Walking along the Crosscut Canal in Papago Park, Tom Hulen points to a sprawling salt cedar tree overtaking a native mesquite. Left unchecked, the salt cedar could kill the mesquite by blocking its sunlight and competing for water and nutrients.
Hulen, conservation director for the Desert Foothills Land Trust, then points out another salt cedar on the canal bank. And another. And another.
It’s a demonstration of what salt cedar, an invasive species brought to the West in the late 1800s, has done along many Arizona rivers, creeks and washes, Hulen says. Able to spread rapidly, salt cedar overwhelms native species including cottonwoods and willows, he says.
“Salt cedar is such a problem, especially along the lower Gila and Colorado rivers,” Hulen says. “In areas where salt cedar dominates, species diversity of plants and animals goes way down.”
Hulen and others advocate the removal of salt cedar on the grounds that it threatens native species, harms ecosystems and creates flood and fire hazards.
Others worry that the plant is a drain on water supplies, although that hasn’t been proved.
Salt cedar, or tamarisk, was imported from Asia as an ornamental plant and was later used to control erosion along riverbanks. Today, its pink and white flowers and reddish bark are a common sight along the Verde, Santa Cruz, San Pedro, lower Gila and Colorado rivers.
Rep. Lucy Mason, R-Prescott, who chairs the House Committee on Water and Agriculture, said she is concerned that salt cedar uses abnormally high amounts of water and could harm municipal water supplies.
“I want to garner volunteer support around the state to identify and eradicate wooded areas made of salt cedar along rivers and streams,” Mason said. “It’s a tremendous amount of work, but I think we need to do that.”
Hulen manages “Bring Back Cave Creek,” an effort to get rid of salt cedar and restore the habitat along Cave Creek, which runs through the northern part of the Valley.
Once a year for 10 days, around 30 people from the Volunteers for Outdoor Arizona chop down salt cedar trees and apply poison to the stumps.
Although completely eliminating salt cedar is an unrealistic goal, widespread removal should still be attempted, Hulen said.
“I don’t think people will spend the money necessary to get rid of it,” Hulen said. “But if we weren’t managing it, it would be the dominant plant.”
Several evolutionary advantages permit salt cedar to choke out native trees. For one thing, salt cedar has growing seasons in the spring and summer, while native trees germinate only in spring, Hulen said.
“Salt cedar has time to outgrow the competition,” Hulen said. “It also grows vegetatively, so I could break off a stem, stick it in the ground and it’ll grow.”
Tamarisks’ deep roots allow them to withstand drought and compete with cottonwoods and willows for sparse water.
Even a small amount of salt cedar in a site during a drought can increase the death of cottonwoods by 60 percent, said Tom Whitham, regents’ professor of biological sciences at Northern Arizona University, citing a study he helped conduct.
When salt cedar overtakes native plants, it displaces native wildlife, said John Brock, an Arizona State University professor of applied biological sciences.
“Studies show that 75 percent fewer birds utilize salt cedar than cottonwoods and willows,” Brock said. However, the endangered southwestern willow flycatcher is one bird that nests in salt cedar. Originally, it nested in cottonwood and willow trees, Brock said.
“The bird has adapted to tamarisks because nothing else is there,” Brock said. Salt cedar is not essential to the flycatcher’s existence if tamarisks are replaced with native vegetation, Whitham said.
Many traits attributed to salt cedar are not necessarily grounded in scientific fact. “Current studies show that salt cedars do not use more water than similar riparian plants such as the cottonwood and willows,” said Julie Stromberg, an associate professor in ASU’s School of Life Sciences.
Salt cedar may or may not affect water supplies, but it does pose potential flood and fire problems, said Herb Guenther, director of the Arizona Department of Water Resources.
Tamarisks clog rivers and cause overflowing while dense thickets are hot spots for wildfires.
Removing salt cedar is intensive and costly, said Guenther, who has helped manage salt cedar on the Colorado and Gila rivers. But so far Guenther said he hasn’t seen any proof that such efforts save water.