A new species of Euclea (Ebenaceae) from ultramafic soils in Sek- hukhuneland, South Africa, with notes on its ecology

Euclea sekhukhuniensis Retief, Siebert & A.E.van Wyk. a new species with a restricted range in Sekhukhuneland, South Africa, is described, illustrated and compared with other members of the genus. It is a gregarious geoxylic sufthitex forming large, much-branched colonies. The species is closely related to the small tree/shrub E. linearis Zeyh. ex Hiem from which it can be distinguished by its larger fruits, broader leaves and exclusively suffrutex growth form. Geographical range and habitat preference also differ between the two taxa. E. sekhukhuniensis is endemic to the Sekhukhuneland Centre of Plant Endemism, where it is confined to the calcareous, heavy-metal soils of the Steelpoort River Valley.

The SCPE lies to the west of the northeastern sec tion of the Great Escarpment of South Africa and is characterized by a heterogeneous geology, topogra phy and climate (Van Wyk & Smith 2001).The core area of the Centre straddles the border of Mpumalanga and Limpopo, around the towns of Burgersfort.Mecklenburg, Roossenekal.Schoonoord and Steelpoort.The SCPE is best demarcated in geological terms as the large, far-eastern outcrop of ultramafic rocks belonging to the Rustenburg Layered Suite of the eastern Bushveld Complex.These rocks are mainly norite, pyroxenite, anorthosite and ferrogabbro, with localized intrusions of magnetitite and chromitite (Viljoen & Schurman 1998).Topographically the SCPE is a mountainous area bor dered by the high ground of the Drakensberg Escarpment in the north and east, the Highveld Escarpment to the south and the Springbok Flats to the west.It lies adjacent to and west of the Wolkberg (Van Wyk & Smith 2001) and Lydenburg (Schmidt et al. 2002) Centres of Plant Endemism, both part of the northeastern Drakensberg Escarpment.
Previously a Euclea taxon with a suffrutex habit and narrow elliptical leaves from Sekhukhuneland was tenta tively considered a hybrid between E. linearis Zeyh.ex Hiem and E. crispa (Thunb.)Giirke subsp.crispa (De Winter 1963)-a suspicion based on the overlapping dis tribution ranges of these two species in Sekhukhuneland.However, subsequent detailed field work and compara tive morphological studies have shown the putative hybrid to be a distinct new species, closely related to E. linearis and endemic to the ultramafic soils of the SCPE.The new species is here described as Euclea sekhukhuni ensis Retief.Siebert & A.E.van Wyk.This is the second Euclea species, after E. dewinteri Retief (Retief 1986), an endemic of the Wolkberg Centre of Plant Endemism, that is strictly confined to the larger northeastern Drakens berg Escarpment.
The genus Euclea comprises ± 20 species, confined to Africa.Arabia.Socotra and the Comoro Islands, with its centre of diversity in southern Africa (Dyer 1975;Bredenkamp 2000).In addition to E. sekhukhuniensis, seven species and infraspecific taxa occur in the SCPE, namely E. crispa subsp.crispa, E. divinorum Hiem.E. linearis, E. natalensis A.DC. subsp.angustifolia F.White.E. daphnoides Hiem.E. schimperi A.DC. and E. undulata Thunb.(Table 1).However, the list is pro visional.for the region is still poorly sampled.All these taxa are evergreen shrubs or trees, except E. sekhuk huniensis.which is an evergreen geoxylic suffrutex.A form of E. crispa (White 1977) along the northeastern Drakensberg Escarpment (but not entering the SCPE) exhibits the same growth form as E. sekhukhuniensis.

