9. A Comparative Review of Forest Dynamics
191
can also occur at the earliest stages of succession, imposing some constraints on the facilitation model ' and on the generality of Cooper's
descriptions of the succession.
In Washington, revegetation following glaciation follows a different
pattern (Oliver, Adams & Zasoski, 1985). As in southern Chile establishment of trees and shrubs proceeds rapidly following deglaciation. Distinct
stages as described by Cooper (1937) do not generally occur. Alnus
sinuata is characteristic of the early stages of succession but Populus is
absent. Vaccinium commonly colonizes in the early stages of the succession
in Washington, but is only part of the succession in Glacier Bay 300 years
or more following deglaciation. Primary forest in Washington is much less
compositionally distinctive, with Tsuga being a part of the succession
from the earliest stages. But in both regions old growth forest generally
develops within 400 to 600 years.
Disturbance by Snow Avalanches
In the southern Andes in subalpine forests of N. pumilio and/or N .
betuloides, infrequent but stand-devastating snow avalanches typically
result in the development of even-aged stands (Veblen et aI., 1981).
Where avalanches occur with greater frequency, however, they may
prevent N. pumilio from successionally replacing N. antarctica. Although
N. pumilio is longer lived and appears to be a better competitor, N.
antarctica's typically more prostrate, shrubby form and effective vegetative
reproduction (through adventitious roots on damaged branches) give it an
advantage at sites frequently disturbed by avalanches (Veblen et aI.,
1977).
Because of the cooler winters in North America, on the mainland and
northern part of the perhumid and subpolar rainforest regions, avalanches
are a widespread and more frequent disturbance agent than in analogous
parts of the Andes. Temperatures are often near freezing during winter in
the perhumid and subpolar rainforest regions which allows for accumulation of wet heavy snow on steep slopes. Forests on avalanche slopes are
usually distinguished from nearby old-growth forest by the presence of
pure even-aged early successional species such as Picea sitchensis or
Pseudotsuga with Alnus and Rubus spectabilis mixtures (Martin, 1989).
These are open forests with a dense shru b layer usually composed of
Alnus, Rubus, Ribes, and Oplopanex . In many cases the species composition of these forests is very similar to that of a riparian floodplain forest.
These forests will succeed to Tsuga or Abies in approximately 100 to 200
years if not subject to additional avalanches. Understory plant communities vary with the frequency of avalanches and can be used to predict
frequency of avalanches (Cushman, 1976). In Washington succession
following avalanches often is rapid and frequently includes the resprouting
or release of Vaccinium spp. and Abies amabilis from the original forest
191
can also occur at the earliest stages of succession, imposing some constraints on the facilitation model ' and on the generality of Cooper's
descriptions of the succession.
In Washington, revegetation following glaciation follows a different
pattern (Oliver, Adams & Zasoski, 1985). As in southern Chile establishment of trees and shrubs proceeds rapidly following deglaciation. Distinct
stages as described by Cooper (1937) do not generally occur. Alnus
sinuata is characteristic of the early stages of succession but Populus is
absent. Vaccinium commonly colonizes in the early stages of the succession
in Washington, but is only part of the succession in Glacier Bay 300 years
or more following deglaciation. Primary forest in Washington is much less
compositionally distinctive, with Tsuga being a part of the succession
from the earliest stages. But in both regions old growth forest generally
develops within 400 to 600 years.
Disturbance by Snow Avalanches
In the southern Andes in subalpine forests of N. pumilio and/or N .
betuloides, infrequent but stand-devastating snow avalanches typically
result in the development of even-aged stands (Veblen et aI., 1981).
Where avalanches occur with greater frequency, however, they may
prevent N. pumilio from successionally replacing N. antarctica. Although
N. pumilio is longer lived and appears to be a better competitor, N.
antarctica's typically more prostrate, shrubby form and effective vegetative
reproduction (through adventitious roots on damaged branches) give it an
advantage at sites frequently disturbed by avalanches (Veblen et aI.,
1977).
Because of the cooler winters in North America, on the mainland and
northern part of the perhumid and subpolar rainforest regions, avalanches
are a widespread and more frequent disturbance agent than in analogous
parts of the Andes. Temperatures are often near freezing during winter in
the perhumid and subpolar rainforest regions which allows for accumulation of wet heavy snow on steep slopes. Forests on avalanche slopes are
usually distinguished from nearby old-growth forest by the presence of
pure even-aged early successional species such as Picea sitchensis or
Pseudotsuga with Alnus and Rubus spectabilis mixtures (Martin, 1989).
These are open forests with a dense shru b layer usually composed of
Alnus, Rubus, Ribes, and Oplopanex . In many cases the species composition of these forests is very similar to that of a riparian floodplain forest.
These forests will succeed to Tsuga or Abies in approximately 100 to 200
years if not subject to additional avalanches. Understory plant communities vary with the frequency of avalanches and can be used to predict
frequency of avalanches (Cushman, 1976). In Washington succession
following avalanches often is rapid and frequently includes the resprouting
or release of Vaccinium spp. and Abies amabilis from the original forest