2.2.5. Stand layering
The crown bulk density is calculated from stand
stocking (trees/ha) and average tree diameter. In multilayered
stands these numbers may not provide an
accurate re¯ection of mass ¯ow rates through the
canopy. The crown ®re spread rate is more a function
of the overstory trees than the total number of trees in
the stand. The crown bulk density may be overestimated
by including the understory trees in the stand
averages. Given various rates of spread (R) this may
represent the difference between a crown ®re spreading
or not spreading. To address this, we have incorporated
a user-de®ned landscape-scale variable,
height of the understory (HU), that allows for ¯exibility
in isolating the overstory trees for crown bulk
density calculations.