Osteospermum namibense Swanepoel, 2021

Swanepoel, Wessel, Cauwer, Vera De & Van Wyk, Abraham E., 2021, A new species of Osteospermum subgen. Tripteris (Asteraceae: Calenduleae) from the Namib Desert, Namibia, Phytotaxa 487 (3), pp. 185-194 : 186-191

Osteospermum namibense Swanepoel , sp. nov. ( Figs 1–3 )

Diagnosis: —Dwarf shrub up to 0.5 m high, related to O. microcarpum , but differing by being perennial (vs. annual in nature, although reported as a short-lived perennial in cultivation); branches initially succulent becoming woody (vs. branches succulent, remaining herbaceous), branching di- or trichotomous, zig-zag in appearance (vs. alternate, virgate); leaves distinctly succulent (vs. not or only slightly succulent), distal leaves arranged in rosettes (vs. alternate and spirally), lamina oblanceolate or narrowly obovate, trigonous in transverse section, sessile (vs. oblanceolate, lanceolate, sublinear or ligulate, crescent-shaped in transverse section, sessile or petiolate); ray florets per capitulum 12–14 (vs. 16–20); achenes dimorphic (vs. achenes monomorphic).

Type: — NAMIBIA. Kunene Region: Skeleton Coast National Park, basalt ridge, 8 km east of Agaatberg Mountain along track to stone circles, 1812 AC, 63 m a.s.l., 25 September 2020 , Swanepoel 396 (holotype WIND!; isotypes PRE!, PRU!) .

Perennial dwarf shrub up to 0.5 m high with di- or trichotomous branching; younger vegetative growth and some reproductive parts glandular hairy and sticky, with adherent sand grains. Branches densely covered with tapering glandular hairs of variable length, glabrescent, initially succulent, becoming woody with prominent leaf scars, short, 13–100 mm long before rebranching. Leaves succulent; older proximal cauline leaves ligulate, semi-amplexicaul, soon withering; younger distal leaves alternate and spirally arranged, appearing as rosettes at branch apices, sessile, lamina oblanceolate or narrowly obovate, in transverse section halfway between base and apex trigonous or vertically compressed trigonous, apex acute or obtuse, base gradually tapering; margins entire, crenate or crenate-serrate towards apex with 2 or 3 teeth each side, midrib slightly prominent abaxially, 10–25 × 5–8 mm, pale green, densely covered by tapering glandular hairs of variable length, usually with additional simple hairs towards base adaxially; withered leaves persistent, sometimes completely covering stem and branches. Inflorescences axillary or terminal, all parts densely covered by tapering glandular hairs of variable length. Peduncle 3–25 mm long, 1.2–2.3 mm diam. Bracts narrowly triangular or clavate, 4–7 × 1–3 mm long. Capitula solitary, radiate, flat, 3.7–4.2 mm diam. (16–18 mm diam. including rays), yellow-flowered, radiate, heterogamous with female ray florets and cylindrical hermaphrodite (female sterile) disc florets. Receptacle convex or flat, foveate, when dry flat or convex, verrucose. Involucre soft, cupuliform to campanulate, reflexing when in fruit, 8.6–9.8 mm long, 6.0– 6.6 mm diam., phyllaries 18–20 in three rows, inner ones longest, narrowly elliptic or lanceolate, 5.0–6.5 × 1.7–2.1 mm long, acute, pale green, with glandular hairs except towards base adaxially, margins membranous, ciliate. Ray florets (12–)13(–14), yellow, ligulate, corolla 6.3–8.3 × 2.5 mm long (including ray), tube cylindrical or slightly widening towards limb, 1.2–2.0 mm long, 0.3–0.4 mm diam.; rays yellow, lanceolate or narrowly elliptic with 4 longitudinal pale yellow-green lines, 5.1–6.3 mm long, apex obtuse, with 3 denticulate teeth; outside of tube and abaxially towards base of limb villose. Ovary oblong or narrowly elliptic, triquetrous pale green, 1.5–1.8 mm long, 0.6 mm diam., glabrous or with short conical hairs towards base and apex, often one ovary densely glandular-hairy. Style terete, ca. 3.3 mm long, 0.2 mm diam.; branches ca. 1.6 mm long, flattened, grooved. Achenes 3-sided, narrowly obovate, winged, smooth or slightly verrucose in places, glabrous or with few short conical hairs, black, 4.7–6.2 mm long, 3.7–4.3 mm wide including wings, apical air-chamber 3-fenestrate, fenestrae ovate, ca. 1 mm long, ca. 20% the length of achene, often one achene lacking wings and with remains of dense glandular hairs. Disc florets 21–38, pale yellow, corolla ca. 3.3–4.0 mm long; tube cylindrical or slightly widening towards limb, ca. 0.8 mm long, 0.4 mm diam. glabrous or villose outside; limb cylindrical or slightly widening towards apex, 2.5–3.2 mm long, 0.8–1.1 mm diam., outside villose towards base; lobes 5, ovate, 0.8 mm long, densely papillate outside. Anthers ca. 1.8 mm long including the ovate apical appendage and sagittate base; filaments terete, ca. 1 mm long. Style simple, sterile, ca. 3.4 mm long, 0.2 mm diam., tipped with a rounded appendage 0.3 mm diam., with short papillae. Ovary sterile, narrowly oblong, laterally flattened, glabrous, 0.7–1.3 mm long.

