Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 64

pathway or ladder fuel needed to move surface fire into the canopy under the right
conditions, but since many forest stands have at least one sapling reaching the can-
opy, it probably isn't appropriate to evaluate CBH at the fine-scale tree-level. The
spatial distribution of ladder fuel across an area is probably more important; clus-
ters of saplings and pockets of dense, low-hanging canopies better facilitate crown
fire transitions. However, few available management-oriented sampling techniques
record tree location. The scale of the fire is also important. Heavy surface fuel load-
ings may foster intense fires that can easily reach the forest canopy via flame or
embers without the need for low fuel ladders, and large running crown fires may
initiate crown fire from adjacent stand making CBH irrelevant.
Many techniques have been used to estimate CBH and CH . Some studies esti-
mated CBH and CH as an average of crown base height and tree height, respective-
ly, across all trees in the sampling frame (Chap. 8). Most studies used the canopy
profile to estimate these two canopy parameters where CBH is estimated as the low-
est height that exceeds a threshold CBH value and CH is the highest height above
a threshold CBD (Fig. 4.4 ). Threshold values are difficult to estimate since there
has been little experimental research in this area, so most studies select a value that
works best for a given application. Sando and Wick ( 1972 ) estimated CBH using
a minimum CBD threshold of 0.037 kg m −3 whereas Reinhardt et al. ( 2006a ) used
0.012 kg m −3 . Agee ( 1996 ) has suggested that a minimum canopy bulk density of
0.1 kg m −3 is needed to ensure active crown fire spread in a horizontal dimension.
The newest version of FuelCalc (Reinhardt et al. 2006a ), a software application
Fig. 4.4 The threshold
method of estimating CBH
and CH from the canopy
profile. Here a threshold of
0.037 kg m −3 is used to detect
the bottom (CBH) and top
(CH)canopy heights at which
CBD exceeds this threshold