The residence time of large woody debris in the Queets River, Washington, USA

Instream large woody debris (LWD) provides several critical functions in ri verine ecosystems, including sediment and nutrient retention, salmonid habi tat enhancement, and stable colonization sites for incipient floodplain veg etation. In this study, the size and species composition of LWD in the Quee ts River, Washington, USA, were examined and compared with the size and spe cies composition of forest trees from which they originated, in order to de termine a depletion rate for LWD in the active channel. Increment cores fro m instream LWD were crossdated against cores from riparian conifers to esti mate the year each LWD piece was recruited to the river channel. Debris pie ces that were decayed or otherwise incompetent to provide cores were dated using standard C-14 techniques. Hardwood species (Alnus rubra, Populus tric hocarpa, and Acer macrophyllum) were better represented among riparian fore sts than among instream LWD, and conifers (Picea sitchensis, Tsuga heteroph ylla, Pseudotsuga menziesii, and Thuja plicata) were better represented amo ng LWD than in the adjacent riparian forest, suggesting that hardwoods were depleted from the channel faster than conifers. The depletion rate of coni ferous LWD from the channel followed an exponential decay curve in which 80 % of LWD pieces were <50 yr old, although some pieces have remained for up to 1400 yr. Although most wood is depleted from the channel within 50 yr, s ome wood is apparently buried in the floodplain and exhumed centuries later by lateral channel migration. The calculated depletion constant of 0.030 i s equivalent to a half-life of <similar to>20 yr, meaning that virtually al l of the wood will have disappeared within 50 yr. This rapid depletion sugg ests that harvesting large conifers from the riparian zones of large stream s could have adverse impacts within three to five decades.