Arizona Thistle - Cirsium arizonicum var. arizonicum
West of Emory Pass, Black Range
New Mexico, USA
July 18, 2015
I photographed this thistle in the upper reaches of Iron Creek, west of Emory Pass, in the Black Range of New Mexico on July 18, 2015. It was near the location of Mitchell’s Cabin (long since gone - I found only one brick at the location). The species found in our area is Cirsium arizonicum (Gray) Petrak var. arizonicum. (Please remember that I am not a botanist, so misidentification is always a possibility.)
David Kell, who is an expert on this genus, and who is responsible for the Flora of North America entries on this genus, has this to say about this species:
“The Cirsium arizonicum complex is widely distributed from the Sierra Nevada, White Mountains, and New York Mountains of eastern California across the mountains of the southern Great Basin and Colorado Plateau to the mountains of eastern Colorado, Arizona, and New Mexico. This group of plants comprises a series of intergrading races with intricately overlapping patterns of variation. For plants that I am treating as C. arizonicum (in the broad sense), F. Petrak (1917) recognized three species, one with a variety and two subspecies plus his unstated type subspecies and variety. R. J. Moore and C. Frankton (1974b) revised the complex, recognizing six species, three of them newly described, for the plants I treat as C. arizonicum plus C. turneri, which I do not include in C. arizonicum. P. L. Barlow-Irick (2002), in a work focused on statistical analyses of variation patterns, recognized six species also, but circumscribed very differently from those of Moore and Frankton. Two of the species proposed by Barlow-Irick have not been formally described.
I have wrestled with how to treat these plants since beginning my research for this treatment. After careful consideration of the complex patterns of variation among members of the C. arizonicum complex, I acknowledged the futility of trying to distinguish more than one species. Any character combinations that I or others have attempted to use to distinguish species break down hopelessly when enough specimens are examined. Instead I have chosen to recognize that in this complex, as in several others, the plants in question are a work of evolution in progress. Cirsium arizonicum is a rapidly evolving, only partially differentiated assemblage of races that have not reached the level of stability that is usually associated with the concept of species. Certainly there is much variation within the group that deserves a level of taxonomic recognition, or at least should be mentioned, but I think it much more prudent to recognize varieties–entities that may be expected to freely intergrade–rather than species. The geographic area where these plants occur, the highlands of the American Southwest, has had a turbulent history in the Quaternary with major shifts in climate, vegetation, and elevational zonation accompanying the vicissitudes of glacial and interglacial episodes. The complicated patterns of variation in C. arizonicum reflect both that history and the geographic and topographic complexity of the region.”
His entry in eflora.org (link above) contains a key for differentiating the varieties of this species - noting the quote above.
The original description of this species was by Asa Gray in the “Proceeding of the American Academy of Arts” 1874, Volume 10, page 44 as:
This species is known as Arizona Thistle. Scientific synonyms for the species include Cirsium nidulum and Cnicus arizonicus.
The Native Plant Society of New Mexico has published a key and identification guide to the thistles of New Mexico.
This species is a composite and a member of the Aster Family (Asteraceae).