MATERIALS AND METHODS
Euclea specimens housed in the National Herbarium (PRE), Pretoria, and H.G.W.J. Schweickerdt Herbarium (PRU), University of Pretoria (acronyms as in Holmgren et al. 1990) were examined to gather data on morphol ogy, phenology and distribution.
Ecological data are based on extensive field obser vations of Euclea linearis and E. sekhukhuniensis in the SCPE (Siebert et al. 2002a, b).Specimens were sampled from the region during all seasons, environ mental factors were noted and plant communities iden tified.The specimens Woody suffrutex, 0.3-1.5 m high, forming large colonies of much-branched clones ± 5 m diam.Plants evergreen, dioecious.Branches ascending, slender and glabrous, except for a rust-brown granular exudate on tinguished from the others by their narrowly oblanceolate-elliptic to linear or linear-talcate leaves and young leaves and twigs that are covered with a granular, rustbrown exudate.E. sekhukhuniensis is most closely related to E. linearis; however, E. sekhukhuniensis is a gregarious, evergreen geoxylic suffrutex (White 1977), whereas E. linearis is a shrub or tree up to 5 m high.
Leaves of E. sekhukhuniensis are straight, broader and longer than the sickle-shaped leaves of E. linearis and its fruits are larger than those of E. linearis (Table 2).
Conservation status'.Euclea sekhukhuniensis has a restricted geographical range within which it is locally fairly common (Figure 3).However, some of its habi tat is under immediate threat of rapid urbanization as a result of increased mining activities in the greater Steelpoort River Valley and the construction of the De Hoop Dam on the Steelpoort River.E. sekhukhunien sis is not formally protected in any conservation area.Populations of this species should therefore be closely monitored and a Red Data List assessment of this spe cies prioritized.Its conservation value is considered rela tively high, as it could possibly be used in the rehabili tation of mine dumps due to its internal mechanism of excluding heavy metals.
Ecology and speciation: both Euclea sekhukhuniensis and E. linearis occur on vegetation anomalies-sparsely vegetated soils that are mineralized (Table 3).This phe nomenon is well reported for populations of E. linearis in wooded grassland on the serpentinites of the Great Dyke in Zimbabwe (Wild 1965).A distant outlier of what has been identified as E. linearis is also found on grassy ridges in fynbos in a limited area in the Calvinia and Vanrhynsdorp region of the Western Cape, where it grows on nutrient-poor soils derived from sandstone of the Table Mountain Group (White 1983).However, the identity of these plants requires verification.In the mountainous regions of the northern provinces of South Africa, E. linearis grows on rocky outcrops and in dry woodlands on slopes and in valleys of serpentenite (Barberton Supergroup) in the Barberton region, acidic sandstone (Waterberg Group) of the Waterberg of Limpopo, and quartzites (Black Reef Formation) along the northeastern Drakensberg Escarpment (White 1983).Thus, it appears that E. linearis tends to colonize habi tats with harsh soil conditions in mountainous regions (acidic, nutrient poor and/or rich in heavy metals).
Euclea sekhukhuniensis appears to be an example of incipient sympatric speciation due to ecological inter actions in a new7 habitat in which restricted gene flow has evolved through selective reproduction between individuals of E. linearis that are adapted to a specific ultramafic substrate.This speculation is supported by the work of Alados et al. (1999).which demonstrates that asymmetry and within-plant variance were higher between specimens of the same species in the contact zone between ultramafic and normal soils.In the SCPE, habitat preference has resulted in the two Euclea species  Euclea linearis mean measurement (30 specimens) 52 28 2.5 1.8 5.9 -5.4 now growing in specific, but different habitats in associ ation with specific plant species (Tables 3, 4).According to Dieckmann & Doebeli (1999), theoretical evidence suggests a prominent role for ecologically driven specia tion in sympatry.Hence, the present study supports the opinion that the ecological species concept is an essen tial part of the biological species concept (Grant 1992).
Generally, ecologically driven speciation is the result of habitat-specific preferences.This has been investi gated and confirmed for an endemic species of Impatiens and its widespread congener (Chung & Kang 1996), as well as for two endemic species of Dicerandra of the same region (Menges et al. 1999).In the case of Euclea sekhukhuniensis, an open niche with an anomalous Carich substrate (14.68%= 146 800 ppm) in an other wise typical environment of brackish soils rich in Mg (19.97% = 199 700 ppm), probably favoured speciation (Figure 4A).Similar trends have been perceived between especially limestone (Ca-rich) and sandstone, once again for an endemic and its widespread congener (Walck et al. 1999), as well as two endemics of the same region (Mustart et al. 1994).Like limestone, the soils inhab ited by E. sekhukhuniensis are Ca-rich (1,95Ca: 1 Mg), more than double that of the soil substrate of E. linearis (Figure 4A).Furthermore, soils in which E. linearis grows have higher concentrations of total Cr and Ni (typical elements of serpentinite) (Figure 4B), with high Fe, Si and Mg levels (lMg:0.34Ca)(Figure 4A), and it accumulates relatively high concentrations of Al and Fe in its roots (Figure 5A).E. sekhukhuniensis accumulates lower levels of Fe in its roots, but with higher concentra tions of Cr and Ni than E. linearis, although these lev els are very low and not regarded as hyperaccumulation (Figure 5A).Overall, it seems that E. sekhukhuniensis is the better excluder of heavy metals, when considering the high concentrations of metals in the associated soil.
It is suggested that Euclea sekhukhuniensis was an ecotype of and has developed from E. linearis as a result of the genetic properties of the latter to adapt to and col onize ultramafic soils such as those derived from serpen tinite (Wild 1974).It is hypothesized that E. sekhukhuni ensis is a 'soil-adapted' neo-endemic which speciated  5B).
Specific epithet and common names: the specific epi thet refers to the geographical area where the species is endemic.Sekhukhuneland is traditionally inhabited by the Pedi (Monnig 1967) and is currently under the reign of K.K. Sekhukhune (Paton 1998).Common names for the taxon include moshigwane (Northem-Sotho), Steelpoort guarri (English) and Steelpoortghwarrie (Afrikaans).

