Rinorea niccolifera is most similar to Rinorea bengalensis by its fasciculate inflorescences and smooth subglobose fruits with 3 seeds, but it differs by its glabrous ovary with shorter style (5 mm long), the summit of the staminal tube sinuate to entire and the outer surface smooth, and its generally smaller leaves (3–8 cm long × 2–3 cm wide) and smaller fruits (0.6–0.8 cm diameter).

TYPE: PHILIPPINES. Luzon Island: Zambales Province, Municipality of Sta. Cruz, Lucapon, in remnant forest on ultramafic soils, along a gully with large boulders, 330 m elevation, flowers and immature fruits, 01 April 2012, Fernando 3016 (holotype LBC; isotypes CAHUP, K, PNH, SING).

Shrub or small tree, 1.5–8 m tall; stem 3–13 cm diameter, outer bark generally smooth, inner bark whitish; young twigs rather zigzag, with prominent stipular scars. Leaves simple, distichous, lamina elliptic to narrowly obovate, (2–) 3–8 (–10) cm long × (1–) 2–3 (–4) cm wide; the margins finely serrate, especially towards the distal half; base acute; apex acute to acuminate; secondary nerves (6–) 8–12 (–13) on each side of the midrib, diverging 40–60° from the midrib; hairy pit domatia very prominent along the midrib on abaxial surface; petiole terete, (–2) 3–5 (–7) mm long; young leaves white or greenish-white, growing in flushes. Stipules narrowly lanceolate, (4–) 6–7 (–8) mm long × 1 mm wide at the base, prominently covering the apical bud, caducous and leaving a distinct scar. Flowers white or cream, bisexual, globose or broadly ovoid, 3.1–3.3 mm long × 3.1–4 mm wide, in dense axillary clusters or fascicles of up to 3–5, sometimes more, rarely solitary; pedicel 2.5–3.2 mm long, 0.7–0.9 mm wide, sparsely covered with fine, short hairs. Sepals 5, free, subequal in size and shape, nearly as wide as long, broadly ovate, 1.3–1.6 × 1.3–1.6 mm, shorter than the petals, light green or greenish-white, distinctly 2–4 (–5) veined, margins entire, ciliate towards the distal half and sometimes covered with brown fine hairy indumentum at the apex. Petals 5, free, subequal in size and shape, broadly oblong to ovate, the apex rounded or obtuse, 2.2–2.7 mm long × 1.3–1.7 mm wide, white or greenish-white, paler towards the apex, the tip slightly deflexed or recurved, margins smooth or sometimes slightly ciliolate near the apex. Stamens 5; anther with 2 thecae, 1 mm long × 0.6 mm wide; connective appendage broadly ovate, 0.4 mm long × 0.7 mm wide, membranous, cream or light orange, the margins fimbriate; filaments as long as the tube, 0.6 mm long × 0.2 mm wide, inserted on the inner surface of the staminal tube surrounding the ovary; staminal tube shallowly 5-lobed, 0.6 mm tall and 0.7 mm thick, the summit sinuate to entire, outer and inner surfaces glabrous, smooth. Ovary ovoid, glabrous, smooth, 1 mm long, 0.9–1 mm diameter, with 3 locules; ovule 1 each per locule; style 0.5 mm long × 0.3 mm wide, erect; stigma pointed, undifferentiated. Fruit a capsule, globose or depressed globose, obscurely 3-angular, 6.5–7 mm long × 6–8 mm wide, green, turning pale green when ripe, glabrous, subtended by the persistent sepals and petals; remnant of stigma prominent, 1–1.5 mm long; 3-locular, dehiscent along three sutures, the locules 7–8 mm long, 5 mm wide and 4 mm deep, folding inwards when seeds are released; pedicel 4–5 mm long, 1–1.5 mm thick. Seeds 3, one in each locule, globose, 3–4 mm long × 3 mm wide, mottled light brown; hilum distinct, white. Seedling with epigeal germination, phanerocotylar; cotyledons foliaceous, 8 mm long × 10 mm wide, apex slightly emarginate, base truncate or obtuse; eophylls simple, elliptic, spirally arranged, 11 mm × 5 mm, margins serrate.

Endemic in the Philippines. Luzon Island, Zambales Province, Municipalities of Sta. Cruz and Candelaria.

This species grows in forests on ultramafic soils, usually along gullies or sloping areas with large boulders or rocks at elevations of 320–825 m. In its type locality, Rinorea niccolifera was observed growing with Syzygium longissimum (Merr.) Merr. (Myrtaceae), Clerodendrum klemmei Elmer (Lamiaceae), Ixora ebracteolata Merr. (Rubiaceae), Severinia disticha (Blanco) Swingle (Rutaceae), Diospyros ferrea (Willd.) Bakh. (Ebenaceae), Calophyllum pentapetalum (Blanco) Merr. (Calophyllaceae), Dillenia luzoniensis (Vidal) Merr. (Dilleniaceae) and Terminalia pellucida C.Presl (Combretaceae), among several other woody plant species. In some other sites within its range in the ultramafic area of Sta. Cruz and Candelaria in northern Zambales, Rinorea niccolifera may occur together with Rinorea bengalensis, the latter also a nickel hyperaccumulator, but is generally a larger tree reaching to 15 m tall and with stem diameter of up to 25 cm (see also further in Key to the species).

Philippines, Luzon Island, Zambales Province, Municipality of Sta. Cruz, Lucapon, along a gully with large boulders, 320 m elevation, flower buds, 19 April 2011, Fernando 2421 (K, LBC, PNH), Fernando 2422 (LBC, PNH); flower buds, 01 April 2012, Fernando 3015 (CAHUP, LBC, PNH), mature fruits and seeds, 26 May 2012, Fernando 3042 (LBC, PNH); Municipality of Candelaria, Malimlim area, on steep slope with rocky soil, 630 m elevation, sterile material, 18 January 2013, Fernando 3072 (LBC), juvenile flower buds, 18 January 2013, Fernando 3073 (LBC, PNH), 605 m elevation, sterile material, 18 May 2013, Fernando 3161 (CAHUP, LBC), on steep slope, 750 m elevation, sterile material, 18 May 2013, Fernando 3181 (LBC), on ridge summit, 825 m elevation, sterile material, 19 November 2013, Fernando 3338 (LBC, PNH); Cultivated: Laguna Province, Los Baños, seedlings grown from seeds of Fernando 3042 germinated in nursery, 21 August 2012, Fernando 3042A (LBC).

The specific epithet niccolifera refers to the ability of this species to hyperaccumulate the heavy metal nickel in its stem and leaf tissues (from niccolum – Neo Latin for nickel, and; fer – to yield, to contain).

Following the IUCN Categories and Criteria (IUCN 2012), we regard this species as Endangered (EN B2ab(ii,iii,iv)). Its habitat is severely fragmented and is so far recorded only from three adjacent localities. Its current known area of occupancy is estimated to be less than 500 km², and a continuing decline is observed, inferred or projected in its (a) extent of occurrence; (b) area of occupancy; and (c) area, extent and/or quality of habitat. Much of the habitat of this new species is subject to open pit mining.