Nordic Journal of Botany

A new species of Acacia (Leguminosae) from the Horn of Africa

Mats Thulin and Abdisalam S. Hassan

Thulin, M. & Hassan, A. S. 1996. A new species of Acacia (Leguminosae) from the Horn of Africa. - Nord. J. Bot. 16: 303-306. Copenhagen. ISSN 0107-055X.

Acacia cernua, sp. nov., is described from open Acacia-Commiphora bushland on the foothills of the north-facing limestone escarpment running along the coast of northern Somalia. The new species is obviously closely related to the widespread A. etbaica that is common on this escarpment, but at considerably higher altitudes. A. cernua differs from A. etbaica in, for example, its smooth, ash grey to white bark, and .in its slender, hanging terminal branches with a glaucous foliage.

M . Thulin, Dept of Systematic Botany, Uppsala University, Villavagen 6, S-752 36 Uppsala, Sweden. - A. S. Hassan, Dept of Botany, University of Reading, Whiteknights, PO. Box 221, Reading RG6 2AS, UK.

Introduction

The genus Acacia in Somalia has recently been revised by Hassan & Styles (1990) and Thulin, Hassan & Styles ( 1993). Thirty-nine native species were recognized in both these papers, but Hassan & Styles also included an insufficiently known species of unknown affinity, and in Thulin, Hassan & Styles (1993) the occurrence of two other obviously undescribed species related to Aca- cia etbaica Vatke in north-east Somalia was pointed out.

During joint field-work in northern Somalia in Janu- ary 1995 we had hoped to be able to collect better mate- rial of some of these incompletely known species. We were successful with the first-mentioned taxon, which we describe below as a new species, A. cernua, the epi- thet alluding to its characteristic slender, hanging, ter- minal branches.

Acacia cernua Thulin & Hassan, sp. nov.

Type: Thulin, Dahir & Hassan 9188, Somalia, Sanaag Region, escarpment S of Laasqoray, 1 1'03". 48" 16'E, 20 Jan 1995 (UPS holotype, FT K isotypes).

Species nova ab A. etbaica cortice cinerea laevi, ramulis

Accepted 16- 10- 1995 0 NORDIC JOURNAL OF BOTANY

terminalibus gracilibus pendulis spinis tenuibus foliis glaucis, et inflorescentiis minoribus differt. - Figg 1, 2.

Slender tree, 2-5 m tall, with a distinct trunk and a rounded to flattened crown with characteristically hanging terminal branches with a glaucous foliage; bark smooth, ash grey to white, not flaking; young branchlets purplish-brown, glabrous or pubescent with f appressed hairs and soon glabrescent, subterete to shallowly sulcate, lenticellate, the epidermis cracking but not flak- ing; internodes 5-18 mm long. Stipules spinescent, in pairs at the nodes, slender, straight, up to 15 mm long and up to c. 0.6 mm thick at the base. Leaves: petioles, rhachides and rhachillae glabrous to puberulous; petiole 4-10 mm long, eglandular; pinnae (1 -)2-4 pairs, the rhachis up to c. 25 mm long, usually with a round slightly raised apically depressed gland c. 0.4-0.6 mm in diam. between the pinnae of one or more pairs; rhachil- lae c. 10--37 mm long; leaflets c. 4-1 I pairs per pinna, 2-4.5(-7.5) x 0.8-2(-3) mm, oblong, glabrous or sparsely ciliate when young, without prominent lateral veins. Inflorescences capitate, axillary, 5-5.5 mm in diam.; peduncle 6-15 mm long (up to 25 mm in fruit), f densely pubescent particularly in upper part, with in- volucel in lower half; bracts of involucel c. 0.5 mm long. Flowers pale yellow. Calyx c. 1.2 mm long, 5- lobed, with pubescent tips to the lobes. Corolla c. 2 mm

Nord. J . Bol. 16(3) 19Y6 303

E

Fig. I . Acacia cernua, from Thulin, Dahir & Hassan 9188. A: Fruiting branchlet (pods pendulous in life), x 1.2. - B: Portion of leaf, showing leaflets and gland between uppermost pinnae, x 12. - C: Node with inflorescences, x 3. - D: Flower, x 12. - E: Seed, x 6 .

3 04 Nurd. J . But. 16(31 1996

long, 5-lobed, glabrous. Stamens c. 30; filaments c. 2.5 mm long: anthers c. 0.15 x 0.25 mm, many with an apical gland. Pistil c. 4 mm long including style; ovary with a c. 0.4 mm long stipe, glabrous, c. 8-lO-ovukite. Pods straight or curved slightly upwards in upper part, linear-oblong, pendulous, compressed, soon dehiscent, purplish-brown, 3 - 7 0 x 8-13 mm, obtuse at the apex, gradually tapering towards the base; valves thin, sparsely to densely puberulous with appressed hairs, with a rather indistinct venation of f longitudinal- oblique anastomosing veins. Seeds c. 4-9, compressed, elliptic to narrowly ovate or almost rhombic in outline, c . 7-8 x 3.5-4 mm. brown: areole central, c. 4.4-4.8 x I .4-1.6 mm, oblong, almost closed. - Figs I , 2.

