- This study examined the effect of fire regime on coarse woody debris (CWD) mass using a combination of field data and modeling. The objectives were to use field sampling to determine how CWD differs between two areas that have had different fire regimes, and investigate how fire frequency and severity, stand growth and development, decomposition, and mortality rates affect the amount of CWD for sites experiencing two different fire regimes.
One regime had infrequent, high severity fires, with a mean fire return interval over 300 years. The second regime exhibited more frequent (120 years), mixed-severity fires which created a mosaic of patches with multiple disturbance cohorts of shade tolerant tree species within stands. A conceptual model of how CWD might be affected by different fire regimes was developed. Then field data were gathered on CWD mass, tree biomass, and site productivity in Douglas-fir forests within stands that have had two different fire
regimes for the past 500 years. Finally, a mass-budget model was developed to compare field data and model results, and was used to better understand the dynamics of CWD with regard to fire regime. While fire frequency and severity established the pattern of CWD succession, it is the interaction of fire regime with the other controlling factors that is responsible for the differences in CWD mass. The study yielded the following findings: 1) Field data indicated that CWD mass was almost twice
as high in stands having an infrequent, stand-replacing fire regime (173 Mg/ha)
compared with stands having a moderately frequent, mixed-severity fire regime (95 Mg/ha). 2) Factors that appear to have the greatest influence in the study area are decomposition rates, fire severity, fire frequency, and fuel consumption, suggesting that environment and stand structure control CWD mass more than fire regime per Se. 3) Site productivity and mortality rates are
similar among sites in the study area, and do not exert a detectable influence for the range of environments examined. 4) CWD distribution was skewed toward the fresher decay classes in the stand-replacing fire regime, but was normally distributed in the mixed-severity regime. 5) CWD levels have greater temporal variability in the infrequent, stand-replacing fire regime than the mixed-severity regime. 6) In the stand-replacing regime, mortality over time
contributes to CWD mass since there is a long span of time between events; whereas, in the mixed-severity regime mortality is more associated with events than with stand mortality over time.