Major plant communities of North America’s most arid region: The San Felipe Desert, Baja California, Me ´ xico M. PEINADO 1 , M. A ´ . MACI ´ AS 2 , J. DELGADILLO 3 & J. L. AGUIRRE 1 1 Departamento de Biologı ´a Vegetal, Universidad de Alcala ´ , Alcala ´ de Henares, Madrid, Spain, 2 Departamento de Ciencias Ambientales, Universidad de Guadalajara, Jalisco, Me ´xico, and 3 Herbario BCMEX, Universidad Auto ´noma de Baja California, Ensenada, BC, Me ´xico Abstract This vegetation study describes the landscape of the San Felipe Desert (Baja California, Me ´xico) based on the phytosociological analysis of its major plant associations, as determined by the Braun-Blanquet method and supported by cluster analysis. Four geomorphological or phytotopographical units were identified in the landscape of the San Felipe Desert (salt marshes, playas or sandy flats, bajadas or alluvial fans, and malpaı ´ses or badlands), each characterized by its own plant communities. Two communities and eleven associations were identified, four of which are described and typified for the first time: Atriplici linearis-Frankenietum palmeri, Parkinsonio microphyllae-Olneyetum tesotae, Errazurizio megacarpae- Ephedretum trifurcae and Pachycereo schottii-Prosopidetum torreyanae. Ecological, phytogeographical, bioclimatic, syntaxono- mical and floristic data are provided for each of these new associations. Key words: Classification, ordination, phytosociology, San Felipe Desert, syntaxonomy, vegetal landscape Introduction In the far southwest of North America, the Baja California Peninsula presents a flora characterized by an unusually high rate of local endemics. Long known to be an area of significant biogeographical differentia- tion (Baird, 1860; Cope, 1873; Dice, 1943), its geological history, abrupt topography, many rainfall regimes and strategic position make it a magnificent refuge, as much for the taxa of the ancient Tertiary geofloras (Madrean or Californian elements), as for those of the Xerophytic-Mexican Region (Sonoran elements). Protected from the climatic oscillations of the Late Wisconsin glacial period, the peninsula’s flora boasts a vast array of plant associations and is an example of the biodiversity that arises when two flori- stic kingdoms – the Holarctic and the Neotropical – come together (Peinado et al., 1994a, 1995a). Over the last 16 years, we have been conducting botanical and phytosociological research in Baja California, chiefly involving the analysis of its flora (Delgadillo, 1989, 1992, 1995; Peinado et al., 1995a), phytogeography (Delgadillo, 1995; Peinado & Delgadillo, 1990; Peinado et al., 1994a), bioclimatic belts and zonobiomes (Peinado et al., 1994b, c; 1997), and making syntaxonomic proposals at the regional level based on the findings of the first phytosocio- logical studies performed on the peninsula accord- ing to the Braun-Blanquet approach (Delgadillo, 1995; Delgadillo et al., 1992; Peinado et al., 1994d, e; 1995b, c). Based on all the information gathered during these years, we are now in a position to focus on more precise studies at the local level. The local studies undertaken so far are the phytosociological analysis of the El Vizcaı ´no Biosphere Reserve, Baja California Sur (Peinado et al., 2005), and the study described in the present article. This article extends the MSc thesis of one of the present authors (Macı ´as, 1998), whose conclusions served to establish the first floristic checklist for San Felipe (Delgadillo & Macı ´as, 2002). Correspondence: Manuel Peinado Lorca, Depto. Biologı ´a Vegetal, Edificio de Ciencias, Campus Universitario, Ctra. Madrid-Barcelona Km 33,600, E-28871, Alcala ´ de Hena ´res (Madrid) Spain. Tel.: þ34 918854945. Fax: þ34 918885066. E-mail: manuel.lorca@uah.es Plant Biosystems, Vol. 140, No. 3, November 2006, pp. 280 – 296 ISSN 1126-3504 print/ISSN 1724-5575 online ª 2006 Societa ` Botanica Italiana DOI: 10.1080/11263500600947715