It took decades for invasive tamarisk trees to overtake waterways across much of the West, and local weed control experts say that it could take decades more for willows and other native trees to adequately recover.
While researchers have come to different conclusions about the impacts that tamarisk has on wildlife and water availability, there is no doubt that the plant has greatly changed the region's streams and rivers.
Tamarisk, also known as salt cedar, was imported to the area from Eurasia in the 1800s and early 1900s as an ornamental plant, windbreaker and riverbank erosion stabilizer. The plant spread and naturalized, and over the years it has displaced between 1 million to 1.6 million acres of native vegetation, according to the U.S. Bureau of Land Management's (BLM's) website.
Part of the reason that tamarisk has had such reproductive success in the West is because the plant was brought here, as the National Park Service (NPS) website puts it, “without its natural enemies.”
One of those enemies – the tamarisk beetle – has since been introduced to the area, and it is probably one of the best biocontrol insects that has been used, according to Michael Johnson, an associate professor at Utah State University’s Grand County Extension Office.
“Before (the beetles were introduced), tamarisk was growing with very little effort being made to control it,” Johnson said. “It took over vast areas, creating monocultures, pushing out other plants and using valuable water resources.”
After decades of study by the U.S. Department of Agriculture (USDA), the release of tamarisk beetle, which feeds almost exclusively on tamarisk plants, was approved for selected sites, including one in Delta, Utah. The first beetles were released there in 2001, according to Tim Graham, an ecologist who lives in Moab and has worked on tamarisk control for Grand County Weed Control.
In 2004, both Johnson and Grand County Weed Control Supervisor Tim Higgs answered an invitation from the USDA's Animal and Plant Health Inspection Service (APHIS) to collect some of the beetles from the Delta site. The beetles were then released on two approved sites in Grand County.
The beetles eat the foliage of the tamarisk, which weakens the plant and eventually kills it. The Nature Conservancy says it takes three to five years for beetles to kill a tamarisk plant, though it could be more or less depending on factors such as the size of the plant’s roots and how frequently it is defoliated by the beetles.
It may also depend on the amount of precipitation and flow of nearby waterways reaching the plants.
One study cited in a report published on the BLM website found that “under a natural flow regime, native trees are competitive with salt cedar (tamarisk)” and also that “during a flood year they have equal or faster growth rates.”
The report also explains another water-related factor that has contributed to the spread and tenacity of tamarisk: increased soil salt levels. Dams, diversions and flow regulation have caused floodplains to dry out and the riparian water tables to decline. Without water flushing out the naturally salty soil, the salt level has further increased. Unlike native plants, tamarisk thrives in these saltier conditions. Tamarisk deposits more salt into the soil when its leaves drop to the ground in the fall and decompose.
By the time Graham started working on the issue, in the autumn of 2007, he said the beetles had already made a noticeable difference.
“By 2009,” he said, “the beetles were browning almost all the tamarisk,” as the once-green plants began to die off.
Higgs reported that today, “there are areas where they’ve got 80 percent control (of the tamarisk population). But in other areas there’s only 10 to 15 percent control.” Higgs said that his office is working with researchers at the University of Denver to find out why this variation exists.
Johnson described the amount of willow and other vegetative regrowth in areas where the beetle has weakened or killed the tamarisk as “phenomenal,” and said he expected this naturally occurring regeneration of native species to continue. He also said that intentional replanting may be helpful in areas that have not experienced regrowth on their own.
Graham agreed that natives are coming back well in some areas.
“Other things are moving in,” he said, but added, “it depends on where, as to what is coming back.”
Graham said that he has observed lots of native plants reclaiming territory once held by tamarisk, such as willow, New Mexico privet and skunkbush – especially very close to waterways. But in other areas, the tamarisk has been replaced by plants that are also considered to be invasive, such as knapweed and Russian olive. Graham said he believes that native plants may eventually return on their own to these places as well, but it is too soon to know.
On some sites, beetle-killed tamarisk has been removed in hopes that doing so will make it easier for native plants to come back. However, Graham believes that does more harm than good.
“I think it’s a waste of time and money for us to go in and cut out the dead tamarisk,” he said. “From an ecological perspective, it’s a negative impact ... If you don’t have any trees, birds won’t come and perch. If there is dead tamarisk, the birds will come and leave the seeds for other plants to establish. You have a better chance of recovery than if you cleared it all out.”
He also added that removing the shade that the dead trees provide can make it more difficult for other species to establish.
The BLM website identifies extensive areas of dead tamarisk as a fire risk, but as Johnson points out, “even green tamarisk burns readily.”
Johnson said that he hears occasionally from residents who believe their plants are being eaten by tamarisk beetles, but it has always turned out to be something else. He explained that “many people confuse the tamarisk beetle with the elm leaf beetle and the cottonwood leaf beetle, all of which have some similar colorization and markings.”
There is also concern for the southwestern willow flycatcher, an endangered bird that has started nesting in tamarisk, in the absence of willow. The Tamarisk Coalition, a regional organization dedicated to the restoration of riparian lands, reports that tamarisk defoliation has lessened chick survival. Its website advises that the tamarisk “may assist in the long-term recovery and resiliency of riparian communities, but potential short-term consequences cannot be disregarded.”
Johnson noted that the tamarisk will not ever disappear entirely.
“In some areas it’s possible it could be lessened by 85 percent, but in the meantime it’s allowed the willows to start growing again,” he said.
He counseled patience with the process.
“It could take decades to get good regrowth,” he said.