Three thinning treatments differing in
residual tree density were applied to each of three stands (total of
nine treated, three control). Treatments were based on historic
forest structure prior to Euro-American settlement and disruption
of the frequent fire regime (circa 1876). Thinning reduced stand
densities 77–88 percent, basal areas 35–66 percent, crown bulk
densities 24–48 percent, and raised crown base height an average of
11 ft. Before thinning, simulated fire behavior under the 97th
percentile of June fire weather conditions was predicted to be
intense but controllable (5.4 ft flame lengths). However, active or
passive crownfires were simulated using crown base heights in the
lowest quintile (20 percent) or winds gusting to 30 mph, representing
the fuel ladders and wind gusts that are important for initiating
crown burning. Under the identical conditions after thinning, all
three treatments resisted crown burning. The degree of resistance
was related to thinning intensity. It is crucial to remove thinning
slash fuels through prescribed burning or other means. If not
removed, slash fuels can cause crownfire behavior in the thinned
stands under severe wildfire conditions. Finally, the crownfire
resistance achieved through thinning will deteriorate over time
unless maintenance burning and/or thinning is continued.
Three thinning treatments differing in
residual tree density were applied to each of three stands (total of
nine treated, three control). Treatments were based on historic
forest structure prior to Euro-American settlement and disruption
of the frequent fire regime (circa 1876). Thinning reduced stand
densities 77–88 percent, basal areas 35–66 percent, crown bulk
densities 24–48 percent, and raised crown base height an average of
11 ft. Before thinning, simulated fire behavior under the 97th
percentile of June fire weather conditions was predicted to be
intense but controllable (5.4 ft flame lengths). However, active or
passive crownfires were simulated using crown base heights in the
lowest quintile (20 percent) or winds gusting to 30 mph, representing
the fuel ladders and wind gusts that are important for initiating
crown burning. Under the identical conditions after thinning, all
three treatments resisted crown burning. The degree of resistance
was related to thinning intensity. It is crucial to remove thinning
slash fuels through prescribed burning or other means. If not
removed, slash fuels can cause crownfire behavior in the thinned
stands under severe wildfire conditions. Finally, the crownfire
resistance achieved through thinning will deteriorate over time
unless maintenance burning and/or thinning is continued.
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