184
T.T. Veblen and P.B. Alaback
root suckering, basal sprouting) typically have an advantage in filling gaps
(Veblen et al. , 1981).
In North American temperate rainforests, broadly analogous patterns
of stand development occur. An interesting contrast is the tendency for
primary succession to include a sequence of dominance by different life
forms (from bryophytes to forbs , shrubs , and then mixtures of all these
life forms plus trees) following disturbance (Cooper, 1937). In contrast, at
most South American sites , dominance by tree life forms seems to develop
earlier, perhaps because of the prevalence of more nutrient-rich volcanic
soils or milder winter temperatures. In the perhumid zone of North
America, the pace and direction of development have been shown to be
highly dependent on soil fertility and drainage: Rapid stand initiation and
a long stage of stem exclusion occur on moderately productive sites,
whereas on nutrient-poor sites, stand initiation is slower and the stem
exclusion phase is shorter or nonexistent (Demeo , Martin, & West,
1992). On nutrient-rich, well-drained sites in the season al rainforest zone ,
by contrast, the stem exclusion stage is shorter than on moderately
productive sites, because of the rapid development of upper-canopy layers
and early self-thinning (Harcombe , 1986). In South America, although
site factors such as drainage also playa major role in controlling the pace
and direction of stand development, research on the role of soil fertility is
scant.
In North America , coarse- and fine-scale disturbances are both
important in controlling forest dynamics , especially in steep terrain , along
the mainland or exposed outer coast of the perhumid zone and throughout
the seasonal rainforest zone (Edmonds, Thomas, & Maybury, 1993; Ott,
1994; Spies & Franklin , 1988). Fire typically is the dominant catastrophic
agent in seasonal rainforests, whereas wind , flooding, avalanches, and
landslides are the key agents in perhumid rainfore sts. In many cases ,
successional trajectories are very long, with directional change continuing
to occur 4 to 5 centuries following disturbance (Alaback, this volume;
Spies, 1991).
Riparian habitats constitute a special case, due to the high frequency of
relatively large flood events and mudslides (debris torrents) . The most
common scenario in large coastal streams appears to be where a large
mudslide or debris torrent creates a lense of material that forms a new
terrace on which Alnus rubra and Picea sitchensis establish over a 200- to
800-year period. After the initial 100 to 200 years of stand development,
gap processes ,become important in creating fine-scale structure. The
riparian stands along large streams are more likely to be characterized by
whole-stand replacement, due to the higher frequency of catastrophic
floods. Less catastrophic disturbance (e.g., blowdown) often results in
local dominance by Rubus species or Alnus species, except on raised
organic surfaces such as old windthrow mounds or logs (Beaudry, 1989;
Clement , 1985; Schrader, 1991). In contrast , the riparian stands along
T.T. Veblen and P.B. Alaback
root suckering, basal sprouting) typically have an advantage in filling gaps
(Veblen et al. , 1981).
In North American temperate rainforests, broadly analogous patterns
of stand development occur. An interesting contrast is the tendency for
primary succession to include a sequence of dominance by different life
forms (from bryophytes to forbs , shrubs , and then mixtures of all these
life forms plus trees) following disturbance (Cooper, 1937). In contrast, at
most South American sites , dominance by tree life forms seems to develop
earlier, perhaps because of the prevalence of more nutrient-rich volcanic
soils or milder winter temperatures. In the perhumid zone of North
America, the pace and direction of development have been shown to be
highly dependent on soil fertility and drainage: Rapid stand initiation and
a long stage of stem exclusion occur on moderately productive sites,
whereas on nutrient-poor sites, stand initiation is slower and the stem
exclusion phase is shorter or nonexistent (Demeo , Martin, & West,
1992). On nutrient-rich, well-drained sites in the season al rainforest zone ,
by contrast, the stem exclusion stage is shorter than on moderately
productive sites, because of the rapid development of upper-canopy layers
and early self-thinning (Harcombe , 1986). In South America, although
site factors such as drainage also playa major role in controlling the pace
and direction of stand development, research on the role of soil fertility is
scant.
In North America , coarse- and fine-scale disturbances are both
important in controlling forest dynamics , especially in steep terrain , along
the mainland or exposed outer coast of the perhumid zone and throughout
the seasonal rainforest zone (Edmonds, Thomas, & Maybury, 1993; Ott,
1994; Spies & Franklin , 1988). Fire typically is the dominant catastrophic
agent in seasonal rainforests, whereas wind , flooding, avalanches, and
landslides are the key agents in perhumid rainfore sts. In many cases ,
successional trajectories are very long, with directional change continuing
to occur 4 to 5 centuries following disturbance (Alaback, this volume;
Spies, 1991).
Riparian habitats constitute a special case, due to the high frequency of
relatively large flood events and mudslides (debris torrents) . The most
common scenario in large coastal streams appears to be where a large
mudslide or debris torrent creates a lense of material that forms a new
terrace on which Alnus rubra and Picea sitchensis establish over a 200- to
800-year period. After the initial 100 to 200 years of stand development,
gap processes ,become important in creating fine-scale structure. The
riparian stands along large streams are more likely to be characterized by
whole-stand replacement, due to the higher frequency of catastrophic
floods. Less catastrophic disturbance (e.g., blowdown) often results in
local dominance by Rubus species or Alnus species, except on raised
organic surfaces such as old windthrow mounds or logs (Beaudry, 1989;
Clement , 1985; Schrader, 1991). In contrast , the riparian stands along