EFFECTS OF SALINITY ON ESTABLISHMENT OF POPULUS-FREMONTII (COTTONWOOD) AND TAMARIX-RAMOSISSIMA (SALTCEDAR) IN SOUTHWESTERN UNITED-STATES

The exotic shrub Tamarix ramosissima (saltcedar) has replaced the nati ve Populus fremontii (cottonwood) along many streams in southwestern U nited States. We used a controlled outdoor experiment to examine the i nfluence of river salinity on germination and first-year survival of P . fremontii var. wislizenii (Rio Grande cottonwood) and T, ramosissima on freshly deposited alluvial bars. We grew both species from seed in planters of sand subjected to a declining water table and solutions c ontaining 0, 1, 3, and 5 times the concentrations of major ions in the Rio Grande at San Marcial, NM (1.2, 10.0, 25.7, and 37.4 meg l(-1); 0 .11, 0.97, 2.37, and 3.45 dS m(-1)). Germination of P. fremontii decli ned by 35% with increasing salinity (P = .008). Germination of T. ramo sissima was not affected. There were no significant effects of salinit y on mortality or above- and belowground growth of either species. In laboratory tests the same salinities had no effect on P. fremontii ger mination. P. fremontii germination is more sensitive to salinity outdo ors than in covered petri dishes, probably because water scarcity resu lting from evaporation intensifies the low soil water potentials assoc iated with high salinity. River salinity appears to play only a minor role in determining relative numbers of P. fremontii and T. ramosissim a seedlings on freshly deposited sandbars. However, over many years sa lt becomes concentrated on floodplains as a result of evaporation and salt extrusion from saltcedar leaves. T. ramosissima is known to be mo re tolerant of the resulting extreme salinities than P. fremontii. The refore, increases in river salinities could indirectly contribute to d ecline of P. fremontii forests by exacerbating salt accumulation on fl oodplains.