- Understanding the causal links between riparian forests, streams, and salmonids is important to help understand the likely effects of forest management practices. Previous studies have identified three major causal pathways (Instream Cover, Light, and Hydrology) through which riparian forests influence streams, and ultimately stream fishes. To evaluate the potential importance of these pathways, I developed a conceptual model of how they can be described. From this I evaluated evidence in support of the influence of these pathways using available data on riparian forest, instream conditions, and size and abundance of age-1 or older coastal cutthroat trout. These data were derived from monitoring conducted in 50 small watersheds across the 110,000 ha of lands managed by the Washington State Department of Natural Resources on the western Olympic Peninsula. The study domain was centered on the Olympic Experimental State Forest (OESF), where forest conditions primary represent previously harvested younger second growth forests (<80 years old). Additional monitoring data from 11 unharvested watersheds were added from the OESF (n=2), the Olympic National Park (n=4), and the Olympic National Forest (n=5) to increase the range of diversity of forest conditions in the sample. Overall the primary objective of this effort was to evaluate evidence in support of causal pathways through which forest conditions influence streams and fish in the study area. Insights gained from this effort are intended to inform future monitoring and management within the OESF.
Results of analyses evaluating statistical relationships among available indicators provided support for all three of the pathways (Instream Cover, Light, and Hydrology) in the model as well as the presence of self-thinning (a density dependent process where fish reach an equilibrium between size and abundance) in age-1 and older cutthroat. Stream depth was one of the most important factors for age-1 or older cutthroat trout on the OESF. Overall, the support for the Light Pathway and instream wood of the Instream Cover Pathway were less important than expected. This may be due to the limited range of conditions in canopy coverage (all watersheds had heavy shading) and instream wood (potentially reduced volumes of wood throughout the OESF). The lack of watersheds with lower canopy cover, higher volumes of instream wood, and deeper stream depths may be an indication of the slow rate of recovery under passive restoration alone.