Simulating riparian vegetation and aquatic habitat dynamics in response to natural and anthropogenic disturbance regimes in the Upper Grande Ronde River, Oregon, USA

We developed state and transition models (STMs) to evaluate the effects of natural disturbances and land-use practices on aquatic and riparian habitats. The STMs consisted of discrete states defined by channel morphology and riparian vegetation. Transitions between states resulted from plant succession and from natural and anthropogenic disturbances. Channel conditions and habitat suitability for anadromous salmonids were ranked by using a qualitative four-factor scale for each state in the STMs. Disturbance probabilities were varied to define both historical and current disturbance regimes. Models were run for 120 years with the current disturbance regime to illustrate changes associated with Euro-American settlement, and then run for an additional 50 years under the historical disturbance regime to illustrate the potential for passive recovery. Results suggested that Euro-American settlement dramatically changed riparian vegetation and channel conditions, which resulted in substantial declines in habitat quality. Passive recovery of channel conditions and habitat suitability was quick in some stream types, but slow in others. Overall, our results underestimate the effects of human land uses on streams and overestimate the rate of recovery under passive restoration because the models do not yet include the effects of many management activities, especially those resulting from forest harvest and roads.