Senecio namibensis Swanepoel & A.E.van Wyk, 2022

Swanepoel, Wessel, Becker, Rolf, Cauwer, Vera De & Van Wyk, Abraham E., 2022, Senecio namibensis (Asteraceae: Senecioneae), a new species from Namibia, Phytotaxa 571 (2), pp. 103-111 : 104-109

Senecio namibensis Swanepoel & A.E.van Wyk , sp. nov. ( Figs 1–3 )

Diagnosis: —Dwarf shrub up to 0.4 m high, morphologically most similar to Senecio englerianus and S. flavus : from S. englerianus it differs in having the leaf blade cordate to suborbicular or reniform (vs. cordate, suborbicular, reniform, ovate or oblate); blade base cordate to deeply cordate (vs. cordate-reniform or truncate); capitula radiate (vs. discoid); involucre usually shorter and narrower: 5.3–7.4 mm long, 3.5–4.0 mm diam. (vs. 6.3–8.0 mm long, 4.4–5.6 mm diam.); from S. flavus it differs in having the capitula distinctly radiate (vs. disciform or obscurely radiate; ray florets scarcely longer than involucre); involucre usually shorter and wider: 5.3–7.4 mm long, 3.5–4.0 mm diam. (vs. 7.0– 9.5 mm long, 2.7–2.9 mm diam.); pappus bristles free, non-fluked and lacking hooklike apical appendages [vs. ca. 33%—according to Coleman et al. (2003) and Milton et al. (2022) —of pappus bristles connate and fluked—see “Materials and methods” for definition of “fluked” bristles—with grappling hook-like apical appendages].

Type: — NAMIBIA. Kunene Region: 1812 (Sanitatas), Namib Desert, gneiss ridge, 3 km north of Khumib River and 4 km east of Skeleton Coast Park boundary (–DA), 377 m a.s.l., 23 May 2022, Swanepoel 585 (holotype WIND!; isotypes PRE!, PRU!) .

Single-stemmed annual or short-lived perennial dwarf shrub, glabrous, herbaceous to succulent, up to 0.4 m high, cushion-forming when perennial. Branches succulent, brittle, pale green, glaucous, short, 10–35 mm long before rebranching. Leaves alternate and spirally arranged, fleshy to succulent, pale green or grey-green, glaucous, lower and middle cauline leaves petiolate with blade cordate to suborbicular or reniform, ca. flat, apex acute or rounded, base subcordate to deeply cordate, margins coarsely dentate with 5 or 9 teeth each side or almost entire, veining palmately, inconspicuous, not prominent, 5–40 × 8–45 mm, fleshy, up to 2.5 mm thick; petiole 3–38 mm long, up to 2 mm diam., brittle, pale green, angle with blade abaxially ca. 120 degrees (ca. patent), blade appearing peltate due to deeply cordate base; upper cauline leaves sessile, sub-amplexicaul to amplexicaul, irregular, up to 12 × 12 mm. Inflorescences axillary or terminal, capitula solitary or cymously twice-forked, 15–95 mm long, pale green. Peduncle 11–50 mm long, 0.6–0.8 mm in diam. (at capitulum 1.0– 1.2 mm diam.), erect, with up to 4 lanceolate or narrowly triangular bracts towards capitulum, ca. 1 mm long, calyculus bracts 5–7, lanceolate or narrowly triangular, 0.8–2.0 mm long. Capitula 2 or 3 per inflorescence, heterogamous with female ray florets and fertile hermaphrodite disc florets, other peripheral florets (between ray florets) similar to disc florets, radiate, slightly convex, 9–11 mm diam. (including rays), yellow-flowered. Receptacle flat, indistinctly alveolate, convex and verrucose when dry. Involucre cupuliform, 5.5–6.3 mm long, 3.5–4.0 mm diam., involucral bracts 10–13, lanceolate, concave in transection, free, 5–6 × 0.7–1.1 mm, herbaceous, margins membraneous, acute, pale green with a purple tinge, glaucous, patent to reflexed when in fruit. Ray florets 3–6, yellow, corolla 5.0– 7.9 mm long (including ray), tube terete, ca. 0.4 mm diam.; ray yellow, elliptic-oblong with 4 or 5 darker longitudinal lines, 2.5–4.4 mm long, 40–55% as long as the involucre, apex rounded or truncate, entire or with 1–4 denticulate teeth, glabrous; ovary oblong, terete, white, densely papillate (not twin hairs), 1.4–1.8 mm long, 0.5 mm diam., ovule oblong, 1.3–1.5 mm long, 0.3 mm diam.; style 2.9–3.2 mm long, ca. 0.2 mm diam., terete; branches ca. 0.8 mm long, flattened, grooved, minutely papillate (sweeping hairs) towards truncate apex; pappus copious, ca. 52, minutely barbellate bristles, 2.5–3.6 mm long, of “ordinary” type (sensu Drury & Watson 1966), erect, persistent, white. Cypselae oblong, terete, angular, grooved, dark brown, white-papillate, ca. 2.4 mm long, ca. 0.5 mm wide. Disc florets ca. 30, pale yellow-green, corolla 3.9–4.5 mm long; tube cylindrical, 1.4–1.6 mm long, 0.4 mm diam.; limb ca. 2.5 mm long, gradually widening from 0.4 mm to 0.8 mm diam. at apex; lobes 5, deltoid-ovate, 0.5–0.8 mm long, thickened abaxially towards apex, glabrous; anthers ca. 1.8 mm long including ovate apical appendage; filament collar balusterform; style ca. 3.8 mm long, 0.2 mm diam., terete, branches as for ray florets; ovary narrowly oblong, densely papillate, ca. 1.8 mm long, 0.5 mm diam., terete, ovary, ovule, pappus and cypselae as for ray florets.

