Assessment of Russian olive as an ecogeomorphic agent on a northern river

The exotic invasive, Russian olive (Elaeagnus angustifolia), may pose a threat to western U.S. rivers because of its broad and increasing habitat suitability. This tree species is more shade and drought tolerant compared to other commonly found riverside genera (native Populus and invasive Tamarix). The distribution and spread of Russian olive have been quantified in the U.S. Southwest. However, its range and impacts remain unknown in more northern regions. Within this context, the Powder River (Montana, U.S.) functions as a model fluvial system for studying the potential impacts Russian olive may have on local species composition and geomorphic processes. Specifically, we measured distribution, percent cover and topographic position of Russian olive, Tamarix and native woody species within the Powder River riparian corridor. Although Tamarix had the widest distribution of elevations relative to the channel, Russian olive had the lowest median elevation. Despite Tamarixs wide range of elevations, it had the least cover (3 % compared to 1819 %) within the riparian corridor and was almost entirely restricted to positions adjacent to the channel. In fact, cluster analysis found woody communities from river's edge to floodplain interior of (1) Russian olive with an herbaceous understory, (2) Populus as a canopy with a Russian olive midcanopy, and (3) Populus with an herbaceous understory. The lack of Tamarix communities may be related to its lower cold tolerance. We additionally measured plant traits known to influence flood hydraulics and associated sediment transport. The best-fit functions for plant bending force (i.e., flexibility) as a function of plant size were exponential and indicated Russian olive is more rigid than both Tamarix and Populus, with Tamarix having intermediate values. We hypothesize that Russian olive, because of its rigidity, high densities, low channel positions, and widespread existence as a shrubby canopy, likely impacts flow and sediment transport more than both invasive Tamarix and native Populus. Additional research will explore relationships between properties of invasive and native woody species and related ecogeomorphic processes, with implications for understanding the associated impacts on river corridors.