A taxonomic study of Stipa nelsonii (Poaceae) with a key distinguishing it from related taxa in western North America

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Abstract
S. nelsonii, more commonly but incorrectly known as S. columbiana, is a wide ranging species of western North America, characterized by a short, pubescent palea and a hirtellous awn. Initial numerical analyses, using unweighted morphological data, suggested that 3 infraspecific taxa existed but with different methods of analysis there was a disturbing instability in the placement of certain operational taxonomic units (OTU), including those based on the holotype of S. williamsii. Because replicate OTU were incorporated in the sample, a character weighting, based on the relative constancy of character states within replicate pairs, was applied. The particular weighting function adopted utilized the added component of variance between groups, i.e., between the replicate pairs. Numerical analyses based on the weighted data proved to be stable with respect to all the OTU. They suggested recognition of 3 infraspecific taxa. Keys to these taxa were devised and applied to about 400 additional speciments allowing the geographical and ecological distributions of the suggested taxa to be examined. The combined morphological and distributional studies suggest that S. nelsonii comprises 2 spp.: ssp. nelsonii and ssp. dorei. Two varieties can be recognized in ssp. nelsonii: var. nelsonii centered in the intermountain region of the USA and var. longiaristata centered in eastern Washington. The range of ssp. dorei extends from southern Yukon to California and eastward to the Saskatchewan-Manitoba border. A key to the infraspecific taxa of S. nelsonii and to related taxa of western North America is included, as well as descriptions and distribution maps of the infraspecific taxa recognized in S. nelsonii.

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