Monday, 17 November 2003: 2:00 PM
Comparative analysis of three silvicultural prescriptions and their effect on crown fire behavior in Southwestern ponderosa pine
Three silvicultural prescriptions of various thinning intensities designed at reducing potential fire behavior in southwestern ponderosa pine were compared in the Ft. Valley Experimental Forest, Peaks Ranger District, Coconino National Forest, located 5 km north of Flagstaff, Arizona, USA. The treatments were: (1) Modified Full Restoration – restore pre-settlement ponderosa pine structure by retaining all pre-settlement trees as well as replace pre-settlement remnants with post-settlement trees. Treatment includes burning of activity generated fuels followed by prescribed burning, (2) Minimal Restoration – achieve a minimal reduction in fire behavior and maintain critical wildlife habitat values. Enhance vigor of older trees by removing all understory trees within a 20-meter radius. Dense conditions would be maintained adjacent to these areas for wildlife corridors. Activity fuels would be piled and burned and followed by broadcast burning, (3) Forests Forever! - designed by the Southwest Forest Alliance (SWFA) was designed to be of minimal impact yet reduce high intensity fire potential. This prescription retained all trees ³ 40 cm as well as representatives of the 20th century regeneration pulse, and reintroduced frequent low intensity prescribed fire. Untreated areas surrounding treatment sites were used as controls. Potential fire behavior was modeled under 97th percentile weather conditions for the month of June, using NEXUS, an EXCEL based spreadsheet which links existing models of surface fire spread and crown fire prediction providing output of site specific indices of crown fire hazard. We used the crowning index, torching index, and fire type (surface fire, passive crown fire, active crown fire, and conditional crown fire) as derived in NEXUS to evaluate the effects of these three treatments on crown fire hazard. Variations in stand structure among treatments, as measured by basal area, crown bulk density and crown base height, contributed to predicted changes in surface and crown fire behavior. However, while all three treatments modified the fuels complex making it difficult for crown fire to initiate within the treated areas, post-treatment stand structure still allowed for conditional crown fire. That is, post-treatment stand structure would support crown fire moving in from adjacent areas. These results suggest that while crown fire initiation (torching index) within site specific areas can be reduced by various thinning intensities under identical weather conditions it is also important to consider the landscape relationship of these treatments. Under severe fire conditions the threat to areas of concern often develops from kilometers away, from large fires with a wide and active fire front. In order to have the greatest impact on fire behavior, intensity and fire growth treatments need to be large enough and placed strategically across the landscape. In addition, weather conditions under which treatments are designed to be effective and the suppression strategy and tactics, which will be appropriate for these areas, are important to identify and consider during their design.
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