Conifer forest stands are comprised of living and dead biomass in four separate fuel strata according to their vertical distribution and effects on fire behavior (see figure 1): ground fuels-principally the duff layer of the forest floor; surface fuels-the litter layer of the forest floor, mosses and lichens, dead down woody debris, herbaceous vegetation, and short to medium-height shrubs; ladder or bridge fuels-tall shrubs, understory conifer trees and loose bark, lichens, and dead branches on tree boles located in the space between the top of the surface fuel stratum and the bottom of the canopy-fuel stratum; and canopy fuels-chiefly the live and dead needle foliage, twigs, small branchwood, and aerial lichens and mosses associated with the overstory tree cover. It is generally accepted that a distinct separation exists between surface fuels and canopy fuels: an open trunk space in which ladder or bridge fuels vary widely in their abundance. Collectively, the four strata constitute a forest fuel complex.