Input and depletion of woody debris in Alaska streams and implications for streamside management

Abstract.-Natural rates ofinput and depletion oflarge woody debris (LWD) in southeast Alaska streams were studied to provide a basis for managing streamside zones to maintain LWDfor fish habitat after timber harvest. Debris was inventoried in a variety of stream types in undisturbed old-growth forest; 252 pieces ofLWD were dated from the age of trees growing on them. Longevity of LWD was directly related to bole diameter: small LWD (10-30 cm in diameter) was less than 110 years old, whereas large LWD (>60 cm in diameter) was up to 226 years old. Assuming equilibrium between input and depletion ofLWD in streams in old-growth forests and exponential decay ofLWD, we calculated input and depletion rates from mean age ofLWD. Input and depletion rates were inversely proportional to LWD diameter and ranged from 1%/year for large LWD in all stream types to 3%/year for small LWD in large, high-energy, bedrock-controlled streams. A model ofchanges in LWDafter timber harvest (which accounted for depletion of LWD and input from second-growth forest) indicated that 90 years after clear-cut logging without a stream-side buffer strip large LWD would be reduced by 70 % and recovery to prelogging levels would take more than 250 years. Because nearly all LWD is derived from within 30 m of the stream, the use of a 30-m wide, unlogged buffer strip along both sides of the stream during timber harvest shoul