SHRUBLAND FORMATIONS AND ASSOCIATIONS IN MEDITERRANEAN-DESERT TRANSITIONAL ZONES OF NORTHWESTERN BAJA-CALIFORNIA

The area between Ensenada and El Rosario (Baja California, Mexico) has long been considered as a transitional zone in which two great ecocli matic regions (Mediterranean and Tropical-Desert) overlap. The florist ic and biotypical diversity of this area was evaluated by analyzing it s shrubland formations from a phytosociological point of view. This ph ytosociological study, carried out according to the Braun-Blanquet met hod and supported by cluster analysis, describes sixteen shrubland ass ociations from Northwestern Baja California. Floristic diversity of th e transitional zone was evaluated using two indices, endemic value (EV ) and endemic community value (ECV), which are related to the degree o f endemism in the flora and plant associations. The phytosociological analysis showed that the high number of shrubland associations found i n this area reflected its transitional character. The closer the assoc iations are to the transitional zone, the higher their biotypical and floristic diversity.