Influences of Riparian Forest Structure and Stand Development on Fish Biomass in Headwater Streams

Riparian forests are dynamic, changing as a result of stand development processes, disturbance, and anthropogenic activities, which in-turn influence large wood recruitment, light availability, and organic matter inputs to adjacent streams. In this study, we evaluated how the age and stage of stand development influenced adjacent stream habitat, light availability and biota in western Oregon headwaters. We assessed changes in stream habitat and fish biomass over time as we revisited five sites where surveys conducted in the late 1970’s had demonstrated increased fish biomass in response to increased light associated with riparian harvest. Surveys in 2014 indicate that trout biomass in these reaches has decreased – likely in response to riparian stand development and canopy closure.  We also used a space-for-time approach to explore relationships between the age/stage of the riparian forest, stream habitat, stream light availability, and vertebrate (fish and salamander) biomass in 9 streams with paired old-growth and second-growth reaches. Canopy openness was positively correlated with total vertebrate biomass (p<0.01) but, surprisingly, large wood and pool area were not. These results highlight the importance of stream light regimes and the potential for riparian forest age controls on light to influence trout and vertebrate populations.