The life-zone theory of plant and animal distribution was proposed by Merriam (1892). Merriam's life-zones have been severely criticized by many authors, especially because an error was made in computing some of the data on temperature. However, zonation of vegetation and animals is obvious in Washington, and the life-zone concept has been employed in Washington by numerous botanists and zoÖlogists. Among them are: Piper (1906), Taylor and Shaw (1927), Jones (1936, 1938) and St. John (1937).

The higher parts of the Cascade Mountains are in the Arctic-alpine Life-zone. This is the area of wind-swept ridges, living glaciers, and permanent snow fields.

Trees are absent but a few shrubs are present; these include: Juniperus sibirica, Salix cascadensis, Salix nivalis, Gaultheria humifusa, Empetrum nigrum, and the heathers, Phyllodoce glanduliflora, Cassiope mertensiana and Cassiope stelleriana. Jones (1938) lists a total of 98 species of plants from the Arctic-alpine Life-zone of Mount Rainier. Many of these plants are most abundant in the next life-zone lower, and are of but incidental occurrence in the Arctic-alpine Life-zone. No mammalian species is resident but individuals of several species regularly visit and occasionally breed there.

Below the Arctic-alpine the Hudsonian Life-zone stretches the entire length of the Cascades. Temperatures are low, especially in winter; then the thermometer does not rise above zero for weeks at a time. The average annual temperature at Paradise, 5500 feet, Mt. Rainier, is 38.6° (all temperatures given here are in degrees Fahrenheit). Snowfall is heavy. The average yearly snowfall, for four years, at Mt. Baker Lodge, at 4200 feet elevation, Whatcom County, was 478 inches; at Goat Lake, 2900 feet, Snohomish County, 261 inches; Tye, Stevens Pass, 3010 feet, King County, 398 inches; Paradise, 5500 feet, Mt. Rainier, 587 inches. The deepest snow recorded at Paradise was 27 feet, 2 inches on April 2, 1917. Following the spring thaws the mountain passes are opened to travel, usually in April or May, although nightly temperatures in April and May are still below zero. Spring precipitation is heavy, the monthly average for a twelve-year period at Paradise being 6.78 inches in April and 5.5 inches in May. Summer temperatures are high in the daytime, when the sun beats down through the rarefied atmosphere, but cool at night when accumulated heat is lost through the thin atmospheric blanket. In summer precipitation is light, averaging, at Paradise, 3.46 inches in June, .9 inches in July, and 3.44 inches in August. In the autumn the temperature, both daily and nightly, drops somewhat, and rain and cloudiness are the rule. At Paradise the average precipitation in September is 8.29 inches and in October 10.02 inches. The winter snows usually arrive by the middle of November.

Trees that are characteristic of parts of the Hudsonian Life-zone include the alpine fir (Abies lasiocarpa), mountain hemlock (Tsuga mertensiana), Alaska cedar (Chamaecyparis nootkatensis) and white-barked pine (Pinus albicaulis). The following shrubs are listed by Jones (1938) as common in the Hudsonian Life-zone on Mt. Rainier: Salix barclayi, Salix commutata, Juniperus sibirica, Alnus sinuata, Ribes howellii, Lutkea pectinata, Potentilla fruticosa, Sorbus occidentalis, Spiraea densiflora, Pachistima myrsinites, Arctostaphylos nevadensis, Arctostaphylos uva-ursi, Cassiope mertensiana, Cassiope stelleriana, Phyllodoce empetriformis, Rhododendron albiflorum and Gaultheria ovalifolia.

There are extensive coniferous forests in the Canadian Life-zone, still lower on the mountain slopes. This is an area of lesser temperature extremes than is the Hudsonian Life-zone. The average annual temperature at Longmire, 2761 feet, Mt. Rainier, is 43.8°. The average temperature for the winter months, however, is below freezing. In July and August the temperatures are high, especially in the daytime. The eighteen year average for Longmire during these months is 60.6°. Snow is regular but the fall is lighter than in the Hudsonian Life-zone. The annual average, over a period of nineteen years, is 184.4 inches at Longmire. Precipitation is similar to that in the Hudsonian Life-zone, averaging perhaps slightly less.

