The Effects of Fuel Treatments on Fire Severity in a Mixed-Evergreen Forest of Southwestern Oregon
Master’s Thesis Abstract by Crystal L. Raymond (2004)
Fuel treatments are now mandated by federal policy to reduce hazardous fuels
on federal forest lands. More information is needed on the effectiveness of fuel
treatments, especially in mixed-severity fire regimes. In this study I had the
rare opportunity to quantify the relationship between fuel structure and fire
severity using pre-fire surface and canopy fuels data and fire severity data
after an intense wildfire. The study area is in a mixed-severity fire regime,
the mixed-evergreen forest of southwestern Oregon. Thinning reduced canopy
fuels, decreasing the potential for crown fire spread, but the presence of
activity fuels increased potential surface fire intensity, so increases in
canopy base height did not decrease the potential for crown fire initiation.
Thinning followed by under-burning reduced canopy fuels and surface fuels,
thereby decreasing both crown fire spread potential and the potential for crown
fire initiation. However, crown fire is not a prerequisite for high fire
severity; damage and mortality of overstory trees in the wildfire was extensive
despite the absence of crown fire, and the low predicted crown fire potential
before and after the fuel treatment. Damage to and mortality of overstory trees
were most severe in thinned treatments (80 - 100% mortality), least severe in
the thinned and under-burned treatment (5% mortality), and moderate in untreated
stands (53-54% mortality) following a wildfire in 2002. Fine fuel loading was
the only fuel structure variable significantly correlated with crown scorch of
overstory trees. Percentage crown scorch was the best predictor of mortality 2
years post-fire. Efforts to reduce canopy fuels through thinning treatments may
be rendered ineffective if not accompanied by adequate reduction in surface
fuels.