7. Biodiversity Patterns in Relation to Climate
117
northern extension of rainforest, conifers are even more dominant than in
the south , perhaps because conifers are more efficient at photosynthesis
at low temperatures (Sprugel , 1989) and are better able to tolerate low
nutrient availability and acid soils. At high latitudes, the multilayered
canopy of conifers may be more efficient at capturing solar radiation,
because of the low angle of sunlight (Kuuluvainen, 1992). The high
density of leaf area may also be more efficient at intercepting low levels
of light resulting from persistently cloudy skies combined with a shallow
sun angle (Alaback & Tappeiner II , 1991; Gholz, Fitz, & Waring , 1976).
The lack of hardwoods in the narrow belt of rainforest on the Pacific
coast may also be attributed to the lack of cold-tolerant evergreen hardwood species in adjacent regions from which they might have migrated.
Although evergreen hardwoods can tolerate the cool winters that typify
the northern rainforest region , few can tolerate the harsh winters that
characterize much of the interior forests of North America. This lack of
hardwoods relative to South America may also reflect the greater development of aridity in the interior of the continent, leading to the early loss
of evergreen broadleaf species in North America.
A high degree of similarity exists between the floras of each of the
major forest zones on the Pacific coast . Most of the flora evolved from
the arcto-tertiary flora and from refugia in the north (Hulten, 1937). Only
in the warm temperate forest zone is there any significant influence from
southern refugia (Whittaker, 1960). In the warm coastal temperate forest ,
more than 80% of the species occur only in that zone or farther east or
south in drier and more continental climates. This high level of endemism
mostly reflects the intrusion of dry local climates and the complex interplay
of oceanic and continental influences in the Klamath Mountain region
near the Oregon-California border. Many species there occur throughout
the mountainous west in xeric to mesic habitats. In addition, the warm
coastal temperate forest also includes several species from the adjacent
Mediterranean zone and closely allied fire-adapted species (e.g., Pinus
ponderosa, P. lambertiana, Lithocarpus densiflorus).
As has been frequently noted with other terrestrial communities, tree
species diversity steadily declines with latitude across the entire Pacific
rainforest zone (see Figure 7.2). (Adams & Woodward, 1989; Rapoport,
1982). A decline occurs most consistently with both conifer and hardwood
tree species. As contrasted with South America, this decline in tree
species is much more gradual in the perhumid and subpolar rainforest
zones , further illustrating the minimal effects of Pleistocene glaciations
(Alaback, 1991; Arroyo et al., this volume).
Partly because of recent fluctuations in glaciers and climate , species in
the north tend to be habitat generalists and are the least likely to be
uniquely adapted to current climatic conditions (Kellison & Weir, 1987).
The latitudinal range in distribution of tree species reflects these patterns
(Little , 1971, 1976, see Figure 7.3). Most species in the subpolar zone
117
northern extension of rainforest, conifers are even more dominant than in
the south , perhaps because conifers are more efficient at photosynthesis
at low temperatures (Sprugel , 1989) and are better able to tolerate low
nutrient availability and acid soils. At high latitudes, the multilayered
canopy of conifers may be more efficient at capturing solar radiation,
because of the low angle of sunlight (Kuuluvainen, 1992). The high
density of leaf area may also be more efficient at intercepting low levels
of light resulting from persistently cloudy skies combined with a shallow
sun angle (Alaback & Tappeiner II , 1991; Gholz, Fitz, & Waring , 1976).
The lack of hardwoods in the narrow belt of rainforest on the Pacific
coast may also be attributed to the lack of cold-tolerant evergreen hardwood species in adjacent regions from which they might have migrated.
Although evergreen hardwoods can tolerate the cool winters that typify
the northern rainforest region , few can tolerate the harsh winters that
characterize much of the interior forests of North America. This lack of
hardwoods relative to South America may also reflect the greater development of aridity in the interior of the continent, leading to the early loss
of evergreen broadleaf species in North America.
A high degree of similarity exists between the floras of each of the
major forest zones on the Pacific coast . Most of the flora evolved from
the arcto-tertiary flora and from refugia in the north (Hulten, 1937). Only
in the warm temperate forest zone is there any significant influence from
southern refugia (Whittaker, 1960). In the warm coastal temperate forest ,
more than 80% of the species occur only in that zone or farther east or
south in drier and more continental climates. This high level of endemism
mostly reflects the intrusion of dry local climates and the complex interplay
of oceanic and continental influences in the Klamath Mountain region
near the Oregon-California border. Many species there occur throughout
the mountainous west in xeric to mesic habitats. In addition, the warm
coastal temperate forest also includes several species from the adjacent
Mediterranean zone and closely allied fire-adapted species (e.g., Pinus
ponderosa, P. lambertiana, Lithocarpus densiflorus).
As has been frequently noted with other terrestrial communities, tree
species diversity steadily declines with latitude across the entire Pacific
rainforest zone (see Figure 7.2). (Adams & Woodward, 1989; Rapoport,
1982). A decline occurs most consistently with both conifer and hardwood
tree species. As contrasted with South America, this decline in tree
species is much more gradual in the perhumid and subpolar rainforest
zones , further illustrating the minimal effects of Pleistocene glaciations
(Alaback, 1991; Arroyo et al., this volume).
Partly because of recent fluctuations in glaciers and climate , species in
the north tend to be habitat generalists and are the least likely to be
uniquely adapted to current climatic conditions (Kellison & Weir, 1987).
The latitudinal range in distribution of tree species reflects these patterns
(Little , 1971, 1976, see Figure 7.3). Most species in the subpolar zone