Flora of the Sierra Bacha, Municipality of Pitiquito, Sonora, Mexico

The Sierra Bacha is in the Central Gulf Coast Subdivision of the Sonoran Desert along the coast of the Gulf of California, south of Puerto Libertad, Municipality of Pitiquito, Sonora. The flora of the Sierra Bacha has 269 species, one additional subspecies, and one hybrid in 57 families and 183 genera. The most species-rich families are Asteraceae (44 species); Fabaceae (23 species); Euphorbiaceae (17 species); Boraginaceae, Cactaceae, Poaceae, and Solanaceae (15 taxa each). Nineteen species (7.0%) are relictual populations of Baja California species, including the only mainland population of Fouquieria columnaris (boojum) at Punta Cirio. Sixteen species (5.9%) are non-native. Eleven packrat (Neotoma sp.) middens were radiocarbon dated from 9,970 to 320 yr B.P. (years before 1950). Plant fossils in the samples were identified to 82 species in 30 families and 69 genera. Ten species no longer occur in the Sierra Bacha. In the Early Holocene (9,970 to 9,270 yr B.P.), F. columnaris was living farther inland on rhyolitic slopes. Vegetation and climate with greater winter rainfall and cooler summers similar to Baja California today were more widespread in western Sonora during the Early Holocene and likely the 15-20 glacial periods in the last two million years of the Pleistocene.