Holocene Migrations of Creosote Bush and Pinyon Pines in the Western United States: Implications for the Next Century.

The biogeographic histories of plant species of the arid western United States are becoming evident as more local paleoecological series are developed and compiled into regional databases. Plant macrofossils from packrat (Neotoma spp.) middens have been especially useful for reconstructing past distributions of arid and semi-arid species such as creosote bush (Larrea tridentata) and one and two-needle pinyon pines (Pinus monophylla, P. edulis). These records document the late Wisconsinan ranges of these species and their subsequent Holocene migrations into their current ranges. Creosote bush grew in the lower Colorado River Valley during the late Wisconsinan (Isotope Stage 2). Starting around 11,000 yr B.P., it migrated northward into its present range. By 6000 yr B.P. it grew at higher elevations than at present in the central Mojave Desert, but did not reach its extreme northern limits until around 4000 yr B.P. Other populations, such as near the shrubs upstream limit along the Colorado River, were not established until the last 2500 years. Its arrival at its most northerly sites lagged well behind other desert thermophiles. Single-needle pinyon (Pinus monophylla) migrated northward from the Mojave Desert into the Great Basin arriving near its current northeastern limit in the eastern Great Basin as early as 7000 yr B.P. It migrated more slowly in the western Great Basin possibly not reaching its northwestern limit until the last 2000 years. Colorado pinyon (Pinus edulis) migrated from near its current southern boundary northward reaching the eastern Grand Canyon as early as 10,600 yr B.P. It is not recorded from central Utah until after 7000 yr B.P. It evidently moved northward slowly, arriving at some northerly and easterly stands only within the last 1000 years. These migrational histories reflect a combination of dispersal limitations and gradual climatic changes. But the long migration times required, coupled with their expansion above their modern elevational limits during the middle Holocene, suggest that the primary factor slowing their response was migrational distance rather than a monotonic trend of warming climates through the Holocene. These results have implications for vegetational effects of the expected climate shifts of the next 100 years. Although this change may be as little as a third as the 6oC warming that occurred near the beginning of the Holocene, the past rates of migration suggest that little equilibration with the new climate can be expected in time spans under 1000 years. Also, mapping of 20 climate variables describing the modern climatic tolerances of these species suggests that they already have significant available potential range, mostly to the north of their current ranges, that should now be suitable for their expansion. These results suggest that either the late Holocene populations had not yet equilibrated with the Pleistocene to Holocene change in climate, or that climate has already warmed so much since the Little Ice Age that many species are no longer in equilibrium with late Twentieth Century climate.