In the Cascades the typical feature of the Canadian Life-zone is the extensive coniferous forest that extends, almost without a break, the entire length of the Cascades on both sides of the main crest. In addition to Douglas fir, the following trees occur in this forest: western hemlock (Tsuga heterophylla), amabalis fir (Abies amabalis), white pine (Pinus monticola) and noble fir (Abies nobilis). Other plants include Vaccinnium ovalifolium, Vaccinnium membranaceum, Menziesia ferruginea, Alnus sinuata, Acer circinatum, Sorbus cascadensis, Cornus canadensis, Clintonia uniflora, Stenauthium occidentale, Galium oreganum, and Prenanthes lessingii. Saprophytes abundant in, if not confined to, this zone are listed by Jones as: Monotropa uniflora, Monotropa hypopitys, Allotropa virguta, Newberrya congesta, Pterospora andromedea, Corallorrhiza maculata, Corallorrhiza mertensiana and Corallorrhiza striata.

West of the Canadian Life-zone in the western Cascades, the coniferous forests merge with the lowland forests of western Washington. To the east of the Canadian Life-zone in the eastern Cascades, there is a distinct change to a more arid climate and flora. At Leavenworth, 1167 feet, Chelan County, the annual average precipitation is but 19.5 inches and at Cle Elum, 1930 feet, but 23.23 inches. Temperatures are higher, the annual average of the above two localities being 47.2° and 45.4° respectively. The winter months are cold, with the average temperature in January and February below freezing. In summer the averages in July and August at Leavenworth are 68.8° and 68.0°, with the average maximum being 87.5° and 86.1°. Snowfall is heavy, the yearly average at Leavenworth being 98.5 inches and at Cle Elum 86.3 inches. The effect of this more arid climate is seen in the vegetation. The dense Douglas fir forest, is replaced by more open forests of yellow pine (Pinus ponderosus). Groves of oak (Quercus garryana) are found near streams. The open forests give way to the extensive grasslands bordering the desert. The transition of vegetation is similar to that occurring in the Yakima Valley Area.

In the Yakima Valley Area, arid conditions prevail. The average yearly precipitation at Yakima is 6.67 inches. Only in November, December and January may more than one inch of precipitation be expected monthly. Snow may be expected in the winter months and the yearly average snowfall is 22.1 inches. Winter temperatures are low, the average for December and January being but slightly above freezing. Summer temperatures are extreme; the July average is 73.1° and the average maximum for the same month is 89°. The highest temperature recorded is 111°.

The open pine forests of the eastern Cascades give way to grasslands. Grasses of several species are common but the bunchgrass (Agropyron spicatum) is most important. Other plants include the primrose (Oenothera pallida), lupines (Lupinus), and Mertensia. In ravines and near watercourses such shrubs as hawthorn (Crataegus douglasii), service-berry (Amelanchier cusickii, Amelanchier utahensis), aspen (Populus tremuloides), syringa (Philadelphus lewisii), snowberry (Symphoricarpos albus), choke-berry (Prunus melanocarpa) and elderberry (Sambucus caerulae) form thickets. Lower in the valley the vegetation is xerophytic, similar to that of the Columbian Plateau. Sagebrush (Artemisiae tridentata) is dominant. Other shrubs include rabbit brush (Chrysothamnus nauseosus, Chrysothamnus viscidiflorus), hop sage (Grayia spinosa), black sage (Purshia tridentata) and greasewood (Sarcobatus vermiculatus).

