Functional equivalency of saltcedar (Tamarix chinensis) and fremont cottonwood (Populus fremontii) along a free-flowing river

Saltcedar (Tamarix chinensis) reportedly has altered ecosystem function in many riparian areas of the American Southwest. I investigated the functional role of this exotic species relative to native Fremont cottonwood (Populus fremontii) along the free-flowing middle San Pedro River in southern Arizona. Floodplains in this river reach are co-dominated by young stands of saltcedar (<44 yrs old) and Fremont cottonwood (<67 yrs old). Mean values for 22 of 30 soil, geomorphology, and vegetation structure traits did not differ significantly between saltcedar and Fremont cottonwood stands. Twenty-six of the 30 traits had similar patterns for change over time (inferred from a succession gradient). Saltcedar was functionally equivalent to Fremont cottonwood for about half of the traits construed as indicators of riparian ecosystem function. Several functions or traits that are theoretically or observably influenced by saltcedar on other rivers (e.g., sedimentation rates, electrical conductivity of soils) did not differ between the two species along the San Pedro. This suggests that the functional role of saltcedar is context-specific and variable among rivers. Also in contrast to the working paradigm, saltcedar appeared to enhance the maintenance of floristic biodiversity. Understory herbaceous cover and species richness were significantly greater than in cottonwood stands, perhaps due to soil differences that developed between the two stand types (e.g., higher clay content in saltcedar soils). Stem densities of velvet mesquite (Prosopis velutina) and other woody successional species did not differ between saltcedar and cottonwood stands. However, stem densities for this group increased with stand age only for cottonwood, raising the possibility that saltcedar may disrupt successional pathways. Other notable differences included greater basal area and canopy height in cottonwood stands, and accumulation of more phosphorus in cottonwood soils than in saltcedar soils.