Potentilla sect. Niveae
in J. M. Coulter and A. Nelson, New Man. Bot. Rocky Mt., 255. 1909.
Perennials, ± tufted or cushion-forming, not stoloniferous; taproots not fleshy-thickened; vestiture of long, short, crisped, and/or cottony hairs, glands absent or sparse to common, sometimes red. Stems ascending to erect, not flagelliform, not rooting at nodes, lateral to persistent basal rosettes, 0.2–2.5 (–4.5) dm, lengths 1.5–5 times basal leaves. Leaves: basal rarely 2-ranked; cauline 0–2 (–3); primary leaves usually ternate, rarely palmate on same plant, (0.5–) 1–12 (–20) cm; petiole: long hairs sometimes absent, ± appressed to spreading, soft to stiff, glands absent or sparse to common; leaflets 3 (–5), at tip of leaf axis, separate to overlapping, usually obovate, sometimes elliptic, suborbiculate, or obtriangular, margins slightly to strongly revolute, rarely flat, distal (1/3–) 1/2 to nearly whole length evenly incised 1/4–3/4 to midvein, teeth (1–) 2–6 (–12) per side, surfaces ± to strongly dissimilar, abaxial gray to white, sometimes yellowish white or reddish, cottony-crisped hairs usually dense, glands absent or obscured, adaxial dark green to grayish white or yellowish white, sometimes reddish, not glaucous, long hairs soft to stiff. Inflorescences 1–7 (–15) -flowered, cymose, ± to very open, or solitary flowers. Pedicels straight in fruit, 0.4–6 (–10) cm, proximal often longer than distal. Flowers 5-merous; hypanthium (1.8–) 2.5–7 mm diam.; petals pale-yellow to yellow, ± obcordate, (3–) 4–10 (–15) mm, longer than sepals, apex retuse; stamens 15–30; styles subapical, narrowly columnar, tapered, or conic, usually papillate-swollen in proximal 1/5 or less, sometimes to proximal 1/3 (–1/2) or not at all, 0.7–1.2 (–1.5) mm. Achenes smooth or slightly rugose.
North America, Eurasia
Species ca. 32 (11, including 1 hybrid, in the flora).
Species of sect. Niveae are primarily trifoliate, with occasional supernumerary leaflets developing on otherwise trifoliate plants (B. Eriksen and J. Nyléhn 1999). Abaxial leaflet surfaces are covered with dense crisped and/or cottony hairs. This is perhaps the most taxonomically difficult section of panarctic Potentilla; the current treatment, which leans heavily on revisions of Russian and Beringian Potentilla by B. A. Jurtzev (1984, 1993, 2001) and J. Soják (1989, 2004), is decidedly provisional. Eastern North American and western montane variation in particular remains poorly known, with more taxa likely to be recognized pending further study. The disposition of some infraspecific names is currently unresolved, notably P. nivea var. macrophylla Seringe, P. nivea var. tomentosa Nilsson-Ehle ex Hultén, and P. chamissonis Hultén var. umanakensis Hultén.
North American species of sect. Niveae have traditionally been treated as Potentilla nivea in the broad sense (including P. hookeriana Lehmann, misapplied), P. uniflora Ledebour (or its illegitimate synonym P. ledebouriana A. E. Porsild), P. vahliana, and P. villosa. The finer taxonomy adopted here is discussed in greater detail elsewhere (R. Elven and D. F. Murray 2008, 2008b; Elven et al., nhm2.uio.no/paf). The species treated here mostly fall into three categories: the P. nivea group (P. arenosa, P. crebridens, P. holmgrenii, P. nivea), the P. uniflora/villosa group (P. subvahliana, P. villosa, P. villosula, P. vulcanicola), and species of presumed intrasectional hybrid origin (P. subgorodkovii, P. vahliana). The North American species comprising the P. nivea group are distinguished by relatively narrow epicalyx bractlets, multi-flowered inflorescences, leaflets with relatively numerous teeth, and either cottony or verrucose petiole hairs. The American taxa of the P. uniflora/villosa group are characterized by relatively broad epicalyx bractlets (often notched or split to base) and smooth, silky, straight hairs on petioles.
