Chile. Tarapacá, near Caña, 3900 m, Philippi s.n (lectotype, designated by Simpson 1979, pg. 46: SGO!; isolectotypes: GH, SGO!).

Trees 1–5 m tall. Leaves slightly congested at the branch tips, imparipinnate with one pair of leaflets, obtrullate in outline, 1.3–1.7 × 0.9–1.2 cm; rachises densely pannose, points of leaflet attachment with a tuft of long hairs; stipular sheaths apically truncate, densely villous on the outer surfaces; leaflets obovate in outline, second pair from the terminal leaflet the largest, one of this pair 0.7–0.8 × 0.3–0.4 cm; margin entire or very slightly crenate with 3–4 teeth, apically obtuse or acute, basally unequally attenuate; upper leaflet surfaces rugose, glabrous, usually covered with a layer of yellowish resinous exudate; lower leaflet surfaces with a dense layer of very short, yellowish pannose hairs. Inflorescences pendant, 0.7–1.5 cm long, bearing 1–2 flowers; floral bracts 3.0–3.5 mm long, narrowly triangular, densely villous on the outer surface; rachises villous. Flowers 5.1–8.0 mm diam.; sepals 4, ovate, green, densely villous outside; stamens 9–13, anthers orbicular, with a dense tuft of straight white hairs on the upper half; styles fimbriate, 2.2–2.9 mm long. Fruits turbinate, with 3–4 irregular flattened ridges with a series of spines, densely villous; 4.1–5.2 × 3.1–7.2 mm including spines. Tetraploid.

Polylepistarapacana is distributed in the volcanic western Andean cordillera from southern Peru to south-western Bolivia, northern Chile and adjacent northwest Argentina at 3400–5013 m elevation (Fig. 116). It has long been considered to be the highest-growing tree species in the Western Hemisphere, with records up to 5200 m by Jordan (1980). However, such high records have not been confirmed and the true upper elevational limit of the species appears to be at around 5000 m. Regeneration of this species is favoured by thermically sheltered and moist microhabitats (Lien et al. 2021). In Oruro (Bolivia), maximum tree height, annual shoot increment and mean tree-ring width decreased with elevation, reaching only 3.3 m of maximum height and 34 cm maximum diameter at 4820 m, where also the highest density of seedlings was found (Hoch and Körner 2005). At higher elevations, there are only scattered individuals of shrubby growth form. Due to the highly seasonal climatic conditions in its distributional range, P.tarapacana forms clearly distinct annual rings, although it is common to find discontinuous rings and reaction tissue (Domic 2005). Tree ring chronologies of up to 705 years have been reconstructed for the Bolivian Altiplano (Argollo et al. 2006) and of up to 536 years in Chile (Moya and Lara 2011). The growth of P.tarapacana is positively related with precipitation in the year prior to the year of growth ring formation (Argollo et al. 2004; Domic 2005; Argollo et al. 2006; Domic and Capriles 2009; Moya and Lara 2011). The species has greater stomatal control when aridity increases and has a decoupling of physiological processes at leaf level versus wood formation depending on their sensitivity to climate (Rodriguez-Caton et al. 2021). The species is sensitive to the soil moisture content (and rainfall), distributing the biomass in small diameters and heights, with preferably multi-stemmed individuals, in order to retain the available moisture (Saavedra 2013). Polylepistarapacana trees have various diffusive and metabolic compensatory adjustments that enable high assimilation rates sustained by a photosynthetic apparatus exceptionally well adapted to the effect of low temperatures and drought (Jaramillo 2015). Extensive gene flow has been found to occur within and between Chilean populations of P.tarapacana (Schmidt-Lebuhn et al. 2006b; Moya and Lara 2011). Still, populations that were subject to less strong climatic fluctuations in the Pleistocene show higher genetic diversity than those in more climatically variable areas (Peng et al. 2015).

