The genus Salix is a large, taxonomically complex, genus of about 450 species worldwide; estimates range from 330-350 to 500. Salix occurs mainly in the Northern Hemisphere. Its center of abundance is in China, where there are about 270 species, and in the former Soviet Union, where there are about 120 species. There are 103 species in North America and 65 species in Europe. The genus also occurs in Japan, Africa, the Middle East, India, and Central and South America. In Australasia and Oceania it occurs only as introductions, some of which are regarded to be weedy.

In a revised classification of New World species of Salix (Argus, G.W., 1997, Systematic Botany Monographs No. 52), based on a numerical taxonomic study, ninety morphological characters were recorded for each of 103 species. A phenetic distance matrix was generated using the Gower metric. The data were analyzed using several clustering methods; the most consistent results were achieved using ISS FLEX. The New World species were grouped into four subgenera. Salix subg. Salix is represented by four sections, S. subg. Longifoliae by one section, S. subg. Chamaetia by eight sections, and S. subg. Vetrix by 15 sections. The species composition of each of the sections was compared with previous taxonomic treatments and problematic taxa were discussed.

The major problem in the classification of Salix into a linear arrangement of subgenera and sections is the presence of polyploidy as well as hybridization and introgression. Many Salix are polyploid, with ploidal levels of 4x-12x. The hypothesis was presented (Argus 1997) that intersectional and intersubgeneric polyploidy is responsible for difficulties in placing some species into sections and sections into subgenera. An indirect method used to identify the possible ancestry of some allopolyploids was to compare species with their nearest neighbors, based on their Gower dissimilarity value. If the nearest neighbors belonged to a different section it was hypothesized that the alloploid may be of intersectional hybrid origin. For example, Salix maccalliana (dodecaploid) has as its nearest neighbors members of S. sect. Salicaster (subg. Salix) and S. sect. Hastatae (subg. Vetrix) and Salix glauca (tetraploid to hexaploid) has nearest neighbors in S. sect. Glaucae, S. sect. Myrtilloides, and S. sect. Hastatae (the first two are in subg. Salix, the third in subg. Vetrix). In the cluster analysis, using different sets of taxa and characters, these species, and other high polyploids, often moved between subgenera. Because the evolutionary pattern of many Salix is reticulate the placement of intersectional allopolyploids in one section or another is arbitrary.

The following classification of New World Salix arranges species into subgenera and sections. Some of the nomenclature in sections Subalbae, Albae, Glaucae, Cordatae Phylicifoliae and Geyerianae has been modified from Argus (1997). The names and citation of introduced and naturalized taxa are marked with asterisk (*) and appear in colored Arial font.

Salix floridana Chapman, Fl. South. U.S. 430. 1860.

Salix amygdaloides Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 114. 1858; Salix bonplandiana Kunth in H.B.K., Nov. Gen. Sp. 2: 24. 1817; Salix caroliniana Michx., Fl. bor.-amer. 2: 226. 1803; Salix gooddingii C. R. Ball, Bot. Gaz. 11: 376. 1905; Salix humboldtiana Willd., Sp. Pl. 4: 657. 1805; Salix laevigata Bebb, Amer. Natu. 8: 202. 1874; Salix nigra Marshall, Arbust. amer. 139. 1785.

Salix sect. *Subalbae Koidz.

Salix lucida Muhl., Ges. Naturf. Freunde Berlin II. 4: 239. 1803; Salix lucida subsp. caudata (Nutt.) E. Murray, Kalmia 15: 11. 1984 "1985" (Basionym: Salix pentandra L. [var.] caudata Nutt., North Am. Sylva 1: 61. 1842); Salix lucida subsp. lasiandra (Benth.) E. Murray, Kalmia 15: 11. 1984 "1985" (Basionym: Salix lasiandra Benth. Pl. Hartweg. 335. 1857); Salix lucida subsp. lucida; *Salix pentandra L., Sp. pl. 2: 1016. 1753; Salix serissima (L. H. Bailey) Fernald, Rhodora 6: 6. 1904 (Basionym: Salix lucida Muhl. var. serissima L. H. Bailey, Geol. & Nat. Hist. Surv. Minn. Bull. 3: 19. 1887).

