Olympic forests are coniferous forests, unique among mesic temperate forests in their almost complete dominance by conifers rather than by hardwoods.
The hardwood:conifer timber volume ratio is 1:1,000 in the maritime Pacific Northwest.

Another unique feature of Olympic forests is the size and longevity of the dominant tree species.
 Every coniferous genus represented here is represented by its largest and longest-lived species, and some of its second and third ranked species as well.
Biomass accumulations are among the highest in the temperate forest zones.

Dominance of conifers is thought to be a result of the amount and timing of precipitation.
The dry summers limit the growth of deciduous trees, such as most hardwoods.
 Evergreens, such as most conifers, are able to take advantage of the winter precipitation by continuing to photosynthesize through fall, winter and early spring, when deciduous trees are not able to photosynthesize.
 No deciduous conifers occur in the Olympics; larch trees occur in the much-drier eastern Cascades, but not in the Olympics or western Cascades.
 There is only one species of evergreen hardwood, the madrone.

The great size and age of conifers here is thought to be a result of the relative lack of frequent windstorms like tropical cyclones.

Along the western flanks of the mountains the increased orographic precipitation supports temperate rain forests in the Quinault, Queets, Hoh, and Bogachiel river valleys at low elevations.
Protection by Olympic National Park, has allowed these rain forests to retain old growth trees, which supports a varied ecosystem.
The Olympic rain forests are composed primarily of Sitka spruce and western hemlock, as are the surrounding lowland Sitka spruce forests, but are distinct in having a relative abundance of groves of bigleaf maple and vine maple, which support large epiphytic communities of mosses, lichens, ferns, and clubmosses; an abundance of nurse logs on the forest floor; a relatively open forest canopy and sparse shrub layer; and a dense moss layer on the forest floor.
 The rain forests are the wintering grounds for herds of Roosevelt elk and it is thought that browsing by the elk is responsible for the open shrub layer and the dominance of Sitka spruce over western hemlock.

The rain forests are just one type of forest found in the Olympic Mountains.
Franklin and Dyrness set out 5 forest zones: Sitka spruce, western hemlock, silver fir, mountain hemlock, and subalpine parkland.
 Other authors include a sixth, Douglas-fir zone.
 Different plant associations are typical of one or more forest zones.
 For instance, the rain forest plant association described above is a member of the Sitka spruce forest zone.

The Sitka spruce zone is a lowland zone dominated by Sitka spruce and western hemlock.
 Precipitation is high, winters are mild, elevations are low.
 This forest zone is typically found at very low elevations on the western coastal plain and not in mountainous areas, although it can be found as high as 1,900 feet (580 m).

The lower elevations of the Olympic mountains, from roughly 480 feet (150 m) up to 1,800 feet (550 m) – 3,700 feet (1,100 m), feature the western hemlock zone, so-called because in most of the zone, the climax tree species would be western hemlock, even though much of the area is dominated by Douglas-fir.
 The reason for this is that Douglas-fir is an early seral species, and reproduces primarily after disturbances such as fire, logging, landslides, and windstorms.
Western hemlock does not reproduce well on disturbed, exposed soil but germinates under the canopy of Douglas-fir, eventually overtaking the forest by shading out the Douglas-fir, which cannot reproduce in shade.
 In this zone, despite the high annual precipitation, drought stress in summer is sufficiently severe to limit growth of many species, such as Sitka spruce.
 Hardwoods such as bigleaf maple, willows, and red alder are limited to disturbed sites and riparian areas.
red-cedar grows in the wettest sites.
A very small area of the northeastern rain shadow contains the
Olympic forests are coniferous forests, unique among mesic temperate forests in their almost complete dominance by conifers rather than by hardwoods. The hardwood:conifer timber volume ratio is 1:1,000 in the maritime Pacific Northwest.
Another unique feature of Olympic forests is the size and longevity of the dominant tree species.  Every coniferous genus represented here is represented by its largest and longest-lived species, and some of its second and third ranked species as well. Biomass accumulations are among the highest in the temperate forest zones.
Dominance of conifers is thought to be a result of the amount and timing of precipitation. The dry summers limit the growth of deciduous trees, such as most hardwoods.  Evergreens, such as most conifers, are able to take advantage of the winter precipitation by continuing to photosynthesize through fall, winter and early spring, when deciduous trees are not able to photosynthesize.  No deciduous conifers occur in the Olympics; larch trees occur in the much-drier eastern Cascades, but not in the Olympics or western Cascades.  There is only one species of evergreen hardwood, the madrone.
The great size and age of conifers here is thought to be a result of the relative lack of frequent windstorms like tropical cyclones.
Along the western flanks of the mountains the increased orographic precipitation supports temperate rain forests in the Quinault, Queets, Hoh, and Bogachiel river valleys at low elevations. Protection by Olympic National Park, has allowed these rain forests to retain old growth trees, which supports a varied ecosystem. The Olympic rain forests are composed primarily of Sitka spruce and western hemlock, as are the surrounding lowland Sitka spruce forests, but are distinct in having a relative abundance of groves of bigleaf maple and vine maple, which support large epiphytic communities of mosses, lichens, ferns, and clubmosses; an abundance of nurse logs on the forest floor; a relatively open forest canopy and sparse shrub layer; and a dense moss layer on the forest floor.  The rain forests are the wintering grounds for herds of Roosevelt elk and it is thought that browsing by the elk is responsible for the open shrub layer and the dominance of Sitka spruce over western hemlock.
The rain forests are just one type of forest found in the Olympic Mountains. Franklin and Dyrness set out 5 forest zones: Sitka spruce, western hemlock, silver fir, mountain hemlock, and subalpine parkland.  Other authors include a sixth, Douglas-fir zone.  Different plant associations are typical of one or more forest zones.  For instance, the rain forest plant association described above is a member of the Sitka spruce forest zone.
The Sitka spruce zone is a lowland zone dominated by Sitka spruce and western hemlock.  Precipitation is high, winters are mild, elevations are low.  This forest zone is typically found at very low elevations on the western coastal plain and not in mountainous areas, although it can be found as high as 1,900 feet (580 m).
The lower elevations of the Olympic mountains, from roughly 480 feet (150 m) up to 1,800 feet (550 m) – 3,700 feet (1,100 m), feature the western hemlock zone, so-called because in most of the zone, the climax tree species would be western hemlock, even though much of the area is dominated by Douglas-fir.  The reason for this is that Douglas-fir is an early seral species, and reproduces primarily after disturbances such as fire, logging, landslides, and windstorms. Western hemlock does not reproduce well on disturbed, exposed soil but germinates under the canopy of Douglas-fir, eventually overtaking the forest by shading out the Douglas-fir, which cannot reproduce in shade.  In this zone, despite the high annual precipitation, drought stress in summer is sufficiently severe to limit growth of many species, such as Sitka spruce.  Hardwoods such as bigleaf maple, willows, and red alder are limited to disturbed sites and riparian areas. red-cedar grows in the wettest sites. A very small area of the northeastern rain shadow contains the 
Olympic Mountains