Rudgea billietiae O. Lachenaud, 2022

Lachenaud, Olivier, Bruniera, Carla P. & Zappi, Daniela C., 2022, The Rudgea hostmanniana complex (Rubiaceae) in the Guiana Shield region, Phytotaxa 561 (3), pp. 219-242 : 223-226

1. Rudgea billietiae O. Lachenaud , sp. nov. Fig. 3 & 4

Foliis glabris crassis venulis inconspicuis, stipulis basi connatis et dorso appendiculatis, fructibusque statu immaturo brunneis deinde rubris Rudgeae pungenti, R. hostmannianae, R. cornigerae et R. tanaosepalae affinis . A Rudgeae pungenti differt bracteis multo minoribus, 3–5 mm longis, inflorescentiis manifeste ramosis et corollae tubo multo breviore, 6 mm longo (vs. 13 mm longo); ab alteris speciebus stipulis longiore tubulosis in vaginam 7.5–12 mm longis (vs. 2–5 mm longis) connatis, praeterea inflorescentiis glabris (vs. puberulis) distinguitur.

Type: — FRENCH GUIANA. Route de Kaw , pk 33, sentier vers les grottes, 4 December 2000 (fl.), F. Billiet & B. Jadin 7456 (holotype, BR! [ BR000000907382 ]; isotypes, CAY! [ CAY014905 , CAY014906 ], MO n.v., NY n.v.) .

Much-branched shrub 1.75–3 m tall; trunk pale grey; branches ascending; twigs glabrous, 1–2 mm thick, soon covered with a pale straw-coloured bark. Stipules 9–15 × 2.5–4.5 mm, glabrous, marcescent and soon corky, consisting of a narrow tubular sheath 7.5–12 mm long (usually split at flower-bearing nodes) bearing ca. 12 terminal linear appendages 1.5–4 mm long, and 6–8 early caducous dorsal appendages 1 mm long, forming a very short decurrent keel inserted in the upper half of the stipule. Leaves opposite; petioles 0.2–1 cm long, glabrous; blades elliptic, 4.7–13.5 × 1.3–5.5 cm, decurrent on petiole at base, gradually acuminate at apex, very thick and easily cracking in the fresh state, entirely glabrous, drying greyish-green to yellowish; midrib concave above; secondary veins 6-11 on each side of midrib, strongly ascending, forming an angle of ca. 45° with the midrib; tertiary veins invisible in fresh material, sometimes faintly prominent in dry material; domatia absent. Inflorescences terminal in rather contracted panicles, 1–4 cm long, erect, white in flower and green in fruit, glabrous; peduncle terete, 0.5–1.6 cm long; branched part 0.5–2.4 × 1.3–3.5 cm; secondary branches 3–4 per node, 0.2–1 cm long; bracts linear or narrowly lanceolate, 3–5 × 0.3–1.5 mm, entire or dentate in lower half, shortly ciliate in upper part. Flowers sessile, 5-merous. Hypanthium obconical, 0.5–0.8 mm long, glabrous. Calyx tube extremely reduced, lobes linear to narrowly triangular, 1.5–2 × 0.3–0.6 mm, very acute at apex, glabrous or ciliate. Corolla white, hypocrateriform; tube funnel-shaped, 6 mm long, 0.5–1 mm wide at base, 2–3 mm wide at mouth, glabrous outside, pubescent inside at the distal portion below the mouth; lobes triangular, 4–5 × 2 mm, glabrous on both sides, with broad, rounded dorsal cornicula 0.7–1 mm long. Stamens white, exserted; filaments exserted beyond the corolla mouth by 3 mm; anthers 1.2 × 0.3 mm. Disk shortly cylindrical, 0.7 mm long, glabrous. Style included, 5 mm long, glabrous; lobes 0.8 mm long, stigmatic surface papillose. Fruits ellipsoid, 6–7 × 5–6 mm when dry, dark red-brown and hard when immature, bright cherry red and soft when mature, glabrous, sessile or with very short pedicel ≤ 0.5 mm long, crowned with persistent calyx 1.5 mm in diameter. Pyrenes plano-convex, hemiellipsoid, 6.5 × 5 mm, dorsal side with 3 hardly distinct longitudinal ridges, smooth between the ridges, ventral side smooth. Seeds with a deep T-shaped ventral furrow.

Distribution and ecology: —This species is endemic to the Kaw mountain in northeastern French Guiana ( Fig. 5 ) where it occurs exclusively in low, open canopy forest on lateritic crust, 150 – 300 m in elevation. It is locally abundant in this habitat, which only covers small areas on the ridge of the mountain.

Phenology: —Flowers have been collected in December (beginning of first rainy season) which is probably the main flowering period; a single inflorescence was also seen in June, among a mostly fruiting population ( Lachenaud 1878 ), which is probably a case of flowering asynchrony. Fruits have been collected in November (once, immature) and were seen in abundance from February to late June at least (OL pers. obs.). They apparently take about six months to mature. They are already full-sized in February, but still hard and brown, and remain so for a long time; they start to turn soft and red in late June (OL pers. obs.).

Eponymy: —This species is named after the Belgian botanist Frieda Billiet, collector of the type. Together with her husband Bernard Jadin (1948–2012), she made important plant collections in French Guiana between 1981 and 2009, several of which have been described as new species, e.g., Philodendron billietiae Croat (1995: 24) , Oryctanthus guianensis Kuijt (2011: 465) and Rudgea jadinii O. Lachenaud ( Lachenaud et al. 2022: 168) .