FIGURE 4 .
FIGURE 4.-Chemical analyses of five soil samples collected from root zone (300 mm deep) for each o f Euclea sekhukhu niensis and E. linearis.
Siebert 935, 937 and Van Wyk & Siebert 13060 were identified as typical E. linearis for comparing ecological traits with E. sekhukhuniensis.Plant material and soil samples were taken at ten sites, five each dominated by either E. linearis or E. sekhuk huniensis.Voucher specimens were taken from these sites; Siebert 937 represents E. linearis and Siebert 938 represents E. sekhukhuniensis (specimens kept at PRU).Soil analysis was done with X-Ray Fluorescence (XRF) Spectrometry, Department of Geology, University of Pretoria and plant analysis with Atomic Absorption Spectrophotometry (AAS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) at the Department of Soil, Climate and Water in Pretoria.

TABLE 1 .
-Distribution o f seven species o f Euclea according to quarter-degree grid squares of SCPE Diagnostic characters: members of Euclea can be divided into two groups (De Winter 1963): 1, species with the corolla shallowly lobed at the apex; and 2, spe cies with the corolla cleft at least halfway down or more.All the Euclea taxa occurring in the SCPE belong to the latter group.E. sekhukhuniensis and E. linearis are dis E. natalensis subsp.angustifolia E. schimperi var.daphnoides E. schimperi var.schimperi E. sekhukhuniensis E. undulata Total m f _. 163367565522 FIGURE 1.-Euclea sekhukhuniensis, Codd 8796, holo.(PRE).young growth; bark grey on older stems.Leaves sim ple, subopposite, subsessile; blade oblanceolate-elliptic, straight, 25-75(-90) x 4-8(-10) mm, glabrous, leathery and smooth, except for a rust-brown granular exudate on younger leaves, usually yellowish green above and pale green below; base tapering into a very short petiole (1 mm), apex acute to rounded, margin entire, main vein and principal lateral veins prominent above and below.Inflorescences axillary, few-flowered, clusters or short spikes.Flowers regular.Male flowers: calyx 4-lobed, ±

TABLE 2 .
-Leaf and fruit measurements of 15 Euclea sekhukhuniensis specimens and only results of 30 randomly selected specimens o f E linearis fromMpumalanga and Limpopo (all in PRE)

TABLE 4 .
(Reeves et al. 1983)b)ed for communities dominated by either Euclea sekhukhuniensis or E. linearis in Steelpoort River Valley(Siebert et al. 2002b), after the Pleistocene(Reeves et al. 1983), and has not yet had the time or routes to migrate out of the Steelpoort River Valley.However, it is doubtful whether this will ever happen, as the species probably prefers the open niches of ultramafic soils where it has a physiologi cal mechanism associated with high plant levels of Ca to tolerate heavy metals (Figure recently