Great Basin Naturalist


During winter 1993, Arizona experienced regional river flooding. Floodwaters at the Hassayampa River eroded floodplains and created a 50-m-wide scour zone available for colonization by pioneer plant species. The slow rate and long duration of the floodwater recession allowed the establishment of spring-germinating native trees (mainly Freemont cottonwood [Populus fremontii] and Gooding willow [Salix gooddingii] as well as summer-germinating species including the introduced salt cedar (Tamarix chinensis and related species). Gooding willow and Freemont cottonwood seedlings showed zonation in the floodplain, while salt cedar was equally abundant in zones with saturated and dry surface soils. Floodplain elevation (and soil moisture) influenced shoot growth rate to different degrees among the 3 species. For example, Gooding willow seedlings were significantly taller in areas with saturated soils than dry surface soils; Freemont cottonwoods were taller in the dry surface soil areas; and salt cedar were equally short in both soil moisture zones. Other factors that differentially influenced abundance or growth rates included competition with herbaceous species (Melilotus spp., an introduced plant, locally preempted salt cedar establishment) and herbivory (selective browsing by livestock at 1 river site reduced the natural height advantage of the native tree species). I draw on the results of this descriptive field study to suggest ways in which stream flows and floodplain land use can be managed to restore ecological conditions that favor native tree species over the introduced and widespread salt cedar.