9. A Comparative Review of Forest Dynamics
189
pioneer species, but often inhibited colonization by other species. It was a
facilitator mostly after the clones died. Unfortunately few studies have
been done of postvolcanic succession in a perhumid or subpolar rainforest
climatic zone in either hemisphere, and no studies have been made of the
early stages of postvolcanic succession in South America. It is unclear
how well these observations on postvolcanic succession would apply to a
wet summer climate or other types of volcanoes in the seasonal rainforest
zone .
Disturbances Associated with Glacial Processes
In the South American rainforest district, glacial activity is an important
agent of disturbance creating apportunities for establishment of shadeintolerant tree species. For example, in northern Patagonia on glaciercovered Mt. Tronador (41°10' S), Nothofagus dombeyi establishes in
abundance on glacial deposits at low elevations in moist hab itats shortly
after the deposits are exposed (Lawrence & Lawrence 1959; Veblen,
A shton, Rubulis, Lorenz & Cortes, 1989a). Even-aged N. dombeyi stands
often develop on moraines that have become stabilized. Nothofagus
dombeyi, Weinmannia trichosperma, and Eucryphia cordifolia are even
capable of establishing and forming dense stands on in-transit moraines
where mass movements have deposited debris on top of the glacial ice
(Veblen et al., 1989a) . At higher elevations and on drier substrates, N.
antarctica and N. pumilio may dominate the colonization of moraines and
outwash deposits.
Further south, at San Rafael Glacier (46°40' S) , Heusser (1964) and
Lawrence and Lawrence (1959) described the establishment of Nothofagus
nitida , N antarctica, and N. betuloides on moraines and outwash. In
southern Patagonia, establishment of N. betuloides, N. pumilio and N.
antarctica on moraines and outwash has been reported for numerous
glacial valleys , (Armesto, Casass a & Dollenz, 1992; Pisano, 1978; Pisano
& Dimitri, 1973). Nitrogen-fixing herbs and shrubs, such as Gunnera
chilensis, Coriaria ruscifolia, and Discaria spp ., are common on freshly
exposed glacial deposits as well as landslide and riparian sites . Although
they undoubtedly play an important role in modifying nitrogen availability
at these sites, this role has not yet been investigated.
Extensive flooding is another disturbance phenomenon associated with
glacial processes in the rainforest district. In southern Patagonia, for
example, rapid glacial melting and partial melting of ice blocks in lakes
periodically flood large areas of Nothofagus forest (Pella & Escobar,
1983). In comparison with drier climates, postglacial succession is generally
more rapid in rainforest climates due to mild summer temperatures and
lack of drought. In North America. Alnus and Picea establish quickly
following glacial retreat in the perhumid and supolar zones. Abies lasiocarpa is a common colonist in drier parts of the perhumid zones or at high
189
pioneer species, but often inhibited colonization by other species. It was a
facilitator mostly after the clones died. Unfortunately few studies have
been done of postvolcanic succession in a perhumid or subpolar rainforest
climatic zone in either hemisphere, and no studies have been made of the
early stages of postvolcanic succession in South America. It is unclear
how well these observations on postvolcanic succession would apply to a
wet summer climate or other types of volcanoes in the seasonal rainforest
zone .
Disturbances Associated with Glacial Processes
In the South American rainforest district, glacial activity is an important
agent of disturbance creating apportunities for establishment of shadeintolerant tree species. For example, in northern Patagonia on glaciercovered Mt. Tronador (41°10' S), Nothofagus dombeyi establishes in
abundance on glacial deposits at low elevations in moist hab itats shortly
after the deposits are exposed (Lawrence & Lawrence 1959; Veblen,
A shton, Rubulis, Lorenz & Cortes, 1989a). Even-aged N. dombeyi stands
often develop on moraines that have become stabilized. Nothofagus
dombeyi, Weinmannia trichosperma, and Eucryphia cordifolia are even
capable of establishing and forming dense stands on in-transit moraines
where mass movements have deposited debris on top of the glacial ice
(Veblen et al., 1989a) . At higher elevations and on drier substrates, N.
antarctica and N. pumilio may dominate the colonization of moraines and
outwash deposits.
Further south, at San Rafael Glacier (46°40' S) , Heusser (1964) and
Lawrence and Lawrence (1959) described the establishment of Nothofagus
nitida , N antarctica, and N. betuloides on moraines and outwash. In
southern Patagonia, establishment of N. betuloides, N. pumilio and N.
antarctica on moraines and outwash has been reported for numerous
glacial valleys , (Armesto, Casass a & Dollenz, 1992; Pisano, 1978; Pisano
& Dimitri, 1973). Nitrogen-fixing herbs and shrubs, such as Gunnera
chilensis, Coriaria ruscifolia, and Discaria spp ., are common on freshly
exposed glacial deposits as well as landslide and riparian sites . Although
they undoubtedly play an important role in modifying nitrogen availability
at these sites, this role has not yet been investigated.
Extensive flooding is another disturbance phenomenon associated with
glacial processes in the rainforest district. In southern Patagonia, for
example, rapid glacial melting and partial melting of ice blocks in lakes
periodically flood large areas of Nothofagus forest (Pella & Escobar,
1983). In comparison with drier climates, postglacial succession is generally
more rapid in rainforest climates due to mild summer temperatures and
lack of drought. In North America. Alnus and Picea establish quickly
following glacial retreat in the perhumid and supolar zones. Abies lasiocarpa is a common colonist in drier parts of the perhumid zones or at high