Landscape configuration and flood frequency influence invasive shrubs in floodplain forests of the Wisconsin River (USA)

1 Invasive species are present worldwide, yet predicting which invasive species will become problematic in which ecosystems remains an important ecological challenge. Floodplains are at particular risk for invasion, especially when subjected to anthropogenic disturbance. 2 We examined how components of flood regime, habitat quality and habitat configuration influenced the presence and abundance of three invasive shrubs in the floodplain of the Wisconsin River. Shrub taxa included two non-natives (Lonicera spp. and Rhamnus spp.) and one native (Zanthoxylum americanum). Observations of taxa presence and abundance were recorded in nine forested river reaches, spanning three physiographic regions. We also compared the productivity of Z. americanum across a flood control levee in one reach. 3 Physiographic region significantly predicted the presence and abundance of these three taxa, acting as a synthetic indicator of differences in climate, geography and topography. Invasion was greatest in regions where modifications to flood regime and land use were most pronounced. Physiographic region was excluded from subsequent analyses to assess more specific predictors of shrub distributions. 4 Habitat configuration and quality both predicted shrub taxa presence and abundance. Shrub taxa were most frequently observed and most abundant in small forest patches, near roads, and in sandy soil with low nutrient content. Edge habitats have many of these characteristics. Edges appear to be favoured by avian shrub dispersers and provide conditions suitable for invasive establishment. 5 Flooding influenced non-native and native invaders differently. Non-native taxa were observed less frequently and at lower abundances in frequently flooded areas, probably because of a sensitivity to flooding. However, the presence, abundance and productivity of the native invader increased with flooding. Anthropogenic modifications to the flood regime limited hydrologic connectivity and may have reduced the competitive advantage of flood-tolerant traits, which allowed the invasion of upland species. 6 Synthesis. In the floodplain of the Wisconsin River, anthropogenic modifications that created edge habitat and altered flood regime facilitated shrub invasions, whereas unfragmented forest and intact flood regime limited invasion. Large patches of floodplain forest, less regulated flooding regimes, and lower road densities may help reduce the spread of invasive shrubs in temperate floodplains.