Most North American plants previously included in Potentilla uniflora are treated here as P. subgorodkovii or P. vulcanicola. The presence of P. uniflora in the narrow sense in North America is unconfirmed, with only one specimen from northwestern Alaska (Ogotoruk Creek, ALA) conforming in diagnostic features. This is about 400 km from the easternmost confirmed Asian occurrence of P. uniflora, which is widespread in northeastern Asia from Taimyr Peninsula eastward to the isthmus of the Chukchi Peninsula (B. A. Jurtzev 1984).
Also treated here is Potentilla tikhomirovii, a presumed hybrid species derived from sections Aureae and Niveae. For species assumed to have originated from crosses between species of sect. Niveae and sect. Pensylvanicae, see 8t. sect. Rubricaules. Hybrids that have not yet stabilized as recognizable taxa are not included in the key, even though putative hybrids are more frequent than the presumed parents in some regions, forming an unresolvable complex that also influences the parental species through back-crosses.
Plants interpreted as hybrids between Potentilla arenosa in the broad sense and P. nivea are particularly common throughout the sympatric ranges of these species. Among the names that have been applied to the resultant hybrids are P. ×drymeja Soják (type from Mongolia), P. ×mischkinii Juzepczuk (type from European Russia), P. ×prostrata Rottbøll (type from Greenland), P. ×subquinata (Lange) Rydberg (= P. nivea var. subquinata Lange; lectotype [J. Soják 1986] from Greenland), and P. ×tomentulosa Jurtzev (type from Siberia); P. ×prostrata is the priority name.
Hair types that are essential in defining species of sect. Niveae are summarized by B. Eriksen and B. A. Jurtzev (1999), modified for the current treatment as discussed in the introduction to the genus. Verrucose or finely warty hairs are most evident at high magnification (for example, 50\x) but can usually be recognized by their dull, frosted appearance, in contrast to the smoother glassy appearance of non-verrucose hairs. Red-tipped glands, characteristic of Potentilla tikhomirovii, should not be confused with red tubercles at the base of some hairs.
|1||Epicalyx bractlets usually 1/2 or less as wide as sepals, margins usually flat; petioles: vestiture either primarily of cottony hairs or of ± stiff verrucose hairs; inflorescences usually more than 1-flowered; central leaflets: distal (1/2–)3/4 to nearly whole length incised less than 1/2 to midvein, teeth (2–)3–8(–12) per side||> 2|
|1||Epicalyx bractlets (1/2–)2/3 to ± as wide as sepals, margins often revolute; petioles: vestiture primarily of soft to weak smooth hairs (or stiff verrucose hairs in P. tikhomirovii); inflorescences often only 1-flowered; central leaflets: distal (1/3–)1/2–2/3(–3/4) incised (1/3–)1/2–3/4 to midvein (1/4–1/2 to midvein in P. villosa), teeth (1–)2–3(–4) per side (3–7 in 77 P. villosa).||> 5|
|2||Petioles: long hairs sparse to abundant, usually stiff, spreading to ± ascending, verrucose, cottony hairs absent; central leaflets usually petiolulate, (petiolules to 5 mm).||Potentilla arenosa|
|2||Petioles: long hairs usually absent, sometimes sparse to common, usually soft, usually ± appressed, smooth, cottony hairs usually abundant to dense; central leaflets subsessile or short-petiolulate||> 3|
|3||Caudex branches columnar, at least partly sheathed with marcescent whole leaves; central leaflets 0.7–1.1 × 0.5–0.6 cm, teeth 2–4 per side, adaxial surfaces grayish green to grayish white, hairs abundant to dense; styles 1.2–1.4 mm, not or ± papillate-swollen in proximal 1/5 or less; mountains of ec Nevada and adjacent Utah.||Potentilla holmgrenii|
