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

Abstract. 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 plots and recorded all perennial species. The data were subjected to multivariate analysis, including Principal Component Analysis (PCA). The floristic variation detected by the PCA was strongly related to altitude and rainfall. The gradient detected by the ordination procedure had Tropical deciduous forest (TDF) and Sonoran Desert (SD) at opposite extremes and Cape sarcocaulescent shrubland (CSCS) in an intermediate position. The numerical classification algorithm detected six distinct groups of species with clearly identifiable field distributions. Vegetation structure also differed significantly between the regions. SD had the lowest species‐richness (α‐diversity) and inter‐site similarity in the CSCS was highest, indicating that this environment is relatively uniform and has low species turnover (β‐diversity). It is concluded that the CSCS is distinctly different from the sarcocaulescent shrublands of the more northern plant communities of the peninsula, where Sonoran Desert floristic affinities prevail. The floristic composition of the CSCS is rich in dry‐tropical affinities, its species richness is higher, 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 unique community.