Taxonomy of the Poa laxa group, including two new taxa from Arctic Canada and Greenland, and Oregon, and a re-examination of P. sect. Oreinos (Poaceae)

18 May 2017

Soreng, Robert J.; Gillespie, Lynn; Consaul, Laurie

The Poa laxa Haenke species group, comprising four alpine-arctic taxa in P. sect. Oreinos, has a complicated and confusing taxonomic history. Here we re-examine the taxonomy of the group and section based on sequences of three plastid and two nuclear ribosomal DNA markers. Poa laxa s.l. resolved in the Poa plastid and nrDNA clades with species of sections Abbreviatae, Oreinos, Stenopoa, and Tichopoa. In plastid analyses Poa laxa s.s. (type of sect. Oreinos) resolved in a subclade with P. glauca and other sect. Stenopoa species, while all other P. laxa s.l. taxa resolved in a subclade with species of sect. Abbreviatae. We maintain P. laxa s.s. (mainly 2n=28) of west-central Europe in sect. Oreinos, while the other P. laxa s.l. taxa from northern Europe and North America (all 2n=42 or higher ploid) are referred to P. sect. Abbreviatae. In North America only one collection of the northern European P. flexuosa subsp. flexuosa is accepted (Greenland, Jensen’s Nunatakker), and P. flexuosa subsp. fernaldiana is accepted for populations in eastern Canada and northeast U.S.A. A new subspecies, P. flexuosa subsp. consauliae, is described from eastern Arctic Canada and Greenland; its variable morphological characteristics suggest introgression with P. glauca and it is predicted to be apomictic. Rocky Mountain P. laxa subsp. banffiana is treated as P. banffiana. Oregon material formerly considered to belong to P. laxa s.l. is named P. wallowensis. Poa sect. Oreinos was determined to be polyphyletic, and is here emended to comprise only three continental European species, including P. laxa. Lectotypes are proposed for P. aspera var. laxiuscula, P. fernaldiana, and P. laxa var. debilior. New chromosome numbers are reported for P. flexuosa subsp. consauliae (ca. 2n=42), and P. glauca (2n=63) from Baffin Island. A key to the species found in North America is given.