Flood and debris flow interactions with roads promote the invasion of exotic plants along steep mountain streams, western Oregon

This study examines the interactions among geomorphic and biogeographic processes that govern the invasion by two contrasting exotic plant species—a shrub, scotch broom (Cytisus scoparius) and an herb, foxglove (Digitalis purpurea), over several decades of road and stream networks in the H.J. Andrews Experimental Forest in western Oregon. Distributions of C. scoparius and D. purpurea were mapped along hillslopes and streams in 1993, 2002, and 2003. The mapped distributions were related to debris flow pathways and changes in stream morphology interpreted from field surveys and air photos over the period 1993 to 2003. Laboratory trials examined the response of seed germination to scarification (to test effects of transport by debris flows), soaking (to test effects of fluvial transport), and substrate texture (to test effects on establishment). C. scoparius and D. purpurea were present along roads and in clearcuts in the Andrews Forest from the 1970s to 2003, but invaded the stream (Lookout Creek) only after debris flows and floods during an extreme storm in 1996. Laboratory trials demonstrated that seeds could germinate on a variety of substrates after scarification and flood transport. Mapping and air photo/GIS analysis indicated that the distributions of exotic plants were located on freshly scoured bars and floodplains adjacent to the active channel, downstream of seed sources along roads that were connected to the main stem of Lookout Creek by road ditch drainage systems, and debris flow paths. This paper outlines a conceptual model for the invasion of exotic plants, highlighting the connectivity between road and stream networks provided by geomorphic processes in steep forested landscapes.