Abstract: Comparison of saltcedar (Tamarix ramosissima) and native trees with respect to stress tolerance, evapotranspiration and ability to establish on western rivers

Tamarix ramosissima (saltcedar) has increased on southwestern U.S. rivers to such an extent that it is now the dominant species on most of the region's major, perennial river systems. It is considered to have a suite of physiological advantages over native vegetation, particularly Populus fremontii (cottonwood) and Salix gooddingii (willow) trees, which have drastically declined on the same rivers. The putative advantages include: high rates of evapotranspiration (ET), by which it desiccates water courses at the expense of native trees; high tolerance to salinity, drought and flooding; ability to regrow following fire; and ability to establish rapidly following disturbance, crowding out native tree seedlings. These advantages are thought to be so overwhelming that, once saltcedar is established, it must be eradicated before native trees can regenerate. Recent studies confirm the greater salt and drought tolerance of saltcedar compared to native trees. However, they also show that rates of ET are within the range reported for cottonwood and willow, and that ET rates along saltcedar-dominated river stretches are low to moderate. Furthermore, saltcedar has lower tolerance to flooding than native trees, and competes poorly with native trees at the seedling stage in cleared plots. On rivers where a pulse-flood regime has been returned, such as in the delta of the Colorado River in Mexico, cottonwoods and willows have successfully regenerated despite the presence of saltcedar. Saltcedar can fulfill some of the same ecological functions as native trees on disturbed rivers. An alternative management strategy to eradication of saltcedar would be to return a more dynamic hydrological regime to regulated rivers wherever possible, allowing saltcedar and native trees to coexist to maximize the habitat value of the riparian zone.

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