Simulating catastrophic and individualistic large woody debris recruitmentfor a small riparian system

Surprisingly little research has been done to partition the contribution of catastrophic disturbance from that of small-scale individualistic mortalit y events on riparian large woody debris (LWD) recruitment. This study compa red the impact of both processes on recruitment through simulation of sever al catastrophic disturbances (a spruce beetle outbreak, a moderately intens e fire, and a clearcut) and undisturbed (individualistic mortality only) ol d growth for a small headwater stream in the Intermountain West of the Unit ed States. All scenarios progressed through a two-stage process, with the F orest Vegetation Simulator growth and yield model controlling forest dynami cs and a postprocessor (CWD, version 1.2) predicting riparian LWD recruitme nt. Projections indicate that individualistic only conditions delivered 2.5 m(3) 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 distu rbance significantly (P < 0.05) increased cumulative (300 yr) LWD recruitme nt over the individualistic-only scenario, whereas clear-cutting significan tly decreased total delivery. In-stream LWD loads, relatively stable in und isturbed riparian zones, fluctuated sharply under catastrophic disturbance. Peak channel loads associated with natural perturbation occurred similar t o 30 yr after the event while debris volumes under clear-cutting immediatel y declined. The postevent recruitment and in-stream LWD stocks of all distu rbance scenarios eventually fell below undisturbed conditions, requiring de cades to recover historical volumes. Catastrophic disturbances induced such steep oscillations in riparian LWD load that the systems experiencing freq uent 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 distu rbance in forests along streams noticeably depauperate of LWD.