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Long considered heavy water users and poor wildlife habitat, non-native saltcedar and Russian olive trees that have spread along streams and water bodies in the West may not be as detrimental to wildlife and water availability as believed.
In a U.S. Geological Survey report requested by Congress and released today, 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 saltcedar (also called tamarisk) and Russian olive. They also assessed the considerations related to wildlife use and the challenges associated with revegetation and restoration following control efforts.
The report was a collaboration among the USGS, the Bureau of Reclamation, U.S. Forest Service, and other federal agencies and universities to assess and summarize a large number of previously published studies.
One notable finding is that native trees such as cottonwoods and willows along western rivers typically consume as much water as non-native saltcedar and Russian olive. Generally, the report noted, removal of saltcedar from floodplain areas along rivers leads to replacement by other vegetation that consumes roughly equal amounts of water. Therefore, removal of saltcedar from these areas is unlikely to produce measurable water savings once replacement revegetation becomes established, report authors wrote.
“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.
Similarly, although 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.
However, according to the report, saltcedar-dominated landscapes do not provide suitable habitat for more specialized birds, such as woodpeckers and birds that live in cavities. 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. Other negative impacts of dense stands of these introduced species can include impeded access to riverside recreational areas, increased wildfire hazard, and clogging of irrigation ditches.
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.
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.
The authors highlight areas where further study could advance understanding of invasive plant control and restoration, including effects on wildlife habitat and water use. “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 provides a summary of the latest science and is expected to be helpful to organizations that undertake the management of saltcedar and Russian olive.
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).