Ecology - Wildland Fuel Fundamentals and Applications - page 92

historical drivers of fire regimes in the southern USA because they created con-
tinuous surface fuelbeds with heavy loadings that facilitated the spread of fire. And
last, wind can create unique patterns of fuelbed properties across the landscape be-
cause wind events and their intensities vary spatially and temporally in forests from
large-scale catastrophic episodes, such as hurricanes and typhoons, to landscape
level occurrences, such as thunderstorms and tornadoes, to fine-scale perturbations,
such as downdrafts and microbursts (Ulanova 2000 ). Each of these wind events can
cause a patchwork of effects from minor branch breakage to minor windthrow dam-
age to complete forest blowdown across landscapes.
6.1.4.2 Fire
Fire is one of the few disturbances with effects that can work both ways; fire can
both reduce and increase fuel loadings depending on the characteristics of the
burn, conditions of the prefire fuelbed and vegetation, and time since burn. Fire
reduces live and dead fuel loadings by consuming them during combustion and
smoldering phases (Chap. 3). The amount of fuel consumed by the fire depends
on many factors, such as the bulk density and depth of the fuelbed along with the
moistures, mineral contents, densities, and SAVRs of the various particles in that
fuelbed (Albini and Reinhardt 1995 ; Call and Albini 1997 ; Chap. 2). Commonly,
wildland fires consume nearly all of the fine fuels (litter, twigs, herbs, shrubs) and
partially consume large branches, CWD, and duff (Call and Albini 1997 ). Fire often
leaves a complex mosaic of residual fuel loadings because of microsite differences
in fuel size, moisture, loadings, and weather (wind, temperature, radiation). Most
forested ecosystems also experience decreases in CBD and increases in CBH after
fire. Crown fires, for example, significantly reduce canopy fuels by direct consump-
tion of all crowns, whereas surface and mixed severity fires may reduce CBD fuels
by either killing those plants that are most susceptible to fire because they were
small or maladapted to survive fire or by directly scorching plant parts. Microsite
differences in surface and canopy fuel consumption become greater when fire in-
tensities are low, but as deep drought, high temperatures, and strong winds increase
fire intensity and spread, microscale consumption and plant mortality often become
more homogeneous (Brown and Reinhardt 1991 ). Keane and Parsons ( 2010 ), for
example, observed that unburned patches after low-intensity prescribed burns were
mostly found in the shade of overstory trees where radiation was reduced so that
fuels did not dry sufficiently to carry the fire.
Fire can also increase surface fuel loadings by killing or damaging plants but
not consuming their biomass. The scorched, dead biomass eventually falls on the
ground over time to increase surface fuel loadings and decrease canopy fuels (Mar-
tin et al. 1979 ; Fig. 6.2c ). Surface fuel loadings may also increase as fires burn or
smolder at the base of live trees and snags to consume enough stemwood so that the
standing boles fall. Increases in surface fuel component loadings after fire have been