| Popis: |
Pollen, spore, and algae records from a variety of depositional contexts provide 12 ka of paleoclimatic proxy data for southwestern North America. Macrofossils and pollen from ancient woodrat nests in the drier interior provide species-level identification and exact spatial and chronometric information. Occasionally, charcoal counts from pollen slides provide fire history data. Vegetation dynamics are primarily recorded on century to millennial timescales, though in a few cases to the near-decade level. During the Pleistocene/Holocene transition, conifer forests were replaced by oak-dominated deciduous forests in more mesic (moister) areas, and in the drier interior, subalpine woodlands retreated upslope and were replaced by juniper woodlands. In some areas, sagebrush steppe was replaced by chaparral and oak woodlands. In the Great Basin and northern desert southwest, retreating sagebrush steppe and juniper woodlands were replaced by drought-tolerant shrub communities. As Mojave and Sonoran desert species became established near their current limits during the early Holocene, pinon pine began its northward expansion from the lower Colorado River area, reaching its northern limits during the middle to late Holocene. During the middle Holocene, more xerophytic plant species dominated the interior, though more mesophytic species were common in southern California. After 6360 cal BP (5500 rcyr BP), more moisture-loving species reexpanded in the region, with their distribution climaxing about 4090 cal BP (3700 rcyr BP). Current plant distributions were established by about 1980 cal BP (2000 rcyr BP) following extensive periods of fire which consumed much of the lower-elevation Neopluvial forest. |
