Systematics of Bouteloua breviseta and B. ramosa (Gramineae)

Abstract

Bouteloua breviseta Vasey and B. ramosa Scribn. ex Vasey were described in 1890, both based on plants from Texas. Although the literature suggests that the type specimens for both species may have been collected in close proximity, this is not the case. The type of B. breviseta is from modern Reeves County, not far from the New Mexico state line, and about 210 airline kilometers north of the type locality of B. ramosa. For some 50 years these two taxa were treated as specifically distinct by agrostologists, but in the mid 1930's both W. A. Silveus and A. S. Hitchcock relegated B. ramosa to synonymy under B. breviseta. With few exceptions, this disposition has been followed by botanists since that time. The present study, which is based on field work throughout their ranges, along with examination of more than 180 herbarium specimens, demonstrates that these two taxa differ in a number of significant ways. Bouteloua breviseta is diploid (2n = 20) and is a gypsophile; B. ramosa is tetraploid (2n = 40) and is characteristic of calcareous soils. Each taxon occupies a different geographical area, and their ranges scarcely overlap. These distinctions, along with significant morphological dissimilarities, indicate that B. ramosa is in no sense a minor variant of B. breviseta but is as distinctive a species as many others commonly recognized in the genus Bouteloua. The original description of Bouteloua breviseta Vasey (1890a) appeared in a report by J. M. Coulter on a collection of plants made by G. C. Nealley in Texas during the years 1887, 1888, and 1889. The type is Nealley 785 [669] from Screw Bean, which Vasey indicated is in Presidio County. McVaugh (1946) attempted to trace Nealley's routes and to pinpoint his collection sites and pointed out that this area was a considerable distance north of modern Presidio County. He stated, Screwbean remains the most obscure of Nealley's localities, not designated on any map I have seen. McVaugh concluded that this locality is actually in Reeves County, about 8 km (5 miles) from the Pecos River and a few kilometers south of Orla. There are gypsiferous soils in this general area from which numerous collections of B. breviseta have been made. A related species, Bouteloua ramosa, was described by the same author (Vasey 1890b) later that same year but credited to F. Lamson-Scribner. No specimen was cited, but Griffiths (1912) stated that it was certainly based on a Nealley collection. It is significant that in the earlier work (Vasey 1890a) the name Bouteloua ramosa Scribner appears as no. 797 on the same page as the original description of no. 785 B. breviseta. Here there is no description of B. ramosa, but there is a note: Chenate Mountains (Presidio county). Although in this publication B. ramosa is a nomen nudum, and the species was not given a description until three months later, it seems reasonable to assume that the specimen cited above served