23.5 Characterization of Dynamics and Evidence Used in Assessment
343
slopes, where limited soil moisture and increased
solar radiation already result in more xerophytic,
flammable vegetation (Harmon et al., 1983). Where
disturbance gradients parallel environmental gradients, distinguishing one from another may prove
challenging. Human-caused disturbances are less
likely to originate in response to environmental gradients. For example, fires are more likely ignited
by humans in low-lying settled areas, and exotic
pests and diseases frequently are introduced near
commercial ports of entry. Once initiated, however,
these disturbances may then be perpetuated by natural processes, thus obscuring their anthropogenic
origins.
These types of interactions, despite their importance, are poorly documented and understood components of disturbance regimes. Although common
in many ecosystems, they have been studied in only
a few. Their presence should be considered in any
detailed assessment of dynamic pattern.
23.4 Ecosystem Parameters
Affected by
Ecosystem Dynamics
Disturbances not only occur at a variety of intensities and frequencies, but they also affect ecosystem composition and structure in different ways.
Many disturbances alter ecosystem composition because species have different susceptibilities to disturbance and different responses to the conditions
that disturbances create. Biotic disturbances, such
as disease or insect outbreaks, often have very specific effects on composition because they affect selected species. For example, the introduction of
chestnut blight in the early 20th century eliminated
the American chestnut as a canopy species throughout the hardwood forests of the Eastern United
States (Woods and Shanks, 1959). The dramatic
impacts of chestnut blight have resulted in a major
shift in forest composition from widespread dominance by chestnut to dominance by a variety of oak
and other species, depending on geography, elevation, and topographic position (Whittaker, 1956).
Disturbance susceptibility also varies with age and
size class, successional state, and topographic position (White et aI., 1999).
Ecosystem structure is also affected in a variety
of ways by disturbance. Within one ecosystem
type, different kinds of disturbance can cause a
wide array of structural changes, and even for a single disturbance type within one ecosystem, a wide
array of structural effects can result as a function
of local disturbance intensity (Lang, 1985). For example, some less intense fires may consume only
minor amounts of organic matter and may even create more fuels than they bum, because some trees
are killed without being consumed by the fire. At
the other extreme are intense fires that consume all
the organic material on the forest floor, cause heavy
mortality in the canopy, and consume most available fuels. Ecosystem structure may be the key consideration for maintaining reproducing populations
of some species. For example, the red-cockaded
woodpecker in southeastern longleaf pine ecosystems depends on large but isolated nest trees, a
structure that requires frequent ground fire (to reduce understory woody plants) and rarer intense
fires (to allow pine reproduction) (Noss, 1989;
Ware et aI., 1993).
Disturbance also has various effects on ecosystem function, specifically the flow of energy, materials, and species among ecosystem components.
On a local scale, storm-related tree gaps experience greater solar radiation, lower relative humidity, and reduced leaf litter, resulting in a shift of
available resources. Larger disturbances, such as
avalanches, landslides, volcanic eruption, flooding, and erosion, all create or destroy substrate
while altering soil chemistry, texture, and drainage
characteristics, all of which will affect ecosystem
function.
23.5 Characterization of
Dynamics and Evidence
U sed in Assessment
Although practicality may require the analysis of
dynamic pattern to be driven by the type, scale, and
quality of the assessment data set itself, the preceding sections serve as a guide to determining a
scale and level of analysis appropriate to the system being examined and the assessment objectives.
