An inselberg was suitable if it had a sufficiently large summit plateau for sampling of vegetation. Using topocadastral maps, 11 potential inselbergs spread 60 km from the northern to central Drakensberg, KwaZulu-Natal, were selected, of which the six that could be sampled were the Sentinel, Eastern Buttress, Inner Horn, Outer Horn, Dragon’s Back and Cathkin Peak (Brand et al. 2015). Position, elevation and surface area of the sampled inselbergs are presented in Armstrong and Brand (2012). Sampling was undertaken during November 2005. All inselbergs surveyed are relatively flat-topped with the exception of the Sentinel, which has three tiers sloping to the east. Sampling was restricted to summits and covered all habitats. All species occurring within 103 Braun-Blanquet sample plots (Brand et al. 2015) were collected. In addition, thorough ad hoc collections of species not found in these plots were made by R. Scott-Shaw and R. Lechmere-Oertel on all habitats of the inselberg summit (e.g. ridges, rocky terrain, wetland seeps, gravel plains and open grasslands). Nomenclature of species is presented in Appendix 1.

Voucher plant specimens were prepared according to standard practices (Alexiades 1996). Initial field identifications were confirmed using the Donald Killick Herbarium (CPF) at Ezemvelo KZN Wildlife, the Geo Potts Herbarium (BLFU) at the University of the Free State, and the National Herbarium (PRE) of the South African National Biodiversity Institute (SANBI) in Pretoria. Voucher specimens (172) were housed at the CPF. A species list was compiled and arranged according to the Englerian system (SANBI, Pretoria). Nomenclature follows Fish et al. (2015) for grasses and Germishuizen et al. (2006) for the remainder modified following consultation with the September 2018 SANBI web database (SANBI/POSA BHRAMS; http://posa.sanbi.org/sanbi/Explore). Red Data species were checked in September 2018 using a SANBI website (http://www.redlist.sanbi.org). Information on endemic or near endemic status (defined according to Carbutt & Edwards [2006]) and distribution, including elevation range, was obtained from published sources (Carbutt & Edwards 2004, 2006; Germishuizen et al. 2006; Pooley 2003; Raimondo et al. 2009; Scott-Shaw 1999).

Growth form was assessed using six relevant categories of Körner’s (2003) adjusted Raunkiaer classification, namely small phanerophytes (Ph; shrubs < 2 m in height), chamaephytes (Ch; dwarf shrubs, thorny cushions), hemicryptophytes (H; perennial herbs and graminoids), geophytes (G; bulbs, rhizomes or tubers), biennial plants (B; mostly rosette plants) and therophytes (annuals, A). Using these symbols, growth form is indicated in Appendix 1 only at the family level except where species within a family differ. Geophyte status for ferns and Limosella was determined according to Pooley (2003) and Germishuizen et al. (2006). Growth form is considered to be genetically determined (e.g. cushion plants remain so when grown from seed at low altitude), whereas life form represents an interaction between growth form and environment (e.g. shrubs may develop into trees when environmental constraints are removed) (Körner 2003).

The potential conservation value of the Drakensberg inselbergs depends in part on whether the flora they harbour is widely distributed or not. We therefore compared their flora with that of the DAC and the Platberg inselberg to the north. On account of the Drakensberg inselbergs forming part of the DAC, it is expected that they should harbour a subset of species, including endemic and near endemic species, occurring within the DAC, which was examined using the study by Carbutt and Edwards (2004). The endemic or near endemic status of plants on Platberg is taken from Brand, Brown and Du Preez (2010). The Platberg inselberg lies 60 km north of the northernmost inselberg sampled in this study (Sentinel); Platberg’s flora was described by Brand et al. (2010) and Brand, Brown and Du Preez (2011). The Platberg supports a large plateau (± 3000 ha) of lower altitude (mean elevation of approximately 2390 m asl), otherwise similar climate and geology to the Drakensberg inselbergs, although grasslands are dominated by C₄ species, especially Themeda triandra (Brand et al. 2011). A degree of similarity of their floras was expected owing to their proximity and environmental affinities.

