GROWTH AND SURVIVORSHIP OF FREMONT COTTONWOOD, GOODDING WILLOW, AND SALT CEDAR SEEDLINGS AFTER LARGE FLOODS IN CENTRAL ARIZONA

During winter 1993, Arizona experienced regional river flooding. Flood waters at the Hassayampa River eroded floodplains and created a 50-m-w ide scour zone available for colonization by pioneer plant species. Th e slow rate and long duration of the floodwater recession allowed esta blishment of spring-germinating native trees (mainly Fremont cottonwoo d [Populus fremontii] and Goodding willow [Salix gooddingii], as well as summer-germinating species including the introduced salt cedar (Tam arix chinensis and related species). Gooding willow and Fremont cotton wood seedlings showed zonation in the floodplain, while salt cedar was equally abundant in zones with saturated and dry surface soils. Flood plain elevation (and soil moisture) influenced shoot growth rate to di fferent degrees among the 3 species. For example, Goodding willow seed lings were significantly taller in areas with saturated soils than dry surface soils; Fremont cottonwoods were taller in the dry surface soi l areas: and salt cedar were equally short in both soil moisture zones . Other factors that differentially influenced abundance or growth rat es included competition with herbaceous species (Melilotus spp., an in troduced plant, locally preempted salt cedar establishment) and herbiv ory (selective browsing by livestock at 1 river site reduced the natur al 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 conditio ns that favor native tree species over the introduced and widespread s alt cedar.