A second important step before analysis is to determine the methods that will be used to characterize ecosystem dynamics. Although many different descriptors of disturbance have already been
identified (e.g., type, shape, magnitude, spatial distribution, temporal distribution, duration), most
disturbance regimes have been typified by in-depth
analysis of just a few of these attributes (Baker,
1992a). These are often limited to size distributions, temporal distributions (particularly interval
between disturbances and frequency per unit area),
and spatial distributions (density per area and topographic position) (Figure 23.4). Mean values of
343
slopes, where limited soil moisture and increased
solar radiation already result in more xerophytic,
flammable vegetation (Harmon et al., 1983). Where
disturbance gradients parallel environmental gradients, distinguishing one from another may prove
challenging. Human-caused disturbances are less
likely to originate in response to environmental gradients. For example, fires are more likely ignited
by humans in low-lying settled areas, and exotic
pests and diseases frequently are introduced near
commercial ports of entry. Once initiated, however,
these disturbances may then be perpetuated by natural processes, thus obscuring their anthropogenic
origins.
These types of interactions, despite their importance, are poorly documented and understood components of disturbance regimes. Although common
in many ecosystems, they have been studied in only
a few. Their presence should be considered in any
detailed assessment of dynamic pattern.
23.4 Ecosystem Parameters
Affected by
Ecosystem Dynamics
Disturbances not only occur at a variety of intensities and frequencies, but they also affect ecosystem composition and structure in different ways.
Many disturbances alter ecosystem composition because species have different susceptibilities to disturbance and different responses to the conditions
that disturbances create. Biotic disturbances, such
as disease or insect outbreaks, often have very specific effects on composition because they affect selected species. For example, the introduction of
chestnut blight in the early 20th century eliminated
the American chestnut as a canopy species throughout the hardwood forests of the Eastern United
States (Woods and Shanks, 1959). The dramatic
impacts of chestnut blight have resulted in a major
shift in forest composition from widespread dominance by chestnut to dominance by a variety of oak
and other species, depending on geography, elevation, and topographic position (Whittaker, 1956).
Disturbance susceptibility also varies with age and
size class, successional state, and topographic position (White et aI., 1999).
Ecosystem structure is also affected in a variety
of ways by disturbance. Within one ecosystem
type, different kinds of disturbance can cause a
wide array of structural changes, and even for a single disturbance type within one ecosystem, a wide
array of structural effects can result as a function
of local disturbance intensity (Lang, 1985). For example, some less intense fires may consume only
minor amounts of organic matter and may even create more fuels than they bum, because some trees
are killed without being consumed by the fire. At
the other extreme are intense fires that consume all
the organic material on the forest floor, cause heavy
mortality in the canopy, and consume most available fuels. Ecosystem structure may be the key consideration for maintaining reproducing populations
of some species. For example, the red-cockaded
woodpecker in southeastern longleaf pine ecosystems depends on large but isolated nest trees, a
structure that requires frequent ground fire (to reduce understory woody plants) and rarer intense
fires (to allow pine reproduction) (Noss, 1989;
Ware et aI., 1993).
Disturbance also has various effects on ecosystem function, specifically the flow of energy, materials, and species among ecosystem components.
On a local scale, storm-related tree gaps experience greater solar radiation, lower relative humidity, and reduced leaf litter, resulting in a shift of
available resources. Larger disturbances, such as
avalanches, landslides, volcanic eruption, flooding, and erosion, all create or destroy substrate
while altering soil chemistry, texture, and drainage
characteristics, all of which will affect ecosystem
function.
23.5 Characterization of
Dynamics and Evidence
U sed in Assessment
Although practicality may require the analysis of
dynamic pattern to be driven by the type, scale, and
quality of the assessment data set itself, the preceding sections serve as a guide to determining a
scale and level of analysis appropriate to the system being examined and the assessment objectives.
A second important step before analysis is to determine the methods that will be used to characterize ecosystem dynamics. Although many different descriptors of disturbance have already been
identified (e.g., type, shape, magnitude, spatial distribution, temporal distribution, duration), most
disturbance regimes have been typified by in-depth
analysis of just a few of these attributes (Baker,
1992a). These are often limited to size distributions, temporal distributions (particularly interval
between disturbances and frequency per unit area),
and spatial distributions (density per area and topographic position) (Figure 23.4). Mean values of