|3||Caudex branches not columnar, not sheathed with marcescent whole leaves; central leaflets 0.5–2(–4) × (0.2–)0.4–1.2(–2) cm, teeth (2–)3–8(–12) per side, adaxial surfaces green to grayish green, hairs sparse to abundant; styles 0.7–1.2 mm, not or ± to strongly papillate-swollen in less than proximal 1/5, rarely in proximal 1/5–1/3; Alaska to Greenland, south to Arizona and New Mexico||> 4|
|4||Epicalyx bractlets linear, 0.2–0.6 mm wide; styles not or ± papillate-swollen in less than proximal 1/5; central leaflets: teeth (3–)5–8(–12) per side, adaxial surfaces green to grayish green; Alaska, Yukon.||Potentilla crebridens|
|4||Epicalyx bractlets narrowly to broadly lanceolate or elliptic, 0.6–1.7 mm wide; styles strongly papillate-swollen at very base, rarely in proximal 1/5–1/3; central leaflets: teeth (2–)3–5(–6) per side, adaxial surfaces usually green, sometimes grayish green; Alaska to Greenland, south to Arizona, New Mexico.||Potentilla nivea|
|5||Epicalyx bractlets: red glands usually common; petioles: long hairs verrucose; central leaflet abaxial surfaces: cottony-crisped hairs common to dense.||Potentilla tikhomirovii|
|5||Epicalyx bractlets: red glands absent; petioles: long hairs smooth; central leaflet abaxial surfaces: cottony-crisped hairs ± dense||> 6|
|6||Carpels: apical hairs sparse to abundant (straight)||> 7|
|6||Carpels: apical hairs usually absent, rarely present (cottony)||> 8|
|7||Central leaflet surfaces ± dissimilar, adaxial grayish green, hairs abundant to dense; inflorescences (1–)2–3(–4)-flowered; Alaska and w Yukon, s in Canadian Rockies.||Potentilla villosula|
|7||Central leaflet surfaces strongly dissimilar, adaxial usually dark green, sometimes grayish green, hairs sparse to abundant; inflorescences 1–2(–5)-flowered; Alaska to Nunavut.||Potentilla vulcanicola|
|8||Inflorescences (1–)2–7(–10)-flowered; plants ± to densely tufted; caudex branches sometimes ± columnar, not sheathed with marcescent whole leaves; central leaflet teeth (2–)3–6(–7) per side, ± approximate to ± distant; leaflets usually ± overlapping, bases mostly broadly cuneate to rounded, adaxial surfaces grayish green, hairs abundant to dense||> 9|
|8||Inflorescences usually 1–2-flowered, rarely to 5-flowered; plants ± densely tufted to cushion-forming; caudex branches usually columnar, often sheathed with marcescent whole leaves; central leaflet teeth (1–)2–3(–4) per side, usually ± distant; leaflets separate to slightly overlapping, bases cuneate, adaxial surfaces dark green to greenish or yellowish gray, hairs sparse to ± abundant||> 10|
|9||Inflorescences (1–)2–7(–10)-flowered; central leaflets incised 1/4–1/2 to midvein, teeth 3–6(–7) per side; filaments 1.8–2.1 mm; rocky habitats, coast and coastal mountains from Alaska to Oregon.||Potentilla villosa|
|9||Inflorescences (1–)2–3(–4)-flowered; central leaflets incised ± 1/2 to midvein, teeth 2–3(–4) per side; filaments 1.1–1.4 mm; various habitats, coast and inland from Alaska to w Yukon, s in Canadian Rockies.||Potentilla villosula|
|10||Petioles: crisped/short-cottony hairs usually absent, sometimes sparse, long hairs ± weak, rarely stiff; plants usually cushion-forming; basal leaves 0.5–2.5(–3) cm; arctic.||Potentilla subvahliana|
|10||Petioles: crisped/short-cottony hairs usually sparse to abundant, long hairs soft to weak; plants ± densely tufted to cushion-forming; basal leaves 1–10(–15) cm; arctic, subarctic, and alpine||> 11|
|11||Epicalyx bractlets elliptic-lanceolate to ovate, (3–)4–6(–7) × (0.8–)1.2–2(–2.5) mm, (1/2–)2/3 to as wide as sepals; sepals 4–6(–7) mm; petals (5–)6–9 mm, lengths 1.5–1.8 times sepals; Alaska to Nunavut, s to British Columbia, Utah, and Colorado.||Potentilla subgorodkovii|
|11||Epicalyx bractlets broadly ovate, 2.5–4(–5) × 1.5–3 mm, ± as wide as sepals; sepals 2.5–5(–6) mm; petals 8–10 mm, lengths 1.8–2.1 times sepals; ne Canada and Greenland.||Potentilla vahliana|
"dm" is not declared as a valid unit of measurement for this property.