Russian olive (Elaeagnus angustifolia) is an exotic tree considered invasive in the western U.S. There is concern it will replace native riparian forests, resulting in a loss of biological diversity. We studied the dynamics of Russian olive invasion on the Marias River in north central Montana and the lower Yellowstone River in southeastern Montana to determine where and in what habitats it occurs, how it is interacting with plains cottonwood (Populus deltoides) and green ash (Fraxinus pennsylvanica), the dominant native riparian trees, and how these interactions will alter future riparian forests. We measured size, density, and age of Russian olive, cottonwood, and green ash as well as vegetative composition and overstory canopy cover in sample plots in alluvial bar and terrace habitats at randomly chosen sites.

Russian olive occurred in multiple-age stands on terraces along both rivers but was rarely found establishing in recently flood-deposited alluvium. Diameter growth rates decreased with increasing overstory canopy cover but did not vary significantly with soil moisture as reflected by ground-layer vegetation. Russian olive grew at nearly 3 times the rate of the native late-successional green ash at sites where both occurred together. Russian olive attained reproductive maturity at ca 10 years of age on both rivers, and, on average, there was less than 1 new plant recruited per mature tree per year. Beaver felled a high proportion of cottonwood within 50 m of both river channels, but Russian olive was little damaged.

Russian olive will establish with increasing frequency in riparian forests as well as wet meadows and along ditches and overflow channels in eastern Montana. Native riparian forests will be replaced by Russian olive as old cottonwoods die on upper terraces or are removed by beaver near active channels. Cottonwood establishment and dominance will not be precluded on rivers where flooding regularly reinitiates primary succession beyond the zone of beaver activity. However, Russian olive may preclude cottonwood recruitment by shading seedlings along streams where flooding does not occur or merely deposits alluvium on top of existing vegetation rather than creating new channels or broad point bars. Because of its long maturation time and low recruitment rate, Russian olive invasion in Montana will proceed slowly compared with many exotics.