Ecology of Russian-olive (Elaeagnus angustifolia) in Western U.S.A.
Patrick B. Shafroth1* and Jonathan M. Friedman2
1U.S. Geological Survey, Fort Collins, CO, USA; shafrothp@usgs.gov
2U.S. Geological Survey, Fort Collins, CO, USA
We review the state of the science regarding various aspects of the distribution and ecology of Russian-olive (Elaeagnus angustifolia L.), a small Eurasian tree widely distributed in the western United States, which by one estimate is the fourth most frequently occurring woody species along rivers in this region. It reaches its greatest abundance along Great Plains, Intermountain, and Colorado Plateau drainages, in relatively close proximity to locations where it was intentionally planted. It is more likely to occur in parts of western North America with a mean annual minimum temperature between -20 and -40 degrees C, and tends to occur at sites that are drier than those associated with native riparian trees and shrubs. Ecological characteristics that likely enable Russian-olive to proliferate include adaptation to the physical environmental conditions that generally characterize semi-arid riparian habitats, lack of intense pressure from herbivores, shade tolerance, and a large seed whose stored energy confers a competitive advantage on seedlings. Further, the success of this species is at least partly due to its ability to take advantage of the reduced levels of physical disturbance and flow homogenization that characterize many riparian habitats downstream of dams. Some studies have indicated a multi-decadal lag before significant expansion of Russian-olive occurs, likely due to its relatively long time to sexual maturity and limited dispersal of its large seeds. Along some western streams, such as the middle Rio Grande, Chinle Creek, and the Escalante River, Russian-olive has been observed to dominate banks and levees adjacent to the main channel, which suggests that fluvial seed dispersal is important and that dense Russian-olive stems may be able to trap sediment and slow channel migration. Russian-olive is an actinorhizal species, forming nitrogen-fixing root nodules. The N content of its leaves and leaf litter have been shown to be considerably higher than that of native cottonwood (Populus deltoides). Thus, Russian-olive may alter soil nutrient status and perhaps related community and ecosystem characteristics. Ample areas for research regarding key aspects of Russian-olive ecology remain under-addressed, including: 1) the effects of Russian-olive on riparian ecosystem structure or function across a range of western riparian systems; 2) the processes and mechanisms that facilitate or inhibit Russian-olive expansion; 3) the conditions or management actions that are required for desirable native vegetation to thrive in place of Russian-olive; and 4) the effects of removal on erosion rates.