112
Paul B. Alaback
Franklin, 1979). It is generally thought that the conifers in this forest zone
have better genetic potential for growth under current climatic conditions
than trees from other geographic areas, because of historical and biogeographic factors. As described earlier, coastal temperate rainforest has
occurred along the northern coastline for at least S million years (and are
composed of many genera 70 million or more years old) and has had
minimal impacts from glaciations, relative to other high-latitude regions,
Genetic selection has apparently favored tree-height growth for many
genera, leading to ideal genetic characteristics for rapid biomass accumulation and high wood product values under current climatic conditions. Forests of the North Pacific are among the tallest in the world
(SOm-70m in the perhumid zone , 70m-100m in the seasonal zone , and
up to 112 m in the warm temperate rainforest zone).
Relatively little work has been done on the genetic structure of dominant tree species of the coastal rainforest zone, with the exception of
Pseudotsuga and Populus. In general, conifers are considered genetically
diverse , reflecting a strong tendency toward heterozygosity and outbreeding (Hamrick, Godt, & Sherman-Broyles, 1992). Large production
of pollen that is efficientl y distributed over long distances helps maintain
this high level of natural genetic diversity (Adams, 1992; Adams &
Birkes, 1990). Because of its economic importance and widespread distribution , much work has been done on the genetics of Pseudotsuga. From
this work it is clear that this conifer has an extremely diverse and complex
genetic structure both throughout and within parts of its range (Campbell ,
1979; Conkle, 1981). In one small watershed in the western Cascades of
Oregon , for example, isoenzyme analysis suggests a high degree of genetic
differentiation according to microtopography and microclimate (Campbell,
1979).
In general, woody plant species have a high degree of genetic variation
relative to other life forms (Hamrick et aI. , 1992). Species with wide
distributions or formerly wide distributions and with outcrossing breeding
systems tend to have the highe st levels of diversity. Animal-dispersed
species tend to have higher levels of diversity than wind-dispersed species .
Endemics typically have 30% or less the genetic variation of widespread
species (Hamrick et al.,1992). The forest tree species of the Pacific forest
zone appear to follow this general trend. Picea sitchensis , Pseudotsuga
men ziesii , and Tsuga heterophylla are the dominant widespread species of
the region and have the greatest levels of genetic diversity. These species
would be expected to have the greatest potential to adapt to climate
change or to be improved for the broadest range of environmental conditions within this biome. Specialized species with limited distributions
such as Cupressids in the warm temperate rainforest , by contrast, would
be expected to have the lowest levels of genetic diversity and at the
greatest risk with rapid climate change.
Paul B. Alaback
Franklin, 1979). It is generally thought that the conifers in this forest zone
have better genetic potential for growth under current climatic conditions
than trees from other geographic areas, because of historical and biogeographic factors. As described earlier, coastal temperate rainforest has
occurred along the northern coastline for at least S million years (and are
composed of many genera 70 million or more years old) and has had
minimal impacts from glaciations, relative to other high-latitude regions,
Genetic selection has apparently favored tree-height growth for many
genera, leading to ideal genetic characteristics for rapid biomass accumulation and high wood product values under current climatic conditions. Forests of the North Pacific are among the tallest in the world
(SOm-70m in the perhumid zone , 70m-100m in the seasonal zone , and
up to 112 m in the warm temperate rainforest zone).
Relatively little work has been done on the genetic structure of dominant tree species of the coastal rainforest zone, with the exception of
Pseudotsuga and Populus. In general, conifers are considered genetically
diverse , reflecting a strong tendency toward heterozygosity and outbreeding (Hamrick, Godt, & Sherman-Broyles, 1992). Large production
of pollen that is efficientl y distributed over long distances helps maintain
this high level of natural genetic diversity (Adams, 1992; Adams &
Birkes, 1990). Because of its economic importance and widespread distribution , much work has been done on the genetics of Pseudotsuga. From
this work it is clear that this conifer has an extremely diverse and complex
genetic structure both throughout and within parts of its range (Campbell ,
1979; Conkle, 1981). In one small watershed in the western Cascades of
Oregon , for example, isoenzyme analysis suggests a high degree of genetic
differentiation according to microtopography and microclimate (Campbell,
1979).
In general, woody plant species have a high degree of genetic variation
relative to other life forms (Hamrick et aI. , 1992). Species with wide
distributions or formerly wide distributions and with outcrossing breeding
systems tend to have the highe st levels of diversity. Animal-dispersed
species tend to have higher levels of diversity than wind-dispersed species .
Endemics typically have 30% or less the genetic variation of widespread
species (Hamrick et al.,1992). The forest tree species of the Pacific forest
zone appear to follow this general trend. Picea sitchensis , Pseudotsuga
men ziesii , and Tsuga heterophylla are the dominant widespread species of
the region and have the greatest levels of genetic diversity. These species
would be expected to have the greatest potential to adapt to climate
change or to be improved for the broadest range of environmental conditions within this biome. Specialized species with limited distributions
such as Cupressids in the warm temperate rainforest , by contrast, would
be expected to have the lowest levels of genetic diversity and at the
greatest risk with rapid climate change.