This species differs from Polylepisflavipila (Bitter) M.Kessler & Schmid-Leb. (2006) in having longer leaflets with dense long hairs on the lower surfaces and crenate margins with more teeth per side.

Peru. Lima: Huarochiri, Carapoma, Bosque de Japani, 11°38'11"S, 076°27'10"W, 3859 m, 11 Nov 2014, T.E. Boza E. & E. Urquiaga 3023 (holotype: USM!; isotype: Z!).

Trees 4–8 m tall. Leaves slightly congested at the branch tips, imparipinnate with one pair of leaflets, obtrullate in outline, (1.9–)2.8–3.6 × 1.9–3.7 cm; rachises densely villous, points of leaflet attachment with a tuft of long hairs; stipular sheaths not spurred, densely villous on the outer surfaces; leaflets obovate in outline, second pair from the terminal leaflet the largest, one of this pair (1.6–)1.9–2.7 × 0.5–0.9 cm; margin crenate with 7–14 teeth, apically acute or emarginate, basally cuneate; upper leaflet surfaces densely pilose; lower leaflet surfaces densely pilose with yellowish hairs 1.0–1.2 mm long. Inflorescences pendant, (1.5–)2.1–5.2 cm long, bearing 3–5 flowers; floral bracts 2.5–3.9 mm long, narrowly triangular, densely villous on the outer surface; rachises villous. Flowers 7.3–8.5 mm diam.; sepals 3–4, ovate, green to reddish, densely villous outside; stamens 11–17, anthers orbicular, with a dense tuft of straight white hairs on the upper half; styles fimbriate, 2.5–3.9 mm long. Fruits turbinate, with 3–4 irregular flattened ridges with a series of spines, densely pilose; 5.4–6.4(–8.8) × 1.9–3.1(–5.1) mm including spines. Tetraploid.

Polylepispilosissima is restricted to the Department of Lima, Peru (Fig. 73). The species occurs in dry and relatively cold areas at 3500–4400 m elevation. It grows in mixed forests with Gynoxyisnitida (Rivera Paucar 2018). The largest forest of P.pilosissima is “Japani forest”, located in Carampoma (Huarochiri, Lima) with 665 ha (Ñingle and Florencio 2013; Rivera Paucar 2018). It hosts a high diversity of birds (74 spp.), including endemic and threatened species, such as Oreotrochilusmelanogaster, Ochthoecaoenanthoides and Conirostrumbinghami (= Oreomanesfraseri) (Sembrero and Valencia 2016; Rivera Paucar 2018). Japani forest stores about 47,000 t of biomass and 24,500 t of carbon in the above-ground woody part (Rivera Paucar 2018).

The species epithet “pilosissima” refers to the characteristic dense, long pilose hairs of the species.

The EOO for P.pilosissima is estimated as 5,129 km², the AOO is assessed at 28 km² and it is known from only five locations. The largest stand of this species is protected within the “Japani” Private Conservation Area. Based on its fragmented and restricted distribution, we assess P.pilosissima as Critically Endangered (CR A2a, B2a).

The populations of Polylepis in Huarochiri Province (Lima) have previously been identified as P.flavipila (Kessler and Schmidt-Lebuhn 2006; Mendoza and Cano 2012). Indeed, P.pilosissima resembles P.flavipila in having one lateral leaflet pair, obovate leaflets with crenate margin with pilose hairs and short inflorescences with few flowers. However, it has leaflets (1.6–)1.9–2.7 cm long, crenate leaflet margins with 7–14 teeth per side, upper leaflet surfaces densely pilose and lower leaflet surface hairs 1.0–1.2 mm long, whereas P.flavipila has leaflets (1.2–)1.6–2.0 cm long, crenate margins with 4–6 teeth per side, upper surface sparsely pilose and lower leaflet surface hairs 0.5–0.6 mm long. Additionally, P.pilosissima is morphologically similar to P.subsericans, with which it shares lower leaflet surface hair density and length. However, P.pilosissima has obovate leaflets with crenate margins, pilose hairs which are dense on both leaflet surfaces and 11–17 flowers per inflorescence, whereas P.subsericans has narrowly elliptic leaflets with entire to slightly serrate apex margins, strigose hairs which are sparse on the upper leaflet surfaces and dense on the lower ones and 9–13 flowers per inflorescence.

Peru. Lima: Huarochiri, Quebrada Yanac in Valle Sta. Eulalia, 11°35'S, 076°27'W, 4000 m, 27 January 1987, Boertmann 9 (AAU!); Carapoma, Bosque de Japani, 11°38'11"S, 076°27'10"W, 3859 m, 11 November 2014, Boza 3023 (USM!, Z!); 10 km NE of Suchi, ca. 61 road km NE of Chosica on road to Huanza, 11°41'24"S, 076°34'48"W, 3900–4000 m, 06 May 1978, Gentry 21638 (MO!, USM!); huacamachay (Alto río Sta. Eulalia), 4000 m, 09 October 1987, Hocking s.n (USM!); Comunidad Campesina de Llacuas, 06 November 1995, Ignacio s.n (USM!); Sta. Eulalia Valley, ca. 15 km NE Huansa, 11°37'S, 076°26'W, 3800 m, 06 September 1991, Kessler 3063 (GOET!); 3064 (GOET!); 3426 (GOET!, LPB, MO!); 3427 (AAU!, GOET!, LPB, MO!); 3428 (AAU!); 3588 (GOET!); 3589 (GOET!); 3590 (GOET!); 3591 (AAU!, GOET!, LPB, MO!); 3593 (GOET!); 3653 (GOET!); arriba de Santa Eulalia, 01 March 1966, Koepcke s.n (USM!). Yauyos, entre Pallaca y Huacracocha a 14 Km. de Tupe, 4000 m, 22 January 1952, Cerrate 1265 (GOET!, MO!, USM!).