Succession of desert plants on debris flow terraces, Grand Canyon, Arizona, U.S.A. Janice E. Bowers, Robert H. Webb & Elizabeth A. Pierson U.S. Geological Survey, 1675 W. Anklam Road, Tucson, AZ 85745, U.S.A. ( Received 16 May 1995, accepted 3 April 1996) Vegetation sampling on 11 debris flow terraces in Grand Canyon National Park, Arizona, U.S.A., showed that plant assemblages changed as age of surface increased. The terraces ranged in age from about 5 to about 3100 years. There were distinct differences among sites in the life history characteristics of the dominant plants. Young terraces (5–55 years) were dominated by short-lived plants that had high reproductive potential. Older surfaces were dominated by species with longer life-spans and lower reproductive potential. Density and cover of long-lived species increased with age of surface; for short-lived plants, density was inversely related to surface age. Species composition was also correlated with site age; however, location, exposure, and other factors ensured that no two debris flows supported identical mixtures of species. Succession on recent Grand Canyon debris flows is driven in part by life-history strategies, particularly life-span and seed dispersal traits, and also by climatic factors, especially those that control germination and establishment of the long-lived dominants. ©1997 Academic Press Limited Keywords: debris flows; desert scrub; life history strategies; longevity; seed dispersal; succession; Grand Canyon National Park Introduction During the past three decades, researchers have documented plant succession at a number of locations in the arid south-western United States. Most of these studies examine succession after recent disturbance such as construction (pipeline corridors, power transmission lines), habitation (abandoned townsites), and agriculture (old fields) (Wells, 1961; Vasek et al ., 1975 a, b; Webb & Wilshire, 1979; Karpiscak, 1980; Lathrop & Archbold, 1980 a, b; Carpenter et al ., 1986; Webb et al ., 1988). A few studies document succession on undisturbed sites such as debris flows and alluvial terraces that are many thousands of years old (Webb et al ., 1987, 1988; McAuliffe, 1991). To date, succession on recent debris flows and alluvial terraces has been little investigated. In this study, we examined debris flow terraces in Grand Canyon, Arizona, for evidence of plant succession. Debris flows are water-based slurries of poorly sorted material ranging in particle size from clay to boulders (Costa, 1984; Webb et al ., 1989). Journal of Arid Environments (1997) 36: 67–86 0140–1963/97/010067 + 20 $25.00/0/ae960197 © 1997 Academic Press Limited