North America is home to approximately 55 species of Ceanothus (Rhamnaceae), more than 30 of them endemic to the biodiversity hotspot known as the California Floristic Province. Although several species of Ceanothus are widespread in North America, most of the species are micro-endemics, known from a small number of occurrences and generally associated with special ecological conditions. As botanical exploration of North America continues, it is expected that more such micro-endemics will be discovered. However, field work is not the only way in which additional Ceanothus diversity will come to light; the strongly variable morphology of Ceanothus means that specimens are difficult to identify, and many biological entities that probably deserve taxonomic recognition are already represented in herbaria but have not yet been recognized as new. We here describe three new taxa of Ceanothus, two initially noted among existing collections, and another discovered during botanical exploration in San Diego County. All three are edaphic micro-endemics found only in the southern California Floristic Province, California. These include 1) Ceanothus foliosus var. viejasensis D. O. Burge&Rebman, restricted to gabbro-derived soils on ViejasMountain in SanDiego County, 2) Ceanothus pendletonensis D. O. Burge, Rebman, &M. R. Mulligan, found on granodiorite-derived soils of the central Santa Margarita Mountains in San Diego County, and 3) Ceanothus thyrsiflorus var. obispoensis D. O. Burge, endemic to rhyolite-derived soils near Hollister Peak in San Luis Obispo County.
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Vol. 42 • No. 3