Reconstruction of past migration rates of Southwestern plant species and their application to future distributions

Temperatures in southwestern North America rapidly increased by at least 4 ^oC around 11,600 years ago. The rate and magnitude of this warming were similar to projections for this region over the next century. Analysis of the ecological consequences of this prior warming can yield projections for future ecological change. Paleoecological data primarily from fossil packrat middens, but also from some pollen studies, suggest that immediately following this shift, early and mid-successional species increased while late-successional species with poor dispersal capabilities decreased. The late-successional species responded with relatively rapid upslope movements within several hundred years, but long-distance equilibration with the new climate took many thousands of years. Accelerator Mass Spectrometry radiocarbon dating of plant macrofossils from Holocene packrat middens representing first arrivals in new regions suggest that pinyon pine populations migrated at average rates of 44 m/yr (Pinus edulis), 19 m/yr (Pinus monophylla – western Great Basin), and 61 m/yr (Pinus monophylla – eastern Great Basin). Creosote Bush (Larrea tridentata) moved more slowly, averaging only 11 m/yr. Because these macrofossils are unlikely to be incorporated into the packrat fossil record until they are abundant across the landscape, these migration rates not only represent dispersal of propagules into new regions but also successional processes. As a result they integrate many of the dynamics of vegetation change and should serve as an indicator of the rate of future migrational responses resulting from climate warming.