Simulating Catastrophic and Individualistic Large Woody Debris Recruitment for a Small Riparian System

Abstract

Surprisingly little research has been done to partition the contribution of catastrophic disturbance from that of small-scale individualistic mortality events on riparian large woody debris (LWD) recruitment. This study compared the impact of both processes on recruitment through simulation of several catastrophic disturbances (a spruce beetle outbreak, a moderately intense fire, and a clearcut) and undisturbed (individualistic mortality only) old growth for a small headwater stream in the Intermountain West of the United States. All scenarios progressed through a two-stage process, with the Forest Vegetation Simulator growth and yield model controlling forest dynamics and a postprocessor (CWD, version 1.2) predicting riparian LWD recruitment. Projections indicate that individualistic-only conditions delivered 2.5 m3 LWD·100 m reach−1·10-yr cycle−1; while the spruce beetle-, fire-, and clearcut-affected stands averaged 2.9, 3.2, and 1.5 m3 LWD·100 m reach−1·cycle−1, respectively. Stands impacted by natural catastrophic disturbance significantly (P < 0.05) increased cumulative (300 yr) LWD recruitment over the individualistic-only scenario, whereas clear-cutting significantly decreased total delivery. In-stream LWD loads, relatively stable in undisturbed riparian zones, fluctuated sharply under catastrophic disturbance. Peak channel loads associated with natural perturbation occurred ∼30 yr after the event while debris volumes under clear-cutting immediately declined. The postevent recruitment and in-stream LWD stocks of all disturbance scenarios eventually fell below undisturbed conditions, requiring decades to recover historical volumes. Catastrophic disturbances induced such steep oscillations in riparian LWD load that the systems experiencing frequent large-scale perturbations never achieved a long-term steady state, as some have postulated. Because of the inflation in cumulative LWD delivery, it may prove advantageous to encourage (or imitate) some catastrophic disturbance in forests along streams noticeably depauperate of LWD.