7. Biodiversity Patterns in Relation to Climate
111
Alaska (an area now occupied mostly by boreal forest or tundra and by
subpolar rainforest on the coast) (Axelrod, Arroyo & Raven , 1991). This
long period of relatively stable climate along the coast (ca. 5 million
years) and the existence of a temperate rainforestlike climate over most
of Alaska and other high latitudes over 20 million years or more appears
to have allowed for the evolution of trees closely adapted to current
climatic conditions with minimal gene depletion during glaciations. The
Pacific coast forest is often considered the most luxuriant conifer forest in
world, reflecting this high level of physiological adaptation (Waring &
Franklin, 1979). Ideal growth conditions occur in this climatic zone
because of abundant moisture, which is available year-round, and
moderate temperatures. Intense selection for height growth and biomass
accumulation has occurred over a long period of time in this zone. As
a consequence, the world's largest and longest-lived species occur in
the Pacific coastal forests of North America for virtually every dominant genus of conifer: Abies, Chamaecyparis Larix, Libocedrus Picea,
Pseudotsuga, Sequoia , Thuja, and Tsuga. The greatest phytomass accumulations in the world also occur here (>2000mt/ha in Sequoia,
> 1000mt/ha in Picea, Tsuga, and Abies forests) . This rich genetic diversity
in dominant trees suggests that there is much potential for adaptation for
climate change. Changes in disturbance regime wrought by changes in
storm frequency and intensity in the perhumid zone, or by changes in
frequency and intensity of fire in the warm temperate and seasonal zones,
may provide the most challenging constraints for trees' adaptation to
climate change in this climatic zone (Alaback & McClellan, 1992;Franklin
et al., 1991).
There is little information on the genetic structure or the history of
understory shrub and herb species that comprise the center of biological
diversity of these forests in terms of plant species richness and plantanimal interactions. Most species are clonal and may have limited recruitment of new genetic individuals (Bierzychudek, 1982; Tappeiner &
Alaback , 1989). Shrubs and herbs also have a much greater dependence
on animal vectors for seed dispersal than dominant trees in these rainforests, making them more sensitive to climate change (Willson, 1991).
Much more information is needed on the genetic architecture and physiology of these shrub and herb species to be able to predict how they might
respond to climate change and to changes wrought by land-use changes.
Genetic Adaptation of Dominant Tree Species
Although it is clear that the northern rainforest climate often provides
ideal growth conditions for trees, climate alone cannot explain the high
level of biomass accumulation and productivity that distinguishes the
Pacific coastal forests from virtually all others in the world (Waring &
111
Alaska (an area now occupied mostly by boreal forest or tundra and by
subpolar rainforest on the coast) (Axelrod, Arroyo & Raven , 1991). This
long period of relatively stable climate along the coast (ca. 5 million
years) and the existence of a temperate rainforestlike climate over most
of Alaska and other high latitudes over 20 million years or more appears
to have allowed for the evolution of trees closely adapted to current
climatic conditions with minimal gene depletion during glaciations. The
Pacific coast forest is often considered the most luxuriant conifer forest in
world, reflecting this high level of physiological adaptation (Waring &
Franklin, 1979). Ideal growth conditions occur in this climatic zone
because of abundant moisture, which is available year-round, and
moderate temperatures. Intense selection for height growth and biomass
accumulation has occurred over a long period of time in this zone. As
a consequence, the world's largest and longest-lived species occur in
the Pacific coastal forests of North America for virtually every dominant genus of conifer: Abies, Chamaecyparis Larix, Libocedrus Picea,
Pseudotsuga, Sequoia , Thuja, and Tsuga. The greatest phytomass accumulations in the world also occur here (>2000mt/ha in Sequoia,
> 1000mt/ha in Picea, Tsuga, and Abies forests) . This rich genetic diversity
in dominant trees suggests that there is much potential for adaptation for
climate change. Changes in disturbance regime wrought by changes in
storm frequency and intensity in the perhumid zone, or by changes in
frequency and intensity of fire in the warm temperate and seasonal zones,
may provide the most challenging constraints for trees' adaptation to
climate change in this climatic zone (Alaback & McClellan, 1992;Franklin
et al., 1991).
There is little information on the genetic structure or the history of
understory shrub and herb species that comprise the center of biological
diversity of these forests in terms of plant species richness and plantanimal interactions. Most species are clonal and may have limited recruitment of new genetic individuals (Bierzychudek, 1982; Tappeiner &
Alaback , 1989). Shrubs and herbs also have a much greater dependence
on animal vectors for seed dispersal than dominant trees in these rainforests, making them more sensitive to climate change (Willson, 1991).
Much more information is needed on the genetic architecture and physiology of these shrub and herb species to be able to predict how they might
respond to climate change and to changes wrought by land-use changes.
Genetic Adaptation of Dominant Tree Species
Although it is clear that the northern rainforest climate often provides
ideal growth conditions for trees, climate alone cannot explain the high
level of biomass accumulation and productivity that distinguishes the
Pacific coastal forests from virtually all others in the world (Waring &