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The analysis of sediments from six wetlands (cienegas) in the Sonoran Desert of Arizona, U.S.A., and Sonora, Mexico, document a marked expansion of wetland taxa-particularly woody plants-about 200 years ago at the beginning of the historic period, following a decrease in charcoal percentages and increased percentages of the dung fungus Sporormiella. The presence of charred seeds and fruits of wetland plants in prehistoric sediment establishes burning of the cienega itself. The charcoal decline ca. 250 years ago precedes the first occurrence of the pollen exotic plants at several sites, the change of cienega sediment from silt to peat, and the increase of percentages of the decay fungus Tetraploa. We conclude that prior to the historic period, burning was frequent enough to exclude most woody plants (Celtis, Cephalanthus, Populus, Fraxinus, Salix) from the wetlands and suppress the abundance of bulrush (Scirpus). The cienegas were probably burned seasonally as a management tool to harvest animals and promote agriculture. Prehistoric agricultural utilization of the cienegas is demonstrated by the presence of corn (Zea) and pre-Columbian weeds. This study also records post-settlement (ca. 200 years ago) change of upland vegetation; i.e. an increase in the abundance of Juniperus, Quercus, Larrea, and Prosopis pollen. Historic fire suppression may have permitted the expansion of these non-wetland woody species. (C) 2002 Elsevier Science Ltd.