Muhlenberg's centaury (Centaurium muehlenbergii COSEWIC assessment and status report: chapter 4

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Distribution

Global range

Centaurium muehlenbergiioccurs from southwest B.C. south to Washington, Oregon, Idaho, Nevada and California (Hitchcock et al. 1959, Hitchcock and Cronquist 1973, Douglas et al. 1999, NatureServe 2005) (Fig. 2). In Washington, C. muehlenbergii was known historically from Kittitas to Spokane Cos. and west through the Columbia River Gorge (Hitchcock and Cronquist 1973); it is still reported to be extirpated from that state (NatureServe 2005) in spite of a 1992 collection of this species from San Juan Island.

Centaurium muehlenbergii has also been reported from Texas, Louisiana, and Mississippi (USDA-NRCS 2005) but these specimens are likely misidentified C. tenuiflorum (J. Pringle, comm. pers. 2001).

Canadian range

In Canada, Centaurium muehlenbergii is known from two isolated sites on southeastern Vancouver Island (one in Oak Bay near Victoria, the other near Nanaimo) and a third from a single Gulf Island, E. Chatham Island, near Victoria (Fig. 3 and Table 1).

The Gulf Islands population was first recorded in 1933, the Oak Bay population in 1961 (HERB Database 2005). Until recently, these were the only two confirmed Centaurium muehlenbergii sites in Canada. However, in 2003, a third, small population was located near Nanaimo, effectively extending the known range of the species northward by approximately 80 km (Table 1).

A single unconfirmed, historical record, dating from 1911, exists from Stanley Park in Vancouver (Douglas et al. 2002), but this report was not based on a voucher collection. The species has not been reported again from this location suggesting that this record may be erroneous (M. Fairbarns, comm. pers. 2003).

Figure 2.  Approximate global range of Centaurium muehlenbergii (possibly extirpated in Washington State). Hatched circles are reported occurrences that are presumed to be incorrectly identified.

Figure 3. Range of Centaurium muehlenbergii in Canada. Solid circles: extant populations. Open triangles: survey locations where C. muehlenbergii was not found.

¹AC: Adolf Ceska; MCM: M.C. Melburn; MM: Mike Miller; HT: H. Toms.

The current Extent of Occurrence (EO) of Centaurium muehlenbergii in Canada is approximately 160 km², and the total area actually occupied is approximately 110 m² (0.0001 km²). Based on a 1x1 km grid, the Area of Occupancy (AO) is 3 km² or 12 km² if based on a 2x2 km grid. This represents << 1% of the species’ global distribution. The recent report of the population at Nanaimo likely represents the discovery of a previously existing and overlooked population.

The nearest U.S. populations of Centaurium muehlenbergiiare in the Columbia River Gorge, over 300 km to the south. A collection of this species was made, however, in 1992 in the San Juan Islands (see accession number WTU 325083, at the University of Washington Herbarium). NatureServe (2007), however, still lists the species as historic (SH).