Spatial organization, position, and source characteristics of large woody debris in natural systems

Abstract

Field data were collected from 150 stream sites with a diverse array of channel types and disturbance patterns within basins relatively unaffected by anthropogenic disturbance to assess the characteristics of natural large woody debris (LWD) organization, size, and riparian areas across forested regions of Washington State. Bankfull channel width was found to be the dominant factor influencing the grouping of LWD pieces. LWD group size as well as stability increased with channel size. Jams (groups ≥10 pieces) contained proportionately similar diameter distributions regardless of size. As bankfull channel width increased the percent of LWD volume decreased in the low-flow channel but increased in the high-flow channel. Both the median LWD lengths and diameters increased with bankfull width and proximity to the low-flow channel, as well as with increasing LWD group size. As channel width increased, the proportion of pieces oriented parallel to the high-flow channel increased while the frequency of perpendicular pieces decreased, especially if rootwads were attached.Forest zones, as defined by climatic influence and fire history, are the best regional predictors of riparian stand attributes such as tree height, basal area, stem diameter and density. Observed riparian stand composition ranged from 8 to 17 tree species within each of six forest zones. Species richness was greater in milder climates and lower in more extreme climates. The greatest observed tree species diversity was within 35 m of the stream channel; however, stream influences on riparian characteristics were observed out to 65 m, the extent of the sample transect. Large streams with active flood plains had a significantly greater deciduous riparian component than channels less prone to fluvial disturbances. LWD volumes did not peak or plateau until adjacent riparian stands reached 550 years in age; however, LWD quantities were also high during the first 150 years of stand origin. The percent of instream wood quantities that could be attributed to an adjacent riparian source decreased with increasing channel size. Where restoration of instream wood and/or riparian areas are warranted, these characteristics offer guidance to the range of conditions found in natural systems to which aquatic and riparian species have adapted.