S Rd (2)
where S is the mass ¯ow rate (kg/m2
/s), R the rate of
spread (m/s), and d the crown bulk density (kg/m3
).
The crown ®re spread equation (Eq. (2)) approximates
mass ¯ow rate (S). The rate of spread (R) can be
adjusted by the user through a dialog box. Regression
equations for predicting crown bulk density (d) were
developed from Brown (1978) in (Agee, 1996) for
ponderosa pine [Pinus ponderosa (Dougl. ex Laws.)],
Douglas-®r [Pseudotsuga menziesii (Mirb.) Franco],
and grand ®r [Abies grandis (Lindl.)] (Appendix A).
Through a network of database queries incorporating
the crown bulk density equations and stand inventory
data, we established a critical ¯ame length for
crown-®re initiation and a mass ¯ow rate for crown
®re spread unique to each stand. Further queries
compared the critical ¯ame length to the ¯ame length
predicted for each stand by BEHAVE, a ®re behavior
program (Burgan and Rothermel, 1984). If the ¯ame
length predicted by BEHAVE exceeded the critical
¯ame length for a given stand, a crown-initiating ®re