The EOO for Polylepistarapacana is estimated as 127,498 km², the AOO is assessed at 448 km² and it is known from 69 locations. The species was categorized as NT in the World List of Threatened Trees (Oldfield et al. 1998). Based on its restricted distribution in these countries, P.tarapacana was categorized as Vulnerable in Chile (Benoit 1989) and Peru (SERFOR 2006). In Bolivia, where it is more widespread, P.tarapacana was categorized as VU (B1b(i,ii,iii)) (Arrázola et al. 2012). Main threats for the species are livestock grazing (camelids), grassland burning, firewood collection and farming activities. In Chile, it is also affected by mining activities which are partly compensated for by conservation measures. It is protected within Sajama National Park in Bolivia. We assess P.tarapacana as Near Threatened (A1+A2a, B1a+B2a, C1).

In southern Bolivia, it is often difficult to differentiate between P.tarapacana and P.tomentella. In fact, Simpson (1979) considered both forms as conspecific, before separating them in a later publication (Simpson 1986). The challenge in distinguishing the two species is posed by a wide transition zone between both taxa, where populations present intermediate characters and do not show typical traits of either species (Simpson 1979; Kessler 1995b). However, throughout most of their distributional ranges, both species are quite distinct, with P.tarapacana having shorter leaflets (0.7–0.8 cm vs. 1.3–2.1 cm), entire to very slightly crenate leaflet margin and obtuse to acute apices (vs. serrate margins and round to emarginate apices) and shorter inflorescences (0.7–1.5 cm vs. 2.8–5.3 cm) with 1–2 flowers (vs. 4–5 flowers).

