2.2.3. Base of the live crown
The regression equation for crown-®re initiation is
based on two variables: overstory foliar moisture; and
the average height of the base of the live crown for the
stand. The foliar moisture can be adjusted as a variable
in the model to mimic seasonal ¯uctuations in water
availability. The height of the BLC presents several
problems. The theory for the crown-®re initiation
model was developed in boreal forests where the stand
canopy is typically a monolayer (Van Wagner, 1977).
In the eastside stands that we used in developing the
®re-risk module, multi-layered stands are common.
Using the average height of the BLC for these stands
may not provide a realistic result. For example, consider
a stand with two distinct layers: an overstory
with an average height of the BLC at 25 m; and an
understory with an average height of the BLC of 1 m.
If the number of trees in each layer is equal, the
average height of the BLC for the stand will be
13 m. The risk of crown-®re initiation may be grossly
underestimated if it is based on this calculation. In
order to address this issue, we used the midpoint
between the minimum and average height of the
BLC instead of the average for calculating crown-®re
initiation risk. In addition, adapting the crown-®re
initiation model to multi-layered stands was problematic
due to the lack of information on the effects of
®re initiation in one layer on the potential for ®re in
another layer. In the two-layered stand example used
above, the ability of a ®re to crown will depend on the
height of the understory trees and the length of the
¯ames generated as the understory trees burn. FVS
growth projections generate a rapid increase in the
minimum height of the BLC under most stand conditions.
Consequently, the risk of crown-®re initiation
inevitably decreases through time unless natural
regeneration is simulated or the fuel model changes
to one with a higher ¯ame length.