Senegalia propinqua L.Bai, 2021

Bai, Lin, Maslin, Bruce & Xia, Nianhe, 2021, Senegalia propinqua (Leguminosae: Mimosoideae), a new species from the Yuanjiang dry-hot valley in south-central Yunnan Province, China, Phytotaxa 522 (1), pp. 38-46 : 39-45

Senegalia propinqua L.Bai , sp. nov. 元江ž合欢【 yuán jiâng jîn hé huân 】 Fig. 1–3 .

Type:— CHINA. Yunnan, Yuxi City , Yuanjiang Hani , Yi & Dai Autonomous County, Xiaohedi River , near the first level of Xiaohedi Hydropower Station , Dry hot valley, elev. 481m, 23°29′46.05″N, 102°21′20.38″E, 29 May 2021, L. Bai & Q. Tian BLSC-029 (holotype IBSC [barcode 0865935]!, isotypes GXMI!, IBK!, IBSC [barcode 0865933 & 0865934]!, K!, KUN!, PE!, PERTH!, SING!)

Diagnosis. Similar to Senegalia kunmingensis in having sessile leaflets (resulting from the squat petiolule that does not extend below the base of the leaflets) and long-pedunculate heads that occur within the upper leaf axils and in elongated, terminal racemes, but differs in having shorter petioles (8–25 mm), more numerous pinnae (13–17 pairs) with more numerous and smaller leaflets (normally 45–60 pairs and 5–7 × 1–1.8 mm), compared to petiole 40–80 mm long, pinnae 3–9 pairs, leaflets normally 21–33 pairs and 6–13 × 1.5–3 mm in S. kunmingensis ; also, while the main vein of the leaflets in both species starts from near the upper margin, in S. propinqua it is close to and parallel with the upper margin, while in S. kunmingensis it runs obliquely to the leaflet apex; furthermore, S. propinqua is a nonlianescent shrub, while S. kunmingensis is commonly a liana or lianescent shrub.

