Barleria mirabilis I.Darbysh. & Q.Luke

Darbyshire, I., Luke, Q. 2016. Barleria mirabilis (Acanthaceae): a remarkable new tree species from west Tanzania. Kew Bulletin 71:13. DOI 10.1007/S12225-016-9622-0

Type

Type: Tanzania, Mpanda Distr., Uzondo Plateau, near Camp 5, W. R. Q. Luke et al. 16253 (holotype EA; isotypes K, NHT).

Morphology General Habit

Tree to 4 - 6 m tall at maturity, widely and intricately branched above a short bole, leafy branches shrubby in appearance; young stems quadrangular, densely buff-sericeous; mature branches soon turning woody, with dense purplish-brown heartwood.

Morphology Leaves

Leaves clustered towards stem apices and with sessile clusters of immature leaves at each node, mature blade subsessile, somewhat fleshy, obovate, 1 - 2 x 0.6 - 0.9 cm, base cuneate or slightly attenuate, margin entire, apex obtuse or rounded, apiculate, surfaces pale-buff to silvery sericeous, hairs longest and most dense along midrib and lateral veins beneath, finer and less appressed between the veins, leaves at distal axils with few to numerous broadly capitate stalked glands between the veins in the proximal half beneath; lateral veins 4 - 5 pairs, prominent beneath, somewhat impressed above, tertiary venation scalariform but usually obscured by the dense indumentum.

Morphology Reproductive morphology Inflorescences

Inflorescences axillary but clustered towards tips of main and short lateral branches and can appear terminal, each cyme single-flowered or rarely 2-flowered, subsessile; bracts foliaceous; bracteoles green, narrowly elliptic or somewhat oblanceolate, 9 - 12 x 3 - 5 mm, condupli­cate and often folded around base of calyx, base cuneate, margin entire, apex acute-apiculate, abaxial surface sericeous with interspersed shorter and more spreading hairs and numerous conspicuous “mush­room” glands to 0.2 mm in diam., these drying orange- brown but pale in fresh material, adaxial surface appressed-pubescent.

Morphology Reproductive morphology Flowers Calyx

Calyx green with somewhat darker palmate-reticulate venation; anterior lobe broadly ovate, 12.5 - 15.5 x 9 - 12.5 mm, posterior lobe as anterior but somewhat larger, to 16 x 14 mm, both with base rounded, margin entire and strongly involute particularly on posterior lobe which folds around anterior lobe, apex obtuse or very shortly attenuate, apiculate, external surfaces convex, silvery sericeous particularly on main veins, and with numer­ous “mushroom” glands towards margins and apex, internal surface appressed-pubescent and glandular- puberulous; lateral lobes pale yellow-green, lanceolate- attenuate, 8.5 - 9 x 4 mm, sericeous towards base and along midrib, glandular-puberulous towards apex.

Morphology Reproductive morphology Flowers Corolla

Corolla pale bluish-lilac with pale yellow-green basal portion of tube, glandular-pubescent externally on upper portion of tube and limb; tube 21 - 23 mm long, cylindrical and declinate below attachment point of stamens where constricted, funnel-shaped above; limb in “2+3” arrangement; abaxial and lateral lobes, c. 11 - 15 x 11 - 12 mm, abaxial lobe broadly obovate with apex emarginate, lateral lobes more elliptic with apices rounded; adaxial lobes c. 5.5 - 6.5 x 6 - 7 mm, somewhat ovate with apices obtuse.

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens attached c. 10.5 mm from base of corolla tube; filaments c. 18 - 20 mm long, glandular-puberulent towards base; anthers 6.3 - 6.6 mm long; lateral staminodes c. 2.7 mm long, glandular-puberulent and with a tuft of long eglandular hairs towards apex, antherodes ab­sent; adaxial staminode shorter, c. 1.3 mm long.

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary c. 2.8 mm long, glabrous, largely enclosed within cupular disc; style sparsely glandular-puberulous to­wards base; stigma linear, c. 1.3 mm long, curved.

Morphology Reproductive morphology Fruits

Capsule c. 20 mm long, oblong-fusiform in face view, 4- seeded, glabrous; mature seeds not seen.

Note

Similar to B. crassa C. B. Clarke and B. nyasensis C. B. Clarke but differs markedly from both in being a tree to 4 - 6m tall at maturity (vs a perennial herb or shrub 0.3 – 2.5 m tall); in having broader, elliptic or oblanceolate bracteoles 3 - 5 mm wide and lacking a spine tip (vs bracteoles linear-lanceolate with a spine tip or reduced to a simple spine, 0.5-3 mm wide);in the bracteoles and posterior and anterior calyx lobes having numerous conspicuous capitate “mushroom” glands externally (these absent in B. crassa and B. nyasensis which only have scattered small glandular hairs on the calyx); in the anterior and posterior calyx lobes being more convex and with markedly involute, entire margins (vs calyx lobes with margin not or hardly involute; denticulate to spinulose-dentate or rarely almost entire); and in the anthers being larger (6.3 - 6.6 mm vs 3 - 5mm long).

