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T.T. Veblen and P.B. Alaback
scale disturbances. Both regions have long-lived forest dominants, a rich
epiphyte layer , and complex canopy structure reflecting both large-scale
and small-scale disturbances. The relative importance of disturbances of
different scales varies significantly, and to a large degree predictably,
from one habitat type to another. Despite the taxonomic differences
between the temperate rainforests of North and South America, the
general patterns of stand development following disturbance are broadly
similar. These patterns are well described by general models of wholestand replacement (e.g. , Oliver , 1981) and fine-scale treefall gap dynamics
(e.g., Watt, 1947). Many areas have a complex history that includes
disturbances at many spatial scales often occurring on the same terrain
but at different points in time. The structure, diversity, and functioning of
these forests appears to be closely tied to the character of these disturbances and their spatial and temporal patterns.
Although the extensive data required to compare the extent and frequency of most disturbances in North versus South American temperate
rainforests are not available, the incomplete information on natural
disturbance regimes in the two areas suggests some significant differences.
Although both areas occur in regions of great tectonic activity, the South
American temperate rainforests appear to be more extensively affected
by , geologic disturbances such as volcanism and earthquake-triggered
mass movements. Disturbance by volcanism is less significant in North
American temperate rainforests because volcanoes mostly occur in the
southeastern part of the seasonal rainforest zone or west of the subpolar
region (Aleutians, Alaska Range , Katmai Range). In contrast, disturbance by snow avalanches and wet snowfalls appears greater in North
America. This reflects the cooler winters and greater snow accumulation
associated with the more continental North American climate.
A major difference exists between North and South American temperate rainforests in terms of the degree of human modification mediated
through introductions of large herbivores. Introduced mammals have had
only a minor impact in the north but have become a dominant influence
in large parts of South American temperate rainforests. Introduced
livestock, deer , and beavers have and continue to playa major role in
modifying forest composition and structure in South America. The greater
impact of large introduced herbivores in South American temperate
rainforests mainly reflects the much greater densities of the animals.
However, in South American forests the dominant plant species appear
to be less resistant to browsing which may reflect the paucity of large
native herbivores in these forests.
Both North and South American temperate rainforests may be particularly susceptible to the effects of climate variation. Relative to other
forest formations ; their distributions are relatively restricted and at their
poleward limits they occur inhabitats where temperature plays a particularly important role in determining tree survival. Thus , global warming is
T.T. Veblen and P.B. Alaback
scale disturbances. Both regions have long-lived forest dominants, a rich
epiphyte layer , and complex canopy structure reflecting both large-scale
and small-scale disturbances. The relative importance of disturbances of
different scales varies significantly, and to a large degree predictably,
from one habitat type to another. Despite the taxonomic differences
between the temperate rainforests of North and South America, the
general patterns of stand development following disturbance are broadly
similar. These patterns are well described by general models of wholestand replacement (e.g. , Oliver , 1981) and fine-scale treefall gap dynamics
(e.g., Watt, 1947). Many areas have a complex history that includes
disturbances at many spatial scales often occurring on the same terrain
but at different points in time. The structure, diversity, and functioning of
these forests appears to be closely tied to the character of these disturbances and their spatial and temporal patterns.
Although the extensive data required to compare the extent and frequency of most disturbances in North versus South American temperate
rainforests are not available, the incomplete information on natural
disturbance regimes in the two areas suggests some significant differences.
Although both areas occur in regions of great tectonic activity, the South
American temperate rainforests appear to be more extensively affected
by , geologic disturbances such as volcanism and earthquake-triggered
mass movements. Disturbance by volcanism is less significant in North
American temperate rainforests because volcanoes mostly occur in the
southeastern part of the seasonal rainforest zone or west of the subpolar
region (Aleutians, Alaska Range , Katmai Range). In contrast, disturbance by snow avalanches and wet snowfalls appears greater in North
America. This reflects the cooler winters and greater snow accumulation
associated with the more continental North American climate.
A major difference exists between North and South American temperate rainforests in terms of the degree of human modification mediated
through introductions of large herbivores. Introduced mammals have had
only a minor impact in the north but have become a dominant influence
in large parts of South American temperate rainforests. Introduced
livestock, deer , and beavers have and continue to playa major role in
modifying forest composition and structure in South America. The greater
impact of large introduced herbivores in South American temperate
rainforests mainly reflects the much greater densities of the animals.
However, in South American forests the dominant plant species appear
to be less resistant to browsing which may reflect the paucity of large
native herbivores in these forests.
Both North and South American temperate rainforests may be particularly susceptible to the effects of climate variation. Relative to other
forest formations ; their distributions are relatively restricted and at their
poleward limits they occur inhabitats where temperature plays a particularly important role in determining tree survival. Thus , global warming is