Salix maccalliana Rowlee, Bull. Torr. Bot. Club 34: 158. 1907.

Salix exigua Nutt. N. Amer. Sylv. 1: 75. 1842; Salix interior Rowlee, Bull. Torrey Bot. Club 27: 253. 1900; Salix fluviatilis Nutt. N. Amer. Sylva 1: 89. 1842; Salix melanopsis Nutt. N. Amer. Sylva 1: 78. 1842; Salix microphylla Schldl. & Cham. Linnaea 6: 354. 1831; Salix sessilifolia Nutt. N. Amer. Sylva 1: 68. 1842; Salix taxifolia Kunth in H.B.K. Nov. gen. sp. 2: 18. 1817. The nomenclature proposed for this section by Dorn (Brittonia 50: 193-210. 1998) has not yet been evaluated.

Salix nivalis Hook. Fl. bor-amer. 2: 152. 1838; Salix reticulata L., Sp. pl. 2: 1018. 1753; Salix reticulata subsp. glabellicarpa Argus, Canad. J. Bot. 43: 1021. 1965; Salix reticulata subsp. reticulata; Salix vestita Pursh, Fl. Amer. Sept. 2: 610. 1814.

Salix setchelliana C. R. Ball, Univ. Calif. Publ. Bot. 17: 410. 1934.

Salix herbacea L., Sp. pl. 2: 1018. 1753; Salix nummularia Andersson in DC. Prodr. 16(2): 298. 1868.

Salix arctophila Cockerell ex A. Heller, Cat. N. Amer. Pl., ed. 3, 89. 1910; Salix chamissonis Andersson in DC. Prodr. 16(2): 290. 1868; Salix fuscescens Andersson, Monogr. Salicum 97. 1867; Salix phlebophylla Andersson in DC., Prodr. 16(2): 290.1868; Salix polaris Wahlenb. Fl. Lap. 261. 1812; Salix rotundifolia Trautv. Nouv. Mem. Soc. Nat. Mosc. 2: 304. 1832; Salix rotundifolia subsp. dodgeana (Rydb.) Argus, Canad. J. Bot. 47: 795. 1969 (Basionym: Salix dodgeana Rydb. Bull. N. Y. Bot. Gard. 1: 277. 1899); Salix rotundifolia subsp. rotundifolia; Salix uva-ursi Pursh, Fl. Amer. Sept. 2: 610. 1814 .

Salix jejuna Fernald Rhodora 28: 177. 1926; Salix ovalifolia Trautv. Nouv. Mem. Soc. Mosc. 2: 306. 1832; Salix ovalifolia var. arctolitoralis (Hultén) Argus, Canad. J. Bot. 47: 795. 1969 (Basionym: Salix arctolitoralis Hultén, Sv. Bot. Tidskr. 34: 373. 1940); Salix ovalifolia var. cyclophylla (Rydb.) C. R. Ball, Proc. Nat. Acad. Sci. 21: 184. 1935 (Basionym: Salix cyclophylla Rydb. Bull. N. Y. Bot. Gard. 1: 274. 1899); Salix ovalifolia var. glacialis (Andersson) Argus, Canad. J. Bot. 47: 798. 1969 (Basionym: Salix glacialis Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 131. 1858); Salix ovalifolia var. ovalifolia; Salix stolonifera Coville, Proc. Wash. Acad. Sci. 3: 333. 1901.

Salix arctica Pall., Fl. Ross. 1: 86. 1788; Salix cascadensis Cockerell, Muhlenbergia 3: 9. 1907; Salix petrophila Rydb. Bull. N. Y. Bot. Gard. 1: 268. 1899; Salix sphenophylla A. K. Skvortsov in Tomatchev, Fl. Arct. URSS 5: 62. 1966.

Salix athabascensis Raup, Rhodora 32: 111. 1930; Salix chlorolepis Fernald Rhodora 7: 186. 1905; Salix pedicellaris Pursh, Fl. Am. Sept. 2: 611. 1814; Salix raupii Argus, Canad. J. Bot. 52: 1303. 1974.

