110
Paul B. Alaback
little more than 2000 years (Brubaker, 1988). Western hemlock and Sitka
spruce have existed in Prince William Sound for only 4000 years and have
yet to reach a stable species mixture ..Subtle changes in global climate
correspond to dramatic changes in northern areas , including glacial surges
or rapid recessions and rapid timberline movement. For example , during
the Little Ice Age approximately 200 years ago, the entirety of Glacier
Bay was filled with ice. Currently, ice remains only in the head of the
bay, more than 95 km from the mouth of the bay. Following deglaciation ,
a cool, dry climate developed that favored the development of an extensive Pinus contorta forest, which now only occurs in isolated patches
in dry local climates usually associated with linkages to dry interior
valleys. Contemporary revegetation patterns following glacial retreat is
dramatically different in rate, composition, and structure than it was in
the past (Chapin, Walker , Fastie, & Sharman, 1994; Fastie, 1990).
Because of this recent history of dramatic climate change, the northern
flora is considered more resilient to changes in climate than more southerly
areas (e.g., Peters , 1990).
Historical linkages to other regions and broad migration corridors, as
well as continuous changes in climate from subtropical to cool temperate,
help explain much of the richness and adaptation of the current forest
flora (e.g. , Axelrod et al. , 1991). Pinus was the first conifer to appear in
the region (90 million years ago). Picea, Chamaecyparis, Pseudotsuga,
Tsuga, Abies, and Thuja first appeared 55 million-65 million years ago.
During the past 25 million years, the climate rapidly cooled, leading to a
major impoverishment and isolation of the flora. During the mid-Miocene
(ca. 25 million years ago), coastal forest was separated from eastern
mesophytic forest by the dry interior. The coastal forest was still continuous with Beringia and other parts of Asia. Late in the Miocene
(15 million years ago) summer temperatures dropped by 7°C, causing
deciduous elements in the flora to retreat to central British Columbia.
This is also when one of the dominant ericaceous understory shrub genera
appeared (Vaccinium) , which is closely associated with acid soils. The
southern portion of coastal forest became more isolated during the
Pliocene, when the Cascades and Coast range rose, eliminating forest
in the interior. Palynological data suggest that when summers became
cooler and drier approximately 7 million years ago, heat became inadequate for most deciduous trees to complete their normal growth and
reproductive cycle before autumn frost (Stephenson, 1990; Waring &
Franklin, 1979). As a consequence, most deciduous elements were lost in
the region at that time. All these reductions in flora were completed well
before the Pleistocene glaciations. This elimination of hardwood trees
distinguishes the flora of rainforests in North America from virtually all
other coastal temperate rainforests in the world.
The Picea-Tsuga coastal forest first appeared 2 million to 5 million
years ago in the Cook Inlet region and extended northward throughout
Paul B. Alaback
little more than 2000 years (Brubaker, 1988). Western hemlock and Sitka
spruce have existed in Prince William Sound for only 4000 years and have
yet to reach a stable species mixture ..Subtle changes in global climate
correspond to dramatic changes in northern areas , including glacial surges
or rapid recessions and rapid timberline movement. For example , during
the Little Ice Age approximately 200 years ago, the entirety of Glacier
Bay was filled with ice. Currently, ice remains only in the head of the
bay, more than 95 km from the mouth of the bay. Following deglaciation ,
a cool, dry climate developed that favored the development of an extensive Pinus contorta forest, which now only occurs in isolated patches
in dry local climates usually associated with linkages to dry interior
valleys. Contemporary revegetation patterns following glacial retreat is
dramatically different in rate, composition, and structure than it was in
the past (Chapin, Walker , Fastie, & Sharman, 1994; Fastie, 1990).
Because of this recent history of dramatic climate change, the northern
flora is considered more resilient to changes in climate than more southerly
areas (e.g., Peters , 1990).
Historical linkages to other regions and broad migration corridors, as
well as continuous changes in climate from subtropical to cool temperate,
help explain much of the richness and adaptation of the current forest
flora (e.g. , Axelrod et al. , 1991). Pinus was the first conifer to appear in
the region (90 million years ago). Picea, Chamaecyparis, Pseudotsuga,
Tsuga, Abies, and Thuja first appeared 55 million-65 million years ago.
During the past 25 million years, the climate rapidly cooled, leading to a
major impoverishment and isolation of the flora. During the mid-Miocene
(ca. 25 million years ago), coastal forest was separated from eastern
mesophytic forest by the dry interior. The coastal forest was still continuous with Beringia and other parts of Asia. Late in the Miocene
(15 million years ago) summer temperatures dropped by 7°C, causing
deciduous elements in the flora to retreat to central British Columbia.
This is also when one of the dominant ericaceous understory shrub genera
appeared (Vaccinium) , which is closely associated with acid soils. The
southern portion of coastal forest became more isolated during the
Pliocene, when the Cascades and Coast range rose, eliminating forest
in the interior. Palynological data suggest that when summers became
cooler and drier approximately 7 million years ago, heat became inadequate for most deciduous trees to complete their normal growth and
reproductive cycle before autumn frost (Stephenson, 1990; Waring &
Franklin, 1979). As a consequence, most deciduous elements were lost in
the region at that time. All these reductions in flora were completed well
before the Pleistocene glaciations. This elimination of hardwood trees
distinguishes the flora of rainforests in North America from virtually all
other coastal temperate rainforests in the world.
The Picea-Tsuga coastal forest first appeared 2 million to 5 million
years ago in the Cook Inlet region and extended northward throughout