9. A Comparative Review of Forest Dynamics
185
smaller streams tend toward continual recruitment, although some of
these smaller systems may be more susceptible to landslides and debris
torrents.
In North America, the species that respond to different disturbances
vary significantly with elevation and topographic position. For example,
at sea level, Tsuga heterophylla is an agressive competitor and excludes
all other tree species on productive sites (Alaback, 1982; Stewart, 1986).
At higher elevations or on poorer sites, the forest is more open, and trees
such as Picea sitchensis, in the perhumid zone, or Abies species, in the
seasonal zone, Pinus contorta, Thuja plicata, and Chamaecyparis nootkatensis can compete more aggressively. Near timberline, Picea sitchensis
can regenerate in single-tree treefall gaps, because of the openness of the
canopy. Tsuga mertensiana, which is less shade tolerant than T. heterophylla, may form nearly pure stands maintained by gap processes at high
elevations where stands are very open and have a persistent snowpack
(Demeo et al., 1993; Lertzman, 1992; Martin, 1989; Matson & Boone,
1984). Analogously, in South America the regeneration of Nothofagus
dombeyi is almost entirely dependent on coarse-scale disturbances in the
more species-rich rainforests but regenerates following fine-scale treefalls
in species-poor mesophytic forests (Veblen, 1989).
In North America on upland sites there is no equivalent understory
dominant to the Chusquea bamboos of South America. However, in
riparian sites and in burned or logged -sites in the seasonal rainforest
region Rubus spectabilis can playa role similar to that played by Chusquea
in that establishment of ' Picea, Pseudotsuga and Tsuga is retarded for
50-100yr. or more (Barber, 1976; Beaudry, 1989; Ruth, 1968; Schrader,
1991). Alnus rubra can also retard successional development, especially
after severe soil disturbance in both the perhumid and seasonal rainforest
zones (Binkley, 1984; Newton, el Hassan , & ZavitKovski, 1968). These
aggressive deciduous species generally do not have the degree of persistence in mature forest as does Chusquea in South America perhaps
because of the more open multi-layered canopies that typify Valdivian
rainforests.
Natural Disturbances
Disturbances of Tectonic Origin
Located at a boundary between two major crustal plates, the southern
Andean region is an area of great volcanic and tectonic activity and ,
therefore, of geologically related ecological disturbances. For example,
on the wet western side of the Andes, in the Valdivian rainforest district,
strong earthquakes . such as the 1960 Valdivian earthquake measuring 8.5
on the Richter scale , periodically, in a single event, trigger thousands of
185
smaller streams tend toward continual recruitment, although some of
these smaller systems may be more susceptible to landslides and debris
torrents.
In North America, the species that respond to different disturbances
vary significantly with elevation and topographic position. For example,
at sea level, Tsuga heterophylla is an agressive competitor and excludes
all other tree species on productive sites (Alaback, 1982; Stewart, 1986).
At higher elevations or on poorer sites, the forest is more open, and trees
such as Picea sitchensis, in the perhumid zone, or Abies species, in the
seasonal zone, Pinus contorta, Thuja plicata, and Chamaecyparis nootkatensis can compete more aggressively. Near timberline, Picea sitchensis
can regenerate in single-tree treefall gaps, because of the openness of the
canopy. Tsuga mertensiana, which is less shade tolerant than T. heterophylla, may form nearly pure stands maintained by gap processes at high
elevations where stands are very open and have a persistent snowpack
(Demeo et al., 1993; Lertzman, 1992; Martin, 1989; Matson & Boone,
1984). Analogously, in South America the regeneration of Nothofagus
dombeyi is almost entirely dependent on coarse-scale disturbances in the
more species-rich rainforests but regenerates following fine-scale treefalls
in species-poor mesophytic forests (Veblen, 1989).
In North America on upland sites there is no equivalent understory
dominant to the Chusquea bamboos of South America. However, in
riparian sites and in burned or logged -sites in the seasonal rainforest
region Rubus spectabilis can playa role similar to that played by Chusquea
in that establishment of ' Picea, Pseudotsuga and Tsuga is retarded for
50-100yr. or more (Barber, 1976; Beaudry, 1989; Ruth, 1968; Schrader,
1991). Alnus rubra can also retard successional development, especially
after severe soil disturbance in both the perhumid and seasonal rainforest
zones (Binkley, 1984; Newton, el Hassan , & ZavitKovski, 1968). These
aggressive deciduous species generally do not have the degree of persistence in mature forest as does Chusquea in South America perhaps
because of the more open multi-layered canopies that typify Valdivian
rainforests.
Natural Disturbances
Disturbances of Tectonic Origin
Located at a boundary between two major crustal plates, the southern
Andean region is an area of great volcanic and tectonic activity and ,
therefore, of geologically related ecological disturbances. For example,
on the wet western side of the Andes, in the Valdivian rainforest district,
strong earthquakes . such as the 1960 Valdivian earthquake measuring 8.5
on the Richter scale , periodically, in a single event, trigger thousands of