In conifer forests, three broad types of fire are commonly recognized on the basis of the fuel stratum or strata controlling their propagation: ground or subsurface fire, surface fire, and crown fire. Ground or subsurface fires burn very slowly in the duff layer with no visible flame and sometimes with only the occasional wisp of smoke. Surface fires spread in the litter and dead-down woody fuel layer of s stand in either the heading direction with the wind and/or upslope, or as backing fires advancing into the wind and/or downslope. Crown fires are dependent on a surface fire and, in some instances, ladder or bridge fuels for both its initial onset and capacity for maintaining flames in the crown space of a conifer forest stand. Thus, a crown fire advances through both the surface and tree canopy fuel layers with the surface and crown fire phases more or less linked together as a single unit. Thus, the term 'crowning' refers to both the fire's ascension into the crowns of trees and the spread from tree to tree.