Ecology - Wildland Fuel Fundamentals and Applications - page 89

6.1.4 Disturbance
Disturbance dictates fuel dynamics in most ecosystems and disturbance history is
often considered the most important factor for predicting and describing fuel char-
acteristics (Brown and Bevins 1986 ; Keane et al. 2012a ). Disturbance is defined
here as an event, force, or agent, of biological or nonbiological origin, that causes
mortality to organisms and changes patterns in ecosystems (Pickett and White
1985 ). Disturbances can be endogenous, originating within the area of interest, or
exogenous, coming from outside the area. Many types of disturbances influence
fuel dynamics and each type has its own unique impacts on fuelbed characteristics.
Disturbances may occur over short periods of time, such as wildland fire, or over
longer periods of years or decades, such as insect and disease outbreaks. A distur-
bance event may impact an entire stand, such as when a crown fire kills all trees or
shrubs, or it may affect only certain species or individuals, such as when mountain
pine beetles kill specific pine tree host species within a stand. In addition, distur-
bances may kill entire plants or simply damage parts of them. Disturbances can also
interact with each other, such as beetles and fire (Jenkins et al. 2008 ), or be totally
independent of each other, such as blister rust and beetles (Six and Adams 2007 ).
A disturbance regime is a general term that describes the temporal and spatial
characteristics of disturbances. Disturbance regimes are fundamentally different
from individual disturbance events because regimes incorporate the cumulative ef-
fects of multiple disturbances over time. Thus, fuel dynamics are governed more by
the disturbance regimes rather than any particular disturbance event. Disturbance
regimes can be generally described by 11 characteristics (Table 6.2 ; Keane 2013 ).
The disturbance agent is the entity that causes the disturbance, such as wind, fire,
or beetles. Often, disturbance agents have a source that triggers the agent. Light-
ning can be a source for wildland fire and heavy snow loads may be the source for
avalanches. Disturbances occur at a particular frequency that is often described over
a period of time depending on scale and objective. Point-level measures, such as
disturbance return interval and occurrence probability, describe the number of dis-
turbance events experienced over time at one point on the landscape. Disturbance
intensity is the level of the disturbance agent as it occurs on the landscape. Insect
and disease intensities are often described by population levels; windthrow intensity
can be described by wind speed. Severity is different from intensity in that it reflects
the impact of that disturbance on the biophysical environment and it is an important
attribute because it directly links to land management. The sizes (area) and patterns
(spatial variability) of disturbance severity and intensity often govern landscape
and fuelbed heterogeneity, which influences a wide variety of landscape character-
istics such as connectivity, fragmentation, and diversity (Turner et al. 1997 ; Knight
1987 ). Pattern refers to the size, shape, and spatial distribution of disturbed patches.
Seasonality reflects the annual time of occurrence typical of a disturbance event,
important because it can influence plant and animal phenology, mortality, and post-
disturbance recovery. Disturbance patterns are often influenced by the duration of
the disturbance agent on the landscape, with durations ranging from seconds (wind)