Input and Depletion of Woody Debris in Alaska Streams and Implications for Streamside Management

Natural rates of input and depletion of large woody debris (LWD) in southeast Alaska streams were studied to provide a basis for managing streamside zones to maintain LWD for fish habitat after timber harvest. Debris was inventoried in a variety of stream types in undisturbed old-growth forest; 252 pieces of LWD 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 of LWD in streams in old-growth forests and exponential decay of LWD, we calculated input and depletion rates from mean age of LWD. 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 of changes in LWD after 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 should maintain LWD.