Distribution and hribitat. Acuciu cernuti is only known from the north-facing limestone escarpment of the Cal Madow Range along the Gulf of Aden in northern Somalia. This escarpment reaches an altitude of about 2400 m, but A. certiucr apparently only grows in a niirrow zone of the open Acrr(.ia-C(~niniiphoru bushland on the arid foothills at 400-625 m altitude (Hassan & Styles 1990 give 200 m, which is obviously an error as the label says 625 m). In this zone the species is ;I

characteristic element of the vegetation on rocky slopes and on rocks along wadis (Fig. 2 ) . It was, in the type locality. often seen growing associated with Boswellio Jieremtr. another rock-inhabiting species, and other woody species in the vicinity included Commiphortr spp. (among others C. myrrhti), Bo.swelli~r siicrci and Omzncwpum muricuturn.

The three collections known of A. cenzuu originate from two localities along the escarpment, separated by a distance of about 120 km. However, i t is most likely that the distribution is more or less continuous, as the intervening part of the Cal Madow Range is virtually uncollected.

V~~rnerculcir name. Cabab (Somali).

I)i.scu.s.sioti. Acacirr cer~iuei appears to be most closely rclated to A. etbiiicri, a common species on the escarp- ments all along the coast of northern Somalia. However. A . rthaica is confined to considerably higher altitudes. 980-1800 m (Thulin, Hassan & Styles 1993). where i t is ;I constituent of BUXUS bushlands and Juniperus wood- lands. A. etbaica, as currently conceived, is a variable taxon, and was subdivided into four subspecies by Rrenan (1957) and Ross (1979). Of these A. ethaicu ssp. u~icinata Brenan is the only one known with certainty from Somalia.

Acucirr cernua and A. etbaicrr agree, for example, in having stipular spines, capitate inflorescences with small. pale yellow flowers, and straight and basally at- tenuate pods. However, A . i'e~riiuri is easily distin-

Fig. 2. At,trcim ccmur. Tree growing S of Laasqoray. Somalia (Thulin. Dahir & Hassan Y024). Photo by M . Thulin.

guished from all forms of A. etbuicci by its slender, hanging. terminal branches with a glaucous foliage, and its smooth, ash grey bark. In A. evbaicri the branches are more robust, not hanging and with a dark green foliage, and the bark is rough and greyish-brown to blackish. Also, in A. cerizuir the spines are very slender, straight and c. 2-15 mm long, and the inflorescences are only 5 - 5.5 mm in diameter. A. rtbaicrr has a variable armature with short to long and hooked to straight spines, but they never quite match the slender spines in A. ceriiuu, and the inflorescences are c. 10-14 mm i n diameter.

Recent data on endemism i n Somalia (Thulin 1994a) shows two major zones particularly rich in endemics, one in the north-east. ranging from 47" in the west to Cape Guardafui in the east, and one in the central part in the Hobyo area. A. ccrriua is confined to the north- eastern area of endemism, and also occurs within the Cal Madow Centre of Plant Diversity as defined by Thulin ( I994b).

Collec~tiorzs other thcirz type. Somalia. Sanaag Region:

305 N u ~ d J . Bol. 1613) I Y Y 6

near Rugi, 10"50'N, 47"18'E, 30 Sep 1987, Fagg & Styles 62 (FHO, K sterile); escarpment S of Laasqoray, 11"04'N, 48"15'E, 15 Jan 1995, Thulin, Dahir & Has- san 9024 (UPS).

Acknowledgements - We are indebted to Ms Louise Petrusson for the drawing in Fig. 1, and to SAREC (Swedish Agency for Research Cooperation with Devel- oping Countries) for financial support of the field work.

References Brenan, J. P. M. 1957. Notes on Mimosoideae: 111. - Kew

Bull. 12: 75-96.

Hassan, A. S. & Styles, B. T. 1990. A conspectus of Somali Acacias. - Chatham Maritime: Natural Resources Institute.

Ross, J. H. 1979. A conspectus of the African Acacia species. - Mem. Bot. Surv. S. Africa 44.

Thulin, M. 1994a. Aspects of disjunct distributions and ende- mism in the arid parts of the Horn of Africa, particularly Somalia. - In: Seyani, J. H. & Chikuni, A. C. (eds), Pro- ceedings of the XIIlth plenary meeting of AETFAT, Zomba. Malawi, 2-1 I April 1991: 1105-1 119. Zomba: National Herbarium and Botanic Gardens of Malawi.

- 1994b. Cal Madow, Somalia. - In: Davis, S. D., Heywood, V. H. & Hamilton, A.C. (eds), Centres of plant diversity, a guide and strategy for their conservation 1: 194-195. Ox- ford: Information Press.

- Hassan, A. S. & Styles, B. T. 1993. Acacia. - In: Thulin, M. (ed.), Flora of Somalia I: 368-384. Kew: Royal Botanic Gardens.

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