A 30000 YEAR RECORD OF VEGETATION DYNAMICS AT A SEMIARID LOCALE IN THE GREAT-BASIN

Plant macrofossils extracted from fossil woodrat (Neotoma spp.) midden s at a single locale in the northwestern Great Basin were used to exam ine vegetation dynamics during the last 30 000 yr. Although the modern assemblage of xeric species at the study site is a recent occurrence, a large proportion of the modern plant taxa near the study locale wer e also found 12 000 - 30 000 yr BP. The persistence of extant species through time was likely facilitated by within-species genetic diversit y and the formation of coenospecies. The diverse topographic and micro habitat features in the northwestern Great Basin also allowed differen t species to coexist during glacial periods. Changes in species compos ition occurred during two time intervals: 20 000 - 30 000 and 10 000 - 12 000 yr BP. Vegetation changes during 20 000 - 30 000 yr BP were cy clic; community composition oscillated between two groups of taxa. Veg etation changes between 10 000 - 12 000 yr BP occurred during the Plei stocene-Holocene transition and were largely directional from the Plei stocene assemblages through two transition assemblages to a Holocene a ssemblage. These changes in species composition generally reflect chan ges in climate. The presence of relatively mesic species during 10 000 - 30 000 yr BP is consistent with the regional late-Pleistocene clima te, and the gradual loss of relatively mesic species during the Holoce ne parallels the change to a more xeric climate. Contrasted with other areas of North America and Europe, the magnitude of vegetation change s at our study area were relatively small. Furthermore, the persistenc e of many species through time at this site in the northwestern Great Basin also differs from results at other study sites in North America and Europe. These differences are probably related to land form charac teristics and genetic diversity within species.