9. A Comparative Review of Forest Dynamics
193
successful (Veblen, 1985b). In general, seedling establishment on logs
and stumps is important throughout the South American rainforest region.
The most shade-tolerant tree species, such as Laureliopsis philippiana,
are able to regenerate beneath a continuous canopy cover but their
growth rates are enhanced by the creation of small canopy gaps (Armesto
& Fuentes, 1988; Veblen , 1985b). 'Somewhat less shade-tolerant species,
both sub canopy and main canopy in potential stature, are totally
dependent on treefall gaps for successful regeneration (Armesto &
Fuentes, 1988). Tree species with some mode of vegetative reproduction
(e.g., root suckering, basal sprouting) typically have an advantage in
filling gaps (Veblen et aI., 1981).
In North America, windstorms are frequent and cause extensive impact
throughout the rainforest region (Harris, 1989; Ruth & Harris, 1979).
Windthrow severity limits stand development on parts of the outer Pacific
coast, but in most regions stem breakage (a combination of fungal attack
and wind) or fungal mortality is of greater frequency than uprooting
(Alaback, unpublished data; Lertzman & Krebs, 1991; Ott, 1994; Taylor,
1990). Maintenance dynamics are common throughout the region , but
stand-replacement level windthrow occurs periodically as well, especially
along exposed coastlines. Snow breakage is also a key disturbance agent.
It can be particularly important in young even-aged stands because of the
susceptibility of tall thin stems to wet heavy snow in early spring (Alaback
& Tappeiner, 1991).
Preliminary studies of treefall gaps in North and South American
temperate rainforests indicate broadly similar patterns of gap sizes, turnover times, and percentages of the forest area in the state of a gap
(Armesto & Fuentes, 1988; Lertzman, 1992; Ott, 1994; Rebertus &
Veblen, 1983b; Spies, Franklin, & Klopsch, 1990; Taylor, 1990; Veblen,
1985b) . The major difference in treefall gap parameters, is the larger
percentage of uprooted trees in South American forests (Figure 9.4).
Possibly this reflects a greater resistance of the angiosperm trees (the
wood of which generally have greater specific densities) to bole breakage,
higher intensity and more frequent storms in the Andes, greater incidence
of root decay fungi in North America, and/or deeper more paludified
soils in North America.
Disturbance by Fire
In South America, the abundance of fire-adapted spro uting species in the
drier deciduous Nothofagus forests (e .g. N. antarctica, N. obliqua, N.
alpina, Lomatia hirsuta, Schinus patagonicus) suggests that fire has been
an important component of these ecosystems for many thousands of
years. However, in the wettest temperate rainforests, prior to human
occupation, natural fires ignited by lightning or volcanism were rare
events. Within the rainforest zone, natural fires appear to occur most
193
successful (Veblen, 1985b). In general, seedling establishment on logs
and stumps is important throughout the South American rainforest region.
The most shade-tolerant tree species, such as Laureliopsis philippiana,
are able to regenerate beneath a continuous canopy cover but their
growth rates are enhanced by the creation of small canopy gaps (Armesto
& Fuentes, 1988; Veblen , 1985b). 'Somewhat less shade-tolerant species,
both sub canopy and main canopy in potential stature, are totally
dependent on treefall gaps for successful regeneration (Armesto &
Fuentes, 1988). Tree species with some mode of vegetative reproduction
(e.g., root suckering, basal sprouting) typically have an advantage in
filling gaps (Veblen et aI., 1981).
In North America, windstorms are frequent and cause extensive impact
throughout the rainforest region (Harris, 1989; Ruth & Harris, 1979).
Windthrow severity limits stand development on parts of the outer Pacific
coast, but in most regions stem breakage (a combination of fungal attack
and wind) or fungal mortality is of greater frequency than uprooting
(Alaback, unpublished data; Lertzman & Krebs, 1991; Ott, 1994; Taylor,
1990). Maintenance dynamics are common throughout the region , but
stand-replacement level windthrow occurs periodically as well, especially
along exposed coastlines. Snow breakage is also a key disturbance agent.
It can be particularly important in young even-aged stands because of the
susceptibility of tall thin stems to wet heavy snow in early spring (Alaback
& Tappeiner, 1991).
Preliminary studies of treefall gaps in North and South American
temperate rainforests indicate broadly similar patterns of gap sizes, turnover times, and percentages of the forest area in the state of a gap
(Armesto & Fuentes, 1988; Lertzman, 1992; Ott, 1994; Rebertus &
Veblen, 1983b; Spies, Franklin, & Klopsch, 1990; Taylor, 1990; Veblen,
1985b) . The major difference in treefall gap parameters, is the larger
percentage of uprooted trees in South American forests (Figure 9.4).
Possibly this reflects a greater resistance of the angiosperm trees (the
wood of which generally have greater specific densities) to bole breakage,
higher intensity and more frequent storms in the Andes, greater incidence
of root decay fungi in North America, and/or deeper more paludified
soils in North America.
Disturbance by Fire
In South America, the abundance of fire-adapted spro uting species in the
drier deciduous Nothofagus forests (e .g. N. antarctica, N. obliqua, N.
alpina, Lomatia hirsuta, Schinus patagonicus) suggests that fire has been
an important component of these ecosystems for many thousands of
years. However, in the wettest temperate rainforests, prior to human
occupation, natural fires ignited by lightning or volcanism were rare
events. Within the rainforest zone, natural fires appear to occur most