ORIGIN OF THE CREOSOTE BUSH (LARREA) DESERTS OF SOUTHWESTERN NORTH AMERICA' PHILIP V. WELLS2 AND JUAN H. HUNZIKER3 ABSTRACT The North American creosote bush (Larrea tridentata) has undergone a simple cyto-geographic differentiation, with the ancestral diploid population in the Chihuahuan Desert,and with tetraploid and hexaploid derivatives in the Sonoran and Mohave Deserts, respectively.The chromosomal races have annectant but largely allopatric distributions, which coincideremarkably well with the boundaries of their respective desert provinces. However, duringthe glacials of the Pleistocene, the lowlands of these provinces were extensively invaded byevergreen woodlands dominated by various species of Juniperus, Pinrs, and Quercus; and thegenerality of the phenomenon during the Wisconsin glacial has been abundantly documentedby some of the most detailed macrofossil evidence of relatively xerophytic plants ever ob-tained. The 'C-dated macrofossil assemblages, preserved in rock-sheltered Neotoma deposits,demonstrate a persistence of woodland conifers and consistent absence of Larrea throughoutmost of its present range in the Southwest during the last major glacial episode of the Pleistocenefrom > 40,000 BP to about 11,000 BP. The oldest reliable record of Larrea in North Americais dated on macrofossils of Larrea itself at 10,580 BP, and the site, near Wellton, Yuma County,Arizona, is one of the lowest (162 m) and most arid in the present Sonoran Desert. Hence,the major features of the modern geographic pattern of Larrea tridentata in North Americamust have originated during the climatic transition from the late Wisconsin to the Holocene,when a desiccating climate gripped the immense lowlands of the Southwest, opening a vastand varied desert niche into which a burgeoning population of Larrea could have expanded anddifferentiated explosively. The biogeography of Larrea during the Pleistocene is a particularly interesting enigmabecause there is an intriguing possibility of a late, intercontinental dispersal fromn South America,where the genus exhibits markedly greater ecomorphological and chemical diversity than itdoes in North America. Also, the genera most closely allied to Larrea are endemic to SouthAmerica. While several "r"-selected, herbaceous genera of Zygophyllaceae have much widerworld-distributions, Larrea is unique among woody Zygophyllaceae in having an amphitropical,intercontinental disjunction in its range. The specialization of Larrea on intensely arid desertswith relatively open plant communities would facilitate establishment after dispersal to anew desert area. However, the migration of Larrea to North America must have lacked con-tinuity. If the extremely xerophytic species, Larrea divaricata, had been able to follow a hy-pothetically continuous route across the wet tropics during a period of drier climate, then amuch greater amphitropical disjunction of American zygophyllaceous genera should be ex-pected. Long distance transport of seeds across the wet tropics is the only feasible alternativefor Larrea. The creosote bush [Larrea tridentata (Sess� & Moc. ex DC.) Cov.] is themost abundant and widely distributed woody plant in the warm or subtropicaldeserts of North America. It forms a massively continuons population throughoutthe Mohave Desert and the Colorado River sector of the Sonoran Desert, and ex-tends eastward on the more arid plains to the "desert grasslands" of southeastern The authors wish to express their gratitude to Prof. T. J. Mabry and the Departmient ofBotany, University of Texas, Austin, for generously bringing us together and fostering dis-cussions leading to this paper. Research supported by NSF grants GB-40306 to University ofKansas and IBP funding to Prof. Otto Solbrig, Harvard University; and by several grants toProf. J. H. Hunziker from Comite Nacional del Programa Biologico Internacional, and otheragencies in Argentina. 2 Department of Systematics and Ecology, University of Kansas, Lawrence, Kansas 66045. 3 Laboratorio de Gen�tica, Departamento de Ciencias Biol�gicas, Facultad de CienciasExactas y Naturales, Universidad Nacional de Buenos Aires, Buenos Aires, Argentina.ANX. MIssouRI BOT. GARD. 63: 843-861.