Holocene vegetation and climate in the Puerto Blanco Mountains, Southwestern Arizona

Van Devender, Thomas

Twenty-one packrat (Neotoma) midden samples were collected from the Puerto Blanco Mountains in Organ Pipe Cactus National Monument. The packrat midden series records a series of plant communities from a Late Wisconsin California juniper-Joshua Tree woodland through a series of Holocene Sonoran desertscrub communities culminating in the modern subtropical brittle bush - foothills palo verde - Mexican jumping bean organ pipe cactus association about 4000 years ago. The major vegetation changes in the lower Puerto Blanco Mountains since the last glacial age recorded in the ancient packrat middens are discussed. Other sections include comparison with other areas, paleoclimates, and discussion. Figures and tables. (Additional ORPI abstract:) Plant macrofossils from 21 pack ratPlant macrofossils from 21 packrat (Neotoma sp.) middens from the 535-605 m from the Puerto Blanco Mountains, southwestern Arizona, provide an excellent history of vegetation and climate for the last 14,120 yr B.P. in the Sonoran Desert. A Late Wisconsin juniper-Joshua tree woodland gave way to a transitional Early Holocene desertscrub with sparse Juniperus californica (California juniper) by 10,540 yr B.P. Important Sonoran Desert plants including Carnegiea gigantea (saguaro) and Encelia farinosa (brittle bush) were dominants. Riparian trees such Acacia greggii (catclaw acacia), Prosopis velutina (velvet mesquite), and Cercidium floridum (blue palo verde) grew on dry, south-facing slopes in a Middle Holocene Sonoran desertscrub in a warm, wet summer climate with frequent winter freezes. Modern subtropical Sonoran desertscrub formed about 4000 yr B.P. as summer rainfall and winter freezes declined. Cercidium microphyllum (foothills palo verde), Sapium biloculare (Mexican jumping bean), Olneya tesota (ironwood) and Stenocereus thurberi (organ pipe cactus) became dominant as riparian trees retreated to wash habitats. The inferences of a latest Wisconsin/Early Holocene summer monsoonal maximum by Kutzbach (1986) are not supported for the Southwest. Apparently the persistence of Late Wisconsin circulation patterns offset any increases in insolation.

Holocene vegetation and climate in the Puerto Blanco Mountains, Southwestern Arizona

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