Growth rates, salt tolerance and water use characteristics of native and invasive riparian plants from the delta of the Colorado River, Mexico

Six riparian plant species representing native and invasive species from the Colorado River delta in the Sonoran Desert of Mexico were tested for salt tolerance and water use characteristics in a greenhouse study in Tucson, Arizona. Negative linear regression equations relating relative growth rates (RGR, g g−1day−1) of each species to mean root zone salinity had high coefficients of determination (r2=0.0573–0.0586,p< 0.05001). Salt tolerance levels, expressed as % reduction in RGR per g l−1NaCl in soil solution, varied widely among species:Allenrolfea occidentalis, 0% reduction;Tamarix ramosissima, 1.058% reduction;Pluchea sericea, 3.055% reduction; andBaccharis salicifolia, Salix gooddingiiandPopulus fremontii, 7–9% reduction (p< 0.0505). Transpiration was proportional to RGR for all species. Contrary to some previous reports,Tamarixdid not have unusually high water use compared to the other species. Differences in salt tolerance among species determined in this study support field observations that soil salinity, which can reach high values along channelized and flow-regulated stretches of south-western United States rivers due to lack of overbank flooding, is a major factor in the replacement of native riparian species by invasive species.