Description. Erect, openly and rather sparingly branched, non-lianescent s hrubs 2–5 m tall, single- or more commonly several-stemmed from ground level, the primary branches ascending to erect (sometimes almost contiguous with main stem); bark on stems and mature branches grey to dark grey or sometimes brown; lenticels numerous, prominent and circular or oblong to linear (transversely orientated). New shoot at first enclosed within a conspicuous bud comprising up to ten, spirally arranged and imbricate scales, the scales coriaceous, longitudinally striate, brown or dark purple and densely short-hairy; when fully emerged the sterile branchlets often short (3–5 cm long), with 3–5(–6) densely arranged leaves, yellowish green and with dense, short, patent or curved hairs but glabrous or occasionally sparsely hairy with age; fertile branches usually longer (to 50 cm) and with up to 7, widely spaced leaves. Prickles internodal, scattered on mature branches, very few or absent on young branchlets, normally absent from leaves (rarely present on lower surface of rachis), often not seen on herbarium material, c. 1 mm long on branchlets but to c. 5 mm long on branches, straight or shallowly recurved. Stipules caducous, narrowly obovate to spathulate, 3–6 × 0.5–2 mm, densely hairy and obscurely veined abaxially or veins not visible, not lobed at base, apex acute to acuminate. Leaves bipinnate; pinnae (10–) 13–17 pairs, 7–12 mm apart (to 20 mm in regrowth plants), 40–60 (–85) mm long on medial pairs, sometimes to 15 mm long on lowermost pair; petiole (including the prominent pulvinus) 8–25 mm long on mature branches, to 36 mm in regrowth plants; rachis 110–180 mm long, densely short-hairy, the hairs curved. Leaflets 45–60 pairs on medial pinnae, to 25 pairs on lowermost pinnae, narrowly oblong, 5–7 mm long, 1–1.8 mm wide, thick, flat, close together (sometimes slightly imbricate), straight or more commonly shallowly curved towards apex of pinna rachis, dark green above and paler green below, glabrous on the upper surface, sparsely appressed hairy on margins and lower surface (densest on main vein), the hairs on basal angle sometimes spreading; apex asymmetric, rounded to obtuse or sometimes broadly acute, sometimes with an apiculum; base truncate, unequal with a square angle on lower edge, the petiolule squat (not extended below base of leaflet) and clearly eccentric; main vein starting very close to upper margin at leaflet base, then close to and parallel with upper margin for most of leaflet length, normally slightly curved away from margin at leaflet apex; lateral veins (on lower surface of leaflets) ±evident, patent or sub-patent, often bifurcating near margin, tertiary veins inconspicuous but appear to form an indistinct and imperfect reticulum (observe dry specimens). Glands: petiole gland – position varies from distal end of pulvinus to immediately below first pair of pinnae, sessile, oblong to obovate or elliptic or sometimes circular in plane view, normally symmetrically thickened but sometimes slightly raised at distal end, 1.2–3 (–4) mm long, 1.2–1.5 mm wide, raised to c. 0.5–1 mm high, slightly concave or longitudinally grooved in the center (rarely flat-topped or, on young leaves, domed), pale green or yellowish green and smooth or more commonly wrinkled when fresh, often wrinkled when dry; rachis glands —situated at base of uppermost 1 (rarely 2) pairs of pinnae, sessile, circular, c. 1 mm diam., slightly raised, concave above; rachilla glands —situated at base of the distal pair of leaflets (often obscured by the leaflet bases), or more often absent, minute, sessile, black. Inflorescences comprising 1–3 pedunculate heads within upper leaf axils and also arranged in elongated, terminal racemes with young leaves or leaf primordia at base of peduncles during anthesis, indumentum on raceme axes comprising short, dense, straight-patent or curved-appressed white hairs (difficult to see in fresh material), also with a dense layer of dark reddish brown glandular hairs in narrow grooves at apex of some raceme axes; peduncles 15–30 mm long, indumentum similar to raceme axes except glandular hairs relatively few and scattered uniformly on some peduncles; peduncular bracts early caducous, often two, situated close to base of heads, filiform, 2–4 mm long, heads globose or oblongoid, 14–17 mm diam. at anthesis when fresh (c. 14 mm when dry), white to cream-coloured (dirty yellow as stamens wither), c. 40-flowered, the flowers densely arranged in the heads (best observed in mature buds); inflorescence buds pale green but aging cream-coloured when fresh, pale yellowish or pale brown when dry. Bracteoles narrowly spathulate, to 2 mm long, laminae less than 0.5 mm wide, often brown and slightly exserted beyond flowers in buds. Flowers 5-merous, sessile or with very short pedicel (0.5 mm long); calyx c. 2 mm long, ¾–⅞ length of petals, gamosepalous, dissected for 1/3–1/2 into triangular or oblong-acute or -obtuse lobes that are cream-coloured when fresh, calyx tube densely hirsutellous (observe at x10 mag.) and 5-veined (veins obscure to visible but not especially prominent); petals 2.5–4 mm long, cream-coloured when fresh, with dense short hairs; stamens very numerous, free to base or a few loosely cohesive at extreme base, exceeding petals by about same length as petals. Ovary densely long hirsute. Pods and seeds unknown.

Other specimens examined:— Yunnan, Yuxi City, Yuanjiang Hani , Yi & Dai Autonomous County, Xiaohedi River , Xiaohedi Hydropower Station , 12 May 1986, H. Zhu-Ge 10424 (SWFC! [barcodes 00007548 & 00007549]); ibidem, 5 June 2019, B.R. Maslin & L. Bai BRM 11038 & BRM 11039 ( KUN!) ; ibidem, 29 May 2021, L. Bai & Q. Tian BLSC-031 & BLSC-032 (IBSC!, KUN!, SWFC!) .

Distribution:— The species is currently known only from the vicinity of the type locality (see Fig. 4 ) in southcentral Yunnan Province, south-western China, where it is represented by less than 10 plants spread over a distance of about 1 km. Further field investigation is needed to determine the frequency and extent of this species.