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

Distribution, habitat, and ecology: — At present Senecio namibensis is known only from ten localities in the Namibian part of the Namib Desert, from the Khumib River in the north to the Rössing Mountains in the south ( Fig. 4 ). This part of the Namib Desert falls mainly in the Namib zone of the Kaokoveld Centre of Endemism ( Van Wyk & Smith 2001). Senecio namibensis occurs approximately 20–32 km from the coast in the north to 30–60 km in the south, with a maximum distance of 75 km in the centre of its distribution at a locality south of the Huab River. It occurs in small colonies of a few plants each in sandy gravel at the base of rocks and boulders in hilly areas, at elevations of 150–500 m a.s.l., with annual average rainfall up to 100 mm ( Mendelsohn et al. 2002).

Conservation status: — Although rare and only known from a small area, Senecio namibensis is probably not threatened at present. The entire known population occurs within either uninhabited or protected areas (Skeleton Coast and Dorob National Parks, including several conservancies). No signs of damage caused by animals or humans were present on any of the in situ specimens examined. The extent of occurrence is estimated at <20000 km ² (7425 km ²) with only 10 subpopulations. However, since no decline or extreme fluctuations in population size or numbers are known, it is here ranked as Least Concern (LC) ( IUCN 2012).

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

Notes: —The distribution of Senecio namibensis does not overlap with that of its two suggested nearest relatives. Senecio namibensis occurs to the east (further inland) of the range of S. englerianus and to the west of S. flavus ( Fig. 4 ). Some of the more prominent morphological features to distinguish among the three species are provided in Table 1 .

