Abstract:
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.
