Late Pleistocene History of Coniferous Woodland in the Mohave Desert

Abstract

Seventeen ancient wood-rat middens, ranging in radiocarbon age from 7400 to 19,500 years and to older than 40,000 years, have been uncovered in the northeastern, north-central, southeastern, and southwestern sectors of the Mohave Desert. Excellent preservation of macroscopic plant materials (including stems, buds, leaves, fruits, and seeds) enables identification of many plant species growing within the limited foraging range of the sedentary wood rat. An approximately synchronous zonal differentiation of vegetation in response to a gradient of elevation on limestone in the northeastern Mohave Desert is apparent from the macrofossil evidence, preserved in wood-rat middens and ground-sloth coprolites, covering a time span bracketed by radiocarbon ages of about 9000 and 10,000 years. XerophilQus juniper woodlands descended to an elevation of 1100 meters, some 600 meters below the present lower limit of woodland (1700 meters) in the latitude of Frenchman Flat. But desert or semidesert shrubs coexisted with the woodland trees throughout much of the span of elevation corresponding to the pluvial lowering of the woodland zone, and the more mesophytic phase of pinyonjuniper woodland was evidently confined to montane habitats at elevations above 1500 meters. Joshua trees, accompanied by desert shrubs, prevailed down to about 600 meters at Gypsum Cave, Nevada, but only the shrubs of the existing warm-desert vegetation occurred at 530 meters near Rampart Cave, Arizona. Pleistocene middens from the southeastern Mohave Desert record a relatively large downward shift of the pinyon-juniper woodland zone, paralleling the remarkably low minimum elevation of the existing woodland zone in that area. The macrofossil evidence speaks for former continuity of the many disjunct stands of woodland vegetation in the Mohave Desert region, at least along the higher divides connecting most of the ranges. However, there is no macrofossil evidence of pluvial continuity of range for the more mesophytic, montane, coniferous-forest zone of ponderosa pine or white fir now occupying islands of relatively mesic environment on the highest mountains of the region. On the contrary, the uneven stocking of the lofty mountains of the Mohave Desert with mesephytic or boreal species and the trend toward endemism suggest a long history of isolation.

Gene flow versus selection pressure and ancestral differentiation in the composition of species: Analysis of populational variation ofJuniperus ashei buch. Using terpenoid data

“…The influence of P l e i s t o c e n e climatic fluctuations on the expansions and c o n t r a c t i o n s of species ranges is well documented. These effects have been amplified r e c e n t l y by the w o r k on rat middens and p a l y n o l o g y in the s o u t h w e s t e r n United States (King, 1973; M e h r i n g e r et al 1970; Van D e v e n d e r & King, 1971;Wells, 1965Wells, , 1966Wells, , 1970Wells & Berger, 1967;Wright, 1970;Whitehead, 1972). The floristic a f f i n i t i e s b e t w e e n the Edwards p l a t e a u of central Texas and the Sierra Madre of Nuevo Leon and C o a h u i l a has existed since the Tertiary (Axelrod, 1958).…”

Late Cretaceous and Tertiary vegetation history of Africa

“…At such a time, sclerophyllous vegetation must have shifted northward, into the present Namib and a good part of the western Karoo, where some relict patches exist today, as well. Such a shift parallels that of the pinyon pine-juniper woodland down into the present deserts of western North America during the last glacial (Wells 1966, Wells & Berger 1967. Inasmuch as there probably were four shifts of this order during the past 2.7 m.y., adaptation to the present desert environment need not be attributed to great antiquity.…”

“…This suggests that as recently as the last pluvial period, the Cape flora may have been far more widespread (Levys 1938), a relation paralleled by the nature of the plants recovered from late Pleistocene wood rat middens of the Mohave Desert and border areas (e.g. Wells & Berger 1967). Levyns (1964) suggested that the youthfulness of the flora in its present mediterranean-climate environment is implied also by the growth rhythms of some of the taxa.…”

A Flora of the Higher Ranges and the Kelso Dunes of the Eastern Mojave Desert in California

“…The flora of the pinyon-juniper woodland is relatively rich in shrubs , cacti, and perennial herbs; annuals are, as in the more mesic white fir-juniper - Figs Gilia, Mentzelia, Nemophila, Phacelia, Thysanocarpus, and Vulpia. Further information on the pinyon-juniper woodlands of the eastern Mojave Desert and their more important components can be obtained from Bradley and Deacon (1967), Castagnoli, de Nevers, and Stone (1981), Hart, Stein, and Warrick (1979), Lanner (1974), Mooney (1973), Prigge (1975), Reveal (1944), Roof (1978), Thome (1976Thome ( , 1979, Trombulak and Cody (1980), Twisselmann (1967) , Vasek (1966) , Vasek and Thome (1977) , Wells and Berger (1967);and West, Rea, and Tausch (1975).…”