Physiological Ecology of Saltcedar:
Why is it a Successful Invader?

Many riparian communities of the arid southwestern United States are dominated by Tamarix ramossima (saltcedar), a naturalized shrub or small tree that outcompetes native riparian taxa and is also reputed to be capable of losing vast quantities of water via transpiration. Our research program over the past 8 years has been aimed at determining

(1) what are the ecophysiological attributes that allow saltcedar to be such a successful invader and subsequent competitor, and

 

(2) if saltcedar does indeed lose significantly higher quantities of water than donative phreatophytes, and why that may be?

 

Our first set of studies were concentrated along the lower Colorado river. The primary objective was a determination of why saltcedar is such a successful invader and competitor in native vegetation. To accomplish this, riparian communities along the Colorado River that were dominated by dense stands of saltcedar were compared to riparian communities along the tributary Bill Williams River, which has a mixture of stands dominated by saltcedar and by native cottonwood-willow (Populus-Salix) forest, as well as mixtures of the two community types. The results of this study showed that

(1) as saltcedar invades riparian communities, it tends to salinize the ecosystem with time,

 

(2) saltcedar is more tolerant of both salinity stress and water stress than are native riparian trees,

 

(3) experimental removal of saltcedar thickets from around mature willow trees resulted in an alleviation of water stress and greater growth of the willow trees, and

 

(4) in post-burn communities, saltcedar resprouts tend to exhibit reduced water stress and greater regrowth than do sympatric resprouts of willow. All of these attributes favor saltcedar over native cottonwood and willow trees in these riparian ecosystems, particularly as the watersheds become more arid due to a lack of spring flooding and declining water tables.

 

Our more recent studies have investigated evapotranspiration (ET) from a variety of scales (leaf to landscape) in saltcedar-dominated floodplains along the lower Virgin river in southern Nevada. Leaf-level gas exchange indicated that saltcedar exhibits similar stomatal conductance as the sympatric phreatophytes arrowweed, mesquite, and willow. However, sap flow in saltcedar was higher per unit sapwood area than the other species due to it having the ability to maintain higher leaf area per unit sapwood area. At the stand level, ET of dense saltcedar stands was found to exceed potential ET early in the summer when soils are moist and the water table is shallow, but in late summer, after floodplain soils dry and the water table drops, saltcedar ET tends to be well below potential rates.

Our studies of individual saltcedar plants in lysimeters indicate that saltcedar can withstand the loss of a water table and can survive on soil water to very low soil water contents. This has been independently verified using the stable isotopes of water in field plants; saltcedar is a facultative phreatophyte that can survive on soil water alone in the summer, whereas willows and cottonwoods are obligate phreatophytes that cannot lose contact with groundwater. As saltcedar stands mature, their extremely high water loss tends to lower water tables to levels that are often below the rooting zone of cottonwoods and willows, particularly small plants.

Over time, this desertification and salinization of the watershed results in localized extinction of the native trees and eventually to complete dominance of the floodplain by monospecific saltcedar thickets. Once this dominance is attained, saltcedar appears to control whole ecosystem processes and to effectively preclude the reintroduction of native species through natural processes. Because of these accumulated ecosystem effects of saltcedar, we conclude that reclamation efforts that remove dense saltcedar stands but do not reintroduce historical flow regimes will not be successful in reestablishing native riparian taxa without extensive follow-up management.

Return to Workshop Home Page or continue on to the next paper "Impacts to Biodiversity (Plants, Fish, Wildlife, Invertebrates) From Expanding Saltcedar."

For information on the outcome of this workshop or integrated weed management in the Pacific Region (Region 1), U.S. Fish and Wildlife Service, Portland, OR, contact: Scott_Stenquist@fws.gov