Russian olive invasion into cottonwood forests along a regulated river in north-central Montana

Russian olive (Elaeagnus angustifolia L.) is an exotic tree that is invasive in western North America where it may replace native riparian cottonwood forests. We mapped the occurrence of Russian olive and measured the size, density, and age of it and cottonwood in sandbar, low-terrace, and high-terrace habitats along the lower Marias River below Tiber Dam in Montana. Russian olive occurs along the entire study reach but is more abundant near domesticated plantings. It establishes in moist lower terrace habitat as well as under mature cottonwood on high terraces. Seventy-seven percent of cottonwood trees in all size classes were damaged by beavers in low terrace sites, while only 22% of Russian olives showed damage. Since construction of Tiber Dam, cottonwood establishment has been restricted to lower terrace sites. Beaver prevent cottonwood from developing a mature canopy close to the river while having little effect on the continued invasion of Russian olive. Riparian cottonwood forests will eventually be replaced by Russian olive as old cottonwoods die on upper terraces and young plants on low terraces are removed by beaver or shaded by the less palatable species.