Effects of Forest Harvesting on Water and Sediment Yields: a Review Toward Better Mitigation and Rehabilitation Strategies

Purposeof Review Paired catchment studies have documented the extent to which forest harvesting impairs forested watersheds. Adverse effects on stream water quality and aquatic ecosystems following ground-based harvesting operations that open forest canopies and compact soils have spurred the implementation of best management practices (BMPs) that are intended to limit runoff volumes (i.e., water yield increase, WYI) and total sediment yields (TSY) into streams. The purpose of this review is to highlight recent insights from water and sediment yield studies, ranging in scale from hillslope plots to catchments, that reveal the sources and mechanisms of hydrological impairment and recovery, and point to specific strategies for developing more targeted BMPs that help prevent, mitigate, and rehabilitate forested watersheds. Recent Findings In the context of forested watersheds, contemporary BMPs have successfully lowered runoff (WYI) and total sediment yields (TSY) relative to previously reported values. Recent research strongly indicates that the delivery of WYIs and TSYs to streams following forest harvesting is still high. Because many factors such as the intensity and extent of management activities and headwater catchment activities interact, improving the effectiveness of generic, inflexible BMPs will be challenging. Recent findings indicate that a site-specific, highly tailored application of a combination of BMP measures before, during, and after the harvesting operation is needed if a further reduction of runoff and sediment delivery to streams is to be achieved. Analyses of 155 paired catchment studies and 39 hillslope plots revealed that forest harvesting resulted in average increases of WYI by 180 mm (+ 46%) and TSY by 477 t km(-2) year(-1) (+ 700%). Smaller hillslope plots established on forest roads, skid trails, and harvested areas underestimate these values (WYI of 8.2 mm, TSY was 42.2 t km(-2) year(-1)). In extreme circumstances such as clearcutting, enhanced WYI and TSY may persist up to several decades before returning to pre-harvest levels. WY increased with increasing precipitation and with, and proportional to, the catchment area harvested, regardless of climatic zone or tree species composition; TSY increased with increasing rainfall and catchment area. Both the impact of harvesting and the time required for natural recovery of hydrologic responses depended on the response but can be shortened when applying contemporary forestry BMPs.