Habitat:— The species grows in thorny succulent shrubland on steep rocky slopes in the Yuanjiang dry-hot valley, at an elevation of 460– 520 m. The Yuanjiang dry-hot valley (see Fig. 4 ) incorporates a unique, geographically restricted ecosystem where about ten species from a range of plant families are known to be endemic (Jin 2002), and from where a new endemic genus of Euphorbiaceae was recently described by Zhou et al. (2017). Such elevated levels of endemism can be seen elsewhere in the world, especially where there exists high plant ecosystem biodiversity and extreme environmental conditions (e.g. García et al. 2002, Mishler et al. 2014, Wagensommer et al. 2017, Perrino et al. 2018, Xu et al. 2019, Laskey et al. 2020). As discussed by Li et al. (2016) and summarized by Zhou et al. (2017), the distinctive hot, dry climate of the Yuanjiang valley results from the rain shadow effect caused by the Ailao- Wuliang Mountains and the Yunnan-Guizhou Plateau which prevent warm moist air from the Indian and Pacific oceans respectively from entering the region.

Conservation status:— Senegalia propinqua occupies a very restricted geographic area, and is currently known from less than ten individuals that are distributed over a distance of about 1 km. In similar cases, species are often considered as Critically Endangered, based on restricted distribution and/or low number of mature individuals (e.g. Wagensommer et al. 2014a, 2014b). At present, we therefore propose to treat S. propinqua as ‘Critically Endangered’ (CR; D) according to the latest IUCN Criteria ( IUCN 2012, 2019). Nevertheless, further field research is necessary to better-assess plant numbers and possible threats to them.

Phenology:— Extant collections of S. propinqua were gathered between late May and mid-June, during which time the specimens were either in very immature inflorescence bud, at peak anthesis or just post-anthesis. Presumably this rather wide range of variation in flowering phenology is due to differing climate conditions in the three years that the gatherings were made. While an accurate determination of flowering phenology is not possible based on current knowledge, we estimate that flowering plants will occur within the period from about early May to mid-July, depending upon seasonal conditions. The fruiting phenology is unknown as pods have not yet been collected.

Distinctive features:— The following distinctive features distinguish S. propinqua from all other species of Senegalia within China and also the broader Asian region. Erect, openly and rather sparingly branched, non-lianescent shrubs 2–5 m tall. Branchlets densely hairy when young, aging glabrous. Prickles scattered on internodes, often absent from young branchlets and herbarium material. Pinnae (10–) 13–17 pairs; rachis 110–180 mm long. Petioles short (8–25 mm long on mature leaves), with prominent pulvinus. Leaflets 45–60 pairs (on medial pinnae), 5–7 × 1–1.8 mm, thick, close together, straight or more commonly shallowly curved towards apex of pinna rachis; apex asymmetric, rounded to obtuse or sometimes broadly acute; main vein very close to and parallel with upper margin for most of leaflet length; petiolule squat (not extending below base of leaflet so that leaflets appear sessile) and clearly eccentric. Rachis gland situated at base of uppermost 1(rarely 2) pairs of pinnae, sessile. Inflorescences comprising pedunculate heads within upper leaf axils and in elongated terminal racemes with young leaves or leaf primordia at base of peduncles during anthesis; flower buds pale green when fresh; heads globose or oblongoid, creamy white. Calyx tube hirsutellous. Pods and seeds unknown.

Affinities:— Senegalia propinqua is perhaps most similar to S. kunmingensis . A comparison of the two species is given in the above diagnosis. Senegalia kunmingensis is widespread in Yunnan province (extending to Guizhou and Guangxi provinces in China, and also northern Vietnam, see Maslin et al. 2019), but is not recorded for Yuanjiang Hani, Yi & Dai Autonomous County. As mentioned by Sun & Chen (1990), Sun (2006) and Maslin et al. (2019), S. kunmingensis occurs mainly in limestone areas ( Fig. 4 ).

