A New Revision of the Grass Genus Bromus in Mexico and Central America

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.

Summit of Sierra Zapaliname above Saltillo, Coahuila, September 2007. (photo by Jeffery M. Saarela)

Highlights of these trips included searching for two rare species (Bromus densus and B. attenuatus) endemic to the Sierra Madre Oriental in northeastern Mexico. Both of these distinctive species were described by former Smithsonian agrostologist Jason Swallen in 1950. Finding B. densus (known from some 18 collections, of which nearly half are Peterson numbers) and B. attenuatus (known from nine collections, of which seven are Peterson numbers), which both grow at high elevations, often required travel through remote mountainous areas along trails that barely pass as roads (Sierra Las Cautivas, Tamaulipas), and long, arduous hikes to mountaintops. The best known sites of B. densus are Cerro Potosí in Nuevo León, a mountain top to which one can drive because there is a radio tower there, and Sierra Zapalinamé, a range just outside of the city Saltillo, Coahuila. Finding the plants in the latter area requires walking up-hill for several hours. Our collections of B. densus and B. attenuatus from Cerro de la Luz, San Luis Potosi in 2010, turned out to be the first records of both species for that state. The mountain hike on which we made these collections is memorable because it rained for several hours as we descended to our camp down a slippery, flowing drainage that was dry just hours earlier.

In addition to being rare and poorly known, Bromus densus, B. attenuatus and the more widespread species B. dolichocarpus, distributed from Central Mexico to Guatemala, are interesting because together they represent a unique lineage (clade) that is the sister group of the rest of the genus. We found this in a phylogenetic study of Bromus published in 2007. Given their evolutionary affinities, we now recognize these three species in their own section, Mexibromus, a new taxon proposed in our revision.

A few native species whose primary distributions are in the southwestern United States are known in Mexico from just one or a few collections. Bromus lanatipes, a species characterized by densely woolly lower leaf sheaths, is distributed primarily in the southwestern U.S., but barely extends into México in Maderas del Carmen (part of the Sierra del Carmen) in northern Coahuila, just south of the Texas border, where it grows in high-elevation pine-oak forests. Five of the seven known collections of this species from this region were made by Peterson on four different visits, and the other two made by a different collector in the early 1970s. Maderas del Carmen is a biosphere reserve, and in addition to its interesting plant diversity, one of the only places in Mexico where black bears are still present (we saw one). Bromus pseudolaevipes, distributed primarily in the Coastal Ranges of California, is known from a single site in Mexico in northern Baja California (Sierra San Pedro Martír), based on a Peterson collection made in 2000.

The rocky road through Sierra Las Cautivas, along the Tamaulipas/Nuevo Leon border, September 2007. (photo by Jeffery M. Saarela)

The most poorly understood Bromus species in Mexico is B. pinetorum, another species described by Swallen in 1950, which is now known from only four collections from the central mountains of Coahuila. Additional field work and collections are needed to better understand the circumscription of the taxon and to collect material for molecular analyses.

Like any taxonomic revision, monograph or flora, our new treatment of Bromus in Mexico and Central America serves as an up-to-date baseline of information for further exploration of the diversity of this group of grasses in these regions, and contributes to broader understanding of this grass genus from a global perspective. Continued exploration will certainly yield new collections that add to our current knowledge of the diversity and distribution of species, and may even yield as-yet-unknown species of Bromus. This kind of knowledge can only be gained by studying and collecting plant diversity in the field and preserving specimens in herbaria, where they become part of the permanent scientific record documenting the distributions of species in time and space.