Local abiotic and biotic conditions can alter the strength of exotic species impacts. To better understand the effects of exotic species on invaded ecosystems and to prioritize management efforts, it is important that exotic species impacts are put in local environmental context. We studied how differences in plant community composition, photosynthetically active radiation (PAR), and available soil N associated with Russian olive presence are conditioned by local environmental variation within a western U.S. riparian ecosystem. In four sites along the South Fork of the Republican River in Colorado, we established 200 pairs of plots (underneath and apart from Russian olive) to measure the effects of invasion across the ecosystem. We used a series of a priori mixed models to identify environmental variables that altered the effects of Russian olive. For all response variables, models that included the interaction of environmental characteristics, such as presence/absence of an existing cottonwood canopy, with the presence/absence of Russian olive canopy were stronger candidate models than those that just included Russian olive canopy presence as a factor. Compared with reference plots outside of Russian olive canopy, plots underneath Russian olive had higher relative exotic cover (exotic/total cover), lower perennial C4 grass cover, and higher perennial forb cover. These effects were reduced, however, in the presence of a cottonwood canopy. As expected, Russian olive was associated with reduced PAR and increased N, but these effects were reduced under cottonwood canopy. Our results demonstrate that local abiotic and biotic environmental factors condition the effects of Russian olive within a heterogeneous riparian ecosystem and suggest that management efforts should be focused in open areas where Russian olive impacts are strongest.
Nomenclature: Cottonwood, Populus deltoides W. Bartram ex Marshall ssp. monilifera (Aiton) Eckenwalder; Russian olive, Elaeagnus angustifolia L.
Management Implications: Understanding how invader impacts vary across the landscape is crucial to developing more efficient management strategies. Current strategies primarily treat landscapes as homogeneous; therefore, management efforts are often inefficient because they do not preferentially target the areas of most concern. Using a more targeted management approach, where early intervention is applied only to areas of the landscape likely to experience strong invader impacts, would be much more cost effective. Based on our results, we suggest that ideally Russian olive should be removed from all habitat types because it is associated with an increase in soil N and proportional exotic plant cover. However, when management funding is limited, we suggest prioritizing control efforts on locations where it is growing in the absence of a cottonwood canopy. In these areas, Russian olive has the largest impact on soil N and proportional exotic cover. Since Russian olive appears to cause an “invasional meltdown” by facilitating the invasion of other exotic species, particularly in open canopies, research on effects of removal is warranted to see if it reverses Russian olive impacts. If the increased soil N associated with Russian olive presence persists after the tree's removal, secondary invasion after the disturbance from the removal process is likely. Additionally, favoring cottonwood establishment over Russian olive can be accomplished by promoting flood disturbance by avoiding channel stabilization (i.e., by riprap) and construction.