Several characters suggest a relationship between S. macrocephala ( Lace 1915: 401) Maslin et al. (2019: 407) and S. propinqua . For example: the two species have a similar number of pinnae that support numerous, ±sessile leaflets of similar size and with asymmetric apices, they also have similar petiole glands, large flower heads, hairy calyx tubes that are obscurely veined or veinless, and dark–coloured glandular hairs on their inflorescence axes. Senegalia macrocephala is most easily distinguished from S. propinqua in having more numerous rachis glands (4–6), and unlike the new species its leaflets are distinctly acute at apices and their main vein is parallel with the upper margin only in the lower ¼ of each leaflet; also, the glandular hairs in S. macrocephala are more obvious and more common (see Maslin et al. 2019 for details). Senegalia macrocephala has a restricted distribution in northeast Myanmar and adjacent regions of far western Yunnan province (Baoshan city and Lincang city), c. 250 km west of where S. propinqua is found ( Fig. 4 ).

Senegalia teniana and S. propinqua are at least superficially similar in being shrubs with young branchlets unarmed or possessing very few prickles (but plants of S. teniana are occasionally lianescent shrubs or trees). These two species also have similarly-sized leaflets and globose or oblongoid inflorescences that are pale green (not red) when in bud and which occur within the axils of the uppermost leaves and also arranged in elongated, terminal racemes. Senegalia teniana also grows in a similar habitat to that of S. propinqua , namely, dry-hot or dry-warm valleys, but not the Yuanjiang dry-hot valley ( Fig 4 ). Senegalia teniana can be easily distinguished from the new species by its leaflets which are fewer in number (25–35 pairs per pinna), distinctly petiolulate (petiolule extending below the base of leaflet) and which have a main vein that extends obliquely from the base to apex (not parallel with upper margin). Senegalia teniana also has generally longer petioles (mostly 20–35 mm) and fewer pinnae (mostly 5–12 pairs) compared to those of S. propinqua .

Etymology:— The species epithet is derived from the Latin propinquus (meaning ‘near’), a reference to the main vein of the leaflet being very close to the upper margin.

Notes:— The labels of the specimens at SWFC recorded this new species as being a liana, however, we did not observe any such growth form among the plants that we inspected. Furthermore, none of the plants possessed tendrils, which might be expected to occur if the shrubs ever developed a lianescent habit.

Figures

FIGURE 1. Senegalia propinqua. A. Leaf showing numerous leaflets per pinnae and position of petiole gland (thick arrow) and rachis gland (thin arrow). B. Leaflets (upper surface—left and lower surface—right) showing indumentum, vernation and main vein close to upper margin. C. Flowering branch with oblongoid heads in axils of mature and immature (arrowed) leaves. D. Stipules (outer surface— left and inner surface—right). E. Branchlet with minute, straight patent hairs and scattered prickles. F. Petiole glands (side view). G. Petiole glands (plane view). H. Flower (calyx and corolla opened artificially to expose gynoecium and numerous, free stamens). I. Corolla (abaxially—upper and adaxially—lower). J. Gynoecium showing densely hirsute ovary. K. Stamen. Drawn by Ding Han Cui (South China Botanical Garden);A, B, D–K are based on holotype and isotype specimens (L. Bai & Q. Tian BLSC-029); C is based on H. Zhu-Ge 10424 (SWFC [barcode 00007548]).

FIGURE 2. Senegalia propinqua. A. Habit. B. Stems (inset: prickles scattered on mature branches). C. Branches. Photos. Lin Bai

FIGURE 3. Senegalia propinqua. A. Fertile branches. B. Terminal raceme with leaf primordia (arrows) at base of peduncles. C. Head (showing elongated receptacle, and flowers with pale-coloured calyx and corolla). D. New shoot showing spathulate stipules (arrows) subtending emergent leaves, and stem with dark reddish brown glandular hairs in narrow grooves. E. Vegetative bud, and branchlet showing numerous lenticels and scattered prickles. F. Leaf (with petiole broken so that petiole gland not visible). G. Leaflets (lower surface) showing main veins close to and parallel with upper margin. H–J. Petiole glands (plane view). K–L. Petiole glands (side view). Photos. Lin Bai

FIGURE 4. Map of southern China showing distribution of Senegalia propinqua and its three closest relatives (also showing major river systems, dry-hot and dry-warm valleys, and carbonate rock regions).