Salix brachycarpa Nutt. N. Am. Sylva 1: 69. 1842; Salix brachycarpa var. brachycarpa; Salix brachycarpa var. psammophila Raup, J. Arnold Arb.17: 230. 1936; Salix glauca L., Sp. Pl. 2: 1019. 1753; Salix glauca subsp. acutifolia (Hook.) Hultén, Ark. f. Bot. 7: 40. 1967 (Basionym: Salix villosa Andersson b [var.] acutifolia Hook. Fl. bor.-am. 2: 145. 1839); Salix glauca subsp. callicarpaea (Trautv.) Böcher, Meddel. Groenl. 147: 19. 1952 (Basionym: Salix callicarpaea Trautv. Nouv. Mém. Soc. Nat. Mosc. 2: 295. 1832); Salix glauca subsp. stipularis (Flod. ex Häyrén) Hiitonen, Suomen kasvio, 272. 1933 (Basionym: Salix glauca var. glauca auct.; Salix stipulifera Flod. ex Häyrén, Mem. Soc. Faun. Fl. Fenn. 5: 133. 1929); Salix glauca var. villosa Andersson, Proc. Amer. Acad. Arts Sci. 4: 68. 1858; Salix niphoclada Rydb. Bull. N. Y. Bot. Gard. 1: 272. 1899.

Salix arizonica Dorn, Canad. J. Bot. 53: 1499. 1975; Salix ballii Dorn, Can J. Bot. 53: 1501. 1975; Salix barclayi Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 125. 1858; Salix boothii Dorn, Canad. J. Bot. 53: 1505. 1975; Salix commutata Bebb, Bot. Gaz. 13: 110. 1888; Salix cordata Michx. Fl. bor.-amer. 2: 225. 1803; Salix eastwoodiae Cockerell ex A. Heller, Cat. N. Am. Pl. ed. 3 89. 1910; Salix farriae C. R. Ball in Standley, Contr. U. S. Natl. Herb. 22: 321. 1921; Salix hastata L., Sp. pl. 2: 1017. 1753; Salix monticola Bebb in Coulter, Man. Bot. Rocky Mt. Reg. 336. 1885; Salix myricoides Muhl. Ges. Naturf. Fr. Neue Schr. 4: 235. 1803; Salix myricoides var. albovestita (C. R. Ball) Dorn, Canad. J. Bot. 54: 2782. 1976 (Basionym: Salix glaucophylla Bebb var. albovestita C. R. Ball, Jour. Wash. Acad. Sci. 29: 429. 1939); . Salix myricoides var. myricoides; Salix myrtillifolia Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 132. 1858; Salix orestera C. K. Schneider, J. Arnold. Arb. 1: 164. 1920; Salix pseudomonticola C. R. Ball in Standley, Contr. U. S. Natl. Herb. 22: 321. 1921; Salix pseudomyrsinites Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 130. 1858; Salix pyrifolia Andersson, Monogr. Salicum 162. 1867; Salix wolfii Bebb in Rothr. Rep. Bot. U. S. Geogr. Surv. 6: 214. 1878.

Salix eriocephala Michx. Fl. bor.-amer. 2: 225. 1803; Salix eriocephala var. eriocephala; Salix eriocephala var. famelica (C.R. Ball) Dorn, Brittonia 47: 165. 1995 (Basionym: S. lutea var. famelica C.R. Ball, Bot. Gaz. 71: 426. 1921); Salix ligulifolia (C. R. Ball) C. R. Ball in E. D. Smith, Amer. Midl. Nat. 27: 236. 1942 (Basionym: Salix lutea Nutt. var. ligulifolia C. R. Ball, Bot. Gaz. 71: 428. 1921); Salix lutea Nutt. N. Amer. Sylva 1: 63. 1842; Salix monochroma C. R. Ball, Bot. Gaz. 71: 431. 1921; Salix prolixa Andersson, Monogr. Salicum 94. 1867; Salix turnorii Raup, J. Arnold Arb. 17: 234. 1936.

