Fuel variability following wildfire in forests with mixed severity fire regimes, Cascade Range, USA

Master's Thesis Abstract by Jessica Hudec (2011)

Fire severity influences post-burn structure and composition of a forest and the potential for a future fire to burn through the area. The effects of fire on forests with mixed severity fire regimes are difficult to predict and interpret because the quantity, structure, and composition of forest fuels vary considerably. This study examines the relationship between fire severity and post-burn fuel characteristics in forests with mixed severity fire regimes. We sampled live and dead canopy and surface fuels across four fire severity classes on three wildfires that occurred on the east side of the Cascade Range, USA, in 2007 and 2008. We used empirical fuels data and stand structure and composition characteristics to calculate potential surface fire behavior for the four fire severity classes. Post-burn average canopy cover is 25-30% in the low severity class and <10% in the high severity class and ranges from 0 to 50% for all fires. All variables representing post-burn canopy fuels differ by fire severity class. The average loading of dead and down woody fuels <7.6 cm diameter and litter is 0.9-1.1 kg m-2 in the low severity class and 0.6-0.8 kg m-2 in the high severity class. Values for fuel loading variables span a wide range of values within and among fires, and substantial overlap exists among severity classes. Fire severity generally does not influence post-burn dead and down woody fuel loading. Estimates of potential fire behavior also cover wide ranges of values, particularly among fires. Flame lengths average 0.4-0.8 m in the low severity classes and 0.3-1.1 m in the high severity classes. The range of potential flame length values, modeled with a 16.1 km h-1 midflame wind speed, varies by up to 2.0 m within a single severity class (0.6-2.6 m). Fire severity does influence potential fire behavior, but typically just one severity class differs from the other three classes. These results indicate that fire severity influences immediate (2-3 years) post-burn canopy fuels and potential fire behavior but does not influence dead and down surface fuel loading for the three fires studied. The wide ranges of values for the fuel components analyzed demonstrate the variability that is characteristic of forests with mixed severity fire regimes and emphasize the need to consider the natural heterogeneity of these forests in fire and fuels management. Quantification of post-burn fuel variability is critical for understanding the ecological significance of mixed severity fires and developing restoration strategies that emulate characteristics of the historical fire regime.