MECHANISMS ASSOCIATED WITH DECLINE OF WOODY SPECIES IN RIPARIAN ECOSYSTEMS OF THE SOUTHWESTERN US

Throughout western North America, riparian ecosystem function has been transformed by anthropogenic influences on riverine environments, Mod ified hood frequency, duration, or intensity; depressed floodplain wat er tables; and increased alluvium salinity have contributed to change in riparian forest communities formerly dominated by Populus fremontii and Salix gooddingii. The invasion of the naturalized arborescent shr ub, Tamarix ramosissima, potentially alters competitive hierarchies an d disturbance regimes in these riparian ecosystems. We evaluated the s tructure and function of two southwestern riparian communities that di ffered in the degree of streamflow perturbation to which they had been subjected: the highly regulated lower Colorado River and the less tig htly regulated Bill Williams River. Ordination analyses provided evide nce that these riparian communities are structured along gradients rel ating to moisture, salinity, disturbance from fire, and community matu rity, with Colorado River sites being more xeric and saline than those on the Bill Williams River. Foliar elemental analyses revealed high s odium concentrations in Tamarix (Na:K ratio = 1.87) and in the native shrub Tessaria sericea (Na:K = 1.56). Evaluation of tissue water relat ions parameters showed that Tamarix had lower osmotic potentials than sympatric woody taxa, helping to confirm that Tamarix is halophytic an d probably capable of greater osmotic adjustment than native species. Carbon isotopic discrimination (Delta) provided evidence for higher wa ter use efficiency in Tamarix than in Populus, Salix, and Tessaria. Ta marix Delta averaged over 1 parts per thousand less than that of the o ther riparian taxa, Experimental removal of Tamarix from stands where Salix was codominant resulted in growth augmentation, less negative wa ter potentials, and higher leaf conductance in Salix, all providing ev idence of interspecific competition. The persistence of Salix, but not Populus, on the Colorado River appears to be due to greater water- an d salinity stress tolerance in Salix than in Populus. A preponderance of senescent Populus along the Colorado River is an indication that th is formerly dominant species is effectively approaching local extincti on in parts of this ecosystem.