Forest Thinning in Pacific Northwest Riparian Areas: Rationale, Risks, and Policy Calibration

Over several decades, both science and evolving forest policies have increasingly recognized the vital importance of maintaining the natural processes and functions of riparian areas in order to prevent, minimize or mitigate the adverse impacts of timber harvest and other management activities on streams and aquatic and riparian-dependent species.   However, the Northwest Forest Plan, recognizing the great many ways that logging operations near surface waters has risks harm to aquatic and riparian habitat, switched the burden of proof, such that logging operations in Riparian Reserves on federal lands is only authorized if it is demonstrably necessary or beneficial on the whole for restoration of natural riparian functions and values.   In recent years, BLM’s Western Oregon Plan Revisions and many timber sale projects of the BLM and the Forest Service have proposed or implemented stepped-up and extensive logging within riparian reserves, contending that thinning is useful or necessary under a wide variety of prevailing forest conditions to hasten growth of some trees.   We contend evidence for net ecological benefit of riparian thinning is sparse and equivocal, while evidence thinning can cause adverse effects is unequivocal and growing.  Among known adverse effects are near-stream soil disturbance and sediment delivery, depletion of near-term and cumulative recruitment of woody debris, risk of thermal and microclimate stress from canopy removal, and risk of pathogen dispersal (e.g., Port-Orford-cedar root disease in Oregon and California).   Moreover, recent research suggests that the diversity of natural disturbances coupled with good growing conditions in riparian areas should lead in most cases to complex, desired riparian forest conditions without thinning.   We submit current science does not justify a standard or common prescription of thinning of Riparian Reserves on Northwest Forest Plan lands.   Policy should limit riparian thinning to cases where the following conditions are met:  1) field inventory and analysis of forest and aquatic conditions justifies a site-specific treatment (such as the need to remove a species alien to the site, or to thin within unnaturally extensive areas of exceedingly dense tree plantations); 2) canopy reduction will not cause stream or wetland warming; 3) the cumulative area of Riparian Reserves impacted by silvicultural treatment, transportation and yarding systems does not exceed ten percent within any ten-year period in any sixth-field subwatershed, 4) the larger logged material is retained on site, and 5) firm agency commitment exists to field monitoring the silvicultural and environmental outcomes, so results will adaptively inform future actions.