Bolivia. La Paz: Pacajes, Laguna Blanca, 14 km hacia la carretera a Tambo Quemado, 17°39'S, 068°43'W, 4110 m, 20 November 2006, Beck 29620 (LPB); Santiago de Machaca, 27 km hacia Berenguela, 4130 m, 24 April 1982, Beck 9008 (GOET!, LPB); ciudad de Piedra, 17°31'35"S, 068°53'17"W, 3888 m, 25 January 2012, Terán 5195 (BOLV). Jardín Botánico La Paz, from seeds collected at La Paz Sajama 1991, Kessler 12628 (GOET!). Oruro: Atahuallpa, Cerro Villa Pucarani en el salar de Coipasa, 19°19'S, 068°18'W, 4670 m, 04 November 1994, Beck 21570 (GOET!). 21569 (GOET!, LPB, MO!). Sajama, Volcán Sajama, 29 April 1987, Arctander s.n (AAU!); al Norte del pueblo de Sajama, 4300 m, 31 May 1991, Beck 19897 (LPB); de Turco 3 km hacia Curahua de Carangas, 3880 m, 18 March 1992, Beck 21044 (GOET!, LPB); 4600 m, 01 August 1989, Driesch s.n (Z!); arriba del pueblo sajama, 4600 m, 01 June 1991, Hensen 2610 (LPB, MO!); localidad Mamaniri proximo a la población de Sajama, 4170 m, 02 April 1991, Huanca 69 (GOET!, LPB); próximo a la población de Sajama a los pies del Cerro Sajama lado nor-este, 4325 m, 02 April 1991, Huanca 74 (GOET!); Cerro Sajama exposición nor-oeste, 4570 m, 03 April 1991, Huanca 80 (GOET!); Cerro Sajama lado Nor-oeste, 4730 m, 03 April 1991, Huanca 81 (GOET!); Tirata, 30 km W C. de Carangas on road to Sajama, 17°52'S, 068°32'W, 4100 m, 27 July 1991, Kessler 2777 (GOET!, LPB, MO!); 2778 (AAU!, GOET!, LPB); 2 km W Tambo Colorado on road to Chile, 18°17'S, 069°02'W, 4500 m, 29 September 1991, Kessler 3284 (GOET!); 3285 (AAU!, GOET!, MO!); 3286 (AAU!, GOET!, LPB); ladera del Río Sururia, 4650–4800 m, 20 April 1979, Liberman 70 (GOET!, MO!); Nevado Sajama, ca. 240 km SSO La Paz, in gelichteten Bestanden an FuBe des Berges, 4300–4800 m, 19 July 1983, Menzel s.n (GOET!); 6 km NE Laguna, foothills of Nevado Sajama, 4200 m, 18°11'26"S, 068°51'13"W, 4200 m, 05 December 1984, Schmitt 173 (MO!, NY); around bae of Volcano Sajama, 4350 m, 18 October 1967, Vuilleumier 316 (GH!, US!); 2 km south of the town of Sajama on the road to Tambo Quemado, 05 September 1986, Zeballos s.n (MO!). Potosí: Enrique Baldivieso, Cerro Chuhuilla on Alota–Lag Hedionda road, 21°29'S, 067°50'W, 4500 m, 13 September 1991, Kessler 3073; 3074; 3075 (AAU!, GOET!); 3076 (GOET!, LPB); ca. 20 km W Alota-Lag Hedionda road, 21°23'S, 067°43'W, 4050 m, 13 September 1991, Kessler 3078 (GOET!). Quijarro, near pass on río Mulatos–Yura road, 19°43'S, 066°28'W, 4300 m, 11 September 1991, Kessler 3065 (GOET!); 3066; 3067 (GOET!, MO!); 3068 (GOET!); al Nor este en linea recta de la población Pulacayo aprox 3 km, 20°22'25"S, 066°41'11"W, 4402 m, 13 March 2010, Zenteno 9828 (LPB). Sud Chichas, 20 mi E of Atoche and 1.5 mi above Santa Barbara on the southwest face of Nevada Choroloque, 4750 m, 15 March 1993, Peterson 12947 (AAU!, GOET!). Sud Lipez, Quetana Chico 18 km hacia el Volcán Uturuncu, 22°13'28"S, 067°13'11"W, 4500 m, 26 September 2006, Beck 32470 (LPB); 32 km E lag Colorada on road to Pena Barrosa, 22°12'S, 062°28'W, 4500 m, 14 September 1991, Kessler 3083; 3084 (AAU!, GOET!, LPB); 3429 (GOET!, USM!); 3430; 3595; 3596; 3597; 3598; 3599; 3600; 3601; 3602; 3603; 3604; 3605; 3606; 3607; 3608; 3610; 3611; 3612; 3613; 3614; 3615; 3616; 3617; 3618; 3619; 3620 (GOET!); Cerro Tapaquillacha, 4600 m, 12 April 1980, Libermann 171 (GOET!, LPB); 2.5 mi S of Yuray (Nuevo) San Antonio de Lipez on road towards Quetena Grande, 4300 m, 19 March 1993, Peterson 13014 (AAU!, GOET!), 26 August 1991, Killeen 2682.

Chile. Parinacota: Putre, Tarapaca, Parinacota, Zapahuira, pres de puente Murmuntani, 18°15'S, 069°35'W, 3400 m, 05 November 1991, Billiet 5467 (MO!). Tarapacá: Parinacota, Lac Chungara, 18°15'S, 069°10'W, 4580 m, 06 November 1991, Billiet 5476 (MO!). ca. 5 km below Putre on road to Arica, 3500 m, 29 December 1995, Landrum 8885 (MO!); Philippi s.n (MO!). Peru. Puno: Puno, El Collao, Santa Rosa, 16°45'35"S, 069°53'11"W, 4131 m, 09 October 2014, Boza 3009 (USM!, Z!); Santa Rosa, 50 miles SSW of Llave, 4600 m, 25 July 1946, Olivera 16 (US!). Tacna: Tacna, Cord. Volcán Tacora, Chislluma, 4500 m, 01 April 1926, Werdermann 1143 (F!, GOET!, MO!, US!, Z!). Tarata, Laguna Casire, en borde y cercanias de la Laguna, 4700–4800 m, 03 April 1998, La Torre 2389 (MO!, US!, USM!); 2440 (MO!, USM!); Chiluyo Chico, 4270 m, 07 December 1997, Roque 503 (USM!); Weddell s.n (F!).