A New Look at the Quantities and Volumes of Instream Wood in Forested Basins within Washington State
Instream wood is recognized as an important feature linked to channel processes that benefit salmonids. Stream channel assessments and restoration/enhancement efforts often associate salmon habitat quality with the quantity and volume of woody debris. Existing wood targets used to assist resource managers do not adequately account for variations in quantity or volume due to differences in geomorphology, ecoregions, or disturbance regimes. To address this issue, field data on instream wood quantities and volumes from 150 stream segments draining unmanaged basins within Washington State (without logging, roads, dams, or other human-induced condition that may influence natural wood loading and retention rates) are used to develop target suggestions for management. Based on the assumption that streams draining unmanaged forest basins incorporate the range of conditions to which salmonids and other species have adapted, wood loads in these systems provide a defensible reference for management. Surveyed sites represent a wide array of geomorphic channel types, channel origins, natural disturbances, and climate regions where the process of wood input and distribution has most likely evolved under a natural rate of disturbance, with the exception of potential fire suppression. Analyses of these data imply that the most consistent predictor of wood volumes and quantities is bankfull width (as a function of basin size) and ecoregion. Wood quantity, volume, and mean piece size increased with channel size due to the increased proclivity for fluvial transport and spatial accretion, along with greater lateral area for wood to accumulate. Forest stand characteristics such as stem density and diameter are influenced by distinctive climates particular to each ecoregion, which in turn influence the size and quantity of instream wood. Percentile distributions describe the range of wood quantities and volumes in streams draining unmanaged basins by discrete bankfull width classes for three distinguishable ecoregion groups. The data also support expanded definitions for minimum volumes of "Key Pieces." Due to both favorable and adverse conditions comprising wood loading ranges; we suggest that the 75th percentiles in each bankfull width class and ecoregion should be used to represent the lower limit for optimum wood quantities and volumes as an index of habitat quality.
- Forestry