Q: How do temperate rainforests differ from tropical rainforests? A: All rainforests are unique in having an overabundance of rainfall for plant growth, either seasonally or throughout the year. In the tropics, rainforests have 2.5 m (100 in.) or more of annual precipitation, but in the temperate zone as little as 1.5 m (60 in.) of rain creates a rainforest climate due to cooler temperatures and lower rates of evaporation. While tropical rainforests are one of the most widespread forest types in the world, spanning all of the continents within 23° of the Equator, temperate rainforests are quite rare and occur only in a few coastal regions at 37-60° latitude. The largest temperate rainforest is on the Pacific coast of North America, from California to southern Alaska. The second largest is the temperate rainforest of southern Chile and Argentine Patagonia. Tropical rainforests are famous as the center of biological diversity for the planet, providing habitat for many animal species. They also exhibit complex relationships between plants and animals, including many specialized pollinators and seed dispersers. Tropical rainforests evolved early in the Earths history, 200 million years or more ago, so they include many ancient and specialized plant and animal species.
Temperate rainforests, by contrast, are more recent in origin, having evolved generally less than 40 million years ago, and they do not have the highest levels of biodiversity when compared with other temperate forest types. Every one to five centuries, they can have massive fires during the dry season. Temperate rainforests generally are dominated by evergreen trees, either needle-leaved conifers in the Northern Hemisphere or broadleaved trees in the Southern Hemisphere. While tropical rainforests have many vertical layers of vegetation, temperate forests usually have a dominant upper tree layer and fewer intermediate layers. Temperate rainforests include some of the fastest-growing trees in the temperate zone and the tallest trees in the world such as the coastal redwoods (Sequoia sempervirens) in Califonia and red gum (Eucalyptus regnans) in Australia. They accumulate more carbon or biomass per hectare than any forest type in the world. The highest biomass occurs on the Pacific coast of North America. Although tropical rainforests are very productive, natural disturbances, diseases, and depredations of animals lead to lower total accumulations of carbon per hectare. However, because of their vast expanse, tropical forests have the greatest aboveground global carbon accumulation of any forest type.
In contrast to most other forest types, in rainforests natural fires play a less prominent role. Windstorms are important natural disturbances in all rainforests. Logging and burning of rainforests is of great concern for conservation for both tropical and temperate types. The greatest percentage of logging has occurred in temperate rainforests, but the greatest rate of harvest and loss of biodiversity is associated with tropical rainforests, especially in Southeast Asia and the Amazon basin.
Paul Alaback, Ph.D. University of Montana School of Forestry and Conservation