Resembles the Ecuadorian species, P.reticulata Hieron. by having 3–5 lateral leaflet pairs and similar type and density of hairs, but differs by its shorter hairs (0.3–0.6 mm vs. 0.6–1.5 mm), shorter inflorescences (2.4–6.7 cm vs. 2.3–13.8 cm) and shorter styles (1.5–2.0 mm vs. 2.6–3.9 mm).

Peru: Piura: Huancabamba, Talanco, 2900 m, 5 Nov 1976, A. Sagastegui A.8635 (holotype: MO!; isotype: UMO!).

Trees 3–15 m tall. Leaves slightly congested at the branch tips, imparipinnate with 3–5 pairs of lateral leaflets, obtrullate in outline, 3.1–4.7 × 1.8–2.9 cm; rachises densely tomentose, points of leaflet attachment with a tuft of long, lanate hairs; stipular sheaths apically acute with spurs, densely lanate on the outer surfaces; leaflets elliptic in outline, second pair from the terminal leaflet the largest, one of this pair 1.1–1.9 × 0.5–0.8 cm; margin entire or slightly crenate at the apex with 3–6 teeth, apically emarginate, basally unequally cordate; upper leaflet surfaces glabrous; lower leaflet surfaces densely tomentose with whitish hairs 0.3–0.6 mm long. Inflorescences pendant, 2.4–6.7 cm long, bearing 4–12 flowers; floral bracts 6.8–7.6 mm long, narrowly triangular, densely lanate on the outer surface; rachises tomentose. Flowers 6.8–7.6 mm diam.; sepals 4, ovate, green, densely tomentose outside; stamens 9–15, anthers orbicular, with a dense tuft of straight white hairs on the upper half; styles fimbriate, 1.5–2.0 mm long. Fruits turbinate, with variable numbers and placement of flattened spines, densely villous; 3.8–4.1 × 5.8–7.1 mm including spines. Diploid.

Polylepisoccidentalis is distributed in the high mountains of western Peru from Huancabamba (Piura) to Pataz (La Libertad) (Fig. 61). The species occurs in relatively dry forest at 2200–3990 m elevation.

The species epithet “occidentalis” (Latin: western) refers to the distribution range occupying the western mountains in Peru.

The EOO for Polylepisoccidentalis is estimated as 12,906 km² and the AOO at 52 km². It is known from 11 locations. It has been found in monospecific forests at the southern margins of the Huancabamba Andean Depression, which forms an important dispersal corridor for Andean tree species (Richter et al. 2009; Peters et al. 2014). However, collections from Cajamarca, La Libertad and Lambayeque are mostly old and the collection areas have undergone heavy deforestation. Based on the reduction of its restricted distribution and its habitat degradation, we assess P.occidentalis as Endangered (A1, B1a+B2a, C1).

The populations of PolylepissectionReticulatae from western Peru have been previously identified either as P.reticulata (Lassermann 2009) or P.weberbaueri (Simpson 1979; Lassermann 2009; Mendoza and Cano 2012; Peters et al. 2014) (Fig. 34). Certainly, P.occidentalis resembles P.reticulata in having 3–5 lateral leaflet pairs and same type and density of hairs. However, it has shorter lower leaflet surface hairs (0.3–0.6 mm long), shorter inflorescences (2.4–6.7 cm long) and shorter styles (1.5–2.0 mm long), whereas P.reticulata has lower surface hairs 0.6–1.5 mm long, inflorescences (2.1–)4.3–13.8 cm long and styles 2.6–3.9 mm long. Additionally, P.occidentalis occurs in western Peru, whereas P.reticulata is distributed in Ecuador. Polylepisoccidentalis is morphologically also similar to P.weberbaueri, with which it shares similar leaflet size and lower leaflet surface hair type and density. The most obvious differences between P.occidentalis and P.weberbaueri are leaflet pair number, inflorescence length, style length and number of stamens, with P.occidentalis having 3–5 leaflet pairs, inflorescences 2.4–6.7 cm long, styles 1.5–2.0 mm long and 9–15 stamens, whereas P.weberbaueri has 2–3 leaflet pairs, inflorescences 8.2–9.7 cm long, styles 2.7–3.2 mm long and 19–21 stamens.

Simpson (1979) considered P.reticulata and P.weberbaueri to occur in both Ecuador and Peru, with the populations of each species separated by the low elevation Huancabamba depression. Interestingly, however, defined in this way in Ecuador, P. “reticulata” occurs at higher elevations than P. “weberbaueri”, whereas in Peru, the reverse is the case. In addition to the morphological traits outlined above, we consider this intriguing distributional pattern to support treatment as four species, with the original P.reticulata restricted to Ecuador and P.weberbaueri to Peru and the respective other populations here described as P.occidentalis (Peruvian P. “reticulata”) and P.simpsoniae (Ecuadorean P. “weberbaueri”).

Peru. Cajamarca: Cajamarca, Dist. La Encañada, San Pedro de Lipiac, 07°03'38"S, 078°20'00"W, 3738 m, 07 September 2010, Beltrán 6933 (USM!). Chota, Llama; caserío Callampampa; afluente de Huarimarca, 2800–3000 m, 19 January 1990, Díaz 4012 (MO!, USM!); Laguna Yahuarcocha (arriba de Incahuasi), 07°34'45"S, 077°58'29"W, 3600 m, 14 September 1985, Sagástegui 12896 (GOET!, IBE, MO!); Tucupampa (Llama-Huambos), 2500 m, 17 March 1997, Sagástegui 15950 (F!, MO!); Miracosta, entre Miracosta y Pampa del Lirio, 06°23'S, 079°15'W, 3250 m, 11 November 2000, Sánchez 10285 (MO!); Huambos, 2000 m, 11 September 1956, Soukup 4457 (F!). Cutervo, carretera entre Llama y Huambos, Tunaspampa, 06°31'52"S, 079°05'26"W, 2600–2900 m, 21 April 1988, Díaz 2879 (F!, MO!). Huambos, 65 km E of bridge over Río Maichil, 25 km W of Huambos, 06°25'S, 079°00'W, 2700 m, 09 February 1988, Gentry 61439 (MO!, USM!). Llama, El Pargo, 4.2 km E of Llama, ca. 1.4 km SE of Tunas Pampas, 06°30'S, 079°31'W, 3000 m, 08 September 1991, Gentry 74575 (MO!, USM!). La Libertad: Pataz, Chirimachay, 07°00'S, 077°00'W, 3450 m, 24 February 1986, Young 2966 (MO!). Lambayeque: Ferrenafe, Ca. 7 km NW of Incahuasi; near Cerro Punamachay on trail to Laguna Hualtaco, 3300–3550 m, 16 November 1984, Dillon 4145 (MO!, USM!). Piura: Huancabamba, 38 km above Canchaque, just below summit, 05°18'08"S, 079°29'54"W, 3120 m, 13 September 1964, Hutchison 6578 (A!, MO!, USM!); Talaneo (Jalca - Cixse), 2900 m, 05 September 1976, Sagástegui 8635 (MO!, UMO). Unknown minor area, La Cruz, 04 June 1961, Acleto 364 (GOET!, USM!); Paso de la Cruz Blanca, 3307–3154 m, 13 November 2007, Lasermann I5-8 (USM!); Paso de la Cruz Blanca, 3307 m, 13 November 2007, Lasermann V/3 (USM!).