Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 61

Fig. 4.2 Example of a typical canopy profile where canopy bulk density ( CBD, kg m −3 ) is plotted
across the vertical profile of a stand. (From Reinhardt et al. 2006b )

There are four basic canopy CBD profiles that generally describe the potential
for crown fires. Multiage, multistrata stands (Fig. 4.3a ) have trees at a wide variety
of heights with mostly shade-tolerant trees comprising the understory so canopy
fuels are dense even at the surface-canopy fuel interface creating a high potential for
crown fires because of the low CBH and high CBD . Two storied stands (Fig. 4.3b ),
commonly having a shade-intolerant, fire-resistant overstory with a shade-tolerant,
fire-susceptible understory, have hourglass-shaped canopy profiles that also have
high CBD and low CBH . These are often the profiles of stands that had frequent
fires but recent fire exclusion has allowed increases in understory biomass. Single-
strata stands have profiles where the majority of biomass is at the top of the canopy
creating a low chance of crown fire initiation (high CBH ) but a high conditional
probability of crown fire spread (high CBD ) (Fig. 4.3c ). The last profile is a forest
stand with trees that are widely spaced and somewhat open so that the CBD rarely
exceeds the threshold for crown fire initiation and spread creating a low chance of
both initiation and spread of a crown fire (Fig. 4.3d ). This is a typical canopy profile
for a stand with a history of frequent fires, woodland, or perhaps a stand with recent
insect- or disease-related tree mortality.

4.3.2 Canopy Bulk Density

As canopy fuel characteristics go, CBD is probably the most important because it
dictates crown fire spread, potential for an active crown fire, and crown fire inten-
sity (Eqs. 4.2, 4.7). CBD is the amount of burnable canopy fuel by canopy volume
(kg m −3 ) and represents the degree of packing of canopy fuels. Below a minimum
CBD threshold value, crown fires have insufficient fuels for crown fire spread.