Hyatt, Timothy. 1998. The residence time of large woody debris in the Queets River, Washington. M.S.
Instream large woody debris (LWD) serves several critical functions in riverine ecosystems, including sediment and nutrient retention, salmonid habitat enhancement, and stable colonization sites for incipient floodplain vegetation. In this study the size and species composition of LWD were compared with the size and species composition of forest trees from which they originated, in order to determine differential depletion rates among species. Increment cores form instream LWD were crossdated against cores from riparian conifers to estimate the year each LWD piece was recruited to the river channel. Debris pieces that were decayed or otherwise incompetent to provide cores were dated using standard 14C techniques. Hardwood species (Alnus rubra, Populus trichocarpa, Acer macrophyllum) were better represented among riparian forests than among instream LWD, and conifers (Picea sitchensis, Tsuga heterophylla, Psuedotsuga menziesii, and Thuja plicata) were better represented among LWD than in adjacent riparian forest, indicating that hardwoods were depleted from the channel faster than conifers. The depletion rate of LWD from the channel followed an exponential decay curve in which 80 percent of LWD pieces were less than 50 years old, although several pieces have remained for up to 1,400 years. A distinct juncture in the decay curve indicates that more than one process is responsible for LWD depletion. Although most wood is depleted from the channel within 50 years, some wood is apparently buried in the floodplain and exhumed centuries later by lateral migration of the channel. The calculated decay constant of 0.061 indicates that the LWD exhibits a half-life of approximately 10 years, in that half of the wood now in the channel within 50 years. An assumed long-term equilibrium between LWD recruitment and depletion suggests that harvesting of large conifers from riparian forests could have adverse impacts to streams within three-to-five decades.