Vegetation physiognomy showed little change across the 60 km range of inselbergs from the Sentinel to Cathkin Peak. It was a mix of short to medium height graminoids, cushion-forming dwarf or low to medium height shrubs, with herbaceous species interspersed. The rugged terrain, alpine plant species and life forms are shown in Figure 1. A total of 200 species representing 45 families and 91 genera (Table 1) were recorded in a total area surveyed of 31.9 ha; a complete list of species is provided in Appendix 1. Of the 200 species, dicotyledons representing 48 genera constituted 66.1%, monocotyledons representing 37 genera constituted 31.2% and five genera of pteridophytes constituted 2.5%. Of the 114 endemic (61) or near endemic (53) species recorded, 75% represented 14 families, of which 42.1% were in the Asteraceae, Poaceae and Ericaceae families. One is a possible new species (Senecio sp. nov.). Averaged across the total area of the six inselbergs sampled, 3.5 endemic or near endemic species were encountered per hectare of inselberg summit.

Table 2 shows the 15 families that contributed the greatest number of species on inselbergs. Asteraceae contributed 27.5% to the total number of species, three times more than the second-ranked Poaceae, the two together contributing 35.5% of the total and 43.8% of the endemic and near endemic species. Other families each contributing at least 5% of the total number of species (≥ 10 species) were Cyperaceae, Ericaceae, Scrophulariaceae, Hyacinthaceae and Iridaceae, totalling 26.5% of the total and 24.6% of endemic and near endemic species. The 61 endemic species belonged to 21 families (6 monocotyledon, 15 dicotyledon), the 53 near endemic species belonged to 20 families (10 monocotyledon, 10 dicotyledon) and the two groups combined belonged in 12 monocotyledon and 19 dicotyledon families. Rank order of the 15 most abundant families for endemic and near endemic species in general mirrored that of their contribution to the total number of species (Table 2). However, families showing a conspicuously high (> 50%) proportion of endemic and near endemic species were the Asphodelaceae, Orchidaceae, Poaceae, Ericaceae, Thymelaeaceae, Scrophulariaceae and Hyacinthaceae. No pteridophyte endemic or near endemic species or any alien plant species were found on any peak.

Red Data status of all except six species was Least Concern, the exceptions being Aponogeton ranunculiflorus (Vulnerable, endemic), Colpodium drakensbergense (Vulnerable, endemic), Helichrysum album (Rare, endemic), Helichrysum pagophilum (Rare, endemic), Muraltia montana (Rare), Berkheya rhapontica (Not Evaluated) and Satyrium longicauda var. jacottetianum (Not Evaluated, near endemic). Accordingly, we place emphasis on endemic and near endemic species for consideration of conservation importance.

Composition of the inselberg flora in terms of growth form is summarised in Table 3. The majority of species were dicotyledenous chamaephytes contributing threefold more species than geophytes. Chamaephytes were made up of dwarf shrubs, herbaceous and filiform forbs. All non-graminoid monocotyledons, representing 10 families, and all four families of pteridophytes were geophytes. Conspicuous within the rich geophyte vegetation (12 genera, 34 species) were the families Hyacinthaceae and Iridaceae, which contributed 28.8% of monocotyledons and 9% of total inselberg plant species. Orchidaceae, represented by three genera and three species, were not as conspicuous on inselbergs as they are in the broader DAC flora (Table 2). Orchid plants occurred singly at low density. Hemicryptophytes were all graminoid monocotyledons. All 16 grass species (Appendix 1) were perennial, tufted species, and all except Eragrostis caesia and Polevansia ridiga use the C₃ photosynthetic pathway. The two exceptions use the C₄ pathway. No Themeda triandra, the most abundant species below 2900 m asl across the Drakensberg, was found. Phanerophytes, about 2 m tall shrubs, were represented by four species, mostly Helichrysum trilineatum, with lesser amounts of Cliffortia nitidula, Passerina montana and Passerina drakensbergensis. No therophytes were found.