Conservation status assessment: —Endangered [EN B1 ab(iii) & 2ab(iii)]. Rudgea billietiae has a very restricted range, being endemic to the ridge of the Kaw Mountain in French Guiana, where it grows in low stunted rainforest on lateritic crust. Its extent of occurrence (EOO) is calculated to be 68 km ², which falls within the limit for Critically Endangered under criterion B1 , while its area of occupancy (AOO) is estimated to be 24 km ², within the limit for Endangered status under criterion B2 (the actual AOO is however certainly <5 km ², because its habitat only occurs as small isolated patches). The species is known from 13 specimens representing six occurrences and one subpopulation; one occurrence is protected in the Réserve Naturelle des marais de Kaw-Roura, the others have no official protection status. The area where it is found harbours important bauxite and gold deposits; mining projects have been abandoned in 2008 but may resurface in the future, and represent the main threat to the species. Another potential threat comes from touristic development and the building of related infrastructures. Forest exploitation is also ongoing in part of its range, but unlikely to represent a major threat to this species, which occurs in areas of low forest with few exploitable trees. In view of all these factors, a decline in habitat extent and quality is projected. The six occurrences represent five locations in the sense of IUCN, and the species qualifies for Endangered status under the conditions B1 ab(iii) & 2ab(iii). The species is recommended for inclusion on the list of protected plants in French Guiana, and particular measures should be taken to protect its habitat, which harbours several other threatened species.

Notes: —Specimens of Rudgea billietiae have for a long time been confused in herbaria with either R. hostmanniana or R. pungens . In the vegetative state R. billietiae is very similar to the latter, which also has stipules connate into a long narrow tube (a character not always easy to see on herbarium specimens, because the stipules of the flower-bearing nodes tend to split early) and can only be separated by the position of the dorsal appendages of the stipules, which are inserted in their upper half in R. billietiae and near their base in R. pungens . The two species differ markedly in their inflorescences, especially in the development of their bracts (much larger in R. pungens ) and in the size of the corolla tube, which is about twice longer in R. pungens . They also seem to have different ranges, R. pungens being apparently absent from the Kaw Mountain. In inflorescence structure and corolla length R. billietiae resembles R. hostmanniana , R. cornigera and R. tanaosepala , but these have much shorter stipular sheaths and puberulous inflorescence axes (glabrous in R. billietiae ). Differences between all these species are summarised in Table 1 .

It is not known whether the flowers of R. billietiae are heterostylous: only two collections with open flowers have been seen (the type and Martin s.n.) and both are of the brevistylous form.

Three specimens collected in the late XVIIIth or early XIXth centuries, Leblond s.n., Martin s.n. and Richard s.n., have no precise locality; the two former are labelled “Cayenne”, but this indication was often used for the whole of French Guiana at that time. They may well have come from the Kaw Mountain, which is only about 50 km from Cayenne and was already accessible at that time. The Richard collection is a mixture, including two branches of R. pungens (one flowering, one fruiting) and one of R. billietiae (fruiting), collected at different times of the year and probably in different places.

Additional Specimens Examined (paratypes): — FRENCH GUIANA. Montagne de Kaw , E end ca. 10 km from end of road, 4°32’N 52°07’W, 10 March 1994 (fr.), L . Andersson, C . Gustafsson, C . Persson & J . Rova 1949 ( CAY); Montagne de Kaw , 13 May 1985 (fr.), C . Feuillet 2244 ( CAY, P); Montagne de Kaw , forêt dense sur versant nord, à proximité de la route, 5 km E de Camp Caïman, 14 June 1979 (st.), J.- J . de Granville 2978 ( CAY, P); Montagne de Kaw , extrémité est, versant sud, 3 November 1985 (imm. fr.), J.- J . de Granville 8244 ( CAY, P, U); Montagne de Kaw , sentier des grottes, 23 February 2014 (imm. fr.), O . Lachenaud 1643 ( BR, CAY, MO, P); same locality, 4°33’15”N 52°10’18”W, 28 June 2014 (fl. buds & fr.), O . Lachenaud 1878 ( BR, CAY, MO, P); Montagne de Kaw , Amazon Lodge, 4°33’35.1”N 52°12’20.7”W, 23 August 2017 (st.), O . Lachenaud 2639 ( BR, CAY, MO); Montagne de Kaw , au pk 30.5, à gauche de la route, 3 January 2019 (st.), O . Lachenaud 2689 ( BR, CAY); Route de Fourgassié , peu après le croisement de la route de Kaw, 3 January 2019 (fallen fl.), O . Lachenaud 2695 (BR, CAY); “Cayenne”, no date [ca. 1800], J.B. Leblond s.n. ( G); “Cayenne”, no date [ca. 1800] (fl.), J . Martin s.n. ( BM [3 sheets], K [2 sheets]); no locality or date (fr.), L. C. M . Richard s.n. ( P [ P04008549 ], mixed with R. pungens ; see notes above) .

Figures

FIGURE 3. Rudgea billietiae. A, flowering branch; B, fruiting branch; C, terminal bud; D, stipules of an older node (dorsal appendages fallen off); E, base of inflorescence and broken stipules of the upper node; F, flower bud; G, flower; H, fruit and portion of infructescence; I, cross-section of fruit. A, E, G from Billiet & Jadin 7456 (BR), B–C and H–I from Lachenaud 1643 (BR), D, F from Lachenaud 1878 (BR). Drawing by O. Lachenaud.

FIGURE 4. Rudgea billietiae. A, inflorescence; B, fruiting branch; C, immature fruits; D, leaf abaxial side; E, terminal bud; F, stipules from older node; G, mature fruits. Voucher collections: A Billiet & Jadin 7456, B–F Lachenaud 1643, G Lachenaud 1878. Photo A by F. Billiet, photos B–G by O. Lachenaud.

FIGURE 5. Distribution of Rudgea billietiae (black triangles), R; bolivarensis (grey circles), R. cornigera (black squares), R. pungens (grey squares) and R. tanaosepala (grey triangles).