The showy tamarisk tree, aka "Saltcedar," and Russian olive long have been reviled as thirsty scourges of Western national park riparian areas. But new research shows these trees are not any thirstier than some native species. However, tamarisk isn't valued as highly as cottonwoods and willows by some bird species, the studies say.
The research, conducted by the U.S. Geological Survey, U.S. Bureau of Reclamation, and the U.S. Forest Service at the request of Congress, do point to problems the trees pose for riverside recreation, noted they could create wildfire problems, and and also can clog irrigation ditches.
The scientists conducted a review of the scientific literature to assess the existing state of the science on the distribution and spread, water consumption, and control methods for tamarisk and Russian olive. They also assessed the considerations related to wildlife use and the challenges associated with revegetation and restoration following control efforts.
Along the Yampa River and Green River corridors through Dinosaur National Monument tamarisk long has been a problem in terms of forcing native species out of the landscape and overgrowing campsites. Volunteer "weed warriors" have been spending dozens of days each summer uprooting the trees to make way for the natives. Uprooted trees are stacked along the rivers for a year to die and dry out, then are tossed into the rivers, where the currents and rapids grind up the trees.
Tamarisk also poses problems along riparian areas in Arches, Canyonlands, and Grand Canyon national parks, as well as Glen Canyon National Recreation Area.
According to the latest report, though, native cottonwoods and willows "typically consume as much water as non-native saltcedar and Russian olive," and so removing the trees with thoughts of saving water is not necessarily a sound strategy.
“None of the published studies to date, which include projects removing very large areas of saltcedar, have demonstrated production of significant additional water for human use," said Curt Brown, director of research for the Bureau of Reclamation.
However, the authors note that saltcedar and Russian olive can also grow on river terraces that are too high and dry for cottonwoods and willows. Some scientists have suggested that, on these sites, revegetation with native dry-site species could save some water for human use. But, the effectiveness of such an approach has not been demonstrated.
The report also noted that while it "has long been assumed that these non-native trees harm streamside habitat and wildlife productivity, research evaluated in the report indicates this isn’t always true. Many reptiles, amphibians, and birds use habitat dominated by saltcedar and Russian olive. Even the endangered southwestern willow flycatcher frequently breeds in saltcedar stands."
Some other species, such as woodpeckers and cavity-dwellers, do not flock to tamarisk-dominated landscapes. Dense tracts of pure saltcedar are typically unfavorable for most wildlife, and the report notes that many birds still prefer native cottonwood or willow habitat.
Saltcedar and Russian olives are now the third and fourth most common streamside plants in 17 western states. The species have been the focus of significant removal efforts along some western rivers, such as the Rio Grande and Pecos River, the report noted.
Plant removal techniques range from use of herbicides and bulldozers to biological controls such as insects. Once the invasive plants are killed or removed, effective restoration depends on replacing them with plant species that meet the specific goals of the planned restoration, the report said.
“The vegetation that replaces saltcedar following its removal, with or without restoration actions, will influence the quality of wildlife habitat, amount of water use and other ecological conditions,” said Pat Shafroth, a USGS scientist and lead editor of the report.
Site restoration, however, can be challenging and costly, depending on the size of the area and the methods used. Restoring key river processes, such as natural patterns of high and low flows, can help re-establish native vegetation and other important ecosystem features over larger areas than is possible with site-specific restoration, he added.
In some areas, biological controls in the form of beetles that attack tamarisk trees have been used with hopes of reducing the spread of the trees. However, scientists need to better understand the processes involved with this approach.
“Research and monitoring could be particularly important in the context of biological control of saltcedar,” Shafroth said. “The beetle that has been released for biological control has been defoliating saltcedar and spreading rapidly in some watersheds. We really need to understand the effects of biocontrol on these ecosystems, to better inform river and riparian restoration.”
The report, Saltcedar and Russian Olive Control Demonstration Act Science Assessment, was completed to fulfill requirements in the Salt Cedar and Russian Olive Control Demonstration Act of 2006 (Public Law 109-320).