9. A Comparative Review of Forest Dynamics
203
to be uniquely adapted to current climatic conditions (Kellison & Weir ,
1987).
Two centuries ago or more in the seasonal rainforest zone some fireadapted tree species such as Pseudotsuga menziesii colonized coastal and
high-elevation sites which now are dominated by cool moist-adapted
species such as Tsuga or Abies (Jan Henderson, V.S .D.A. , Forest Service,
Olympic National Forest , personal communication). Natural regeneration
following logging at mid-elevations in the Oregon Cascades may also
result in major shifts in dominant species, perhaps also suggesting a
somewhat different climate two to five centuries ago when dominant
trees from the original forest established. Similarly, in southern Oregon
reforestation following logging on southwest-facing slopes is often slow or
unsuccessful unless manipulation of competing vegetation, shade cards
and other intensive reforestation techniques are applied , suggesting a
possible climatic shift from when these forests established two to three
centuries ago (Minore, 1978; Perry, Amaranthus, Borchers et al., 1989).
Since the late 1970s a 0.1 degree per decade warming has been documented in interior Alaska, providing a possible model of the effects of
global warming on northern landscapes (Trenberth, 1990). Frequency of
severe snow accumulation has declined dramatically in the perhumid
rainforest zone , resulting in dramatic increases in large mammal populations which were formerly constrained by extreme winter conditions.
This has resulted in reduced tree regeneration by the most palatable
species , such as Chamaecyparis nootkatensis and Thuja plicata (Hennon,
1992). Frequency of snow avalanches has also dramatically declined in
coastal regions . This has resulted in a rapid colonization of alpine tundra
by Tsuga mertensiana , or of Cassiope- or Carex-dominated meadows by
Rubus spectabilis.
Conclusions
The disturbance regimes of the temperate rainforests of North and South
America show some important similarities related to their locations in
areas of often rugged topography and great tectonic activity, and in cool
moist climates. In comparison with other types of forest ecosystems, fire
is a relatively infrequent natural disturbance in the subpolar and perhumid
rainforests. In the seasonal rainforests, however , fire often is a major
factor in their dynamics. In both North and South American temperate
rainforests, windstorms and landslides are important agents of disturbance. Towards higher latitudes and altitudes, disturbance by glacial
activity is also important.
In general, many characteristics of the temperate rainforests of the west
coasts of the Americas reflect the interplay between fine-scale and coarse-
203
to be uniquely adapted to current climatic conditions (Kellison & Weir ,
1987).
Two centuries ago or more in the seasonal rainforest zone some fireadapted tree species such as Pseudotsuga menziesii colonized coastal and
high-elevation sites which now are dominated by cool moist-adapted
species such as Tsuga or Abies (Jan Henderson, V.S .D.A. , Forest Service,
Olympic National Forest , personal communication). Natural regeneration
following logging at mid-elevations in the Oregon Cascades may also
result in major shifts in dominant species, perhaps also suggesting a
somewhat different climate two to five centuries ago when dominant
trees from the original forest established. Similarly, in southern Oregon
reforestation following logging on southwest-facing slopes is often slow or
unsuccessful unless manipulation of competing vegetation, shade cards
and other intensive reforestation techniques are applied , suggesting a
possible climatic shift from when these forests established two to three
centuries ago (Minore, 1978; Perry, Amaranthus, Borchers et al., 1989).
Since the late 1970s a 0.1 degree per decade warming has been documented in interior Alaska, providing a possible model of the effects of
global warming on northern landscapes (Trenberth, 1990). Frequency of
severe snow accumulation has declined dramatically in the perhumid
rainforest zone , resulting in dramatic increases in large mammal populations which were formerly constrained by extreme winter conditions.
This has resulted in reduced tree regeneration by the most palatable
species , such as Chamaecyparis nootkatensis and Thuja plicata (Hennon,
1992). Frequency of snow avalanches has also dramatically declined in
coastal regions . This has resulted in a rapid colonization of alpine tundra
by Tsuga mertensiana , or of Cassiope- or Carex-dominated meadows by
Rubus spectabilis.
Conclusions
The disturbance regimes of the temperate rainforests of North and South
America show some important similarities related to their locations in
areas of often rugged topography and great tectonic activity, and in cool
moist climates. In comparison with other types of forest ecosystems, fire
is a relatively infrequent natural disturbance in the subpolar and perhumid
rainforests. In the seasonal rainforests, however , fire often is a major
factor in their dynamics. In both North and South American temperate
rainforests, windstorms and landslides are important agents of disturbance. Towards higher latitudes and altitudes, disturbance by glacial
activity is also important.
In general, many characteristics of the temperate rainforests of the west
coasts of the Americas reflect the interplay between fine-scale and coarse-