Ridsdalea backanensis C.H.Nguyen & Aver., 2024

Van Phung, Khoa, Averyanov, Leonid V., Maisak, Tatiana V., Krupkina, Ludmila I., Cao, Hai Xuan & Nguyen, Cuong Huu, 2024, Checklist of the genus Ridsdalea (Rubiaceae, Gardenieae) in Vietnam with description of the new species R. backanensis, PhytoKeys 239, pp. 1-12 : 1

Ridsdalea backanensis C.H.Nguyen & Aver. sp. nov.

Figs 1 , 2

Diagnosis.

Ridsdalea backanensis differs from R. daweishanensis mainly in the smaller leaves, 7-11 cm long (vs. leaves 10-14 cm long in R. daweishanensis ), the 1-3-flowered inflorescence (vs. inflorescences uniflorous), the much shorter pedicels, 3.5-7 mm long (vs. pedicels 20-25 mm long), the shorter subulate calyx lobes, 6.5-11 mm long (vs. calyx lobes narrowly oblanceolate, 14-16 mm long), the corolla outside sparsely hairy (vs. corolla outside glabrous), and the oblanceolate anthers, 2-3 mm wide (vs. anthers linear, about 1.5 mm wide).

Type.

Vietnam, Bac Kan Province: Ba Be District, Ba Be National Park, dry evergreen broad-leaved old secondary forest with domination of Burretiodendron hsienmu Chun & F.C.How, Streblus asper Lour., and Arenga pinnata (Wurmb) Merr., on crystalline highly eroded rocky limestone near the boat station at elev. 250-300 m a.s.l., tree 10-15 m on shady rocky steep slope, not common, 18 November 2022, Nguyen Huu Cuong, Cao Xuan Hai, L. Averyanov, T. Maisak, AL 1682 (Holotype: VNF NHC20221118006!; Isotypes: LE LE01169974! https://en.herbariumle.ru/?t=occ&id=160358, LE LE01169975! https://en.herbariumle.ru/?t=occ&id=160361, LE LE01169983! https://en.herbariumle.ru/?t=occ&id=163681; photos of living material made before preparation of type herbarium specimens: LE LE01123672! https://en.herbariumle.ru/?t=occ&id=155829, LE LE01123671! https://en.herbariumle.ru/?t=occ&id=155828) .

Etymology.

The species is named after the area of origin (Bac Kan Province, northeastern Vietnam).

Description.

Evergreen tree, (4)5-20(22) m tall with obscurely ovoid crown and solitary trunk to 30(35) cm DBH. Bark lenticellate, smooth greenish gray. Young twigs dichotomous, slightly flattened or angular, older ones terete, glabrous. Leaves entirely glabrous, shortly petiolate, opposite, or in whorls of 3; petiole obscurely half circular in cross section, adaxially shallowly grooved, (1.5)2-5(7) mm long, (2.5)3-5(5.5) mm wide; stipules interpetiolar, broadly triangular (1)1.5-2.2(2.5) mm long, (2.5)3-5(5.5) mm wide, broad and slightly fused at the base, shortly acuminate at apex, adaxially densely hairy with short hairs; leaf blade chartaceous to slightly coriaceous, narrowly ovate or narrowly obscurely rhomboid, (7)8-10(11) cm long, (3)3.5-5(5.5) cm wide, tapering from the middle to cuneate base and to shortly acuminate, obtuse apex; both surfaces dark green, glossy, median vein shallowly canaliculate on adaxial side, prominent abaxially, secondary (lateral) veins (4)5-6(7) pairs. Inflorescence cymose, 1-3-flowered, terminal or pseudoaxillary; peduncle (1.5)2-8(9) mm long, glabrous or sparsely hairy with stiff adpressed hairs, with 1-2 very small adpressed triangular alternate bracts, (1)1.2-1.8(2) mm long and wide. Pedicels (3.5)4-6(7) mm long, sparsely hairy with sericeous adpressed hairs, with 1(2) bracteoles; bracteoles alternate, triangular, (1.5)2(2.5) mm long and wide, acute to apiculate, glabrous outside, densely hairy with stiff hairs inside. Calyx green, deciduous; calyx tube and lobes glabrous or sparsely hairy with sericeous adpressed hairs outside, inside densely hairy with stiff, yellowish gray hairs; calyx tube (1.2)1.5-2(2.2) mm long, (4)4.5-6(7) mm in diameter; free calyx lobes recurved, subulate, tapering from base to obtuse apex, (6.5)7-10(11) mm long, (0.7)0.8-1.2(1.4) mm wide, without distinct veins. Corolla campanulate, outside pure white, sparsely hairy with small sericeous adpressed hairs, inside glabrous, finely transversally folded (seen when fresh, Fig. 2Q ), white with light yellow tint and many purple speckles and longitudinal streaks, lasting 3-4 days, (5)5.5-6(6.5) cm long, (2.5)2.8-3.2(3.5) cm in diameter at the middle, abruptly contracted at the base into a short, narrow, 2.5(3-3.5(4) mm wide tube over a length of (7)8-10(11) mm; corolla lobes 5, overlapping to left, glabrous or sparsely hairy with small sericeous adpressed hairs outside, glabrous inside, triangular broadly ovate, (1.2)1.4-1.6(1.8) cm long and wide, blunt to obtuse at apex, white outside, purple-mottled inside, recurved. Stamens 5, alternate to corolla lobes, subsessile, inserted on the upper part of the corolla tube; anthers oblanceolate, narrowing and obtuse at the apex, (1.8)1.9-2.1(2.2) cm long, (2)2.2-2.5(3) mm wide, longitudinally dehiscent by linear thecae. Disc prominent, annular, (2.4)2.5-2.8(3) mm in diameter, glossy yellow. Style and stigma erect, (4)4.5(5) cm long; style white, terete, about 2.5-3 cm long, 0.8-1 mm in diameter; stigmatic part pale yellow, narrowly ellipsoid, longitudinally ribbed, 1.5-2 cm long, 2-2.5 mm in diameter; ovary inferior, unilocular, with many ovules in two rows. Fruit berry-like, indehiscent, dark green to dull olive-brownish, glabrous, globular to broadly ellipsoid or ovoid, (4.5)5-7(7.5) cm long, (3.5)4-4.5(5) cm in diameter, with semi-woody wall; seeds (15)20-55(65), white, flattened, obscurely triangular to lenticular, (7)8-12(13) mm long and wide, (2.5)3-4(4.5) mm thick, sunk in a dark brown or black pulpy matrix.

