174
T.T. Veblen and P.B . Alaback
Although the much greater size of the North American land mass at
temperate latitudes results in greater continentality at the same latitude
and a more poleward location of rainforests in North America, this
tripartite subdivision shows the broad similarity of climatic and vegetation
gradients along the west coasts of the temperate latitudes of both
continents:
Subpolar rainforests occur along the coasts at the highest latitudes.
They are characterized by the most homogeneous annual distribution
of precipitation and the coolest summers (Almeyda & Saez , 1958;
Farr & Hard, 1987; Far & Harris, 1979). These rainforests have the
wettest summers (>20% annual precipitation during summer), the
coolest temperatures «12°C mean for the warmest month), the greatest
snowfall, and the least stature and diversity of the rainforests. Subpolar
forests also occupy the smallest fraction of the landscape, which is
also characterized by large areas of bog, tundra, and ice . In South
America, subpolar forests (known as Magellanic rainforests) include the
coastal forests extending from approximately 48° southward to Tierra
del Fuego , as shown in Figure 9.1. In North America, subpolar forests
(also known as subboreal rainforests) occur west and north of Yakutat
Bay, mostly in the Prince William Sound region, 58°N-61°N, as shown
in Figure 9.2.
Perhumid rainforests are the modal, or more typical, form of temperate
rainforest and are also the most common type globally. Summer precipitation is generally within the range of 10% to 20% of annual rainfall,
and the mean temperatures of the warmest month are usually 12°C-16°C.
In North America, perhumid rainforests occur primarily in Southeast
Alaska, from Yakutat Bay south to the northern tip of Vancouver Island
(north of the Brooks Peninsula) . In South America, this includes the area
from approximately the Taitao Peninsula (ca. 47°30' S) north to the
Chilean province of Valdivia (c. 40
0S).
The seasonal rainforest is a transitional type between rainforest and
mesophytic temperate forest. Annual rainfall exceeds 1400 mm , but
summer temperatures are often much higher (especially in the more
continental climate of North America, where mean July temperatures
may exceed 20°C) and summer droughts are common. Fire is often an
important aspect of the ecology of these regions (Agee, 1990, 1991;
Heusser, Rabassa, Brandani , & Stuckenrath, 1988; Veblen, 1985b).
Species richness is greatest in seasonal rainforests, since they include a
rich representation of rainforest species as well as species from adjacent
biogeographic regions (e.g. , the warm temperate forest zone (deciduous
in South America, evergreen in North America) , the mesophytic forest
zone, as well as sclerophyll species from the region of Mediterranean-type
climate in central Chile and northern California; Arroyo et aI. , this
volume ; Alaback, this volume). Seasonal rainforests are well represented
in North America, including the coastal forests of Oregon , Washington,
T.T. Veblen and P.B . Alaback
Although the much greater size of the North American land mass at
temperate latitudes results in greater continentality at the same latitude
and a more poleward location of rainforests in North America, this
tripartite subdivision shows the broad similarity of climatic and vegetation
gradients along the west coasts of the temperate latitudes of both
continents:
Subpolar rainforests occur along the coasts at the highest latitudes.
They are characterized by the most homogeneous annual distribution
of precipitation and the coolest summers (Almeyda & Saez , 1958;
Farr & Hard, 1987; Far & Harris, 1979). These rainforests have the
wettest summers (>20% annual precipitation during summer), the
coolest temperatures «12°C mean for the warmest month), the greatest
snowfall, and the least stature and diversity of the rainforests. Subpolar
forests also occupy the smallest fraction of the landscape, which is
also characterized by large areas of bog, tundra, and ice . In South
America, subpolar forests (known as Magellanic rainforests) include the
coastal forests extending from approximately 48° southward to Tierra
del Fuego , as shown in Figure 9.1. In North America, subpolar forests
(also known as subboreal rainforests) occur west and north of Yakutat
Bay, mostly in the Prince William Sound region, 58°N-61°N, as shown
in Figure 9.2.
Perhumid rainforests are the modal, or more typical, form of temperate
rainforest and are also the most common type globally. Summer precipitation is generally within the range of 10% to 20% of annual rainfall,
and the mean temperatures of the warmest month are usually 12°C-16°C.
In North America, perhumid rainforests occur primarily in Southeast
Alaska, from Yakutat Bay south to the northern tip of Vancouver Island
(north of the Brooks Peninsula) . In South America, this includes the area
from approximately the Taitao Peninsula (ca. 47°30' S) north to the
Chilean province of Valdivia (c. 40
0S).
The seasonal rainforest is a transitional type between rainforest and
mesophytic temperate forest. Annual rainfall exceeds 1400 mm , but
summer temperatures are often much higher (especially in the more
continental climate of North America, where mean July temperatures
may exceed 20°C) and summer droughts are common. Fire is often an
important aspect of the ecology of these regions (Agee, 1990, 1991;
Heusser, Rabassa, Brandani , & Stuckenrath, 1988; Veblen, 1985b).
Species richness is greatest in seasonal rainforests, since they include a
rich representation of rainforest species as well as species from adjacent
biogeographic regions (e.g. , the warm temperate forest zone (deciduous
in South America, evergreen in North America) , the mesophytic forest
zone, as well as sclerophyll species from the region of Mediterranean-type
climate in central Chile and northern California; Arroyo et aI. , this
volume ; Alaback, this volume). Seasonal rainforests are well represented
in North America, including the coastal forests of Oregon , Washington,