Similar to Pseudobombax longiflorum (Mart.) A.Robyns by its long petiolules, obovate to suborbicular leaflets, and maculate seeds, but differing by the smaller leaves, flowers and fruits, slender branches, petioles 4 times the length of the petiolules (vs. a petiole/petiolule ratio of 6–12 in Pseudobombax longiflorum), and fruits acuminate for the distal 20% of their length (vs. 3%–5% in Pseudobombax longiflorum).

BOLIVIA. Santa Cruz: 42 km E of Curuyuqui, 18°45'56"S, 62°13'59"W, 350 m, 25 October 1991 (lf, fr), A. Gentry, R. Foster & M. Peña 75227 (holotype: MO!; isotypes: F!, LPB!, USZ!, WIS!).

Treelets3–8 m, deciduous; branches glabrous, relatively slender, often covered with pale wax; brachyblasts absent. Stipules not seen. Leaves palmately compound, clustered at apex of the branches; petioles (17–) 34–87 mm long, flattened, slender, glabrous, usually glaucous with pale wax at the ends, bases slightly thickened, apices slightly thickened to 3–4 mm diam.; petiolules 14–22 mm long; leaflets (4) 5 (7), chartaceous, proximal leaflets 17–38 × 9–30 mm, distal leaflets 45–57 (–75) × 29–51 mm, obovate, broadly elliptic to suborbicular, apices retuse, rarely acuminate, bases obtuse, rounded, truncate to slightly cordate, margins entire, glabrous on both surfaces, except for sparse, peltate microtrichomes, abaxial surface dull-brown in dried state, midrib prominent abaxially, 8–12 secondary veins inconspicuous, intersecondary veins present, tertiary veins reticulate. Complete inflorescences not seen; pedicels 19 mm long (–26 mm when in fruit). Flowers c. 65 mm long; receptacle with single whorl of c. 5 glands; calyces 8–9 × 11–15 mm, cupular to campanulate, truncate, outwardly glabrous except for peltate microtrichomes, internally sericeous; petals (50–) 61–70 × 6–8 mm, linear to lanceolate, apex acute, dark-brown externally, covered with tufted, rigid hairs, inwardly pilose to glabrescent towards the base, covered mainly by verrucose microtrichomes; stamens c. 150–200, staminal tube 9–10 × 4 mm, pubescent, phalanges absent, filaments free for 40–58 mm, anthers hippocrepiform c. 2 mm long; ovary 5 × 2 mm, oblong-obovoid, glabrous except for peltate microtrichomes, style c. 70 mm long, glabrous, stigma inconspicuously 5-lobed. Capsules c. 90 mm long, woody, oblong-obovoid, conspicuously acuminate for distal 15 mm or so, valves coriaceous, glabrous, kapok abundant, golden brown. Seeds c. 5 mm diam., subglobose to pyriform, maculate, glabrous.

Pseudobombax pulchellum is known from only four sites in the Department of Santa Cruz, Bolivia, and appears to be endemic to Chiquitano dry forest at elevations of 230 to 520 m.

Flowers of this new species are known from a single collection made in June; fruiting material was collected in October.

The specific epithet refers to the small, delicate leaflets and long, slender petiolules of this species, diagnostic even in sterile specimens. The epithet also honors the Brazilian botanist Aline Costa da Mota for her important insights and collaboration on the systematics of Bombacoideae.

Although Pseudobombax pulchellum occurs in the Kaa-Iya del Gran Chaco National Park, itmust be considered Near Threatened because it is known from only four sites and “there are plausible events that may cause the species to decline, but these are unlikely to make the species Extinct or Critically Endangered in a short time” (IUCN 2010). Extensive fieldwork in other areas of dry forests in Bolivia is necessary to properly survey and to assess the status of this species.

BOLIVIA. Santa Cruz: Chiquitos, 19–23 Dec 1993 (fr), G. Navarro Sanchez 2192 (LPB!); Cordillera, 09 January 1993 (lf), G. Navarro Sanchez 1713 (MO!, USZ!); ibidem, 18°29'20"S, 61°07'06"W, 230 m, 17 June 1998 (fl), Alfredo F. Fuentes & G. Navarro Sanchez 2436 (MO!); Ñuflo de Chavez, 17°05'00"S, 61°47'00"W, 400 m, 24 October 1995 (lf), Alfredo F. Fuentes 1132 (LPB!, USZ, WIS!).

Pseudobombax pulchellum is a remarkable species by its diminutive aspect, especially the relatively small, retuse, obovate to suborbicular leaflets, and the flowers; the petiolules are markedly long in relation to the petiole, and fruits are conspicuously acuminate. It seems to be closely related to Pseudobombax longiflorum (Mart.) A. Robyns, a sympatric congener in Bolivian Chiquitano dry forest. The two are similar because of their glabrous aspect, long petiolules, leaflets with retuse apices, truncate to cordate bases, and maculate seeds.

The new species is also similar to Pseudobombax croizatii A.Robyns and Pseudobombax minimum Carv.-Sobr. & L.P. Queiroz; both of these species share small flowers (60–65 mm long), glabrous staminal tubes, androecia with relatively few stamens (c. 150–200), and glabrous fruits. The following key can be used to separate these four species of Pseudobombax.