190
T.T. Veblen and P.B. Alaback
elevations in the seasonal zone. Dense even-aged Picea or Abies forest
develops within 200 years of deglacation . These forests are distinguished
from forests growing on older terrains by less dense canopies and the
presence of deciduous hardwoods (Populus trichocarpa , Alnus, or Betula) ;
they are also characterized by Rosaceous shrubs and semisaprophytic
herbs , such as Pyrola, and the absence of Ericaceous shrubs and Tsuga.
Ericaceous understory shrubs are the slowest to colonize postglacial substrates (after at least 300 years). Tsuga colonizes pure Picea forests
approximately 300 years or more following deglaciation. There are many
possible paths of succession following deglaciation, however, few of these
alternative pathways have been studied in detail (Fastie , 1990; Reiners ,
Worley & Lawrence, 1971)
The first work on postglacial succession in the temperate rainforest
zone focused on the eastern arm of Glacier Bay, Alaska (e.g., Cooper,
1937; Crocker & Major, 1955, Reiners et al., 1971). A simple chronosequence was used to characterize this succession from bryophytes (e.g.,
Rhacomitrium spp.), through a shrub stage (Salix spp.) , to a Picea sitchensis forest, and finally to a steady-state Tsuga heterophylla forest.
Alnus sinuata is widely recognized for playing an important role succession
following deglaciation by fixing atmospheric nitrogen (Crocker & Major,
1955). Following establishment of Alnus forests , Picea seedlings grow
slowly in the understory, eventually overtopping the short-lived Alnus.
The nitrogen-rich soils then allow for rapid growth of Picea and the
establishment of a pure Picea forest within two centuries of deglaciation
(Cooper, 1937). Each of the principal stages of this succession is associated
with significant changes in soil chemistry and structure (Crocker & Major,
1955).
Recent work on postglacial succession in North America has emphasized
plant life-history strategies and competitive relationships to examine
whether succession truly follows the facilitation model of Connell and
Slatyer (1977). Preliminary results of some of this work suggest that
postglacial succession is a far more complex and multifaceted process
than is usually presented. For example in Glacier Bay, the original work
was done only on the southern and eastern arm of the Bay where glacial
retreat occurred rapidly (Cooper, 1937). Farther up the eastern arm and
in the western arm, where a wide variety of parent materials predominate,
a wide range of plant successional trajectories develop following deglaciation (Fastie , 1990). The speed of deglaciation and physical processes
associated with it such as scour, fill , flooding, sediment texture , and other
edaphic changes lead to divergent successional pathways. For example
Pinus contorta plays a key role in early succession on sandy sites or dry
sites. Sheperdia is also characteristic of these sites, especially on limestone .
On others Picea comes in first with Salix and Populus. On coarse textured
soils there is only limited colonization by Alnus resulting in lower concentrations of nitrogen and a slower successional pattern. Late seral species
T.T. Veblen and P.B. Alaback
elevations in the seasonal zone. Dense even-aged Picea or Abies forest
develops within 200 years of deglacation . These forests are distinguished
from forests growing on older terrains by less dense canopies and the
presence of deciduous hardwoods (Populus trichocarpa , Alnus, or Betula) ;
they are also characterized by Rosaceous shrubs and semisaprophytic
herbs , such as Pyrola, and the absence of Ericaceous shrubs and Tsuga.
Ericaceous understory shrubs are the slowest to colonize postglacial substrates (after at least 300 years). Tsuga colonizes pure Picea forests
approximately 300 years or more following deglaciation. There are many
possible paths of succession following deglaciation, however, few of these
alternative pathways have been studied in detail (Fastie , 1990; Reiners ,
Worley & Lawrence, 1971)
The first work on postglacial succession in the temperate rainforest
zone focused on the eastern arm of Glacier Bay, Alaska (e.g., Cooper,
1937; Crocker & Major, 1955, Reiners et al., 1971). A simple chronosequence was used to characterize this succession from bryophytes (e.g.,
Rhacomitrium spp.), through a shrub stage (Salix spp.) , to a Picea sitchensis forest, and finally to a steady-state Tsuga heterophylla forest.
Alnus sinuata is widely recognized for playing an important role succession
following deglaciation by fixing atmospheric nitrogen (Crocker & Major,
1955). Following establishment of Alnus forests , Picea seedlings grow
slowly in the understory, eventually overtopping the short-lived Alnus.
The nitrogen-rich soils then allow for rapid growth of Picea and the
establishment of a pure Picea forest within two centuries of deglaciation
(Cooper, 1937). Each of the principal stages of this succession is associated
with significant changes in soil chemistry and structure (Crocker & Major,
1955).
Recent work on postglacial succession in North America has emphasized
plant life-history strategies and competitive relationships to examine
whether succession truly follows the facilitation model of Connell and
Slatyer (1977). Preliminary results of some of this work suggest that
postglacial succession is a far more complex and multifaceted process
than is usually presented. For example in Glacier Bay, the original work
was done only on the southern and eastern arm of the Bay where glacial
retreat occurred rapidly (Cooper, 1937). Farther up the eastern arm and
in the western arm, where a wide variety of parent materials predominate,
a wide range of plant successional trajectories develop following deglaciation (Fastie , 1990). The speed of deglaciation and physical processes
associated with it such as scour, fill , flooding, sediment texture , and other
edaphic changes lead to divergent successional pathways. For example
Pinus contorta plays a key role in early succession on sandy sites or dry
sites. Sheperdia is also characteristic of these sites, especially on limestone .
On others Picea comes in first with Salix and Populus. On coarse textured
soils there is only limited colonization by Alnus resulting in lower concentrations of nitrogen and a slower successional pattern. Late seral species