Ecology - Wildland Fuel Fundamentals and Applications - page 96

so that they become fuel breaks thereby impeding the spread of fire, but as less pal-
atable woody plants become established in heavily grazed areas, canopy and surface
FWD may increase, and fire spread may eventually be possible, and the subsequent
effects may be more severe than a grassland fire.

6.1.4.5 Disturbance Interactions

While effects of individual disturbances may be important to fuelbed characteristics
in some ecosystems, it is really the interaction of disturbances that most influences
fuel dynamics (Schoennagel et al. 2004 ; Bigler et al. 2005 ). These interactions may
be direct and immediate, such as trees surviving mixed severity fire becoming more
susceptible to subsequent windthrow because of their increased wind exposure, or
they may be indirect and long term, such as mountain pine beetle mortality in pine
forests allowing shade-tolerant species in the understory to increase CBD and lower
the canopy layer (CBH), thereby creating the potential for more severe and intense
crown fires. Interactions can be simple, such as where grazing reduces flashy fuels
and thus limits fire spread and decreases fire intensity (Bachelet et al. 2000 ), or
they can be quite complex, such as increases in woody plant density and size as a
result of fire exclusion, which then increases transpiration to reduce water avail-
ability making the ecosystem more susceptible to tree mortality from insects and
pathogens in heavy droughts that then fosters more intense fires (Bigler et al. 2005 ).
The origin, condition, and future dynamics of most fuelbeds can only be under-
stood by addressing the multiple interactions of disturbances with the biophysical
environment.
There are many interesting examples of multiple disturbance interactions that
influence fuel dynamics (Keane et al. 2014 ). Bark beetles and the fungus Phaeolus
schweinitzii may interact with fire to alter lodgepole pine fuel dynamics in Oregon,
USA (Geiszler et al. 1980 ). Windthrown trees may facilitate an increase in Ips spp.
beetle populations that then kill surrounding live trees resulting in even heavier
loadings and deeper fuelbeds (Goheen and Hansen 1993 ). Trees that remain after
fuel reduction treatments may be more prone to wind damage that results in canopy
fuel decreases and surface fuel increases (Reinhardt et al. 2008 ). Matson and Bart
( 2013 ) noted that root disease of spruce caused by Inonotus tomentosus may con-
tribute to increased mortality from the spruce beetle ( Dendroctonus rufipennis ) in
boreal and subboreal spruce forests of British Columbia but the interaction is highly
governed by the condition of the vegetation. Bachelet et al. ( 2000 ) found that the
interaction of fire with grazing maintained the pine savanna in the Black Hills of
South Dakota USA. In southwestern US ponderosa pine forests, dwarf mistletoe
( Arceuthobium vaginatum ) weakens pine trees making them more susceptible to
mountain pine beetle attack thereby increasing tree mortality and surface fuels that
will then tend to foster more intense wildfires that will kill even more pines (Parker
et al. 2006 ).