Rudgea graniticola O. Lachenaud, 2022

Lachenaud, Olivier, Bruniera, Carla P. & Zappi, Daniela C., 2022, Six new and a little-known species of Rudgea (Rubiaceae-Palicoureeae) from the Guianas, Phytotaxa 531 (3), pp. 154-174 : 163-166

Rudgea graniticola O. Lachenaud , sp. nov. ( Fig. 5 )

Ramis minute puberulis, foliis coriaceis, fructibus aurantiacis ellipsoideis, pyrenis dorso costatis, corolla tubo breve (4.5-5 mm) et intus villoso, stipulisque parvis (3-4.5 mm) dorso carina appendiculata munitis Rudgeae crassilobae ( Bentham 1841: 233) B.L. Robinson (1910: 408) affinis, sed differt foliis glaberrimis (vs. in axillis nervorum pubescentibus) costa mediana supra prominente (vs. impressa) venulis laxiore reticulatis, corollaque lobis intus glabris (vs. villosis).

Type: — FRENCH GUIANA. Montagne des Nouragues , 4°03’N, 53°42’W, October 1989 (fl.), D.Larpin 722 (holotype,CAY [CAY078936]; isotype, P [P04008509]) .

Shrub, much-branched, or small tree, to 10 m tall, to 15 cm at dbh; main trunk often leaning or branched from base, the branches erect; bark grey, rugose; terminal branchlets 1 – 1.5 mm thick, minutely puberulous (appearing glabrous to the naked eye), soon covered with a grey bark. Stipules minutely puberulous, 3 – 4.5 × 0.8 – 3 mm, consisting of a persistent truncate to round sheath 1 – 2 mm long, bearing on each side a dorsal keel with 3 – 5 terminal, corky and soon caducous appendages 1 – 2 mm long. Leaves opposite, petiolate; petioles 0.3 – 0.8 cm long, minutely puberulous; blades elliptic or slightly obovate, 2.7 – 10 × 1 – 5 cm, decurrent on petiole at base, shortly acuminate at apex, coriaceous with recurved margin, entirely glabrous, dark green above and paler green below in life, variously drying green, yellowish or brown; midrib convex on both sides; secondary veins 5 – 7 on each side of midrib, moderately ascending, forming conspicuous loops 0.5 – 3 mm from the margin; tertiary veins conspicuously and rather laxly reticulate; domatia absent. Inflorescences terminal, white, trichotomously paniculate or rarely reduced to a glomerule, 0.8 – 6.8 cm long, minutely puberulous; peduncle terete, 0.5 – 3 cm long; flowering portion 0.3 – 3.8 × 0.5 – 3.5 cm; main ramifications, if present, 2 per node, 0.1 – 1.2 cm long; bracts shortly triangular, up to 1.5 mm long, acute, corky, whitish. Flowers sessile, 4-merous, heterostylous. Hypanthium tronco-conical, 0.5 mm long, puberulous. Calyx cupuliform, 0.5 – 1 mm long, minutely puberulous, cleft at least to the middle (often nearly to the base) into elliptic to triangular lobes, these acute or obtuse at apex. Corolla white; tube narrowly cylindrical, 4.5 – 5 × 1 – 1.5 mm, glabrous outside, pubescent at the distal portion inside; lobes narrowly triangular, 2.5 – 3.5 × 1 – 1.3 mm, apex with dorsal cornicula <0.2 mm long, round, minutely puberulous outside, otherwise glabrous. Stamens half-exserted in long-styled flowers, or fully exserted, with filaments exserted 2 mm beyond corolla mouth in short-styled flowers; anthers elliptic, 1 – 1.3 × 0.3 mm, dorsifixed. Disk hemispherical, entire, 0.5 mm, glabrous. Style exserted, 7 mm long in long-styled flowers, or included, 4 mm long in short-styled flowers, papillose towards apex; branches c. 0.7 mm long. Fruits ellipsoid, 7 – 9 × 5 – 6 mm when dry, green when immature, orange when mature, glabrous, sessile, the calyx not enlarging. Pyrenes ellipsoid, 6.5 – 7 × 4.5 mm, with 3 – 4 strong dorsal ridges; seeds with a deep T-shaped ventral furrow.

Distribution and ecology: — This species has a scattered distribution in northern and central French Guiana (Chutes Voltaire, Nouragues, la Trinité, Massif des Emerillons) and northern Suriname (Bakhuis Mts) ( Fig. 2 .) It grows exclusively in low forest on granitic inselbergs, at 150 – 410 m elevation, and is locally very common in this habitat.

Phenology: — Flowering specimens were collected in March and October; specimens with immature fruits were collected in January, and with mature ones from February to April.

Conservation status assessment: — Endangered [B1ab(iii)+2ab(iii)]. Rudgea graniticola occurs on inselbergs in French Guiana and Suriname, and is known from 17 collections representing five occurrences. Its extent of occurrence (EOO) is calculated to be 44,550 km ², exceeding the limit for Vulnerable status under subcriterion B1, and its area of occupancy is estimated to be 24 km ², within the limit for Endangered status under subcriterion B2. Of the four occurrences in French Guiana, three occur in protected areas (Parc Amazonien de Guyane, Réserve Naturelle des Nouragues, and Réserve Naturelle de la Trinité); the fourth one (near Chutes Voltaire) may be impacted by touristic activities but the current level of threat is rather low. The only occurrence in Suriname occurs in a mining concession, which leads us to expect a decline in habitat extent and quality. The five occurrences represent five locations in the sense of IUCN, and the species qualifies for Endangered status according to the conditions B1ab(iii)+B2ab(iii). Considering however that its range and habitat are not well prospected, the species may be more widespread than records suggest, in which case this assessment will have to be revised.

