Tuesday, 18 November 2003: 10:30 AM
Inventory and classification of wildland fire effects in silviculturally treated vs. untreated forest stands of New Mexico and ArizonaPoster PDF (181.3 kB)
Ninety years of aggressive fire suppression and fire exclusion coupled with heavy grazing and prolific conifer regeneration around the turn of the century have combined to change forest structure, understory and overstory composition, and fuel biomass conditions in southwestern forests. Stand-replacement fires, particularly in ponderosa pine forests (Pinus ponderosa), have displaced high frequency low intensity historical fire regimes. We hypothesized forest stands treated recently (<15 years) using silvicultural practices were less likely to experience stand-replacing crown fires compared to untreated stands. We compared understory and overstory fire damage and fireline intensity (kcal/s/m) in silviculturally treated vs. untreated forest stands in New Mexico and Arizona. Study sites ranged in elevation between 1,900 and 2,800 m. Silvicultural treatments included: lop, pile, burn; lop and scatter; harvest and burn; prescribed fire; commercial thin; and shelterwood. Ground char was estimated following Ryan and Noste (1985), fire damage was estimated following Omi and Kalabokidis (1991), and fireline intensity was estimated using scorch height and Van Wagner’s (1973) equations. Preliminary results indicate fire severity in middle elevation (circa 2,350 m) montane coniferous forests is reduced when the fuel leg of the fire behavior triangle is abridged. Under extreme conditions created by drought, high winds, and suitable topographical conditions, we observed treated forest stands that, although suffering less severe fire and ground char damage than adjacent untreated stands, were still subjected to near stand-replacement type damage. However, this illustrates that even under extreme conditions, fire severity can be mitigated by fuel reduction, and further that more aggressive treatments would likely have faired better. In particular, we observed prescribed fire in combination with mechanical thinning had the greatest impact toward mitigating fire severity. Silvicultural prescriptions designed to reduce stand susceptibility to crown fire must consider slope and aspect, slash treatment, and residual tree and stand characteristics. Specifically, as density (stems/ha) and basal area (m2/ha) decrease and mean diameter at breast height (cm) increase, fire severity, ground char, and fireline intensity decrease. Further, a threshold in canopy bulk density of 0.10 kg/m3 on stands with 0–5 percent slope was identified beyond which initiation of a crown fire was possible and below which it did not occur.