Phenology: —Flowers and fruit were recorded from January to September.

Distribution, habitat and ecology: — At present Osteospermum namibense is known only from the northern part of the Skeleton Coast National Park, Namibia ( Fig. 4 ) where it occurs in small colonies of a few plants each. This part of the Skeleton Coast National Park falls within the Namib Desert zone of the Kaokoveld Centre of Endemism, a biogeographical region known for its many restricted-range plants and animals, and extending from northwestern Namibia to southwestern Angola ( Van Wyk & Smith 2001). Osteospermum namibense occurs approximately 5–20 km from the coast at elevations of 60–200 m a.s.l. between the Engo and Khumib rivers.

Environmental conditions in the general distribution area of Osteospermum namibense are extremely harsh, with low rainfall, high temperature variation and strong winds. Average annual rainfall in the area is less than 57 mm, occurs in summer and is highly erratic. Annual mean temperature is 19°C ( Fick & Hijmans 2017). However, the area regularly receives fog from the bordering Atlantic Ocean with relative occurrence of fog and low clouds varying from approximately 40% to 12% (mean 25%) ( Andersen & Cermak 2018) from west to east in the distribution area. Fog occurrence is seasonal and at this latitude (18°– 19°S), with minimum occurrence during December and January and maximum occurrence during August and September ( Andersen et al. 2019). The wind is nearly always blowing and calm for only 14% of the time as measured at Möwe Bay to the south of the new species’ range ( Mendelsohn et al. 2002).

The new species grows on basalt, gneiss and latite in soil-filled rock fissures. The persistent withered leaves on the stems are probably an adaption to protect the fleshy younger branches against the elements until these have been lignified. The potential role, if any, of these leaves in the collecting of water from fog or dew as a source of additional water (e.g. Henschel & Seely 2008) needs investigation. Gland-covered parts of the plants are sticky and trap sand grains ( Fig. 3B, C ). This capture of sand grains by especially desert plants has been proposed as a potential defence (so-called “sand armour”) against larger herbivores. Sand should wear down the teeth of larger herbivores, thus discouraging browsing of the plants ( Farmer 2014). Despite the presence of gemsbok and springbok in the range of the new species, we have never encountered any signs of the plants being eaten by these antelope, but more observations are needed to test the validity of this defence hypothesis. The only potential pollinators recorded were flies of the family Mythicomyiidae (A. Kirk-Spriggs pers. com.; Fig. 3A ).

Osteospermum namibense and O. microcarpum both occur at Okau Spring in the Skeleton Coast National Park, the morphological distinction between the two species being quite obvious when the two are compared. They also occupy different habitats; O. namibense grows on the rocky ridges, and O. microcarpum on the sandy areas adjacent to the spring.

Conservation status: — Although rare and known from a small area, Osteospermum namibense is probably not threatened at present as the entire known population occurs within the boundaries of the Skeleton Coast National Park, a remote and protected area with limited access by humans. No signs of damage caused by animals or humans could be found on any of the in situ specimens examined. It should be considered as Vulnerable (VU D1) due to the small known population size ( IUCN 2012).

Etymology: —The specific epithet refers to the Namib Desert which, in its broadest definition, stretches along the Atlantic Ocean from San Nicolau in Angola through Namibia to the Olifants River in the Western Cape, South Africa ( Seely 2004, Goudie & Viles 2015).

Notes: —The nearest relative of Osteospermum namibense appears to be O. microcarpum , a widespread species from which it differs in habit, branches, leaf and floral characters. Distribution ranges of the two species overlap in the northern Namib. However, they utilize different habitats with O. namibense occurring in rocky areas, whereas in the area of overlap O. microcarpum occupies sandy washes along drainage lines and the beds of ephemeral rivers. Elsewhere O. microcarpum is found from southwestern Angola throughout arid western Namibia to the Northern and Western Cape Provinces of South Africa ( Norlindh 1943). Some of the more prominent morphological features to distinguish between the two species are provided in Table 1.

Additional collections (paratypes): — NAMIBIA. Kunene Region:—1812: Opuwo District , (– AA) , 16 June 1997, Burke 97155 ( WIND!) ; Strandloper settlement on road from Cape Fria to Sanitatas, (–AC), ca. 200 m, 18 April 1985, Moss & Jacobsen ( WIND!) ; Agaatberg omgewing, Kaap Fria, (–AC), 15 April 1986, Schoeman s.n. ( WIND!) .

Figures

FIGURE 1: Osteospermum namibense: Habit and habitat. A. Plant, ca. 0.4 m high, with dead leaves persisting on older stems. B. Plant partially buried by windblown sand; note leaves arranged in rosettes. Photographs by W. Swanepoel.

FIGURE 2. Osteospermum namibense: di- and trichotomous branching. A. Live plant. B. Woody remains of a dead plant. Photographs by W. Swanepoel.

FIGURE 3. Osteospermum namibense: flower and fruiting heads, as well as leaves.A. Flower head viewed from above, with two visiting flies in the family Diptera: Mythicomyiidae. B. Leaves and side view of flower head; note succulence of leaves and sand grains sticking to gland-covered surfaces. C. Fruiting head with mature achenes. Photographs by W. Swanepoel.

FIGURE 4. Known distribution (black dots) of Osteospermum namibense.