Origin of the Creosote Bush (Larrea) Deserts of Southwestern North America

Abstract

• â-* The North American creosote bush (Larrea tridentata) has undergone a simple cytogeographic 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 coincide remarkably well with the boundaries of their respective desert provinces. However, during the glacials of the Pleistocene, the lowlands of these provinces were extensively invaded by evergreen woodlands dominated by various species of Juniperus, Pinus, and Qaercus; and the generality of the phenomenon during the Wisconsin glacial has been abundantly documented by some of the most detailed macrofossil evidence of relatively xerophytic plants ever obtained. The u C-dated macrofossil assemblages, preserved in rock-sheltered Neotoma deposits, demonstrate a persistence of woodland conifers and consistent absence of Larrea throughout most of its present range in the Southwest during the last major glacial episode of the Pleistocene from > 40,000 BP to about 11,000 BP. The oldest reliable record of Larrea in North America is 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 America must 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 vast and varied desert niche into which a burgeoning population of Larrea could have expanded and differentiated explosively.The biogeography of Larrea during the Pleistocene is a particularly interesting enigma because there is an intriguing possibility of a late, intercontinental dispersal from South America, where the genus exhibits markedly greater ecomorphological and chemical diversity than it does in North America. Also, the genera most closely allied to Larrea are endemic to South America. While several "r"-selected, herbaceous genera of Zygophyllaceae have much wider world-distributions, Larrea is unique among woody Zygophyllaceae in having an amphitropical, intercontinental disjunction in its range. The specialization of Larrea on intensely arid deserts with relatively open plant communities would facilitate establishment after dispersal to a new desert area. However, the migration of Larrea to North America must have lacked continuity. If the extremely xerophytic species, Larrea divaricata, had been able to follow a hypothetically continuous route across the wet tropics during a period of drier climate, then a much greater amphitropical disjunction of American zygophyllaceous genera should be expected. Long distance transport of seeds across the wet tropics is the only feasible alternative for Larrea.The creosote bush [Larrea tridentata (Sesse & Moc. ex DC.) Cov.] is the most abundant a...

Molecular Phylogeny of Larrea and Its Allies (Zygophyllaceae): Reticulate Evolution and the Probable Time of Creosote Bush Arrival to North America

“…At present, the arid rain shadow pocket of the Extorax-Tula drainage in eastern Querétaro (southern Mexico) harbors an isolated area of landscape dominated by Larrea, where the observed cacti endemisms strongly suggest a considerable antiquity for the desert plants confined there (Wells and Hunziker, 1977). Ancestral diploid populations probably established and remained restricted to the proximities of this region, which could have also served as a refugium throughout the Pleistocene glacial advances due to its relatively mild, frost-free climate during this period.…”

“…According to this hypothesis, present-day diploid populations of both L. divaricata and L. tridentata probably descended from a common South American ancestor and started to diverge allopatrically when migrants reached North America. The dispersal probably occurred through long-distance migratory birds (Wells and Hunziker, 1977).…”

“…First, long-distance dispersal of Larrea across the tropics may have occurred through an intermediate series of appropriate "way stations" (Peru, Bolivia) (Raven, 1963), a situation which might have arisen as a result of the expansion of the world's arid climates in the late tertiary and early Quaternary (10 -1.8 mybp) (Axelrod, 1970;Porter, 1974). A detailed discussion on the identity of the putative carriers has been presented by Wells and Hunziker (1977). Second, 30 species in five orders of insects have been found to be associated with L. tridentata, many of them being monophagous (Schultz et al, 1977).…”

“…Most of the present areas of the warm deserts in North America were occupied by pluvial woodlands until the recessional phases of the Wisconsin glacial, when a desiccating climate opened a vast and varied desert niche into which Larrea populations could have expanded and differentiated explosively (Wells and Hunziker, 1977).…”

“…There are two general hypotheses concerning the geographical origin of Larrea: (1) Johnston (1940) proposed a South American origin for Larrea, a hypothesis further sustained by Hunziker et al (1972), Hunziker (1975), and Wells and Hunziker (1977) based on the fact that the genera of Zygophyllaceae most likely to be closely related to Larrea have contemporary distributions that suggest a northern South American origin for most of them, and (2) Turner (1972) proposed that L. tridentata developed as a diploid population in North America millions of years ago and subsequently became established in South America through longdistance dispersal. Porter (1974) also supported this view originally, but later admitted (1979) that it was no longer tenable and concluded that the South American origin "had been proved convincingly in a series of papers by Hunziker and his associates.…”