Distribution.

The new species is only known from Nam Mau Commune, Ba Be District (in the limits of the territory of Ba Be National Park), Bac Kan Province, northeastern Vietnam. It is likely endemic to a very limited area of karstic rocky limestone associated with Ba Be National Park. This location lies near the northeast border of the generic area in Indochina.

Habitat.

Primary and old secondary dry evergreen broad-leaved forest with domination of Burretiodendron hsienmu Chun & F.C.How, Streblus asper Lour., and Arenga pinnata (Wurmb) Merr. on crystalline highly eroded rocky limestone at elevations of 150-300 m a.s.l., commonly on shady rocky steep slopes or in rocky valleys between limestone hills. Occasional.

Phenology.

Flowering from November to December, fruiting from December to January next year.

Preliminary IUCN conservation status.

Only two subpopulations were discovered on rocky karstic limestone in the middle part of Ba Be National Park, with few mature trees. The species meets the following conditions (Red List IUCN 2023): extent of occurrence (EOO) <100 km2 (B1) and area of occupancy (AOO) <10 km2 (B2) with one known location (a), and continuing decline of quality of habitat (b). We estimate that in the two subpopulations currently known, fewer than 50 mature individuals occur. These conditions identify the conservation status of the new species as globally Critically Endangered CR B1ab(iii) + B2ab(iii) As a result, Ridsdalea backanensis deserves the highest priority for conservation.

Notes.

The new species is most similar in its morphology to Ridsdalea daweishanensis described from SE Yunnan (Maguan & Hekou counties) and also reported for NW Vietnam, Lao Cai Province ( Zhang et al. 2007; Chen and Taylor 2011; Bui and Nguyen 2015). The new species differs from R. daweishanensis in its height to 22 m tall (vs. height of tree to 15 m), the leaf blades 7-11 cm long and 3-5.5 cm wide (vs. 10-14 cm long, 4-5 cm wide), the 1-3-flowered inflorescences (vs. inflorescences uniflorous), the 3.5-7 mm long pedicels (vs. pedicels 2-2.5 cm long), the free subulate calyx lobes, tapering from base to apex and broadest at the base, 6.5-11 mm long, without distinct veins (vs. calyx lobes narrowly oblanceolate, broadest above the middle, 14-16 mm long, 1-nerved), the 5-6.5 cm long corolla, sparsely hairy with small sericeous adpressed hairs outside and glabrous inside, contracted at the base into a narrow tube, 7-11 mm long, 2.5-4 mm in diameter (vs. corolla 5-5.5 cm long, entirely glabrous, contracted at the base into 6-7 mm long tube, 5 mm in diameter), the corolla lobes glabrous or sparsely hairy with small sericeous adpressed hairs outside and glabrous inside, 1.2-1.8 cm wide (vs. corolla lobes entirely glabrous, 2.2-2.3 cm wide), the oblanceolate, 2-3 mm wide anthers (vs. anthers linear, about 1.5 mm wide), and the globular to broadly ellipsoid or ovoid fruits, containing 15-65 seeds (vs. fruits globular, containing fewer than 20 seeds).

The newly discovered plant species will doubtless be of considerable interest for cultivation as an ornamental, since it is a nice-flowering tree that may be effectively used for gardens and urban green architecture.

