Abundance of non-native and noxious plant species in riparian areas of the Western US

Plants that are non-native (introduced to US) or noxious weeds have become a major concern of land management agencies.  Riparian zones experience frequent natural disturbance (high flows, flooding, erosion, etc.), as well as anthropogenic disturbances in many cases (livestock grazing, diversions, logging, roads, etc.), which may make them vulnerable to colonization by non-native or noxious species.  However, the high productivity and lack of open niche space in riparian areas may also make them resistant to invasion by non-native and noxious species.  We evaluated data from three large riparian vegetation data sets from mountainous areas of the western US (covering major portions of Washington Oregon, Idaho, Nevada, California and Montana) to estimate the abundance of non-native and noxious species in riparian zones on Forest Service and BLM land.  Some of the non-native species with the highest cover on the streambanks were Poa pratensis, Taraxacum officinale, Trifolium repens, and Phleum pratense.   Some of the noxious weeds with the highest cover were Cirsium vulgare, Cirsium arvense and Elymus repens.  We found that cover values for the non-native and noxious species were typically less than 5%, suggesting that riparian areas resist invasion and domination by non-native and noxious species.  However, many sites did have some cover of non-native or noxious species (87% of sites on one National Forest), suggesting that these non-native and noxious species are spreading across much of the landscape, even if at low cover values.  These non-native and noxious species may increase if conditions that favor them were to increase, such as disturbance or climate change that favored them.  It will be important to determine what conditions prevent the expansion of these non-native and noxious species in riparian zones, so that land managers can seek to prevent such conditions where possible.