*Salix atrocinerea Brotero, Fl. Lusit. 1: 31. 1804; *Salix aurita L., Sp. pl. 2: 1019. 1753; *Salix caprea L., Sp. pl. 2: 1020. 1753; *Salix cinerea L., Sp. pl. 2: 1021. 1753; Salix discolor Muhl. Ges. Naturf. Freunde Berlin II. 4: 234. 1803; Salix hookeriana Barratt ex Hook. Fl. bor.-amer. 2: 145. 1838; Salix humilis Marshall, Arbust. amer. 140. 1785; Salix humilis var. humilis; Salix humilis var. tristis (Aiton) Griggs, Proc. Ohio Acad. Sci. 4: 301. 1905; Salix paradoxa Kunth in Humboldt and Bonpland, Nov. Gen. Pl. 2: 20. 1817; Salix scouleriana Barratt ex Hook. Fl. Bor.-amer. 2: 145. 1838.

Salix bebbiana Sarg., Gard. & For. 8: 463.1895.

Salix drummondiana Barratt ex Hook. Fl. bor.-am. 2: 144. 1838; Salix pedunculata Fernald, Rhodora 28: 188. 1926; Salix pellita Andersson, Monogr. Salicum 139. 1867; Salix planifolia Pursh, Fl. Am. Sept. 2: 611. 1814; Salix pulchra Cham. Linnaea 6: 543. 1831; Salix tyrrellii Raup, J. Arnold Arb. 17: 231. 1936

Salix arbusculoides Andersson, Monogr. Salicum 147. 1867.

Salix candida Flüggé ex Willd. Sp. Pl. 4: 708. 1806; Salix wiegandii Fernald, Rhodora 35: 243. 1933 (Treated here as a species of hybrid origin, S. calcicola ´ S. candida.).

Salix calcicola Fernald & Wiegand, Rhodora 13: 251. 1911; Salix richardsonii Hook. Fl. bor.-amer. 2: 147; Salix tweedyi (Bebb ex Rose) C. R. Ball in J. M. Coulter, Bot. Gaz. 40: 377. 1905 (Basionym: Salix barrattiana Hook. var. tweedyi Bebb ex Rose, Contr. U. S. Natl. Herb. 3: 572. 1902).

Salix alaxensis (Andersson) Coville, Proc. Wash. Acad. Sci. 2: 280. 1900 (Basionym: Salix speciosa Hook. & Arn. b [var.] alaxensis Andersson in DC. Prodr. 16(2): 275. 1868); Salix alaxensis var. alaxensis; Salix alaxensis var. longistylis (Rydb.) C. K. Schneider, J. Arnold Arb. 1: 225. 1919 (Basionym: Salix longistylis Rydb. Bull. N. Y. Bot. Gard. 2: 163. 1901); Salix barrattiana Hook. Fl. bor.-amer. 2: 146. 1838; Salix silicicola Raup, J. Arnold. Arb. 17: 236. 1936.

Salix argyrocarpa Andersson, Monogr. Salicum 107. 1867.

Salix cana Mart. & Gal. Bull. Acad. Roy. Sci. Bruxelles 10: 344. 1843 (This seems to be the proper section for this taxon according to usage, but the type specimen has not yet been studied.); Salix geyeriana Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 122. 1858; Salix lemmonii Bebb in S. Watson, Bot. California 2: 88. 1879; Salix petiolaris Sm. Trans. Linn. Soc. 6: 122. 1802.

Salix irrorata Andersson, Öfvers. Förh. Kongl. Svenska Vetensk.-Akad. 15: 117. 1858; Salix lasiolepis Benth. Pl. Hartw. 335. 1857; Salix lasiolepis var. lasiolepis; Salix lasiolepis var. bigelovii (Torr.) Bebb in S. Watson, Bot. Calif. 2: 86. 1879 (Basionym: Salix bigelovii Torr. Pacific Railway Reports 7: 139. 1857); Salix mexicana Seemen, Bot. Jahrb. 21 (52): 9. 1896; Salix riskindii M. C. Johnston, Madroño 28: 150. 1981.

Salix breweri Bebb in S. Watson, Bot. Calif. 2: 88. 1879

Salix sericea Marshall, Arbust amer. 140. 1785

Salix delnortensis C. K. Schneider, J. Arnold. Arb. 1: 96. 1919; Salix jepsonii C. K. Schneider, J. Arnold Arb. 1: 89. 1919; Salix sitchensis Sanson ex Bong. Mem. Acad. St. Petersb. 6. 2: 162. 1833.