Diarrhena obovatain the Potomac River Watershed: A New Invader?

Abstract

Diarrhena obovata (Gleason) Brandenburg is a woodland grass primarily of the central United States in the Mississippi River drainage (Brandenburg et al. 1991). Isolated occurrences of D. obovata in the Appalachian and Piedmont regions of the mid-Atlantic states have been found in the last several decades, and the species is or has been considered to be a rare native species in Pennsylvania (Rhoads and Klein 1993); in West Virginia, Virginia, and New York (NatureServe 2011); and (as Diarrhena americana Beauv.) in Maryland (Maryland Natural Heritage Program 2003). The taxonomic concept of D. obovata was first recognized by Gleason (1952a, 1952b) as Diarrhena americana Beauv. var. obovata Gleason. Brandenburg et al. (1991) elevated var. obovata to species rank by proposing the new combination D. obovata, after discovering consistent morphological differences between D. americana var. americana and var. obovata that corresponded to largely allopatric ranges for the two taxa. Most recent floristic authors (e.g., Barkworth et al. 2007, Brandenburg 2007, Rhoads and Block 2007, Weakley 2011) follow Brandenburg et al. (1991) in recognizing D. obovata as a distinct species. D. americana (in the narrow sense of Brandenburg et al. [1991], and as is used in this paper, unless otherwise noted) occupies mostly the Ohio River basin in the central Appalachians and interior plateaus, with a disjunct distribution in the Ozark region. Although it overlaps the range of D. americana, D. obovata occurs primarily in the central Mississippi and lower Missouri River drainages of the upper midwestern United States (Brandenburg et al. 1991, Barkworth et al. 2007, Brandenberg 2007). The former treatment of D. obovata under D. americana (in the broad sense) occasionally has created ambiguity about which one of the two taxa has been represented when D. americana has been recorded in literature and databases. D. obovata apparently was first reported in Maryland, as D. americana (in the broad sense) without note of variety, by Rodney Bartgis in 1988 (Maryland Natural Heritage Program, unpublished data), several years prior to the recognition of D. obovata as a species by Brandenberg et al. (1991). At the time of Bartgis’ report, Brown and Brown (1984) had listed D. americana as potentially occurring [only] for Maryland. Shortly after this discovery, Bartgis showed me this occurrence along the Potomac River near Plummers Island in Montgomery County. In ensuing years, additional records of Diarrhena, reported as D. americana (in the broad sense), were made from along the Potomac and the Chesapeake and Ohio (CO Wiegand and Becker 1995). In 1995, I reported a colony of D. americana (in the broad sense) from along the CO Accepted March 24, 2012. DOI: 10.2179/11-044