I0
Czh
1:5 (1)
where I0 is the critical surface intensity (kW/m),
C0.010 (Van Wagner, 1977), z the height of the base
of the live crown (BLC) (m), and h the heat of ignition
(largely a function of overstory foliar moisture)
(kJ/kg).
Critical surface intensity (I0) can be estimated for a
range of values for foliar moisture content and height
to base of live crown (BLC). Values of I0 represent
minimum levels of ®reline intensity necessary to
initiate crown ®re (Agee, 1996) and can be used to
directly calculate critical ¯ame lengths using Byram's
equation (Byram, 1959). A regression equation was
developed from tables in Agee (1996) to relate foliar
moisture content and BLC to the critical ¯ame length
necessary to initiate crown ®res (Appendix A).
The crown-®re initiation equation (Eq. (1)) was
developed in boreal forests where the majority of
vegetational biomass exists in a single canopy layer.
Due to the strati®ed nature of many of the eastside
forest canopies, the movement of ®re from the ground
into the crown may be more complicated than that
proposed in the equation. However, lacking any other
quanti®cation of crown-®re initiation based on stand
structural attributes, we have adopted this method with
the understanding that it is a ®rst approximation.
The second equation relates crown bulk density and
rate of spread to the ability of a crown ®re to spread: