114
Paul B. Alaback
In England, for example, Picea sitchensis is one of the primary species
used for reforestation or afforestation (Cannell, 1984). At low elevations,
seeds from Washington state (40
0N)
grow the best, but they are frost
susceptible. At higher elevations or a farther north, seed from the Queen
Charlotte Islands in British Columbia (54°N) appear to be better adapted
(greater frost tolerance) . In Oregon , Washington, and southern British
Columbia, P. sitchensis is not normally planted, because of its susceptibility to the white pine weevil (Ruth & Harris , 1979). The narrow
distribution of Picea along the coast (often 1 km or less) in the seasonal
and warm temperate rainforests may be because it is the only microclimate
in those regions with cool-enough summer temperatures to deter weevil
development. From the Queen Charlotte Islands north, there are insufficient growing degree days for weevils to complete their life cycle, so
they do not affect leader growth of P. sitchensis (McMullen, 1976).
Tsuga heterophylla also appears to have a high level of genetic diversity,
suggesting significant potential for improvement through breeding or for
adaptation to climate change (Foster, Campbell , & Adams , 1985; Meager,
1976). Tsuga has a strong tendency for outcrossing and is in the same
general range of genetic diversity as Pseudotsuga and Picea. Tsuga is
considered most closely related to Picea but rarely hybridizes with other
species. Hybrids between the closely related. T. mertensiana are very
rare, even though they often occur in mixed stands together. Coastal
populations of Tsuga in Oregon form a north-south cline of variation;
however, interior populations have a more complex pattern that cannot
be predicted by latitude (Kuser , 1980; Kuser & Ching, 1980). Significant
variation in frost hardiness, date of bud burst and growth characteristics
occur along this gradient as well as along the whole latitudinal gradient
from Alaska to Oregon.
Cedars appear to have less genetic diversity than Tsuga or Picea as
reflected by their relatively narrow distributions and narrower ecological
amplitude . Cupressids also tend to grow slower but have much better
defenses against wood decay, allowing for them to grow to great ages
(1000 years or more; Hennon & Loopstra, 1991; Waring & Franklin ,
1979). Cupressids tend to be stress tolerators and specialize in colonizing
extreme sites, such as wet microsites, steep rocky terrain, limestone substrates, or riparian areas. Because of this, they appear to be particularly sensitive to changes in climatic and soil conditions (Demeo , Martin, & West,
1992; Hennon, Hansen , & Shaw, 1990; Martin , 1989). A major anomaly
is Thuja plicata, which has a relatively wide distribution yet appears to
have low genetic diversity both in terms of isoenzyme analysis and leaf oil
terpenes (Copes, 1981; Von Rudloff & Lapp, 1979). Tests of Thuja in
Scandinavia suggest that frost tolerance and leaf blight are genetically
controlled (Seegaard, 1966). In plantation trials in Poland and Denmark,
Alaskan seed sources show much poorer growth but generally show more
resistance to frost than southerly seed sources. In the northern limit of
Paul B. Alaback
In England, for example, Picea sitchensis is one of the primary species
used for reforestation or afforestation (Cannell, 1984). At low elevations,
seeds from Washington state (40
0N)
grow the best, but they are frost
susceptible. At higher elevations or a farther north, seed from the Queen
Charlotte Islands in British Columbia (54°N) appear to be better adapted
(greater frost tolerance) . In Oregon , Washington, and southern British
Columbia, P. sitchensis is not normally planted, because of its susceptibility to the white pine weevil (Ruth & Harris , 1979). The narrow
distribution of Picea along the coast (often 1 km or less) in the seasonal
and warm temperate rainforests may be because it is the only microclimate
in those regions with cool-enough summer temperatures to deter weevil
development. From the Queen Charlotte Islands north, there are insufficient growing degree days for weevils to complete their life cycle, so
they do not affect leader growth of P. sitchensis (McMullen, 1976).
Tsuga heterophylla also appears to have a high level of genetic diversity,
suggesting significant potential for improvement through breeding or for
adaptation to climate change (Foster, Campbell , & Adams , 1985; Meager,
1976). Tsuga has a strong tendency for outcrossing and is in the same
general range of genetic diversity as Pseudotsuga and Picea. Tsuga is
considered most closely related to Picea but rarely hybridizes with other
species. Hybrids between the closely related. T. mertensiana are very
rare, even though they often occur in mixed stands together. Coastal
populations of Tsuga in Oregon form a north-south cline of variation;
however, interior populations have a more complex pattern that cannot
be predicted by latitude (Kuser , 1980; Kuser & Ching, 1980). Significant
variation in frost hardiness, date of bud burst and growth characteristics
occur along this gradient as well as along the whole latitudinal gradient
from Alaska to Oregon.
Cedars appear to have less genetic diversity than Tsuga or Picea as
reflected by their relatively narrow distributions and narrower ecological
amplitude . Cupressids also tend to grow slower but have much better
defenses against wood decay, allowing for them to grow to great ages
(1000 years or more; Hennon & Loopstra, 1991; Waring & Franklin ,
1979). Cupressids tend to be stress tolerators and specialize in colonizing
extreme sites, such as wet microsites, steep rocky terrain, limestone substrates, or riparian areas. Because of this, they appear to be particularly sensitive to changes in climatic and soil conditions (Demeo , Martin, & West,
1992; Hennon, Hansen , & Shaw, 1990; Martin , 1989). A major anomaly
is Thuja plicata, which has a relatively wide distribution yet appears to
have low genetic diversity both in terms of isoenzyme analysis and leaf oil
terpenes (Copes, 1981; Von Rudloff & Lapp, 1979). Tests of Thuja in
Scandinavia suggest that frost tolerance and leaf blight are genetically
controlled (Seegaard, 1966). In plantation trials in Poland and Denmark,
Alaskan seed sources show much poorer growth but generally show more
resistance to frost than southerly seed sources. In the northern limit of