192
T .T. Veblen and P.B . Alaback
(Oliver et al., 1985). In this case forest development is much less compositionally distinctive than it is in coastal Alaska.
Disturbance by Wind
The mid- to high-latitude zone of western South America is well known
for its strong winds, and blowdowns are a common disturbance throughout
the temperate rainforest zone (Agostini , 1941; Eskuche, 1973). Windstorms on Tierra del Fuego, causing the windthrow of individual trees as
well as the blowdown of entire stands, are particularly important in
controlling the structure of Nothofagus forests (Puigdefabregas , del
Barrio, Gallart et al., in press; Rebertus & Veblen 1993a, 1993b). On
Tierra del Fuego, most old-growth forests of Nothofagus pumilio and N .
betuloides have > 10% gap area (surface area beneath treefall-caused
openings) and complete turnover rates through gap processes (i.e., the
time required for treefalls to disturb an entire stand) range from 300 to
500 years (Rebertus & Veblen, 1993b), which is similar to more speciesrich Valdivian rainforests in Chile (Veblen, 1985b). Uprooting and stem
breakage are the predominant modes of treefall . Most gaps involve multiple treefalls and typically exhibit two or more events of gap creation and
subsequent expansion (Rebertus & Veblen, 1993b).
On Tierra del Fuego , under certain circumstances of wind exposure,
topographic position , and substrate, gradual gap enlargement results in a
wave-like pattern similar to well-documented fir waves in the northeastern
United States (Puigdefabregas et al., in press) . Well-developed mortality
waves in N. betuloides forest occur at right angles to the coast along the
Beagle Channel and Isla de los Estados , where strong coastal winds and
possibly salt spray may be the driving forces. These mortality waves are
conspicuous because of the bleached white dead-standing trees and logs
pointing in a leeward direction . Further inland, waves do occur, but they
are less conspicuous and more irregular in shape and movement (Rebertus
& Veblen , 1993a). Large-scale blowdowns, sometimes involving hundreds
of hectares, are a conspicuous feature in the Fuegian landscape. Indeed,
some areas, where soils are thin , are a complete mosaic of blowdowns of
different ages.
Farther north in the Valdivian rainforest region, fine-scale treefalls
have been studied on the Island of Chiloe (Armesto & Figueroa , 1987;
Armesto & Fuentes, 1988; Veblen, 1985b) and in the Andes in the
Province of Valdivia (Veblen , 1985b). These studies have shown that the
shade-intolerant species such, as Nothofagus dombeyi and Weinmannia
trichosperma, typically do not regenerate in gaps created by single or
small groups of trees. The exception is where a large nurse log allows a
N. dombeyi seedling to establish above the bamboo-dominated understory. If the seedling's roots develop a secure and sustaining anchorage in
the soil before the nurse log decays, this regeneration strategy may be
T .T. Veblen and P.B . Alaback
(Oliver et al., 1985). In this case forest development is much less compositionally distinctive than it is in coastal Alaska.
Disturbance by Wind
The mid- to high-latitude zone of western South America is well known
for its strong winds, and blowdowns are a common disturbance throughout
the temperate rainforest zone (Agostini , 1941; Eskuche, 1973). Windstorms on Tierra del Fuego, causing the windthrow of individual trees as
well as the blowdown of entire stands, are particularly important in
controlling the structure of Nothofagus forests (Puigdefabregas , del
Barrio, Gallart et al., in press; Rebertus & Veblen 1993a, 1993b). On
Tierra del Fuego, most old-growth forests of Nothofagus pumilio and N .
betuloides have > 10% gap area (surface area beneath treefall-caused
openings) and complete turnover rates through gap processes (i.e., the
time required for treefalls to disturb an entire stand) range from 300 to
500 years (Rebertus & Veblen, 1993b), which is similar to more speciesrich Valdivian rainforests in Chile (Veblen, 1985b). Uprooting and stem
breakage are the predominant modes of treefall . Most gaps involve multiple treefalls and typically exhibit two or more events of gap creation and
subsequent expansion (Rebertus & Veblen, 1993b).
On Tierra del Fuego , under certain circumstances of wind exposure,
topographic position , and substrate, gradual gap enlargement results in a
wave-like pattern similar to well-documented fir waves in the northeastern
United States (Puigdefabregas et al., in press) . Well-developed mortality
waves in N. betuloides forest occur at right angles to the coast along the
Beagle Channel and Isla de los Estados , where strong coastal winds and
possibly salt spray may be the driving forces. These mortality waves are
conspicuous because of the bleached white dead-standing trees and logs
pointing in a leeward direction . Further inland, waves do occur, but they
are less conspicuous and more irregular in shape and movement (Rebertus
& Veblen , 1993a). Large-scale blowdowns, sometimes involving hundreds
of hectares, are a conspicuous feature in the Fuegian landscape. Indeed,
some areas, where soils are thin , are a complete mosaic of blowdowns of
different ages.
Farther north in the Valdivian rainforest region, fine-scale treefalls
have been studied on the Island of Chiloe (Armesto & Figueroa , 1987;
Armesto & Fuentes, 1988; Veblen, 1985b) and in the Andes in the
Province of Valdivia (Veblen , 1985b). These studies have shown that the
shade-intolerant species such, as Nothofagus dombeyi and Weinmannia
trichosperma, typically do not regenerate in gaps created by single or
small groups of trees. The exception is where a large nurse log allows a
N. dombeyi seedling to establish above the bamboo-dominated understory. If the seedling's roots develop a secure and sustaining anchorage in
the soil before the nurse log decays, this regeneration strategy may be