Temperate rainforests are most commonly found on the windward side of coastal mountain ranges. In such places warm, moisture-laden winds blowing from over the ocean are forced upward, where they cool, form clouds, and release their moisture as large quantities of rainfall. These forests have developed in high-rainfall, temperate regions along the west coasts of North and South America, New Zealand, and elsewhere.
There are many variants of temperate rainforests. In northern California, coastal rainforest can be dominated by stands of enormous redwood (Sequoia sempervirens) trees older than 1,000 years. More extensive old-growth rainforests elsewhere on the western coast of North America are dominated by other conifer species, especially Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla), along with sitka spruce (Picea sitchensis), red cedar (Thuja plicata), and fir (Abies concolor). Rainforests also occur in wet, frost-free, oceanic environments of the Southern Hemisphere, for example, in parts of New Zealand, where this ecosystem type is dominated by southern beech (Nothofagus spp.) and southern pines (Podocarpus spp.).
Relatively few species have an obligate need for old-growth temperate rainforest as their habitat. In other words, most species that occur in old-growth temperate rainforests also occur in younger but mature forest of a similar tree-species composition. In the temperate rain-forests of the Pacific coast of North America, the spotted owl (Strix occidentalis), marbled murrelet (Brachyramphus marmoratus), and some species of vascular plants, mosses, and lichens appear to require substantial areas of this ecosystem type as a major component of their habitat. However, the numbers of species dependent on temperate old-growth rainforests are very much smaller than in tropical rainforests. With respect to biodiversity issues, the importance of temperate rainforests is substantially associated with their intrinsic value as a natural type of ecosystem, and somewhat less so with the number of dependent species.