Ecology - Wildland Fuel Fundamentals and Applications - page 91

and minutes (avalanches) to days (fire) and years (insects). The variability of dis-
turbance characteristics such as severity, frequency, and size, coupled with their
interaction with existing patterns, duration, and seasonality of disturbance, as well
as climate and vegetation, govern landscape dynamics and ultimately controls wild-
land fuels (Schoennagel et al. 2004 ).
The following sections provide simplistic descriptions of the effects of four com-
mon western North American disturbance regimes on fuelbed properties. It is im-
portant to note that these disturbances also interact with climate, vegetation, and
other disturbances at the landscape level to uniquely modify fuel dynamics and
create locally unique fuelbeds (Keane et al. 2014 ).
6.1.4.1 Wind
Wind influences fuel deposition in most ecosystems because of its high frequency,
variable intensity, and disparate severities (Mitchell 2013 ). Many fuel particles
would eventually fall to the ground without wind, but because wind is such a
frequent phenomena, its action often dislodges most plant material long before it
would fall on its own and then transports the detached fuel particles various dis-
tances. Wind effects range from chronic to acute. Trees respond to chronic wind
exposure by thickening stems and structural roots, and by reducing shoot length
thereby resulting in shorter and thicker branches and trunks that ultimately make
unique surface fuel particles. Moderate acute wind may cause minor acute effects
by freeing dead or dying plant material and depositing it over small areas. Strong
winds, however, may strip live foliage and branch material from upright plants
and deposit them great distances from the source plant (Schoennagel et al. 2012 ).
Severe wind may also cause plant breakage or uprooting, a disturbance process
known as “windthrow” or “blowdown,” and these major effects create significantly
different fuelbeds (Mitchell 2013 ). Hurricanes and typhoons may have winds that
are so strong that they can topple most trees thereby moving canopy fuels directly to
the ground surface over a short time period (Busing et al. 2009 ; Turner et al. 1997 ).
The severity and frequency of windthrow depends on many site factors including
soils, plant conditions (health, size), stand conditions (densities, size distribution),
topography, management history, and, of course, wind characteristics (e.g., speed,
duration, and direction).
Windthrow disturbances generally decrease canopy fuels, increase surface fuel
loadings, and change surface fuelbed properties (Fig. 6.2b ). Wind rarely reduces
surface fuel loading as with some other disturbances. As a result, wind serves sev-
eral ecological roles in fuel deposition. First, winds affect spatial distributions of
fuels by blowing fuel particles either evenly or unevenly across an area, depending
on particle size, wind speed and direction, and vegetation condition (species, size,
morphology, health). Second, strong winds may collapse canopy fuels into the sur-
face fuel layer thereby decreasing CBH, CBD, and CC but increasing surface fuel
component loadings, bulk density, depth, and fuel component diversity (Vihnanek
et al. 2009 ). Schoennagel et al. ( 2012 ) mention that hurricane winds were important