From Plant Press, Vol. 18, No. 1, January 2015.
By Jeffery M. Saarela and Paul Peterson
The grass genus Bromus includes about 160 species distributed in temperate regions around the world. In North America, the bromes of Canada and the United States are well known, but the taxonomy of the group has been much less clear in adjacent Mexico and Central America. Eugène Fournier (Mexicanas Plantas, 1886) recognized three species of Bromus in México, including one with eight varieties. In an early twentieth century revision of Mexican grasses, Albert S. Hitchcock (Mexican Grasses, 1913) recognized seven Bromus species in México, and he later recognized two of these in Central America (The Grasses of Central America, 1930). Thomas R. Soderstrom and John H. Beaman (The Genus Bromus in México & Central America, 1968) produced the first revision of Bromus in México and Central America recognizing 16 species. The different names and, in some instances, taxon concepts in these previous revisions have resulted in considerable confusion. Alan A. Beetle (Las Gramíneas de México, 1988) and Adolfo Espejo-Serna et al. (Poaceae in Las Monocotiledóneas Mexicanas, 2000) recognized 25 and 26 species, respectively. We recently published a revised taxonomic treatment for Bromus in México and Central America (Phytotaxa 185: 1-147). We accept 22 species in the flora, of which twelve are native and ten introduced. We include a key to the species in English and Spanish, descriptions, synonymies, complete illustrations of all species, distribution maps, images of representative herbarium specimens, and lists of all specimens examined.
Our revision is based on examination of over 2000 herbarium specimens, including over 400 numbers of Bromus collected in Mexico by Paul Peterson on numerous trips. Many of these collections have provided important new knowledge on the distribution of Bromus species in Mexico, serving as a reminder of the critical importance of new fieldwork as part of revisionary work. Searching for species of Bromus in Mexico was one of the main focuses of our expeditions to Mexico in 2008, 2009 and 2010. Studying the plants in the field proved critical for understanding, clarifying and developing species concepts for some taxa. For example, plants that we now recognize in a single species, B. richardsonii, are morphologically variable in Mexico, and we experienced considerable difficulty understanding their variation in the context of previous species concepts applied to this variation. Based on careful study of this variation in the herbarium, we concluded that only a single, variable taxon could be recognized.