The effect of fire regime on coarse woody debris in the west central Cascades, Oregon

This study examined the effect of fire regime on coarse woody debris(CWD) mass using a combination of field data and modeling. The objectiveswere to use field sampling to determine how CWD differs between two areasthat have had different fire regimes, and investigate how fire frequency and
severity, stand growth and development, decomposition, and mortality ratesaffect the amount of CWD for sites experiencing two different fire regimes.One regime had infrequent, high severity fires, with a mean fire return intervalover 300 years. The second regime exhibited more frequent (120 years),mixed-seventy fires which created a mosaic of patches with multipledisturbance cohorts of shade tolerant tree species within stands. A conceptualmodel of how CWD might be affected by different fire regimes was developed.Then field data were gathered on CWD mass, tree biomass, and siteproductivity in Douglas-fir forests within stands that have had two different fireregimes for the past 500 years. Finally, a mass-budget model was developedto compare field data and model results, and was used to better understandthe dynamics of CWD with regard to fire regime.
While fire frequency and severity established the pattern of CWDsuccession, it is the interaction of fire regime with the other controlling factorsthat is responsible for the differences in CWD mass. The study yielded thefollowing findings: 1) Field data indicated that CWD mass was almost twiceas 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 studyarea are decomposition rates, fire severity, fire frequency, and fuelconsumption, suggesting that environment and stand structure control CWDmass more than fire regime per se. 3) Site productivity and mortality rates aresimilar among sites in the study area, and do not exert a detectable influencefor the range of environments examined. 4) CWD distribution was skewedtoward the fresher decay classes in the stand-replacing fire regime, but wasnormally distributed in the mixed-severity regime. 5) CWD levels have greatertemporal variability in the infrequent, stand-replacing fire regime than themixed-severity regime. 6) In the stand-replacing regime, mortality over timecontributes to CWD mass since there is a long span of time between events;whereas, in the mixed-severity regime mortality is more associated with eventsthan with stand mortality over time.