14. Patch Dynamics for Forested Ecosystems in the Pacific Northwest
295
to view these processes from two perspectives of the temporal domainfrequency and duration. This distinction falls out naturally when viewed
from a time series perspective; pattern can be described in terms of the
peak structure (i.e., frequency) and in terms of the length or width of the
peak (i.e., duration). Frequency and duration of disturbance will affect
the type and rate of recovery in the ecosytem.
The frequency and duration characterizing a disturbance event may not
always be commensurate or dependent on the same variables. Duration
of a wildfire is primarily a function of several factors.jncluding fuel load,
topography, and present climate conditions (i.e., precipitation and wind;
Agee and Huff, 1987; Morrison and Swanson, 1990). Although fire
frequency is also a function of the same factors , it is constrained by
frequency of initiation events , such as lightning strikes (i.e., elevation and
combustible snags). Similarly, the frequency and rate of propagation of
bark beetle infestation in PNW forests is strongly influenced by the order
of disturbance events; large concentrations of downed wood or stressed
live trees resulting from windthrow events and drought conditions,
respectively, can initiate epidemic outbreaks. Frequency of infestation
events (approximately every 20 to 30yrs) is related to the probablities
of these two types of disturbances and the order in which they occur
(Bradshaw & Garman, 1994). In contrast, the duration of an infestation
event is more dependent on the overall landscape composition and connectivity (i.e ., an outbreak will only be sustained if there is available
appropriate material) . However, although infestation initiation may
require a forest stand which has sustained physiological stress (e.g.,
drought) , the spread can be sufficiently severe as to attack apparently
healthy, live trees (i.e. , a threshold condition may be breached) (Bedard,
1950). Duration of an outbreak may be expected to be dependent on the
area and perhaps spatial pattern of -the stand and landscape composition.
Modeling Anthropogenic Effects on Landscape Heterogeneity
In the previous section, landscape heterogeneity in the Pacific Northwest
temperate forests was described in terms of pattern-generating processes
within a spatial and temporal context. However, a glance at most regional
satellite imagery reveals that the most striking source of landscape heterogeneity is anthropogenically derived patterns of historic land-use and
land-conversion patterns. With the intensive and extensive resource
extraction sustained over the recent decades, the temperate forests are
markedly fragmented (Franklin and Forman , 1987). The implementation
of a national policy of fire suppression has also contributed to the alteration of landscape dynamics by favoring species composition to fire intolerants and increasing the probability of insect outbreak (Borchers, 1994) .
In contrast to natural disturbance patterns, such human activities as
295
to view these processes from two perspectives of the temporal domainfrequency and duration. This distinction falls out naturally when viewed
from a time series perspective; pattern can be described in terms of the
peak structure (i.e., frequency) and in terms of the length or width of the
peak (i.e., duration). Frequency and duration of disturbance will affect
the type and rate of recovery in the ecosytem.
The frequency and duration characterizing a disturbance event may not
always be commensurate or dependent on the same variables. Duration
of a wildfire is primarily a function of several factors.jncluding fuel load,
topography, and present climate conditions (i.e., precipitation and wind;
Agee and Huff, 1987; Morrison and Swanson, 1990). Although fire
frequency is also a function of the same factors , it is constrained by
frequency of initiation events , such as lightning strikes (i.e., elevation and
combustible snags). Similarly, the frequency and rate of propagation of
bark beetle infestation in PNW forests is strongly influenced by the order
of disturbance events; large concentrations of downed wood or stressed
live trees resulting from windthrow events and drought conditions,
respectively, can initiate epidemic outbreaks. Frequency of infestation
events (approximately every 20 to 30yrs) is related to the probablities
of these two types of disturbances and the order in which they occur
(Bradshaw & Garman, 1994). In contrast, the duration of an infestation
event is more dependent on the overall landscape composition and connectivity (i.e ., an outbreak will only be sustained if there is available
appropriate material) . However, although infestation initiation may
require a forest stand which has sustained physiological stress (e.g.,
drought) , the spread can be sufficiently severe as to attack apparently
healthy, live trees (i.e. , a threshold condition may be breached) (Bedard,
1950). Duration of an outbreak may be expected to be dependent on the
area and perhaps spatial pattern of -the stand and landscape composition.
Modeling Anthropogenic Effects on Landscape Heterogeneity
In the previous section, landscape heterogeneity in the Pacific Northwest
temperate forests was described in terms of pattern-generating processes
within a spatial and temporal context. However, a glance at most regional
satellite imagery reveals that the most striking source of landscape heterogeneity is anthropogenically derived patterns of historic land-use and
land-conversion patterns. With the intensive and extensive resource
extraction sustained over the recent decades, the temperate forests are
markedly fragmented (Franklin and Forman , 1987). The implementation
of a national policy of fire suppression has also contributed to the alteration of landscape dynamics by favoring species composition to fire intolerants and increasing the probability of insect outbreak (Borchers, 1994) .
In contrast to natural disturbance patterns, such human activities as