Distinct wetland vegetation associated with numerous seasonal pools and seeps supported a number of endemic or near endemic species of OBW, FACW or FAC plant species (Appendix 1). Hydrophytes endemic to the DAC included Aponogeton ranunculiflorus and Limosella vesiculosa, near endemics included Crassula gemmifera and Limosella grandiflora, while Aponogeton junceus and Potamogeton thunbergii were widespread. Facultative wetland and FAC plants associated with seeps included Aster erucifolius, Crassula dependens, Psammotropha obtusa, the DAC endemic Rhodohypoxis rubella and the wide-ranging Rhodohypoxis baurii var. baurii. Facultative plants associated with wetland communities included three DAC near endemic taxa, the geophytes Wurmbea elatior and Dierama robustum, and the biennial, caespitose grass Pentaschistis airoides.

The inselberg flora of 200 species was not a simple subset of the DAC flora of 2520 species as seen by differences in the rank order of families by number of species (Table 2). Although Asteraceae was the dominant family of both, Scrophulariaceae and Iridaceae within the DAC were replaced by Poaceae and Cyperaceae on inselbergs as the second and third most dominant families; some families such as Apocynaceae and Lamiaceae were absent from inselbergs but were conspicuous in the DAC, whereas Crassulaceae, Polygalaceae, Apiaceae and Rosaceae were more conspicuous on inselbergs than in the DAC.

At the genus level, the endemic Polevansia (Poaceae), Dracomonticola (Orchidaceae) and Glumicalyx (Scrophulariaceae) occurred on inselbergs and within the DAC, whereas Heteromma (Asteraceae) and Strobilopsis (Scrophulariaceae) were not recorded on inselbergs. The near endemic genera Craterocapsa (Campanulaceae), Guthriea (Achariaceae) and Rhodohypoxis (Hypoxidaceae) occurred on inselbergs and within the DAC, whereas Glekia (Scrophulariaceae), Huttonaea (Orchidaceae) and Thamnocalamus (Poaceae) occur in the DAC but were not found on inselbergs. Comparing the 61 endemic species recorded on inselbergs (Appendix 1) with a list for the DAC (Carbutt & Edwards 2006) reveals that Aponogeton ranunculiflorus, Senecio cristimontanus, Senecio parentalis and Senecio sp. nov. (Scott-Shaw C.R., Brand R., Lechmere-Oertel R., 14363 (CPF)) are 4 of 47 DAC species identified as possibly holo-endemic (a species known from only one or a few highly localised sites; Carbutt and Edwards 2006:108).

Of the 61 endemic and 53 near endemic species recorded on inselbergs, Helichrysum (Asteraceae) contained the highest number of all 91 genera with 8 endemic, 11 near endemic and only 3 widespread species. Respective numbers of endemic or near endemic species for other genera were 6 and 2 of 11 Erica species (Ericaceae), 1 and 1 of 4 Cliffortia species (Rosaceae), 1 endemic of 2 Passerina shrub species (Thymelaeaceae), 1 near endemic of 2 Macowania shrub species (Asteraceae), while both Euryops species (Asteraceae) were endemic. This study recorded a higher maximum elevation than previously recorded (shown in parentheses) for the following DAC endemic or near endemic species: Cliffortia filicaulis (1586 m), Helichrysum albanense (1675 m), Helichrysum cymosum (1200 m), Passerina drakensbergensis (1981 m), Thesium costatum var. juniperinum (1500 m) and Thesium nationae (1350 m).