Additional material studied

(paratypes). Vietnam, Bac Kan Province: Ba Be District, central part of Ba Be National Park, dry evergreen broad-leaved old secondary forest with domination of Burretiodendron hsienmu , Streblus asper and Arenga pinnata on crystalline highly eroded rocky limestone along Ba Be River at elev. c. 250 m a.s.l., tree 10-15 m tall on rocky slope in humid valley between limestone hills, occasional, 17 November 2022, Nguyen Huu Cuong, Cao Xuan Hai, L. Averyanov, T. Maisak, AL 1680 (LE LE01169964 https://en.herbariumle.ru/?t=occ&id=163682, VNF-NHC 20221117004, photos of living material made before preparation of voucher herbarium specimens: LE LE01123669 https://en.herbariumle.ru/?t=occ&id=155826). Vietnam, Bac Kan Province, Ba Be District, Ba Be National Park, dry evergreen broad-leaved secondary forest on steep rocky slopes composed by white crystalline marble-like highly eroded limestone near Dau Dang Waterfall of Nang River around point 22°27'09"N, 105°34'16"E, at elevation of about 150 m a.s.l., tree about 5 m tall on shady steep rocky slope, young flower buds green, locally common, 20 October 2023, Nguyen Huu Cuong, Chu Ngoc Quan, L. Averyanov, Nguyen Van Ly, T. Maisak, AL2330 (LE LE01253796 https://en.herbariumle.ru/?t=occ&id=212995, photos of living material made before preparation of voucher herbarium specimens: LE LE01124442 https://en.herbariumle.ru/?t=occ&id=207144, VNF) . Vietnam, Bac Kan Province, Ba Be District, Ba Be National Park, dry evergreen broad-leaved secondary forest on steep rocky slopes composed by white crystalline marble-like highly eroded limestone near Dau Dang Waterfall of Nang River around point 22°27'09"N, 105°34'16"E, at elevation of about 150 m a.s.l., tree about 8 m tall on shady steep rocky slope, fruits broadly ellipsoid, dirty green to almost black, locally common, 20 October 2023, Nguyen Huu Cuong, Chu Ngoc Quan, L. Averyanov, Nguyen Van Ly, T. Maisak, AL2331 (LE LE01253792 https://en.herbariumle.ru/?t=occ&id=212991, LE LE01253793 https://en.herbariumle.ru/?t=occ&id=212992, photos of living material made before preparation of voucher herbarium specimens: LE LE01124443 https://en.herbariumle.ru/?t=occ&id=207145, VNF) . Vietnam, Bac Kan Province, Ba Be District, Ba Be National Park , dry evergreen broad-leaved secondary forest on very steep rocky slopes composed by white crystalline marble-like highly eroded limestone along lake shore on point 22°24'58"N, 105°36'48"E, at elevation of about 250 m a.s.l., tree about 10 m tall on very steep rocky slope, not common, 29 October 2023, Nguyen Huu Cuong, Chu Ngoc Quan, L. Averyanov, Nguyen Van Ly, T. Maisak, AL2369 (LE LE01253794 https://en.herbariumle.ru/?t=occ&id=212993, LE LE01253795 https://en.herbariumle.ru/?t=occ&id=212994, photos of living material made before preparation of voucher herbarium specimens: LE LE01124480 https://en.herbariumle.ru/?t=occ&id=207183, VNF) .

Figures

Figure 1. Ridsdalea backanensis A mature tree in natural habitat B part of crown with two main trunks C trunk of mature tree D bark at DBH E flowering and fruiting branches F, G ripe fruits H ripe fruits, sagittal section I ripe fruit, cross section J fruit, sagittal section, seeds removed K ripe seeds. Photos by C. H. Nguyen (A-E) and L. Averyanov (F-K) from plant used for preparation of the paratype voucher specimens (AL 1680), photo-correction, and design by L. Averyanov and T. Maisak.

Figure 2. Ridsdalea backanensis A part of crown of flowering tree in natural habitat B leaves, adaxial and abaxial surface C flowering branches D flower bud and opening flower, side view E Opening flower, frontal view F, G flowers, frontal view H flowering branch showing leaves from abaxial side I flowering branch showing leaves from adaxial side J corolla, sagittal section outside and inside K peduncle, pedicels and calyxes, side view L peduncle and pedicel, side view M peduncle, pedicel, calyx and base of style, side view, corolla removed N calyx, frontal view, corolla removed O separated, upper part of calyx tube, calyx lobes, and nectary disc, frontal view P indumentum of abaxial surface of calyx on dry specimen Q portion of adaxial surface of corolla, sessile stamen and apical part of stigma R stamen S pedicel, calyx and pistil with magnified stigma, side view T apical part of stigma U middle part of stigma Photos by L. Averyanov (A, C, F-K, P) and C. H. Nguyen (B, D, E, L-O, Q-U) from plant used for preparation of the type voucher specimens (AL 1682), photo-correction and design by L. Averyanov and T. Maisak.