Salt cedar: Villain or scapegoat when it comes to water use?
|Title||Salt cedar: Villain or scapegoat when it comes to water use?|
|Publication Type||Feature Articles|
|Journal||Southwest Climate Outlook|
|Alternate Journal||Southwest Climate Outlook|
|Full Text|| |
Salt cedar’s reputation as a high water user has made it the bane of water agencies for many decades. When the drought slowed the flow of many southwestern rivers down to a trickle in 2002, its presence along New Mexican waterways even made it a target of then-gubernatorial candidate Bill Richardson.
Upon his election, Richardson followed through with his plan to eradicate salt cedar stands lining the state’s riverbeds. In 2003, the state spent $4 million to spray the herbicide Arsenal from helicopters onto stands of salt cedar, also known as tamarisk because of its scientific name (Tamarix species, mainly ramosissima). About 25,000 acres of salt cedar had been so treated by spring of this year, according to an April 1 op-ed piece in the Albuquerque Journal by Assistant Secretary of the Interior Rebecca Watson, who touted the eradication effort as an outstanding example of water conservation in the West.
Yet there are some who consider salt cedar to be a scapegoat. One of these skeptics is Edward Glenn, a senior research scientist with the University of Arizona’s Environmental Research Laboratory. Glenn mentored then-graduate student Pam Nagler in research estimating water use of salt cedar compared to other species based on their leaf area indices and other remotely sensed data for a roughly 200-mile stretch of the Lower Colorado River.
“Particularly, salt cedar doesn’t seem to be the big hog, the biggest water user, that people have given it credit for,” Glenn said. “For years and years, peoplewould quote these figures that they were using 3 to 4 meters of water a year, but they didn’t have good methods for measuring it.”
More recent techniques using sophisticated technology have found that salt cedar trees were using comparable amounts of water as the native cottonwood and willow trees they are seen as replacing.
“They (researchers) found that it actually uses less water than Bermuda grass. So your back lawn is actually using more water than salt cedar,” Glenn said. Nagler, Glenn and others reported in a 2004 paper in Agricultural and Forest Meteorology that salt cedar actually appeared to consume less water than cottonwood, based on leaf area indices.
A year-long study conducted by Steve Hansen, an assistant area manager for the Albuquerque office of the U.S. Bureau of Reclamation (USBR), and colleagues found that salt cedar at the site they measured in the late 1990s used about 4 feet of water a year. This is about one third of the 4 meters it had been accused of consuming, although values would vary somewhat by site. Salt cedar used about the same amount of water as alfalfa, and roughly 20 percent more water than cottonwood, Hansen’s research indicated.
Glenn credited Juliet Stromberg, an associate professor at Arizona State University, with launching the effort to examine the salt cedar issue objectively.
Stromberg explained by telephone that she falls into the camp of researchers who suspect salt cedar has proliferated because of changes in streamflow patterns, livestock grazing, water availability, and water quality. Given sufficiently high water tables and natural flood regimes (which reduce soil salinity) and protection from grazing, cottonwoodand willow will grow taller than salt cedar and therefore maintain dominance in stands, her research indicates.
“There is an assumption that salt cedar has contributed to changes in stream hydrology and geomorphology that has, in turn, reduced the ability of cottonwood and willow to survive,” she explained.
However, seeds from both native species are distributed and nourished by a natural flood regime, which typically is lacking in the dam-regulated environment of western rivers. In addition to salt cedar, houses tend to line the rivers, and it’s doubtful many residents would welcome annual floods. Also, the ongoing water use by the growing population of people and by long-time farmers may be lowering the water table beyond the tolerance of cottonwood and willow.
“If salt cedar is not the cause—if it’s just sort of a symptom—then if you clear the salt cedar you haven’t addressed the root cause of vegetation change,” she added. Rather than native vegetation, salt cedar is likely to return, unless changes occur in the management of rivers and floodplain lands.
New Mexico planners have not yet moved fully into the stage of re-establishing native vegetation to replace the Arsenal-killed salt cedar stands. Although thousands of salt cedars lining the Middle Rio Grande River are “deader than a hammer,” many of them remain standing on the landscape while officials confirm their demise, Hansen said. State officials are trying to figure out what to do with all the dead wood, which can act as a fuel source in case of fire, or transform into dangerous woody debris in case of floods. Until then, little can be done to re-establish native species, he indicated.
Streamflow in river stretches in which salt cedar was killed are not showing clear signs of an increase in water yield since eradication, said Hansen, who attributed this to an inability to measure water levels accurately enough to detect a difference. He compared the concept of measuring a difference to trying to detect how much water a person has consumed based on a change in their weight. Instead, he suggested it is more accurate to measure the actual amount of water the person consumed, as with studies like his that document how much water a salt cedar tree consumes.
Given the relatively small portion of the water allotment consumed by “phreatophytes” like salt cedar, cottonwood, and willow—which the USBR estimates at about 7 percent of its total water budget along the lower Colorado River from Hoover Dam to Mexico—it’s even more understandable that a difference would be difficult to detect. Based on the 7 percent proportion, even if salt cedar represented all the phreatophytes and was completely replaced with cottonwood stands that used 20 percent less water, the best the Bureau could hope for would be about a 1.4 percent increase in total available water along this stretch.
Still, in the Middle Rio Grande, the savings estimated from the approximately 60,000 acres covered by salt cedar in 2002 potentially would amount to about 40,000 acre-feet of water, Hansen noted. However, if riverside trees follow water use patterns similar to mountaintop trees, the water yield increase may be more obvious during wet years than dry ones. (See related story in this issue.)
Also, it’s a bit more complicated than a one-to-one replacement of salt cedar with native vegetation because cottonwood and willow trees won’t necessarily be able to survive in the same places occupied by salt cedar, noted Fred Nibling, a research botanist for the USBR’s Denver office.
“The difference is the footprint on the terrain that salt cedar is capable of occupying is much greater than that of cottonwood and willow,” he elaborated. So the eradication program could help the USBR in its mission to deliver the allotted water to its clientele, which includes farmers near New Mexico’s Elephant Butte Reservoir who have not received their full allotment for several years.
Salt cedar is considered an invasive species by most ecologists. It was introduced to the West from Asia, in part to help control erosion. Its ability to live along relatively dry channels that do not support other riparian species does help prevent erosion, but salt cedar is also accused of making the soil more saline via leaffall, and of contributing to flood risk by narrowing channels.
Nibling acknowledged that the situation posed an environmental challenge, with the goal of controlling invasive plants (salt cedar) competing with the goal to protect endangered species (including the willow flycatcher, which does well in salt cedar stands).
“It’s an interesting quandary,” Nibling said. “It’s really a challenge to our scientific skills to make it work for both groups.”