Platberg, with 100-fold greater area than the combined area of six inselberg summits, supported 649 species compared with the 200 species recorded on inselbergs, of which 80 species (40% of the inselberg flora) were shared (Table 4). Representation of families was broadly similar, with Asteraceae, Poaceae and Cyperaceae contributing (in order) the most species at Platberg (35.5%) and on inselbergs (41.5%). Ericaceae were well represented on inselbergs but were less prominent on Platberg, although eight species were supported. Asteraceae, Crassulaceae, Iridaceae and Hyacinthaceae were families better represented on DAC inselbergs than on Platberg, with four of six Crassulaceae species shared with Platberg; the converse pattern shown by the Fabaceae, Lobeliaceae and Campanulaceae. Shared species comprised three ferns, 25 monocotyledons (11 DAC endemic or near endemic), including 6 of 12 Cyperaceae species shared, and 52 dicotyledons (13 DAC endemic or near endemic). Grassland on Platberg was dominated by Themeda triandra, a species not recorded on inselbergs.

Inselbergs supported a small proportion of the plant diversity found in the DAC, but they supported a large proportion of high altitude endemic and near endemic species (Table 2). Sampled inselbergs supported significant plant diversity of 200 species on a surface area of only 31.9 hectares, of which 114 were endemic or near endemic. A large number of endemic and near endemic taxa in an alpine environment is usually explained by numerous small-scale microhabitats and a wide range of edaphic conditions created by slope and aspect (Hedberg, 1955, 1964; Körner 2003), although the level summits of most Drakensberg inselbergs probably offer less habitat diversity than mountain peaks. Endemic and near endemic species offer a strong focus for conservation owing to their limited ranges, small population sizes and a limited but significant genetic pool (Bangert et al. 2005; Carbutt & Edwards 2006). Assessing the value of inselbergs for conservation of plant diversity depends on the threats and changes inselbergs are likely to face relative to those faced within the DAC on the adjacent escarpment.

Inselbergs and the escarpment portion of the DAC differ markedly in terms of exposure to the threats of land use, climate change, alien vegetation and resource extraction by humans. Sustained, severe grazing by livestock is the most important contemporary threat to DAC flora on the escarpment plateau (Nüsser 2002; Quinlan & Morris 1994), a threat to which inselbergs are unlikely to ever be exposed owing to their inaccessibility. Certain non-graminoid growth forms are vulnerable to local extirpation by sustained livestock grazing (O’Connor et al. 2010; Scott-Shaw & Morris 2015), an impact that is possibly occurring on the escarpment. It is not known if sustained grazing on the escarpment has altered fire-return interval; grazing may have promoted grasses at the expense of shrubs, hence increasing the likelihood of burning, or it may have decreased this likelihood through reducing fuel load. The current healthy representation of heath-like elements on inselbergs is, however, evidence of relatively long fire-return intervals compared with other Drakensberg grasslands (De Villiers & O’Connor 2011; Killick 1963). Alien plants pose an increasing threat to the main DAC system, in which 166 taxa, or 6% of its total angiosperm flora, has been recorded as alien (Carbutt 2012; Carbutt & Edwards 2004). In contrast, aliens were absent from inselbergs, a fact attributed to their extreme climate and isolation that restricts arrival of plant propagules associated with humans and livestock, or from natural means. Inaccessibility of inselbergs protects them from harvesting of plants for medicinal purposes, a practice resulting in a substantial local loss of species elsewhere in KwaZulu-Natal (Scott-Shaw 1999). Inselbergs are therefore spared three threats facing the DAC, namely livestock, alien plants and harvesting, all threats set to continue if not increase within the DAC. Inselbergs may contain only a small proportion of DAC flora but they afford this flora strong protection.