The species epithet “mirabilis” (miracu­lous, astonishing) refers both to the fact that this is a truly remarkable species, being the only tree Barleria recorded to date, and to the fact that such a striking and locally frequent species has not been found previously despite considerable past botanical explo­ration at the collecting locality.

The above description is based on a single population of plants. It would be desirable to gather further flowering material of this species to provide a more complete description of the corolla and androecium, as only three flowers are available on the type collection. This new species falls within Barleria sect. Barleria (see e.g. Balkwill & Balkwill 1997; Darbyshire 2010). It is clearly closely allied to a small group of species that share the combination of few- or single ­flowered cymes, bracteoles lacking marginal spines, corollas with a “2+3” limb arrangement (i.e. a weakly defined 2-lobed upper lip and 3-lobed lower lip), a pistil that lacks short crisped hairs at the base of the style and a linear stigma formed by one of the two stigma lobes being considerably elongat­ed. This group of species corresponds to Obermeijer's (1933) sect. Barleria subsect. “Aculeatae”. It is represented by a number of species in East Africa including B. crassa and B. nyasensis with which B. mirabilis is most likely to be confused in herbaria, but from which it clearly differs in the characters listed in the Recognition section above. This group of species has not been included within any molecular phylogenetic analyses to date. Other Acanthaceae tree species are rare but not unheard of in Africa. For example, Justicia adhatodoides (E. Mey. ex Nees) V. A. W. Graham from South Africa is described as a “shrub or small tree 3 - 6 m high” (Manning & Getliffe-Norris 1995: 17, as Duvernoia adhatodoides E. Mey. ex Nees), several species of Anisotes Nees are capable of becoming small trees (Baden 1981; Vollesen 2010) and the mangrove tree genus Avicennia L. is now widely included within the Acanthaceae (see Schwarzbach & McDade 2002).

Outside of Africa, the tropical American genera Bravaisia DC. and Trichanthera Kunth are often trees including at least two mangrove species, and B. integerrima (Spreng.) Standl. can grow to 20 m tall (Daniel 1988;Trippet al. 2013). Keen observers will note that the habit photograph in Fig. 2B shows a species of Loranthaceae (Erianthemum sp. [Luke et al. 16254]) parasitising the upper branches of Barleria mirabilis; this is believed to be the first record of an Acanthaceae species hosting a loranthaceous parasite, at least in Africa (R. Polhill, pers. comm.). The mature heartwood of Barleria mirabilis has been studied by one of the authors (Q.L.) and found to have a notably high density, calculated as 1.015 g/cc (1015 kg/m3) which is more dense than that of ""African Teak"" (Pericopsis elata (Harms) Meeuwen). It has a somewhat silky texture reminiscent of Dalbergia melanoxylon Guill. & Perr. and is a rich dark purplish- brown colour. The fact that this locally frequent tree species has previously gone un-noticed despite significant plant collecting efforts in the type locality is surprising. That said, it is almost certainly due to the late flowering of the species which was in full flower well into the dry season, at a time when botanical interest in the dry woodlands of western Tanzania is rather limited. Most of the botanical exploration of the Uzondo Plateau has occurred at the end of and immediately after the main rains, in April to May.

Distribution

Known only from the Uzondo Plateau of West Tanzania.

Ecology

Barleria mirabilis is recorded from sandstone rock outcrops, set in Brachystegia woodland, with an open shrub and small tree community within which this species dominates together with Keetia ferruginea Bridson (also a tree), Kotschya bullockii Verdc., Euphorbia cooperi N. E. Br. ex A. Berger var. ussanguensis (N. E. Br.) L. C. Leach and Aloe chabaudii Schonland. Similar rock outcrops are frequent but scattered across the Uzondo Plateau. The new species flowers towards the end of June, and perhaps beyond, which is well into the dry season in this region of Tanzania. This late flowering season is shared by only a few Barleria species in west Tanzania, for example B. mpandensis I. Darbysh. (Darbyshire & Ndangalasi 2009). Peak fruiting time is uncertain; the collection made in November was post the main fruiting time and only old fruits remained on the plants.

Conservation

Human settlement on the Uzondo Plateau is currently very sparse and the natural habitats there remain largely undisturbed. Furthermore, this species is confined to areas of rock outcrop, unsuitable for agriculture. That said, the main north-south highway of western Tanzania bisects the plateau, connecting Mpanda town with the northwest of the country. Human populations contin­ue to grow rapidly in western Tanzania; Katavi Region experienced a 3.26% growth rate between 2002 and 2012, increasing by c. 156,000 people (Brinkhoff 2015). Whilst the north-south highway is not heavily used at present, it is quite likely that human settle­ments will expand along this obvious access route in light of this population growth. If so, it is quite feasible that the demand on natural resources, and in particular on woody plant species as a source of timber and firewood, will pose a plausible future threat to this species. With only a single location known and with an Area of Occupancy considerably less than 20 km2 this species is therefore assessed as Vulnerable under criterion D2 (VU D2) of IUCN (2012).