by Barry Rhea and Bill Goosmann
The Animas River is the largest river in southwestern Colorado. It is a major tributary to the San Juan River, which it joins in Farmington, New Mexico, roughly twenty-five miles south of the state line. Like so many other rivers in the southwestern U.S., the Animas has seen dramatic changes since the arrival of the Europeans almost three centuries ago. Water development, trapping, agriculture, grazing, mining, timber harvesting and urbanization have all left their mark on these river systems. Perhaps some of the most profound changes have occurred over the last half century as a result of invasions by non-native trees and shrubs. While saltcedar (Tamarix spp.) has been the most notable of these exotic invasive species, increasingly Russian olive (Elaeagnus angustifolia L.) is exerting its influence over many of these important ecological systems. Fortunately, unlike a number of other area streams, the Animas River, at least in Colorado, is in a relatively early stage of invasion. There is still a window of opportunity to alter this process but that window is quickly closing. While other woody species, such as saltcedar and Siberian elm threaten this river system, Russian olive probably represents the most immediate threat to its ecological integrity.
Russian olive is a medium-sized shrub or tree typically ranging in height from 10-25 feet. However, individuals may attain heights of up to 40 feet. It has narrow leaves, 2-3 inches long covered with minute scales that give it a silvery or grey-green appearance. Its dark reddish brown smaller branches are armored with thorns and tipped with clusters of small, fragrant, yellow flowers that bloom in May and June. The fruit, which gives this shrub its name, is olive shaped and is silvery when first formed and tan to brown at maturity in fall. It is closely related to three native shrubs: silverberry (Elaeagnus commutate) silver buffaloberry (Shepherdia argentea) and Canada buffaloberry (S. canadensis).
This exotic was originally introduced into the United States in the early 1900s from temperate areas of Asia. As an attractive, hardy and easily established ornamental it was a popular choice in the west for ornamental plantings, windbreaks and as a source of food and cover for wildlife. As recently as the 1980s and 1990s, state and federal agencies were still subsidizing the distribution of Russian olive in the United States. Since its introduction, however, it has escaped cultivation in many areas of the U.S. and is especially problematic in the Southwest and the inter-mountain West in seasonally wet riparian habitats, wet meadows, and the margins of ponds and ditches.
Russian olive possesses a number of characteristics that make it an aggressive competitor in riparian vegetative communities. These include tolerance to drought, tolerance to salinity, rapid germination rates, symbiotic nitrogen fixation, rapid growth rates, shade tolerance, root sprouting, low palatability for livestock and wildlife browsing, and early-age and abundant seed production. The seeds of Russian olive are also attractive to wildlife, particularly birds, and survive the digestive process. This serves as a primary vector for its spread to previously unaffected areas. All of these characteristics allow Russian olive to become established during periods of stress for native vegetation, such as droughts, heavy grazing pressure, or significant soil disturbance. Once established, this deep-rooted species can out-compete other more water dependent plant species, such as willow and cottonwood, for moisture. Russian olive can also inhibit the regeneration of these native species through moisture competition, as well as through shading. Although the most obvious changes occur within the tree and shrub layers, the herbaceous layers may also experience profound adverse effects.
Broader changes in the dynamics of western river systems may also advantage Russian olive. Dam-building and diversions for irrigation and other uses have reduced the flood frequency and intensity of most rivers in western North America. This has altered the natural regime of disturbance within these systems, reducing or eliminating forces that scour and remove exotics like Russian olive, the same forces that create and maintain the open areas of sand and gravel on which new willows and cottonwoods germinate. Russian olive, on the other hand, is not dependent on such processes.
The impacts of Russian olive on riparian systems are manifold. It can alter successional dynamics of riparian forests, alter hydrodynamics of such systems, and alter wildlife use and habitat. As the previous discussion indicates, much of the native riparian forests of the interior West is dominated by pioneer species, i.e., cottonwoods and willows. In the absence of the natural disturbance regime, such systems usually succeed to prairie grasslands or sagebrush steppe. Under these conditions, invasion by Russian olive represents a new functional guild that often forms self-replacing stands. In other situations, Russian olive also aggressively competes with native species that are seral to cottonwoods, such as green ash (Fraxinus pennsylvanica) and box elder (Acer negundo).
A wide variety of faunal species have adapted to the western riparian ecosystems and are dependent wholly or, in part, on the unique habitat conditions found in these relatively water-rich zones. However, little is known about the long-term effects of Russian olive invasion to riparian-dependent wildlife populations. It is clear that as Russian olive begins to dominate native vegetation the overall diversity in structure and composition of the habitat is diminished. When cottonwoods are replaced, certain bird guilds, such as cavity nesters, appear to be consistently absent from Russian olive stands. Research in Wyoming found that overall bird numbers and species diversity declined in riparian habitats dominated by Russian olive. Invertebrate populations (insects) have been observed to decline as this species begins to dominate the vegetative community. This has implications for all species that depend on insects including birds, fish, and small mammals and could result in significant impacts throughout the food chain.
