surface is covered by rocks, with a mean size o f >10 m in diameter (Table 3).Species group U (Table 1).
Environmental data: widespread throughout the Roossenekal Subcentre on north, south and west aspects of footslopes, midslopes and scarps of undulating norite hills.It lies on gentle slopes (1-3°) and is found pre dominantly on sandy soils.Approximately 70-80% of the soil surface is covered by rocks, with a mean diame ter of >10 m (Table 3).Notes on floristic diversity : a strong floristic relation ship exists w ith Subassociation 11.2 in species group W and Association 5 species group X (Table 1).The mean number of species encountered per sample plot is 30 and the total number of plant species is 66 taxa (six relevés) (Table 3).None of the ten plant taxa of conservation value, namely two SCPE endemics, six near-endemics and five Red Data List taxa.are restricted to the sub association (Table 2).
Environmental data: this community is widespread throughout the SCPE.It occurs on rocky flats with 'humus-rich', sandy soils.It covers moderately sloped footslopes.midslopes and scarps of 3-9° on all aspects of undulating norite and pyroxenite hills.Approximately 60-90% of the soil surface is covered by large rocks, w ith a mean size of >10 m in diameter (Table 3).Notes on floristic diversity: floristic relationships are similar to that of Subassociation 11.1 (Table 1).The mean number of species encountered per sample plot is 27 and the total number of plant species is 75 taxa (eight relevés) (Table 3).There are nine taxa of conservation value in the subassociation, namely three SCPE ende mics, five near-endemics and four Red Data List taxa (Table 2).(Siebert et al. 2002c) Species group AF (Table 1).

IV. Combreto erythrophylli-Celtion africanae
Environmental data: this well-documented alliance (Coetzee 1975;Van der Meulen 1979;Matthews et al. 1992) represents forests and dense woodlands in rocky refugia of the SCPE.It is a rare vegetation type and can be found on southern aspects of valleys, and mountain footslopes, midslopes and crests.The habitat is charac terized by large norite boulders of minimum 2 m high and the mean rock diam eter ± 0.5-2.5 m and covers ± 10-70% of the soil surface.It is characterized by gentle to moderate slopes (1-7°).Soil types are a red or black clay base on unconsolidated material and include the Mayo (lithocutanic B-horizon) and the Oakleaf (neocutanic B-horizon) Forms.Notes on floristic diversity: a strong floristic affinity exists with Association 4 in species group AF (Table 1), which indicates its relationship with the tall bush clumps of rocky ridges.The mean number of species encountered per sample plot is ± 36 and the total number of plant species is 79 taxa (11 relevés) (Table 3).There are five taxa of con servation value associated with this major group, namely two SCPE endemics and three SCPE near-endemics (Table 2).O f these taxa, an undescribed Gymnosporia, is restrict ed to the alliance.
Environmental data: in the Roossenekal Subcentre this subassociation represents wooded rocky refugia, mostly associated with boulders around caves, in kloofs and below cliffs, or stone walls of old kraals.It is a vegetation unit on red clay soils of the Mayo and Oakleaf Forms.These units occur on footslopes and midslopes of undulating norite hills.The gentle slopes vary from 3-7° and east-south-west aspects are predominant.Rock cover percentage varies from 10 to 40% and rock diameter is 1-1.5 m (Table 3).Notes on floristic diversity: strong floristic links exist with Subassociation 12.2 in species group AC and a spe cific Afromontane link with Association 13 in species group AE (Table 1).Species shared with other rock habi tats are few.The mean number of species encountered per sample plot is a high 41 (Table 3).The total number of plant species for this subassociation is 68 (six relevés).Two taxa of conservation value occurs in this subassoci ation, namely an undescribed endemic Cyphostemma species (Siebert 1383) and the near-endemic Euphorbia lydenburgensis.
Environmental data: this vegetation type is slightly transformed woodlands of rocky banks along the larger rivers in the valleys.The habitat lies between norite out crops on black and red clay soils derived from alluvium.It lies on gentle slopes of 3-5°.Soils are predominantly the Mayo and Oakleaf Forms.Approximately 20-70% of the soil surface is covered by rocks, with an average diameter of 0.5-2.5 m (Table 3).Notes on floristic diversity: the subassociation shows the same floristic links as Subassociation 12.1 (Table 1).The mean number of species encountered per sample plot is 33 and the total number is 64 (three relevés) (Table 3).It has two taxa of conservation value, namely a locally common near-endemic and an undescribed en demic Gymnosporia (Siebert 45S) (Table 2).
Environmental data: this association comprises two relict Afromontane Forest patches on the summit of the Leolo Mountains (1 8(X) m a.s.l.).It is associated w ith norite substrates and boulders of 2-6 m high.The habitat has a southern aspect and a gentle slope of 1-3°.Approximately 20-40% of the soil surface is covered by rocks with a mean diameter of 500-750 mm (Table 3), which can be up to 3 m high.Soil is black clay of the Oakleaf Form.Notes on floristic diversity: strong floristic affinities exist with Association 12 in species groups AE and AF (Table 1).It also shares woodland species with Asso ciation 4 in species group AF (Table 1).However, this is not a true vegetation type of rock habitats, but due to the subsequent undersampling and existence of only two forest patches (2 relevés).The community is described here for lack of better placement.The mean number of species encountered per sample plot is 35 and the total number of plant species is 65 taxa (two relevés) (Table 3).This association has one taxon with conservation status, namely the undescribed, near-endemic Gymnosporia sp.nov.(Van Wyk & Siebert 13351) (Table 2).Another note worthy taxon is Nemesia zimbabwensis with its disjunct distribution shared with the Eastern H ighlands of Zimbabwe.Both these taxa are restricted to the associa tion.Its relict status gives the community special conser vation significance as a rare plant community (perhaps the rarest in the SCPE).

