Ecology - Wildland Fuel Fundamentals and Applications - page 94

6.1.4.3 Insects and Disease

Fuel dynamics after insect and disease outbreaks will vary with the agent and the
intensity and severity of the outbreak (Table 6.1 ), but in general, insects and dis-
eases kill or damage their hosts thereby reducing live canopy fuels and increasing
surface fuels (Augusiak et al. 2013 ; Parker et al. 2006 ). Dead and damaged canopy
material eventually falls to the ground and becomes part of the surface fuelbed, as
has been shown after spruce budworm infestations (Azuma 2010 ), blister rust infec-
tions (Campbell and Antos 2000 ), gypsy moth invasions (Collalti et al. 2014 ), and
bark beetle outbreaks (Parker et al. 2006 ). Unlike wind and fire, insects and patho-
gens usually target specific plant hosts and sizes for infection, and as a result, the
distribution and abundance of host plants on the landscape becomes a major factor
in subsequent fuel dynamics. This section will focus on the bark beetles and fungal
pathogens as examples of two significant disturbances because it would be nearly
impossible to cover the wide variety of insects and disease species that influence
forest and rangeland fuel complexes throughout the world.
The insect species most extensively studied for their influence on fuel dynam-
ics are bark beetles, and more specifically, the mountain pine beetle (Jenkins et al.
2008 ). Surface fuels after major mountain pine beetle outbreaks appear to change in a
number of ways (Fig. 6.2d ). First, there is a flush of fine fuels, primarily needles and
small twigs, occurring in the few years following mortality (Hoffman et al. 2012 ).
These decompose relatively quickly as the large branches and eventually the beetle-
killed snags fall over the next several decades (Page and Jenkins 2007 ). Klutsch
et al. ( 2014 ) found that FWD and CWD increased by 10 % for every 1.0 m 2 ha −1
of basal area loss from endemic beetle mortality. Dead trees eventually topple as
decomposition rots the root structure thereby increasing 100-h and 1000-h fuels.
Canopy fuel characteristics also change after a beetle outbreak. Foliage on fatally
attacked pine trees first turns red then falls to the ground over the next 2-5 years.
These red needles may have lower moistures and higher flammability than green fo-
liage, possibly creating a higher potential for crown fires (Schoennagel et al. 2012 ;
Jolly et al. 2012 ). But when these red needles fall leaving only dead pine tree bole
and branches, which is often called the gray stage (4-10 years after an outbreak),
the canopy characteristics of CBD, CBH, and CC decrease to reduce fire hazard, but
the magnitude of the reduction in canopy fuels depends on the severity of the beetle
outbreak (Hicke et al. 2012 ). Meanwhile, the space vacated by the beetle-killed
trees may be quickly occupied by opportunistic herb, shrub, and most importantly,
seedlings and saplings of shade-tolerant tree species to increase live fuel biomass
and change canopy fuel characteristics (Kovacic et al. 1985 ). Many factors control
the timing and magnitude of the live and dead fuel dynamics after beetle outbreaks.
First, the level of tree mortality (severity) and tree species composition (pre-out-
break conditions) of the host stand dictates the potential amount of biomass that can
be actually deposited on the ground (Hicke et al. 2012 ). Mixed species stands where
50 % of the trees are pine host species and all host species were killed deposit less
fuel than stands composed entirely of host species that experienced 100 % mortal-
ity. Second, the biophysical setting and host species characteristics often dictate the