Climate change is an impending threat against which inselbergs are poorly protected compared with the escarpment owing to differences in topography. Changes in temperature regime are expected to exert a strong impact on plant distribution in the general region (Jewitt et al. 2015a). Specifically, current altitudinal distribution of C₃ and C₄ grasses in the Drakensberg appears to be strongly controlled by temperature (Bentley & O’Connor 2018; Morris 2017), with a transition from C₄ to C₃ grasses at an altitude of approximately 200 m below the average altitude of inselberg summits (Morris 2017; Morris, Tainton & Boleme 1993). An expected altitudinal increase of C₄ grasses in response to temperature increases resulting from climate change (Stock, Chuba & Verboom 2004) might displace C₃ grasses from inselberg summits, as they have no opportunity for contraction into an elevation refuge or aspect-related topographic refuge owing to the flat-topped nature of inselbergs (Sentinel excluded). In contrast, such opportunities for range adaptation exist on the main escarpment for C₃ grasses and other temperature-sensitive species. A first threat is that all except two of the 16 grasses sampled are C₃ species; representation of this key group may therefore decrease with a consequent reduction of endemic or near endemic grasses, three of which are represented by monotypic genera. On account of C₃ grasses currently providing a matrix for inselberg vegetation, a novel threat might arise if these grasses were to be replaced as this would probably considerably alter matrix structure. This threat would be acute if a novel matrix created an inappropriate or poor environment for other species, especially considering a large proportion of these are endemic and near endemic species that generally display a narrow environmental distribution.

Uncertainty exists about likely changes in rainfall of this region resulting from climate change (Engelbrecht, McGregor & Engelbrecht 2009), but if the region became drier perhaps the most conspicuous threat would be to the hydrophilic flora, including endemic and near endemic species, found in pools and seeps because desiccation would become commonplace.

The potential contribution of Drakensberg inselbergs to conservation of DAC flora will be aided to some degree by other nearby inselbergs such as Platberg. Although 60 km distant, at lower elevation and with differing vegetation, Platberg nonetheless shared a proportion of Drakensberg inselberg species (Table 4) including 24 DAC endemic and near endemic species, illustrating that some DAC endemics may be more widespread than currently known (Table 4). A number of such inselbergs exist within the KwaZulu-Natal lowlands, eastern Free State and extending north archipelago-like along the main escarpment forming connections with Afro-alpine vegetation to the north (Carbutt & Edwards 2001; White 1983). Phytogeographic affinities also extend to the south into the Cape Floristic Region (Goldblatt & Manning 2000) via the Sneeuberg (Clark, Barker & Mucina 2009) and Zuurberg (Carbutt & Edwards 2001; Van Wyk et al. 1988), as well as into the interior Hantam-Roggeveldberge and Nuweveldberge (Clark et al. 2011a, 2011b). Notwithstanding that comparison among these would be compounded by the response of temperature to the combined effects of altitude and latitude, a spatially explicit assessment of phytogeographic affinities between the DAC and its inselbergs with these other montane regions, especially surrounding inselbergs similar to Platberg, would provide a broad geographic context for working toward conservation of the Drakensberg hotspot and centre of endemism of plant diversity.

Inselbergs supported a preponderance of chamaephytes, geophytes and hemicryptophytes, a paucity of phanerophytes, while therophytes were absent (Table 3), showcasing growth forms well or not adapted for this high altitude environment (Gröger & Barthlott 1996; Körner 2003; Parmentier et al. 2006; Wesche, Miehe & Kaepell 2008) that experiences severe temperatures, freezing conditions, desiccation including through freezing, and high wind speeds (Tyson et al. 1976). The common forms observed of dwarf and sclerophyllous shrubs, cushion plants, caespitose graminoids with filiform xeromorphic leaves and acaulescent rosette plants (Appendix 1) are characteristic of alpine regions (Körner 2003; White 1983). By contrast, geophytes below-ground bulbs ensure protection in fire-prone grasslands and high altitude alpine conditions of freezing temperatures and heave soils (Mucina & Rutherford 2006). However, fire does occasionally penetrate inselbergs, as revealed by burned Helichrysum trilineatum shrubs (R. Brand pers. obs., November 2005).

