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

Abstract

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.

Wood placement in river restoration: fact, fiction, and future direction

“…Therefore, recruitment distances vary with species and age of trees in the riparian zone (McDade et al 1990;Van Sickle and Gregory 1990). For example, recruitment distances of old-growth conifer trees that reach heights of more than 70 m are much greater than those of smaller alder trees, which reach heights of only about 35 m (Beechie et al 2000;McDade et al 1990).…”

Red Alder-Conifer Stands in Alaska: An Example of Mixed Species Management to Enhance Structural and Biological Complexity

“…The presence of red alder did not inhibit the production of large-diameter conifers, an important source of LWD for streams, and both alder and conifers provided small woody debris for fishless headwater streams in southeastern Alaska [39]. Additionally, Beechie et al [145] found in LWD-recruitment and pool-formation models for NW Washington streams, that the time necessary for red alder to attain first recruitment of pool-forming LWD is 50% shorter than for Douglas-fir, although alder LWD declines rapidly after 70 years due to species' short life span.…”

Grays River Watershed and Biological Assessment, 2006 Final Report.

“…In some cases, however, interim LWD inputs and man-made solutions may be necessary until the forest structure is restored to a self-sustaining level (a process that can take decades). This is especially critical where LWD is a limiting factor inhibiting salmonid recovery (Beechie et al 1996;Beechie et al 2000;Abbe et al 2003;Gregory et al 2003).…”