180
T .T. Veblen and P.B . Alaback
ones and robust ones with stem diameters>15em, are typical of Valdivian
and north Patagonian rainforests, which is an important contrast to North
American rainforests. Following the strong west-to-east precipitation
gradient from the windward side of the Andes to the rainshadow in their
lee is the gradient from temperate rainforests , through mesic fore sts,
xeric woodlands, to the Patagonian steppe of bunchgrasses and shrubs.
North America
The gradient from 44°N to 62°N along the Pacific slope of North America
extends from a cool, wet climate with summer drought to perpetually wet
rainforests and tundra, analogous to the one described for the equivalent
latitudinal range in South America. As contrasted with South America,
the gradient in North America is more gradual, and all the principal
rainforest types occur approximately 7° to 10° farther north (see Figure
9.2) . This latitudinal shift is due to the decrease in land area with latitude
in temperate South America versus the poleward increase in land area in
North America. Due to the greater continentality of the North American
climate, summer temperatures are substantially greater in North America
at the same latitudes. The rainforest zone is also somewhat broader in
South America, owing to the greater zonal flow in the broad belt of
westerly circulation over the largely landless South Pacific Ocean, although
isolated pockets of marine-influenced climate and rainforest do occur as
far east as the Caribou Mountains in British Columbia and west of the
Bitterroot Divide in Idaho in North America (Lorain, 1988). Winters are
more severe in North America, with freezing temperatures recorded at
least annually throughout the whole region .
Subpolar forests are dominated by Tsuga mertensiana, T. heterophylla ,
and Picea sitchensis (Cooper, 1942; Borchers, Wattenbarger, & Ament,
1989). These forests have strong compositional affinities to the subalpine
subzone of the perhumid rainforest region . Terrains in this zone have
been ice-free for no more than 4000 years and tend to be much less
compositionally stable than in South America. Proportionately less subpolar rainforest occurs in North America than in South America, perhaps
as a result of less persistent winds and more localized ocean influence in
North America. North American perhumid forests include PiceasitchensisTsuga heterophylla forests on well-drained sites and some mixture of
Chamaecyparis nootkatensis, Thuja plicata, and Pinus contorta on .poorer
sites (Alaback & Juday, 1989; Martin , 1989). In seasonal rainforests,
Larix lyallii, Pseudotsuga menziesii, Abies amabilis, several species of
Acer, and Alnus occur in addition to all the species of the perhumid forest
region (Henderson, Peter, Lester, & Shaw, 1989; Klinka, Pojar, &
Meidinger, 1991; McKee, LaRoi, & Franklin, 1982; see Table 9.2).
Riparian forests are much more compositionally unique in North America
T .T. Veblen and P.B . Alaback
ones and robust ones with stem diameters>15em, are typical of Valdivian
and north Patagonian rainforests, which is an important contrast to North
American rainforests. Following the strong west-to-east precipitation
gradient from the windward side of the Andes to the rainshadow in their
lee is the gradient from temperate rainforests , through mesic fore sts,
xeric woodlands, to the Patagonian steppe of bunchgrasses and shrubs.
North America
The gradient from 44°N to 62°N along the Pacific slope of North America
extends from a cool, wet climate with summer drought to perpetually wet
rainforests and tundra, analogous to the one described for the equivalent
latitudinal range in South America. As contrasted with South America,
the gradient in North America is more gradual, and all the principal
rainforest types occur approximately 7° to 10° farther north (see Figure
9.2) . This latitudinal shift is due to the decrease in land area with latitude
in temperate South America versus the poleward increase in land area in
North America. Due to the greater continentality of the North American
climate, summer temperatures are substantially greater in North America
at the same latitudes. The rainforest zone is also somewhat broader in
South America, owing to the greater zonal flow in the broad belt of
westerly circulation over the largely landless South Pacific Ocean, although
isolated pockets of marine-influenced climate and rainforest do occur as
far east as the Caribou Mountains in British Columbia and west of the
Bitterroot Divide in Idaho in North America (Lorain, 1988). Winters are
more severe in North America, with freezing temperatures recorded at
least annually throughout the whole region .
Subpolar forests are dominated by Tsuga mertensiana, T. heterophylla ,
and Picea sitchensis (Cooper, 1942; Borchers, Wattenbarger, & Ament,
1989). These forests have strong compositional affinities to the subalpine
subzone of the perhumid rainforest region . Terrains in this zone have
been ice-free for no more than 4000 years and tend to be much less
compositionally stable than in South America. Proportionately less subpolar rainforest occurs in North America than in South America, perhaps
as a result of less persistent winds and more localized ocean influence in
North America. North American perhumid forests include PiceasitchensisTsuga heterophylla forests on well-drained sites and some mixture of
Chamaecyparis nootkatensis, Thuja plicata, and Pinus contorta on .poorer
sites (Alaback & Juday, 1989; Martin , 1989). In seasonal rainforests,
Larix lyallii, Pseudotsuga menziesii, Abies amabilis, several species of
Acer, and Alnus occur in addition to all the species of the perhumid forest
region (Henderson, Peter, Lester, & Shaw, 1989; Klinka, Pojar, &
Meidinger, 1991; McKee, LaRoi, & Franklin, 1982; see Table 9.2).
Riparian forests are much more compositionally unique in North America