Fuel bed characteristics of Sierra Nevada conifers

A study of fuels in Sierra Nevada conifer forests showedthat fuel bed depth and fuel bed weight significantly varied by tree species and developmental stage of the overstory. Specific values for depth and weight of woody, litter, and duff fuels are reported There was a significant positive relationship between fuel bed depth and weight. Estimates of woody fuel weight using the planar intercept method were significantly related to sampled values. These relationships can be used to estimate fuel weights in the field. West. J. Appl. For. 13(3):73-84. The accumulation of hazardous fuels throughout the Sierra Nevada over the past 60 yr has become an issue of recent •ncreased concern. In 1993, Congress directed that a study be conducted to assess what information is needed to make decisions for the future management of the Sierra Nevada ecosystems. One of the critical findings of the Sierra Nevada Ecosystem Project was that live and dead fuels in the conifer forests today are more abundant and continuous than in the past (SNEP 1996). Combined with the growing density of homes, these increases in fuels exacerbate the threat of fire to the human and natural resources in the Sierra Nevada. Accurate methods are needed to assess fuel conditions in order to develop fire protection and fuel mitigation plans. Lack of such accurate information reduces the reliability of these plans. Prescribed fire has been proposed as one of the most effective tools for reducing fuel hazards (van Wagtendonk 1996). The use of prescribed fire requires reliable estimates of fuel weight in order to safely accomplish the burns and to evaluate their effectiveness in meeting management objectives (Reeberg 1995). In addition, fuel bed weight and depth are used in the Rothermel (1972) rate of spread equation that •s central to the National Fire Danger Rating System (Deemmg et al. 1977) and the fire behavior prediction system used NOTE: Jan W. van Wagtendonk is the corresponding author and can be reached at (209) 379-1885; Fax: (209) 379-1886; E-mail: jan_van_wagtendonk@usgs.gov. The authors thank all of the people who helped on this project. Joe Coho oversaw the data entry, and Diane Ewell spent many hours in the field collecting fuels. She was ably assisted by many volunteers including Charisse Sydoriak, Kay Beeley, Kathy Tier, and Karen Kolbeck. Liam Bickford volunteered to enter all of the data. by fire behavior analysts (Rothermel 1983). Fuel weight and depth are specified for standardized models that are used in both of these systems (Anderson 1982, Albini 1976). Fuel weight plays a dual role in Rothermel' s(1972) spread rate equation. In the numerator, it is multiplied by fuel heat content to produce a source of heat for the calculation of reaction intensity. Fuel weight is divided by fuel depth in the denominator of the equation to determine fuel bed bulk density, which acts as a heat sink. Increases in fuel weight usually cause reaction intensity to increase more than rate of spread. In fact, all else being equal, as weight increases, rate of spread may actually decrease because more fuel must be raised to ignition temperature (Burgan and Rothermel 1984). Brown (1981) identifies fuel bed bulk density as one of the most difficult properties to measure in the field. Deeming et al. (1977) categorized woody fuels by size classes and duff fuels by depth classes that both correspond to fuel moisture timelags (Table 1). Timelag is the amount of time necessary for a fuel component o reach 63% of its equilibrium moisture content (Lancaster 1970). Deeming et al. (1977) stress that the duff designations are very rough approximations and should be used with caution. For exTable 1. Timelag classes and corresponding woody fuel size classes and duff fuel depth classes (Deeming et al. 1977). Timelag class Woody fuel size class Duff fuel depth class (cm) 1 hr 0.004).64 0.00-0.64 10 hr 0.64-2.54 0.64-1.91 100 hr 2.54-7.62 1.91-10.16 1000 hr >7.62 >10.16