HOLOCENE VEGETATIONAL DISEQUILIBRIUM SUGGESTED BY SLOW DISPERSAL OF LATE-SUCCESSIONAL TREES AND SHRUBS OF WESTERN NORTH AMERICA

AMS radiocarbon dating of plant macrofossils from packrat (Neotoma spp.) middens verifies migrational timing of western North American trees and shrubs such as creosote bush (Larrea tridentata) and pinyon pines (Pinus monophylla, P. edulis). These records document their late Wisconsinan ranges and 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 and upslope into its present range. By 6000 yr B.P. it had spread upslope to above its current limits in the northern Mojave Desert. Despite evidence of cooling late Holocene climates from isotope and tree-ring proxies, it continued to migrate northward, albeit at a slower rate. Three northerly populations did not arrive at their current limits until around 4000, 2500, and 2000 yr B.P. Its migration lagged well behind other desert thermophiles suggesting that migrational distance played a key role in its rate of dispersal. Single-needle pinyon (Pinus monophylla) migrated northward from the Mojave Desert into the Great Basin arriving near its current northeastern limit as early as 7000 yr B.P. but more slowly in the western Great Basin where it reached its northwestern limit around 2000 yr B.P. 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 periodic climatic changes. But the long migration times required suggest that the primary factor slowing their response was migrational distance. These results have implications for vegetational effects of the expected climate shifts of the next century. The observed differential migration rates over short upslope distances versus long latitudinal distances, and between early and late successional plant species, suggest that these data may be of greater use in modeling the dynamics of differential vegetational response to future warming than in reconstructing Holocene paleoclimates