Ceanothus (Rhamnaceae)



Diversification of Ceanothus L., subgenus Cerastes (Rhamnacae)

The genus Ceanothus L. (Rhamnaceae) comprises 50 species of shrub to tree-like perennial plants restricted entirely to North and Central America, with a center of diversity in the California Floristic Province (1). Classically, Ceanothus has been divided into two subgenera (2), Cerastes (21 spp.) and Ceanothus (29 spp.). Molecular phylogenetic studies have shown that the subgenera are reciprocally monophyletic (3), and that they probably represent recent (< 5 mya) but independent diversifications (2, 3, 4, 5) following ancient (18-39 mya) divergence (3), an idea that is supported by the fossil record (2). As a whole, Ceanothus is an ecologically dominant component of many western North American plant communities, particularly in the California Floristic Province (CFP), where the genus comprises more than 20% of native shrub diversity (6). Species of Ceanothus also contribute to ecosystem nitrogen budgets through their symbiotic relationship with nitrogen-fixing bacteria of the genus Frankia (7). My dissertation research is focused on the Cerastes subgenus of Ceanothus, an ecologically and morphologically diverse group exhibiting an array of growth forms, from prostrate shrubs to small trees, and a broad spectrum of habitat associations, from coastal dunes to sub-alpine forest (2, 3, 4, 6). The Cerastes subgenus is strongly associated with the mediterranean-type climate of the CFP, with 20 of the 21 species restricted to this region. In addition to adaptations for drought, many Cerastes taxa are adapted to unusual, often harsh soil conditions. 11 of the 21 recognized species are edaphic-endemics, being restricted to soils derived from particular geological parent materials. My dissertation research focuses on the systematics of the Cerastes subgenus of Ceanothus, with an emphasis on understanding the evolution of edaphic ecology in this group.

References Cited

1. Takhtajan, A. 1986. Floristic Regions of the World. (translated by T.J. Crovello & A. Cronquist). UC Press.

2. Van Rensselaer, M., H.E. McMinn, and H.L. Mason. Ceanothus. Santa Barbara Botanic Garden.

3. Hardig, T.M., P.S. Soltis & D.E. Soltis. 2000. American Journal of Botany 87: 108-123.

4. Fross, D. & D. Wilken. 2006. Ceanothus. Timber Press, Portland, Oregon.

5. Ackerly, D.D., D.W. Schwilk, & C.O. Webb. 2006. Ecology 87: S50-61.

6. Hickman, J.C. (ed.). 1993. The Jepson Manual: higher plants of California. University of California Press.

7. Delwiche, C.C., P.J. Zinke, & C.M. Johnson. 1965. Plant Pathology 40: 1045-1047.