Plant growth form and vegetation structure of alpine regions is commonly ascribed to the temperature regime of the coldest period (Hedberg 1970; Körner 2003; Körner et al. 2011). A consequent explanation for the dominance of chamaephytes and hemicryptophytes (Table 3) is the extent to which their growth forms enable decoupling of a plant’s climate from the ambient climate through increasing heat accumulation in the leaf canopy (Körner 2003) that allows physiological processes to proceed when ambient temperature may be at or below freezing for extended periods (Wesche et al. 2008). An apparent contradiction is provided by the large number of non-graminoid monocotyledon genera with relatively large, semi-succulent, water-rich leaves (e.g. Agapanthus, Albuca, Eucomis, Gladiolus, Hesperantha, Kniphofia, Ledebouria and Moraea) recorded on inselbergs; however, these long-lived perennial geophytes are in leaf only during the warmer summer season. Leaves, stems and flowers die after summer, leaving no visible, above-ground trace, while the bulbs are protected below ground from freezing, desiccation and heave soils (Wesche et al. 2008).

Growth habit contributes to graminoid species meeting the demands of a high altitude climate (Gibbs-Russell et al. 1991; Körner 2003). Most graminoid species recorded possessed a caespitose growth form (Appendix 1), which not only protects meristems against fire or herbivory (Briske & Richards 1995) but also confers a temperature advantage to a plant. Thermal advantages also accrue to graminoids with a cushion form (e.g. Pentaschistis spp.), rhizomatous species (e.g. Koeleria capensis and, Festuca spp.), species with a combination of a decumbent growth form, rhizomes and stolons (e.g. Colpodium, Polevansia) and even to loosely tufted stoloniferous species (e.g. Karroochloa, Schoenoxiphium and Scirpus). Graminoids, especially caespitose forms, therefore appear well adapted to the climate challenge of this high altitude environment. Furthermore, nearly all graminoid species possessed a C₃ photosynthetic pathway, enabling efficient photosynthesis at the temperatures of high elevation (Pitterman & Sage 2000; Sage 2001, 2004). In correspondence, C₄ grasses are dominant on the main escarpment up to 2950 m asl on north and 2750 m asl on south aspects, above which C₃ grasses are dominant (Morris 2017; Morris et al. 1993), and altitude exerts a strong influence on the distribution of individual C₃ grass species (Bentley & O’Connor 2018).

Water balance may become critical for evergreen plants receiving seasonal precipitation, especially as water availability may be further interrupted by soil freezing (Tranquillini 1979). Effective coping strategies for non-deciduous species (84% of those recorded) include succulence owing to water-use efficiency, stomatal control and, for mosses, desiccation tolerance (Körner 2003; Rutherford & Westfall 1994). There is no knowledge about stomatal control within species of this flora. Although succulent species were evident on inselbergs, they contributed only 1.1% of the flora (6 Crassulaceae; 2 Aizoaceae; 2 Euryops; 12 Senecio; Othonna burttii; Psammotropha obtusa). Crassulaceae and Aizoaceae species also benefit through use of the crassulacean acid metabolism (CAM) photosynthetic pathway that is effective at low temperatures (Pitterman & Sage 2000). The conspicuous presence of Crassulaceae species on Drakensberg inselbergs by comparison with the DAC (Table 2) is consistent with a pattern evident for inselbergs in west (Parmentier et al. 2006), central and east Africa (Hauman 1955; Hedberg 1970), suggesting their abundance owes to reduced exposure to fire.

Thick roots capable of withstanding the shearing forces exerted by frost heaving of soils (Körner 2003; White 1983) were possessed by Orchidaceae, Apiaceae, many Asteraceae, Brassicaceae, Caryophyllaceae, Fabaceae, Gentianaceae, Scrophulariaceae and the genera Craterocapsa, Cyphia, Epilobium, Geranium, Polygala, Scabiosa and Valeriana. Poor representation of phanerophytes suggests an influence of strong winds (Grace 1977), growth rates inhibited by low temperatures (Tranquillini 1979) or even the inhibiting effect of high ultraviolet radiation on stem extension (Hedberg 1964), ecological factors that have been largely ignored in the Drakensberg.