Temperate Rainforest

Introduction. Rainforests of the mid-latitudes exist in both the Northern and Southern hemispheres. The largest area is found along the Pacific coast of North America from California north through British Columbia into Alaska. At the northern extreme it transitions into a boreal rainforest. An inland band of western rainforest, separated from the coastal communities, stretches along the Columbia and Rocky mountains. Some authors identify the coniferous forests of the Atlantic coast of eastern Canada from Nova Scotia to southern Labrador, on Newfoundland, and in the mountains of eastern Quebec as temperate and boreal rainforest. Elsewhere in the Northern Hemisphere, mid-latitude rainforest can found in Japan and Korea. Relict patches of temperate rainforest survive in Europe, in the Alps, Great Britain, Iceland, Ireland, and Norway.

Temperate rainforest borders the Pacific Ocean in southern Chile and Argentina. Covering an area less than half the size of the North American Pacific coast rainforests, the largest expanse is the Valdivian rainforest, the norrthernmost (37°45′ S─43° 20′ S) of three rainforests and the most species-rich. It abuts the Northern Patagonian rainforest (43° 20′ S─47° 30′ S). Southernmost is the Magellanic rainforest, extending southward from 47° 30′ S into Tierra del Fuego . Other significant Southern Hemisphere rainforests grow in Australia and New Zealand.

Northern Hemisphere forests are all composed of the same genera of needleleaf trees in the pine family: Abies (firs), Picea (spruces), Pinus (pines) Thuja (cedars), and Tsuga (hemlocks). They tower above a dense understory of broadleaf shrubs and herbs and a thick ground layer of ferns, mosses, and liverworts. Lichens growing on rocks, fallen logs, and tree trunks and hanging from tree branches are abundant and species-rich.

In the Southern Hemisphere, broadleaved evergreens emerge above a many-layered understory of both evergreen and deciduous trees and shrubs. In the South American forests the genus Nothofagus (southern beeches) dominates. Other Gondwanan genera also occur there and in Australia and New Zealand, but most species are restricted to a particular continent. The pine family is completely absent from the Southern Hemisphere, but Cupressaceae (cypresses), an ancient family that was in existence before the breakup of Pangea, is represented in both hemispheres.

Southern Hemisphere temperate rainforests evolved in isolation from northern hemisphere mid-latitude expressions of the biome. They instead exhibit evolutionary ties to tropical species, and as a consequence most trees produce edible fruits and rely upon animal pollinators and seed dispersers.

Some of the world’s tallest trees grow in temperate rainforests. Douglas fir (Pseudotsuga menziesii var. menziesii) and coast redwoods (Sequoia sempervirens) in North America attain heights of more than 325 ft. as does mountain ash (Eucalyptus regnans) on Tasmania, Australia. Alerce or Patagonian cypress (Fitzroya cupressoides) in the Valdivian forest of Chile and Argentina in the 19th and 20th centuries, before widespread logging, reportedly also grew this tall. Temperate rainforests everywhere are threatened by deforestation and climate change.

Climate. Cool, moist marine west coast climates are associated with temperate rainforests growing in coastal areas. These climates are marked by a low annual temperature range with both summers and winters being relatively mild. Inland northwestern North American and eastern Canadian forests are influenced by continentality and experience greater temperature extremes, particularly in winter when snows may be deep. Coastal areas in the high latitudes also receive snow.

Precipitation is year round, though much is concentrated in the autumn and winter months. Fourteen to 25 percent of annual precipitation occurs during the three driest months, enough to maintain a definitely humid climate type. Even so seasonality is sufficient to affect fire regimes, species composition—including abundance of epiphytes, and drought tolerance. Total annual amounts vary widely, due in part to the great latitudinal extent of the Pacific coast forests of both hemispheres. Recorded totals range from 33−220 inches a year.

Soils. A deep humus layer with large amounts of woody debris develops in these forest and only slowly decomposes. It is rich in invertebrates and supports extremely diverse macrofungi (mushrooms). Nutrient cycles in coastal forests are linked to the sea by fish-eating birds (e.g., marbled murrelet) as well as bears and sometimes wolves that feed heavily on salmon migrating upstream to spawn, as in the Great Bear Rainforest of British Columbia, Canada).

A “pit and mound” microtopography often develops on the forest floor. The pits result from holes left by uprooted trees (windthrows); the mounds are hummocks formed by slowly decaying logs, other woody debris, and roots cloaked in a dense covering of mosses, liverworts, and ferns.

Vegetation. Since plant life in the various temperate rainforests differs from continent to continent and even on the same continent, vegetation and other details specific to each major region will be discussed on separate pages. See:

North America
Pacific Coast rainforests
Inland Northwest rainforests
Eastern Canada’s rainforests

South America
Valdivian, Patagonian, and Magellanic Rainforests

Australia
Gondwana Rainforests of Australia
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