38th Annual AASP MEETING, Sept. 2005, St. Louis, MO.
TWENTIETH-CENTURY VEGETATION CHANGE IN THE AMERICAN SOUTHWEST

Owen K. Davis, John Logan, Ryan Wicks & Chris Eastoe
University of Arizona, Tucson, Arizona.

      Palynological investigations of wetland sediments from southern Arizona have established a general pattern of historic expansion of woody plants such and juniper and mesquite in the uplands, and the expansion of trees such as ash, buttonbush, hackberry and willow in the wetlands . Both are accompanied by decreased charcoal abundance in the sediment (reduced fire frequency), and increased frequency of the spores of dung fungi (increased abundance of grazing animals ).
     Palynological investigations of coastal California wetlands record the same decreased charcoal abundances and increased abundances of dung fungi spores, but typically do not record the invasion of trees - with the exception of tamarisk.
     Here we present three new records of historic vegetation change as a comparison with the southern Arizona and coastal California pollen records. The first is a 6.4 m core from Sentenac Cienega, elevation 623 m, on the western boundary of the Anaza Borego Desert, San Diego Co., CA. Three radiocarbon dates confirm rapid sedimentation during the historic period. Like the southern Arizona records, Sentenac Cienega has been invaded by trees, oak woodland has expanded in the uplands, and the frequency of dung fungal spores has increased .
     The second record is from the forests of northern Arizona. Boneyard Fen (2620 m) occupies an opening in the mixed coniferous forest in Apache Co., AZ. The historic decrease in charcoal is recorded, but the frequency of dung fungal spores decreases rather than increases as in the southern Arizona sites. The vegetation remains forested during the historic transition, but the forest composition shifts from fir and douglasfir to spruce.
     The third record is from the grasslands of northwestern New Mexico. This record from sediment of a dry cave (1860 m, San Juan Co., NM) in Chaco Culture National Historic Park, shows a distinct decline in charcoal, an increases in shrubs (Chenopodiacea) and juniper, but only a modest increase in dung fungal spores.
     The records from three sites share with the southern Arizona chronology the decrease in charcoal abundance (decreased fire frequency); however, the Boneyard Fen record indicates a change in forest composition rather than forest expansion, and the abundance of grazing animals appears to have decreased rather than decreased.










THE SOUTHWEST
NEXT Montezuma Well Boneyard Fen San Pedro Coastal Sites Sentenac Cienega Tahoe










COASTAL SOUTHERN CALIFORNIA
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Erodium Corn spirogyra










    CONCLUSIONS

     SouthwestCoastalSentanacBoneyardChaco
    Fire (Charcoal)--- weak--
    Grazing (Sporormiella)+++ weak-+ weak
    Riparian VegetationWoody +Herb +Both +Herb ++ weak
    Upland VegetationWoody +Cheno, late+, lateTransform.Cheno, Woody +

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