Natalie M. West1, Ann Marie Reinhold2, Geoffrey C. Poole2 and Erin K. Espeland1
1United States Department of Agriculture – Agricultural Research Service Pest Management Research Unit, 1500 North Central Avenue, Sidney, Montana 59270, USA
2Department of Land Resources and Environmental Sciences, Montana State University, Post Office Box 173120, Bozeman, Montana 59717, USA
Riparian plant communities are shaped by hydrology, and seed deposition is one process through which fluvial dynamics influence invasion pressure. Hydrochory may increase invader dispersal, but is difficult to measure and its importance can be obscured by demographic lags or establishment limitations. Linking fine-scale spatial patterns of inundation frequency to riparian invader distribution provides an initial evaluation of the relationship between likely areas of seed deposition from hydrochory and current invasions. We examined the importance of spatial patterns of flooding frequency (i.e. the distribution of flood return intervals through space) in predicting the distribution of Russian olive (Elaeagnus angustifolia L.) at the floodplain scale.
This nitrogen-fixing shrub or small tree is a prominent invader of riparian zones in western North America. Broad establishment tolerance and a persistent seedbank gives Russian olive an advantage over co-occurring woody species and facilitates its invasion along western rivers. Large vertebrate dispersed seeds are likely to be important to this competitive advantage, but often cited as a key factor limiting dispersal and slowing its invasion front. Hydrochory is assumed but not explicitly considered in spread dynamics. Specific data are mainly anecdotal or inferred from studies of other factors such as habitat limitation and large-scale range determinants. Current theory suggests an invasion lag of 30 years, but does not consider how increased propagule movement through water may relax dispersal limitations. This process should become even more important as Russian olive increases in the landscape, reducing the relative importance of demographic dynamics.
As a pilot study, we related inundation models generated using the United States Army Corps of Engineers Hydrological Engineering Center’s River Analysis System (HEC-RAS) to a Russian olive geospatial dataset across a 10.6-km-portion of the Yellowstone River floodplain in Montana, USA. Our results demonstrate that the majority of Russian olive is found within a narrow range of flooding intervals. Russian olive cover was highly spatially associated with portions of the floodplain wetted under relatively frequent return intervals: 80.9% occurred within the 10-year inundation zone, three-quarters (i.e. 61.6% of total) of which occurred within the 5-year zone. Both density and occupancy was greatest in the 1.5-5 year zones; occupancy was moderate in the 10 year and lowest in the 20 year and greater inundation zones. As dispersal rather than establishment limitation is thought to be a priority filter for Russian olive invasions into riparian areas, these results highlight the importance of considering the potential contributions of hydrochory. Tying flooding frequency to likely spatial distributions would allow us to prioritize vulnerable sites and timelines for management and begin to tease apart how dispersal might interact with demography to drive the spatial and temporal spread of Russian olive along riparian corridors.