Notes: — This species has previously been confused with the more widespread and rather similar Rudgea crassiloba . Both have relatively small and coriaceous leaves, minutely puberulous twigs, very short stipules with a dorsal keel bearing terminal appendages, usually paniculate inflorescences (sometimes glomerulate in R. graniticola ), a short corolla tube, orange to red ellipsoid fruits, and dorsally costate pyrenes. Both are also commonly associated with granitic rocks (hence the specific name of R. graniticola ) although R. crassiloba may also be found on other substrates. They differ by the characters given in the diagnosis.

Rudgea graniticola also resembles R. ayangannensis (see above) in vegetative and inflorescence characters. Differences between the three species are summarised in Table 3 . The distribution of R. graniticola is separate from both R. ayangannensis , which is endemic to Guyana, and R. crassiloba , which is mostly found in the Amazon/Orinoco basin and only reaches the southern part of Suriname and French Guiana.

According to Bruniera (2015), Rudgea crassiloba belongs to the informal “ lanceifolia clade”, being closely related to R. reticulata Bentham (1850: 458) . In view of its morphology, R. graniticola probably also belongs to this clade.

The inflorescences are very variable in this species. Collections from the Nouragues inselberg (including the type specimen) have lax and many-flowered panicles, while those from Chutes Voltaire and Massif des Emerillons have few-flowered glomerules. Specimens from Bakhuis Mountains and La Trinité are intermediate, and since no other differences have been found, we recognize only one variable taxon.

The oldest collection of this species (Collector unknown in A.P. de Candolle & C.L. L’Héritier s.n.) is labelled “Cayenne”, which at that time often referred to the whole of French Guiana. In view of its habitat, the species is unlikely to occur in the close vicinity of Cayenne. Neither Candolle nor L’Héritier ever visited French Guiana, so they must have obtained this material from another, unknown collector.

Additional specimens examined (paratypes): — FRENCH GUIANA. “Cayenne” (see notes), no date [c.1800] (fr.), Collector unknown in A.-P. de Candolle & C.L. L’Héritier s.n. (G); Massif des Emerillons, zone nord, savane roche au sommet d’une colline sur la gauche de la Haute Approuague, 29 September 1980 (fallen fl.), J.J. de Granville 3949 (CAY, P); Montagnes de la Trinité, sommet nord, 12 January 1984 (imm. fr.), J.J. de Granville, C.C. Berg, M.J. Jansen-Jacobs & J. van Setten 5909 (BR, CAY, K, P, U) & 5938 (BR, CAY, K, P, U); Montagnes de la Trinité, inselberg Nord-Ouest, 16 January 1984 (fallen fl.), J.J. de Granville, C.C. Berg, M.J. Jansen-Jacobs & J. van Setten 6016 (CAY, U); Station des Nouragues, 4°03’N, 52°42’W, 22 February 1991 (fr.), J.J. de Granville 11164 (CAY, P, U); Montagne des Nouragues, 4°03’N, 52°42’W, April 1989 (fr.), D. Larpin 496 (CAY, P); Inselberg des Chutes Voltaire, 20 December 2015 (fallen fl.), O. Lachenaud 2166 (BR, CAY); without locality, 1842 (fr.), E. Mélinon s.n. (P); Nouragues Field Station, 4°05.289’N, 52°40.774’W, 16 February 2002 (fr.), S.A. Mori, F. Blanchard & T.A. Lobova 25381 (CAY); Réserve des Nouragues, Station de recherches, 4°03’N, 52°42’W, 14 March 2004 (fl. & fr.), O. Poncy, L. Barrabé, P. Petronelli, J.Y. Serein & J.G. Jourget 1878 (CAY, P); Montagne des Nouragues, 4°03’N, 52°42’W, 17 April 1988 (fr.), C. Sarthou 275 (CAY); Station des Nouragues, 13 March 1996 (fr.), P. Solano K331 (CAY); same locality, 14 March 1996 (fr.), P. Solano K394 (CAY); inselberg Trinité, 4°35’N, 53°21’W, 9 May 1998 (st.), J.F. Villiers & C. Sarthou 6293 (CAY).

SURINAME. Bakhuis Mountains , concession BMS, zone 22, centre, 4°29’45”N, 57°00’59”W, 5 April 2006 (fr.), B. Bordenave, S. Dourga, F. van Troon, I. van Troon & J. James 8365 ( CAY) .

Figures

FIGURE 2. Distribution of Rudgea approuaguensis (triangles), R. ayangannensis (stars) and R. graniticola (circles).

FIGURE 5. Rudgea graniticola. A, flowering branch; B, fruiting branch; C, portion of stem, node and stipules in side view; D, detail of lower leaf surface; E, flower bud; F, short-styled flower; G, long-styled flower; H, fruit and portion of infructescence; I, pyrene, dorsal view; J, pyrene, basal view; K, seed, cross-section. A, D–F from Larpin 722, B, H–K from Larpin 496, C from Poncy et al. 1878. Drawn by O. Lachenaud.