Russian olive invasion also has its human costs. Dense, essentially impenetrable stands can develop along the riverbank making access to the river difficult or impossible for recreationists such as boaters or fishermen. Invasion can spread to low lying pastures reducing forage production and utilization by livestock. Russian olive will invade irrigation channels increasing the maintenance requirements for these structures. The eventual impacts to the cottonwood component results in, not only an ecological loss but also an esthetic loss within the river corridor.
Management and Control
As with many exotic species, manual removal of seedlings (with roots) before maturity is easier and more effective than treating mature trees. Research has shown that mechanical treatments, including fire, have not been effective due to this species ability to re-sprout. Mowing, cutting, girdling, chaining, burning, and bulldozing may temporarily suppress Russian olive, but this only represents a short-term solution. These treatments have actually shown a tendency toward a denser re-growth of the plant after mechanical treatment.. Additionally, mechanical approaches often involve indiscriminate removal of desirable species and can result in soil disturbance that opens areas for other non-native weed species. Biological controls may hold some promise in the future and there are specific pathogens that attack Russian olive. These pathogens, however, tend to weaken the tree but generally do not kill it. This approach, even if an effective agent could be found or developed, could have the undesirable effect (in terms of liability) of killing ornamental trees outside the treatment area. Effective control appears to require cutting or chopping coupled with immediate chemical treatment.
Chemical treatments with herbicides have been the focus of several researchers and probably hold the best current approach to Russian olive control. Of special note is a rather comprehensive though informal study conducted by Extension Service at Utah State University. This study examined the effects of a variety of herbicides and application techniques on Russian olive. The research found that very effective control could be achieved with glyphosphates, such as RoundupTM or its wetland approved counterpart, RodeoTM applied at a rate of 2cc’s/inch of trunk diameter to cut frills at the base of the tree. Although this approach is somewhat labor intensive it allows for specific placement of a relatively safe herbicide and minimizes potential impacts to adjacent vegetation and surface water that could occur through such techniques as foliar applications.
In 2000, Barry Rhea of Rhea Environmental Consulting began experimenting with some of these techniques on the Animas. While conducting willow flycatcher surveys on the San Juan River and lower Animas River in New Mexico, he became alarmed at the extent to which these river systems had been altered by both Russian olive and saltcedar. And he could not help but notice while driving along the river back to Durango that the same process, was occurring in Colorado, albeit to a much lesser degree than was occurring in New Mexico. It was obvious that it was just a matter of time before the upper reaches of the Animas would suffer the same fate as its lower reaches. Barry’s initial efforts were limited in scope, primarily as a result of limited manpower and funding, but he did what he could to spread the alarm to whomever would listen. He contacted and was interviewed by local radio and newspaper. In 2002, he contacted a local river advocacy group — Friends of the Animas River — to expand the effort. The group’s non-profit status might be more funding available while also supplying a broader range of volunteer support. In 2002 and 2003, volunteer efforts lead to the treatment of over 2 miles of some of the worst “hot spots” on the river. In 2005, with a grant from the Colorado Division of Wildlife’s wetlands program, the Southwest Youth Conservation Corp was hired to help with the effort. We have also had other partners that aided the effort. We have also had volunteer days on the river using Fort Lewis College students, local arborists, and concerned citizens, and the City of Durango has begun policing and eradicating Russian olive on city-owned property.
In addition to these eradication efforts, there is also an educational piece. Much of the problem on the river is the result of escaped ornamental trees. We have developed a brochure explaining the problem and letting the public know what role they can play in this effort.
The spread of Russian olive poses a significant problem for the health of Colorado’s riparian habitat. However, the success of the community eradication efforts in Durango proves that there is still time, although likely not as much as we would like.
References
- Dewey, S. and D. Miner.
- Russian olive control from frill cut herbicide applications, unpub. report. Extension Service, Utah State Univ. Logan, UT.
- DiTomaso, J.M. and E.A. Healy.
- Aquatic and Riparian Weeds of the West. University of California, Agriculture and Natural Resources, Publication 3421. Oakland, CA.
- Katz, G.L., and P.B. Shafroth.
- “Biology, Ecology and Management of Elaeagnus angustifolia L. (Russian Olive) in Western North America.” Wetlands, v23 n4. December 2003.
- Whitson, T.D. et al.
- Weeds of the West. Western Society of Weed Science and University of Wyoming. Pioneer of Jackson Hole. Jackson Hole, WY.