Modeling Recovery Rates and Pathways for Woody Debris Recruitment in Northwestern Washington Streams

We modeled large woody debris (LWD) recruitment and pool formation in northwestern Washington streams after simulated stand-clearing disturbance using two computer models: Forest Vegetation Simulator for stand development and Riparian-in-a-Box for LWD recruitment, depletion, and pool formation. We evaluated differences in LWD recruitment and pool formation among different combinations of channel size, successional pathway, and stand management scenario. The models predict that time to first recruitment of pool-forming LWD is about 50% shorter for red alder Alnus rubra than for Douglas-fir Pseudotsuga menziesii at all channel widths. Total LWD abundance increases faster in red alder stands than in Douglas-fir stands but declines rapidly after 70 years as the stand dies and pieces decompose. Initial recovery is slower for Douglas-fir stands, but LWD recruitment is sustained longer. Total LWD abundance increases faster with decreasing channel size, and pool abundance increases faster with decreasing channel width and increasing channel slope. The models predict that thinning of the riparian forest does not increase recruitment of pool-forming LWD where the trees are already large enough to form pools in the adjacent channel and that thinning reduces the availability of adequately sized wood. Thinning increases LWD recruitment where trees are too small to form pools and, because of reduced competition, trees more rapidly attain pool-forming size. On channels less than 20 m wide, thinning of red alder and underplanting shade-tolerant conifers will reduce near-term alder recruitment and increase long-term conifer recruitment. However, the same treatment on channels more than 20 m wide may increase both near-term and long-term recruitment. Compared with the natural fire regime, timber harvest rotations of 40–80 years during the past century have reduced the percentage of riparian stands that can provide LWD of pool-forming size to streams, especially in channels at least 20 m wide.