9. A Comparative Review of Forest Dynamics
187
destroy critical salmonid habitat (Swanston & Marion, 1991). Notable
exceptions occur in the Queen Charlotte Islands in British Columbia,
where rugged terrain and high intensity storms lead to widespread slope
failures (Schwab, 1983). Expansible clay soils derived from volcanic ash
in the Cascades, loose volcanic ash, and interbedded mudstone and siltstones are also prone to frequent slope failures following intense storms
in the seasonal rainforest zone (Swanston & Swanson, 1976). These slope
failures often dominate the terrain in the southern Cascades. Logging and
associated road construction dramatically increase probability of slope
failure (Schwab, 1983; Swanston & Swanson, 1976). Slope failures are
particularly frequent when slopes exceed 70% in high rainfall zones. In
general, slope failures in logged and roaded regions are smaller, occur at
lower elevations, develop on gentler gradients , and tend to travel shorter
distances than those in natural undisturbed landscapes (Swanston &
Marion, 1991).
There is some evidence that earthquakes, although infrequent, may still
have important implications for stand dynamics in the perhumid and
subpolar rainforest zones in North America in as much as they may
trigger massive slope failures. In the Queen Charlotte Islands, for
example, surveys of landslides suggest that a large portion of the landslides
that are now visible on the terrain occurred at about the same time,
approximately 85 years ago, and were most likely triggered by earthquakes
(Steve Chatwin, B.C. Ministry of Forests Research Branch , Victoria,
B.C. personal communication) . There are several large landslides in
northern Southeast Alaska that appear to be of the same age, and may
also have been the result of an.earthquake in the area (Douglas Swanston,
V.S.D.A., Forest Service, Forestry Sciences Laboratory, Juneau, AK ,
personal communication).
Volcanism
In South America, volcanic ash deposition is important in creating conditions favorable to the establishment of shade-intolerant tree species
(especially Nothofagus spp.) that otherwise tend to be successionally
replaced . Nothofagus spp. are frequently observed growing on recent
volcanic ash and lava deposits, but specific studies of Nothofagus stand
development patterns on recent volcanic deposits are lacking. At relatively
high elevations in areas of high rainfall, Fitzroya cupressoides often is the
first tree species to colonize volcanic ash or lava, or may cocolonize with
Nothofagus betuloides (Lara, 1991 ; Veblen, Delmastro, & Schlatter,
1976). At mid elevations in competition with Shade-tolerant tree species,
the regeneration of F. cupressoides may be dependent on volcanism
and/or landslides (Lara, 1991).
Volcanic ash deposition is not common in North American perhumid
rainforests, but is common in the southern seasonal rainforest zone and
187
destroy critical salmonid habitat (Swanston & Marion, 1991). Notable
exceptions occur in the Queen Charlotte Islands in British Columbia,
where rugged terrain and high intensity storms lead to widespread slope
failures (Schwab, 1983). Expansible clay soils derived from volcanic ash
in the Cascades, loose volcanic ash, and interbedded mudstone and siltstones are also prone to frequent slope failures following intense storms
in the seasonal rainforest zone (Swanston & Swanson, 1976). These slope
failures often dominate the terrain in the southern Cascades. Logging and
associated road construction dramatically increase probability of slope
failure (Schwab, 1983; Swanston & Swanson, 1976). Slope failures are
particularly frequent when slopes exceed 70% in high rainfall zones. In
general, slope failures in logged and roaded regions are smaller, occur at
lower elevations, develop on gentler gradients , and tend to travel shorter
distances than those in natural undisturbed landscapes (Swanston &
Marion, 1991).
There is some evidence that earthquakes, although infrequent, may still
have important implications for stand dynamics in the perhumid and
subpolar rainforest zones in North America in as much as they may
trigger massive slope failures. In the Queen Charlotte Islands, for
example, surveys of landslides suggest that a large portion of the landslides
that are now visible on the terrain occurred at about the same time,
approximately 85 years ago, and were most likely triggered by earthquakes
(Steve Chatwin, B.C. Ministry of Forests Research Branch , Victoria,
B.C. personal communication) . There are several large landslides in
northern Southeast Alaska that appear to be of the same age, and may
also have been the result of an.earthquake in the area (Douglas Swanston,
V.S.D.A., Forest Service, Forestry Sciences Laboratory, Juneau, AK ,
personal communication).
Volcanism
In South America, volcanic ash deposition is important in creating conditions favorable to the establishment of shade-intolerant tree species
(especially Nothofagus spp.) that otherwise tend to be successionally
replaced . Nothofagus spp. are frequently observed growing on recent
volcanic ash and lava deposits, but specific studies of Nothofagus stand
development patterns on recent volcanic deposits are lacking. At relatively
high elevations in areas of high rainfall, Fitzroya cupressoides often is the
first tree species to colonize volcanic ash or lava, or may cocolonize with
Nothofagus betuloides (Lara, 1991 ; Veblen, Delmastro, & Schlatter,
1976). At mid elevations in competition with Shade-tolerant tree species,
the regeneration of F. cupressoides may be dependent on volcanism
and/or landslides (Lara, 1991).
Volcanic ash deposition is not common in North American perhumid
rainforests, but is common in the southern seasonal rainforest zone and