Milton et al. (2022) report S. englerianus as being dimorphic for capitulum-type (mostly non-radiate, occasionally radiate), citing as reference Jürgens et al. (2021), which refers to the “https://southernafricanplants.net” website. On the Senecio englerianus (as “ Senecio engleranus ”) page of this website, a photograph of a plant with yellow rays is shown (FotoID: 10798, E. Erb, Namibia, 16-08-2006), probably the basis for the statement that the capitula in S. englerianus are dimorphic. However, the plant in the photograph almost certainly represents S. namibensis , although it is not possible to confirm this based on a photograph alone. Senecio vulgaris Linnaeus (1753: 867) from the British Isles has also been reported as being either radiate or non-radiate. In this case, the polymorphism arose by introgression of a cluster of regulatory genes from the radiate S. squalidus Linnaeus (1753: 869) (originating from Sicily) into the non-radiate S. vulgaris after the introduction and spread of the former from Europe to the British Isles ( Kim et al. 2008). Due to the absence of related species of Senecio in the area of distribution of S. englerianus , from which it could have obtained radiate capitula, the possibility of S. namibensis being a radiate variant of S. englerianus is ruled out as highly unlikely. In support of separate species status for S. namibensis is also the combination of other morphological differences between it and S. englerianus , notably the shorter and narrower involucre ( Table 1 ).

Unrelated asteraceous species within the range of Senecio namibensis with which it can be confused, are Dauresia alliariifolia ( Hoffmann 1888: 280) Nordenstam & Pelser (2005: 76) and Engleria africana Hoffmann (1888: 273) , due to similarities in leaf characters. However, Dauresia alliariifolia has white branches (vs. green), an ecalyculate involucre (vs. calyculate) and discoid white- or yellow-flowered capitula (vs. radiate, yellow-flowered). Engleria africana has much larger capitula, ca. 12 mm long (vs. 5.3–7.4 mm), the involucres consist of three rows of involucral bracts (vs. one row) and the peripheral florets lack rays (vs. ray florets present).

Additional specimens examined (paratypes): — NAMIBIA. Kunene Region: 1812 (Sanitatas): Khumibrivier se droë loop (–DC), 7 May 1962, Kotze 124 ( WIND!) . 2013 (Unjab mouth): Huab River area north of Gaias (–DB), 27 August 1977, Craven 510 ( WIND!) ; Foot of mountains along Huab River (–DC), 12 August 1979, Müller & Loutit 1164 ( WIND!) . 2014 (Welwitschia): Damaraland , between Huab and Mikberg (– CA) , 11 September 1993, Günster 9391 ( WIND!) . Erongo Region: 2113 (Cape Cross): 30 km east of Ugabmond (– BB) , 16 August 1979, Müller & Loutit 1213 ( WIND!) . 2114 (Uis): Messum crater (– AC) , 15 May 2000, Hachfeld 119244 ( WIND!) . 2214 (Swakopmund): Rössingberge (–DB), 27 February 1958, Merxmüller & Giess 1735 ( WIND!) ; Westseite Rössing (–DB), 8 May 1969, Homann, Benseler & Mittendorf 23 ( WIND!) .

Figures

FIGURE 1: Senecio namibensis, habit of plant just past peak flowering (at type locality). Leaf blades moderately fleshy, suborbicular with base cordate and margins dentate; capitula radiate. Photograph by W. Swanepoel.

FIGURE 2. Senecio namibensis, plants in flower, showing habit and variation in leaf morphology (at type locality). A. Leaves succulent; blade with base strongly cordate, margin conspicuously dentate; capitula radiate. B. Leaves succulent; blade with base weakly cordate, margin weekly dentate. Photographs by W. Swanepoel.

FIGURE 3. Senecio namibensis, morphology of capitula. A. Capitulum viewed from above, showing female ray florets and fertile hermaphrodite disc florets. B. Capitulum viewed from the side, showing calyculate and involucral bracts. C. Capitulum viewed obliquely from below, showing calyculate and involucral bracts. Scale bar = 2 mm. Photographs by W. Swanepoel.

FIGURE 4. Known distribution of Senecio namibensis (solid circles) and S. englerianus (open triangles). Also plotted are the known distribution of S. flavus (open circles) in Namibia; its range extends further south (arrow) into the Northern Cape, South Africa, but those localities, as well as its disjunct distribution in Northern Africa, are not shown. Source: herbarium records in WIND.