( ) Journal of Arid Environments 1998 40: 133155 Article No. ae980438 Dynamics of Fremont cottonwood ( ) Populus fremontii and saltcedar ( ) Tamarix chinensis populations along the San Pedro River, Arizona J. Stromberg Department of Plant Biology, Arizona State University, T empe, AZ 85287-1601, U.S.A. ( ) Received 10 February 1998, accepted 26 June 1998 ( ) Woodlands of the exotic saltcedar Tamarix chinensis have replaced forests of ( ) ( ) native Fremont cottonwood Populus fremontii and willow S alix spp. along many rivers of the American South-west. In the middle basin of the San Pedro River, saltcedar dominates only at the drier sites where the surface and ground-water conditions no longer support cottonwood  willow forests. At ( ) sites with perennial or near-perennial stream flow, saltcedar is co-dominant with Fremont cottonwood. However, saltcedar has been declining in importance at these sites, perhaps due to recent occurrence of conditions that ( favour cottonwood establishment frequent winter flooding, high rates of ) stream flow during spring, exclusion of livestock . This shift provides evidence of capacity for self-repair in degraded Sonoran riparian ecosystems. In the upper basin, in contrast, saltcedar has increased in relative abundance at sites that show evidence of ground-water decline, signaling a need for vigilance in river management. Saltcedar is generally sparse in the upper basin, probably due to the combination of cool temperatures and persistence of perennial or near-perennial stream flows in most areas. Throughout the San Pedro River, saltcedar and cottonwood both have been influenced by changing flood patterns. Expansion of Fremont cottonwood populations and initial colonization by saltcedar both correlate with post-1960 increases in fall and winter flood frequency and decreases in summer flood size.  1998 Academic Press Keywords: population dynamics; Populus fremontii; T amarix chinensis; hydrologic regimes Introduction Hydrologic regimes strongly influence the structure and function of wetland and ( ) riparian vegetation Bedford, 1996; Poff et al., 1997 . Relationships between flow ( ) regimes and dynamics of cottonwood Populus populations are particularly well known, mainly because hydrologic threats to cottonwood-dominated rivers have ( ) stimulated extensive research Braatne et al., 1997 . Studies of the A igeiros section of 0140196398020133  23 $30.000  1998 Academic Press