There are three North American species of deciduous shrubs in the genus Dirca L. Dirca palustris L. occurs in the eastern United States and in southeastern Canada (Vogelmann 1953). Although its distribution is broad, D. palustris typically is obscure locally and is restricted to moist soils as an understory species along wooded slopes (Kurz 1997). Dirca occidentalis Gray also occurs on slopes (McMinn and Forderhase 1935), but its distribution is narrowly restricted to six counties of California near the San Francisco Bay. Within this area, D. occidentalis inhabits cool niches with frequent fog and relatively uniform soil moisture (Johnson 1994). Dirca mexicana Nesom & Mayfield was discovered in 1994 as one population of 800 to 1000 plants along a slope in the Sierra Madre Oriental of Tamaulipas, Mexico (nesom and Mayfield 1995).