A transitional xerophytic tropical plant community of the Cape Region, Baja California

Our main objective was to use comparative floristic and structural criteria in order to understand the differences in assemblages brought about by the meeting of arid and dry-tropical environments. We sampled 33 vegetation pl ots and recorded all perennial species. The data were subjected to multivar iate analysis, including Principal Component Analysis (PCA). The floristic variation detected by the PCA was strongly related to altitud e and rainfall. The gradient detected by the ordination procedure had Tropi cal deciduous forest (TDF) and Sonoran Desert (SD) at opposite extremes and Cape sarcocaulescent shrubland (CSCS) in an intermediate position. The num erical classification algorithm detected six distinct groups of species wit h clearly identifiable field distributions. Vegetation structure also diffe red significantly between the regions. SD had the lowest species-richness ( ce diversity) and inter-site similarity in the CSCS was highest, indicating that this environment is relatively uniform and has low species turnover ( P-diversity). It is concluded that the CSCS is distinctly different from the sarcocaulesc ent shrublands of the more northern plant communities of the peninsula, whe re Sonoran Desert floristic affinities prevail. The floristic composition o f the CSCS is rich in dry-tropical affinities, its species richness is high er, it is homogeneous in its species content and turnover and is more dense with a taller canopy than the northern desert scrubs. It is proposed that the boundary line separating the Cape sediments of granitic origin from the basalt-derived sediments of the northern-lying Sierra de la Giganta should be used as an easily identifiable landscape trait to delimitate this uniqu e community.