Taxonomy of the Ceanothus vestitus Complex (Rhamnaceae)

The Ceanothus vestitus complex comprises three closely-related shrub species restricted to the southwestern United States and northern Mexico: C. paucifloms, C. perplexans, and C. vestitus. These plants are an ecologically important component of shrubland habitats at moderate to high elevations over a large portion of North America, and are strongly represented in herbaria. However, taxonomy in the C. vestitus complex is complicated by highly variable, intergrading morphology, leading to identification problems. We use multivariate morphometries to explore variation, test circumscriptions, and search for diagnostic traits. Analyses are based on a suite of nine characters scored for 248 specimens. Results show that only C. perplexans is significantly divergent from other members of the complex. Furthermore, variation is continuous among species for all nine characters. Cluster analyses suggest alternative groupings, but none are discrete. Given the lack of significant divergence between C. vestitus and C. paucifloms, we advocate the use of one name for these species. The earliest available name is C. paucifloms. We reduce C. vestitus to synonymy with C. paucifloms. We maintain C. perplexans, and present a case for recognition of this species in the absence of categorical identifying features.