Trivalvaria recurva Chaowasku & D. M. Johnson 2021, sp. nov.

Type:— THAILAND. Kanchanaburi: Phuye, Thong Pha Phum District, ca. 350 m, 8 August 2010 (flower), Nakorn-Thiemchan NTC 13 (holotype: BKF!).

This new species most resembles T. casseabriae Y.H.Tan, S.S.Zhou & B.Yang in Yang et al. (2018: 5) in its small flowers with short white petals, but the petals are only ca. 3 mm long and strongly recurved versus 6–10 mm long and spreading, and the leaves are elliptic to oblanceolate with a maximum width of 6.8 cm versus lanceolate to oblong with a maximum width of 5.5 cm (Table 1).

Shrubs with twigs persistently short-pubescent. Leaves elliptic to oblanceolate, 13.8–17.2  4.8–6.8 cm, base cuneate, apex sharply acuminate, the acumen 5–16 mm long, glabrous except for hairs along midrib above and below, secondary veins eucamptodromous, 7–10 per side, arcuate; petiole 5.5–9.0 mm long. Inflorescences internodal, 1–2-flowered, peduncles 1.5–3.0 mm long, pedicels ca. 1.2 mm long, bearing 1 minute bract; buds with petals separated and erect early in development, exceeded by androecium. Flowers bisexual or staminate. Sepals semicircular, ca. 2 mm long, obtuse, sericeous. Petals white, recurved at anthesis, glabrate on both surfaces, outer petals ovate, ca. 3  2.7 mm, acute, inner petals oblong, ca. 3.0  2.0 mm, rounded. Stamens 50–100, forming a globose boss ca. 3.0 mm [fluidpreserved] in diameter, clavate, ca. 1.7 mm long, anther connective apex truncate, glabrous to sparsely pubescent. Carpels 4–7, ovaries not examined, stigmas exceeding the tops of the stamens, irregular, pubescent. Receptacle not visible. Fruit of ca. 9 monocarps borne on a short pedicel. Monocarps red in vivo, oblong, acute at the apex.

Distribution and Ecology:— Known only from the type collection from Kanchanaburi Province in southwestern Thailand, where the plant occurs in evergreen forest at ca. 350 m.

Phenology:— Flowering August.

Etymology:— Named for the petals recurved at anthesis.

Vernacular names:— Ma khao dok lek (มะขาวดอกเล็ก) (Kanchanaburi), sakao phrai (สกาวไพร) (General).

Conservation status:— Trivalvaria recurva occurs within the boundaries of a national park, an area not yet widely surveyed for the species. At present, the status of the species should be considered data deficient.

Notes:— Trivalvaria recurva is distinctive among Thai species in having short petals that are separated at the tips from an early stage of development and recurved at anthesis. A photograph of the fruit (Fig. 2F, and Chalermglin (2001: 131) under Polyalthia Blume (1830: 68)) shows that the monocarps are red when mature, nearly sessile, oblong and slightly acute at the apex. Trivalvaria recurva was collected from an area of Kanchanaburi Province where other narrowly distributed Annonaceae species occur, for example Pseuduvaria phuyensis (Saunders, Su & Chalermglin, 2004: 42) Y.C.F.Su & R.M.K.Saunders in Su et al. (2010: 37). Trivalvaria ochroleuca is the only other Trivalvaria species known from this area and can be distinguished from T. recurva by its shorter blunt leaf acumen 3–8 mm long, inflorescences sub-opposite the leaves and greater numbers of stamens and carpels.