Ordination and environmental factors
The naturally sparsely vegetated appearance of rocky habitats can be ascribed to the relatively high surface cover of rock, comprising many taxa typical for this habitat in the northern provinces of South Africa.When compared with other habitats of the SCPE (Siebert 2001), its environmental factors are relatively homoge neous.A combination of factors such as terrain type (slope), soil texture (clay/sand content) and rockiness (rock size and rock cover), affect the species composition of these plant communities.The ordination supported a gradient which is mainly the consequence of rockiness.
The scatter diagram displays the distribution of relevés along the first and second ordination axes (Figure 2).The vegetation units are represented as groups, their distribution on the scatter diagram corresponding with certain physical environmental conditions.The rocki ness.slope and soil texture determine a definite gradient that is depicted by both the first (eigen value = 0.669) and second axis (eigen value = 0.456).Rockiness, slope and soil texture determine the moisture retention and drainage of the habitat.The gradient on the x-axis expresses rock cover as a percentage of the soil surface, with the left extreme of the scatter diagram representing rocky flats with its continuous layers of rock at the soil surface and the right depicting the large boulders with large areas of open soil between them, which are typical for rocky refugia.On the y-axis.the gradient indicates higher moisture availability over the long term at the top of the diagram, because clayey soils on moderate slopes with large areas covered with rock remain moist over a longer period.Steep slopes with sandy soils and low rock cover dry out quickly and are at the bottom of the dia gram.The x-axis also exhibits a gradient with deep soils at the right and shallow soils at the left.
All these gradients correlate closely with each other and have a strong influence on the vegetation structure and species composition.The three most dominant and conspicuous taxa of each growth form (trees/shrubs/suffrutices. forbs/sedges and grasses) are given for each of the four major vegetation types (alliances) depicted in the scatter diagram (Table 4).
A vegetation key is presented to aid with the identifi cation of the various plant communities (Table 5).The definitions are broad indications of typical groups and should be seen as a guideline.A diagnostic characteristic of the vegetation or habitat is given, followed by a most diagnostic and a most visual species of the plant com munity.The first species is restricted to the specific com-  munity only, and the second is dominant, but also occurs in other communities.Where one species is given, no species was restricted to the group only.

Diversity and conservation
Much of the vegetation distribution of the SCPE is attributed to the vegetation dynamics and historic evolu tion of the floristics of the region, w hich is influenced by three factors, climate, topography and soils (Siebert 1998).It has been shown that the vegetation of rocky outcrops is specifically adapted to the chemical environ ment (Morrey et al. 1989: Bredenkamp & Deutschlander 1995;Tyler 1996).When considering the large number of associations described in this paper, it is speculated that the soil chemistry of the rock habitats in the SCPE is partly responsible for the unique communities and plant diversity that occur there (Siebert 2001).
Rock habitats are characterized by high spatial het erogeneity due to the range of differing aspects and slopes (topography), all of which result in differing soil, light and hydrological conditions.Variation in aspect and soil drainage has proven to be an important predictor of plant diversity in the SCPE (Siebert 2001).This is attrib uted to landscapes with spatially heterogeneous abiotic conditions, which provide a diversity of potential niches for plants.Plant species richness and diversity is signifi cantly higher in sites with high geomorphological het erogeneity (Burnett et al. 1998).However, rock habitats are usually isolated, which means that should they be disturbed, the species richness will be influenced nega tively due to the long distances between similar plant communities (Bruun 2000).
Vegetation units or rock habitats in the SCPE require special protection against disturbances and should be considered for conservation purposes in the light of the rapidly developing mining industry o f the region.Already, some plant communities of rock habitats in the SCPE (especially the rocky refugia) are not as diverse as they were in the past, due to extensive harvesting of fire wood and building material (Crookes et al. 2000).These Afromontane forests of rocky refugia are of continental conservation importance (White 1981).In addition, cer tain SCPE plant endemics of rock habitats are restricted to specific communities, and once removed, will proba bly not return.If it is considered that 'disclim ax' in dry woodlands (<1 (MX) mm/annum) created through past land use activities, will not develop into stands similar to the previous state (Roth 1999).and given the present state of fragmentation and continued disturbance taking place w ithin SCPE ecosystems, the rock habitats of the SCPE require immediate attention.All the plant communities o f rock habitats presented in this paper are of conserva tion value, as they provide important islands to sustain biodiversity.

CONCLUSIONS
The classification obtained by TW INSPAN and refined by Braun-BIanquet procedures resulted in 17 veg etation units (plant communities) that can be related to environmental factors.These vegetation units should be considered as ecologically interpretable plant communi ties for the area concerned.The classification of these vegetation units as associations is supported by the results of the ordination that pointed towards meaningful rela tionships between the vegetation and habitat gradients.
It is hoped that the classification and description of the different vegetation units will make a significant con tribution tow ards the understanding of the plant com m u nities of rock habitats of the SCPE and in southern Africa as a w hole.The information supplied in this paper can be meaningfully applied in the management and conserva tion of the respective areas.Proper and sound future assessment of the region's vegetation should include aspects such as species richness, rarity and habitat pref erence.Such studies will further contribute and benefit decisions on land use management and conservation, and might hold solutions for the rehabilitation of areas dis turbed by mining activities in this region.

FIGURE
FIGURE 1.-Extent o f occurrence o f rock habitats of Sekhukhuneland Centre o f Plant Ende mism in Limpopo [Northern Province] and Mpumalanga, South Africa.
FIGURH 2.-Relative positions of all relevés along first and second axis of ordination of rock habitat vegetation in Sekhukhuneland Centre of Plant Endemism.Numbers refer to plant communities inTable 1.