7. Biodiversity Patt erns in Relation to Climate
109
related to forest yield and productivity (Farr & Harris, 1979). Because of
persistent westerly winds and warm Kuriosho currents, the entire west
coast has a distinctly maritime climate, with moderate temperatures in
both winter and summer. Variation both seasonally and diurnally is
greatest in the north (see Figure 7.1). Fog plays a key role in depressing
spring and summer temperatures, particularly in the warm temperate and
seasonal zones. The moderating influence of the ocean is most constricted
along the southern coast, often extending to within as little as 1 km, and is
most pervasive along the northern coast,where moderated temperatures
may extend 150 km inland. Mean annual temperatures decline sharply
with latitude , while seasonal range in: temperature increases. Thermal
energy appears to be a critical limiting resource in the perhumid and
subpolar zones for both plantgrowth,decomposition, nutrient release;
and animal activity (Alaback & McClellan, 1992; Parker, 1988; Wallmo
& Schoen, 1980).
History
The pollen record is extensive and relatively well known along the Pacific
coast of North America, especially along the northern coast and interior
(e.g. , Axelrod, 1976; Axelrod , Arroyor, & Baven, 1991; Brubaker, 1988;
Heusser, 1960). Vegetation has been in a continual state of flux during
the entire pollen record (ca. 14,000 years). As with other regions,
these fluctuations demonstrate that plant species tend to vary in an
individualistic fashion . They respond differentially to stress and vary in
their speed of migration to suitable habitat areas. Life-history strategies,
reproductive mechanisms, and physiological tolerances vary among the
principal dominant species and determine migration rates. For example,
many tree species in Alaska are physiologically adapted to grow much
farther north or west than the limit of their present-day distribution
(e.g., A amabilis, Picea glauca , P. sitchensis). In the Southern Hemisphere
rainforests, even more dramatic asymmetries between local populations
and exotics occur. Exotic tree species such as Pinus contorta, because of
their adaptation to the current cool temperature regime, can extend
timberline as much as 500 m or dramatically increase wood productivity
over that of any native species (Hawkins & Sweet, 1989a). Other Southern
Hemisphere endemics appear to grow best at temperatures much higher
than current climatic conditions (Hill, Read , & Busby, 1988; Read &
Hill , 1985). In the coastal temperate rainforest of southern Scandinavia,
Picea abies grows well much farther west and north than its natural
distribution , suggesting a large time lag in its migration and equilihration
to current climatic conditions.
Many historical plant assemblages have no modern counterparts. The
current forest species mix of the perhumid zone has been in existence for
109
related to forest yield and productivity (Farr & Harris, 1979). Because of
persistent westerly winds and warm Kuriosho currents, the entire west
coast has a distinctly maritime climate, with moderate temperatures in
both winter and summer. Variation both seasonally and diurnally is
greatest in the north (see Figure 7.1). Fog plays a key role in depressing
spring and summer temperatures, particularly in the warm temperate and
seasonal zones. The moderating influence of the ocean is most constricted
along the southern coast, often extending to within as little as 1 km, and is
most pervasive along the northern coast,where moderated temperatures
may extend 150 km inland. Mean annual temperatures decline sharply
with latitude , while seasonal range in: temperature increases. Thermal
energy appears to be a critical limiting resource in the perhumid and
subpolar zones for both plantgrowth,decomposition, nutrient release;
and animal activity (Alaback & McClellan, 1992; Parker, 1988; Wallmo
& Schoen, 1980).
History
The pollen record is extensive and relatively well known along the Pacific
coast of North America, especially along the northern coast and interior
(e.g. , Axelrod, 1976; Axelrod , Arroyor, & Baven, 1991; Brubaker, 1988;
Heusser, 1960). Vegetation has been in a continual state of flux during
the entire pollen record (ca. 14,000 years). As with other regions,
these fluctuations demonstrate that plant species tend to vary in an
individualistic fashion . They respond differentially to stress and vary in
their speed of migration to suitable habitat areas. Life-history strategies,
reproductive mechanisms, and physiological tolerances vary among the
principal dominant species and determine migration rates. For example,
many tree species in Alaska are physiologically adapted to grow much
farther north or west than the limit of their present-day distribution
(e.g., A amabilis, Picea glauca , P. sitchensis). In the Southern Hemisphere
rainforests, even more dramatic asymmetries between local populations
and exotics occur. Exotic tree species such as Pinus contorta, because of
their adaptation to the current cool temperature regime, can extend
timberline as much as 500 m or dramatically increase wood productivity
over that of any native species (Hawkins & Sweet, 1989a). Other Southern
Hemisphere endemics appear to grow best at temperatures much higher
than current climatic conditions (Hill, Read , & Busby, 1988; Read &
Hill , 1985). In the coastal temperate rainforest of southern Scandinavia,
Picea abies grows well much farther west and north than its natural
distribution , suggesting a large time lag in its migration and equilihration
to current climatic conditions.
Many historical plant assemblages have no modern counterparts. The
current forest species mix of the perhumid zone has been in existence for