A 30000 year record of vegetation dynamics at a semi-arid locale in the Great Basin

Plant macrofossils extracted from fossil woodrat (Neotoma spp.) middens at a single locale in the northwestern Great Basin were used to examine vegetation dynamics during the last 30 000 yr. Although the modem assemblage of xeric species at the study site is a recent occurrence, a large proportion of the modem plant taxa near the study locale were also found 12 000 30 000 yr BP. The persistence of extant species through time was likely facilitated by within-species genetic diversity and the formation of coenospecies. The diverse topographic and microhabitat features in the northwestern Great Basin also allowed different species to coexist during glacial periods. Changes in species composition occurred during two time intervals: 20 000 30 000 and 10 00012 000 yr BP. Vegetation changes during 20 000 30 000 yr BP were cyclic; community composition oscillated between two groups of taxa. Vegetation changes between 10 000 12 000 yr BP occurred during the Pleistocene-Holocene transition and were largely directional from the Pleistocene assemblages through two transition assemblages to a Holocene assemblage. These changes in species composition generally reflect changes in climate. The presence of relatively mesic species during 10 00030 000 yr BP is consistent with the regional late-Pleistocene climate, and the gradual loss of relatively mesic species during the Holocene parallels the change to a more xeric climate. Contrasted with other areas of North America and Europe, the magnitude of vegetation changes at our study area were relatively small. Furthermore, the persistence 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 characteristics and genetic diversity within species.