Euphorbia otjipembana Leach subsp. okakoraensis Swanepoel, 2013

Swanepoel, Wessel, 2013, Euphorbia otjipembana subsp. okakoraensis (Euphorbiaceae), a new subspecies from the Kaokoveld, Namibia, with an amplified description of Euphorbia otjipembana, Phytotaxa 117 (2), pp. 51-57 : 53-56

Euphorbia otjipembana Leach subsp. okakoraensis Swanepoel , subsp. nov. ( Figs. 1 & 2 )

Differs from the typical subspecies in the branches being consistently grey-green; spine shields more slender, 2.7–4.4 times as long as broad (at apex), usually rectangular between upper and lower spine pairs; peduncular and cyme branch bracts not leaving prominent scars; cyathia occasionally with six glands and six involucral lobes; glands erect, slightly fleshy; male flowers occasionally arranged in six fascicles; female flowers occasionally with ovary 4- lobed and four styles; capsule occasionally 4-locular.

Type: ― NAMIBIA. Kunene Region: Stony northeastern slopes of Okakora Mountains near Okombambi , 1712 BB, 930 m, 24 March 2007 , Swanepoel 271 (holotype WIND!; isotype PRE!) .

Phenology: —Cyathia were recorded on subsp. okakoraensis from August to October.

Distribution and habitat: — At present Euphorbia otjipembana subsp. okakoraensis is known only from the type locality, in the northern part of the Kaokoveld, northwestern Namibia ( Fig. 3 ). This locally falls within the Kaokoveld Centre of Endemism, a biogeographical region extending from northwestern Namibia to southwestern Angola ( Van Wyk & Smith 2001). It occurs 112 km from the coast, near Okombambi, in a narrow gorge between the Baynes Mountains to the east and the Okakora Mountains to the west at an elevation of 930 m. Average annual rainfall in the area is ± 200 mm ( Mendelsohn et al. 2002). The subspecies is locally uncommon and grows in rock crevices on the northeast-facing aspect of the Okakora Mountains in full sun and in partially shaded areas. The substrate is stony soil derived from greywacke of the Mulden Group of the Damara Sequence ( Schreiber et al. 2002).

Conservation status: — Although rare and known from a single locality only, subsp. okakoaraensis is probably not threatened as it occurs in a remote, almost inaccessible area. The local Ovahimba do not distinguish among E. otjipembana subsp. okakoraensis , E. otjipembana subsp. otjipembana , E. kaokoensis and E. subsalsa subsp. fluviali s and use the vernacular Otjiherero name ‘ ohahi ’ for all four taxa. The subspecies okakoraensis is thus regarded as being poisonous and probably without any use, other than for poison. No signs of damage caused by animals or humans could be found on any of the in-situ specimens examined.

Etymology:— The specific epithet refers to the Okakora Mountains, the Ovahimba name for the mountains where the new subspecies is found.

Additional specimen examined (paratype): — NAMIBIA. Kunene Region: Kaokoveld, Otjomborombonga , main kloof to the south, 1712 BB, 14 July 1976 , Leistner , Oliver, Steenkamp & Vorster 155 ( PRE!) .

Discussion:— The subspecies okakoraensis differs from the typical subspecies in the branches and in some of the floral characters. In subsp. okakoraensis the stem epidermis is grey-green whereas in subsp. otjipembana the epidermis is usually fresh (bright) green, rarely grey-green. Spine shields in subsp. okakoraensis usually are much narrower at the apex (2–3 mm wide) and—while separate—longer (7–15 mm), usually ± rectangular between the upper and lower spine pairs and ± parallel-sided or narrowly cuneate towards the base, appearing drawn out. In the typical subspecies the spine shields are 2.8–5.2 mm wide, 5– 9 mm long and usually narrowly cuneate from apex to base.

Differences in the floral characters include the inflorescences, which in subsp. okakoraensis are shaded in red, yellow and green and the cyathia which occasionally have six glands and six lobes instead of the usual five. In subsp. otjipembana the inflorescences are yellow and the cyathia always have five glands and five lobes. In subsp. okakoraensis the bracts on the peduncle and cyme branches lack thickening towards the base and do not leave prominent scars as in the typical subspecies. The glands in subsp. okakoraensis are always erect and not particularly fleshy, whereas in subsp. otjipembana they are sub-erect or spreading and fleshy. Male flowers in subsp. okakoraensis are arranged in 5 or 6 fascicles whereas in subsp. otjipembana the flowers are arranged in 5 fascicles only. Female flowers in subsp. okakoraensis have the ovary 3- or 4-lobed and the pedicel is triangular, quadrate or pentagonal and the three or four ascending-spreading styles are united in a slightly shorter column than in subsp. otjipembana . In the latter, the ovary is always 3-lobed, the pedicel is pentagonal and the three styles are spreading-recurved. The capsule in subsp. okakoraensis is 3- or 4-locular whereas it is always 3-locular in subsp. otjipembana .

Apart from the morphological differences, the two subspecies have different flowering times. Subspecie s okakoraensis flowers from August to October and subsp. otjipembana from June to August. The different flowering times were maintained even in cultivation in Windhoek, Namibia, where the two subspecies were grown in close proximity during the study. Prominent morphological features to differentiate among subsp. okakoaraensis and the typical subspecies are supplied in Table 1.

Euphorbia otjipembana subsp. okakoraensis can also be confused with E. kaokoensis and E. subsalsa subsp. fluvialis , which too occur in the mountainous area along the Kunene River in northwestern Namibia. These species share the shrubby habit, the grey-green colour of the branch epidermis and the spine shields with two pairs of spines with subspecies okakoraensis. The latter, however, can be readily distinguished from E. subsalsa subsp. fluvialis by its branches 18–35 mm in diam., which are 5–7-angled (3–5-angled on branchlets), compressed or wing-like. In E. subsalsa subsp. fluvialis the branches are thinner (10–25 mm) and consistently 4-angled with the angles compressed. The branches in E. kaokoensis are ± 20 mm in diam. and 5– 7-ribbed. The spine shields in E. kaokoensis furthermore differ from the other species in that the four spines are more closely set thus appearing to arise almost from a common point.

Figures

FIGURE 1. Euphorbia otjipembana subsp. okakoraensis: (A) plant in natural habitat, ± 0.8 m tall; (B) branch indicating spine shields.

FIGURE 2. E. otjipembana subsp. okakoraensis: (A) flowering branch; (B) branch in transverse section; (C) bisexual cyathium from above; (D) cyme with old central male cyathium and two bisexual cyathia; (E) peduncular bract; (F) cyme branch bract; (G) involucral lobe (adaxial side); (H) male flowers (rarely fused at base, left); (I) fascicular bract with bracteole; (J) female flower; (K) seed. Scale bar 5 mm (A and B), or 1 mm (C–K). All from Swanepoel 271. Illustration by Lesley Deysel.

FIGURE 3. Known distribution of E. otjipembana: subsp. otjipembana (open circle), both subspp. okakoraensis and otjipembana (black dot).