Exotic plant invasion from roads to stream networks in steep forested landscapes of western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6t053j18t

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  • This project explores mechanisms governing the invasion of fluvial systems by two contrasting exotic species in stream networks of steep, relatively remote forested landscapes of western Oregon. This research addresses hypotheses relating limitations of seed source locations, seed transport processes, and sink availability to the distributions of scotch broom (Cytisus scoparius) and foxglove (Digitalis purpurea) in the H.J. Andrews Experimental Forest (Andrews). Cytisus scoparius and Digitalis purpurea distributions were mapped along hillslopes and in streams in the Andrews and compared to maps and aerials photographs of flood and geomorphic disturbances. Effects of seed scarification, soaking, and substrate texture on seed germination were investigated using laboratory trials. Distributions of Cytisus scoparius and Digitalis purpurea in the Andrews differed greatly and were controlled by interactions between seed sources along roads and invasion sites along streams The distribution of Cytisus scoparius and Digitalis puipurea in streams is consistent with limitation by upgradient hillslope seed source locations. The results of this study suggest that Cytisus scoparius and Digitalis purpurea were present along roads and in clear-cuts in the Andrews from the 1970s to 2003, but invaded the stream network only after geomorphic processes during a large flood in 1996 overcame barriers to stream invasion. Debris flows and flood flows overcame seed dispersal barriers by transporting seeds into the valley bottoms from hilislope source locations along roads and in clear-cut patches. Furthermore, geomorphic processes acting along the stream corridor created suitable conditions for seed deposition, germination, and establishment by removing vegetation and creating sites protected from scour. Seeds were deposited in valley bottoms (sinks) away from scour in middle stream reaches and plants established in these protected locations. This paper outlines a conceptual model highlighting the role of 1996 sinks as seed sources between 1996 and 2003, enabling the invasion to propagate downgradient as a wave into lower reaches of the stream network.
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