Climatic conditions in the Olympic Mountains are, in general, similar to those of the higher Cascade Mountains. There is a limited area of Arctic-alpine Life-zone on Mount Olympus. The principal life-zone is the Hudsonian. The Canadian merges with the lowland Humid-Transition and is difficult to ascertain as a separate zone. In the Hudsonian Life-zone the average temperatures are low. Winter climate is bitter and the snow lies deep. In the absence of government weather stations in the Olympics, detailed descriptions of climatic conditions can not be given. Vegetation of the Hudsonian Life-zone of the Olympic Mountains is, in general, similar to that of the same life-zone of the Cascades (see Jones, 1936, Botanical Survey of the Olympic Peninsula).

The lowlands of western Washington have a cool, humid climate. The average annual temperature of the area varies little from 50°. In winter the temperature, especially in January and February, commonly drops below the freezing point at night. Summer temperatures are moderate, rarely reaching 90°. Snowfall is light, averaging about 10 inches. The prevailing winds are from the west and are moisture-laden. They rise over the Olympic Mountains and loose heavy rains along the coastal area of the lowlands. The average annual precipitation at La Push, Clallam County, is 97.9 inches; at Clearwater, Jefferson County, 124.98 inches; at Aberdeen, Grays Harbor County, 81.58 inches; at South Bend, Pacific County, 83.35 inches. In contrast, the more inland areas receive less than half as much precipitation. The yearly average at Bellingham is 31.09 inches; at Seattle, 30.07 inches; at Tacoma, 39.53 inches; at Vancouver, Clark County, 37.24 inches.

The outstanding feature of the vegetation of western Washington is the coniferous forest. Previous to the logging activities a dense cover of Douglas fir, western hemlock and red cedar spread almost unbroken over the area. The openings in the forest and the marshy ravines and river valleys supported growths of underbrush and deciduous trees so dense and luxuriant as to compare with a tropical jungle. In the dense rain forests along the coast, mosses and lichens develop an understory vegetation many inches deep and clothe the branches of the forest trees. The mild temperature and excessive rainfall cause some species that usually are of bush or shrub size to reach the proportions of small trees. In some places one can climb twenty feet from the ground in a huckleberry tree, the trunk of which is five inches in diameter. The coniferous forest is made up of several species of trees. Most important among these are the western hemlock (Tsuga heterophylla), Douglas fir (Pseudotsuga taxifolia), and red cedar (Thuja plicata). Locally the western yew (Taxus brevifolia), lodgepole pine (Pinus contorta) and spruce (Picea sitchensis) may be common. Deciduous trees are numerous and include several willows (Salix sp.), aspen (Populus tremuloides), hazel (Corylus californica), alder (Alnus oregona), oak (Quercus garryana), broadleaf maple (Acer macrophyllum), vine maple (Acer circinatum), and flowering dogwood (Cornus nuttallii). Mosses and ferns are abundant. The sword fern (Polystichum munitum) and bracken (Pteridium aquilinum) are especially common. Space prevents listing all but a fraction of the typical shrubs but these include huckleberry (Vaccinium parvifolium, Vaccinium ovatum), Oregon grape (Berberis nervosa), salal (Gaultheria shallon), rose (Rosa gymnocarpa), thimbleberry (Rubus parviflorus), salmonberry (Rubus spectabilis), blackcap raspberry (Rubus leucodermis) and wild blackberry (Rubus macropetalus).

The higher parts of some of the ranges of northeastern Washington are in the Hudsonian Life-zone, but most of the mountains are in the Canadian Life-zone. The valleys are in the Transition Life-zone. Climatic conditions are similar to those of the eastern slopes of the Cascades. Winter temperatures are low, the average for December, January and February being below freezing. Summer temperatures are high, the July average for Colville being 67.2° and the July average maximum being 87.4°. Vegetation consists principally of coniferous forests in the mountains and deciduous woods in the valleys. Among the interesting features of the vegetation are the extensive stands of almost pure larch (Larix occidentalis). In most respects the flora closely resembles that of the Blue Mountains.

The Blue Mountains of southeastern Washington differ from other ranges in Washington in their relative aridity. There are few streams and a single river drains the area. There are no government weather stations in the Blue Mountains. The winter temperatures are low and the snow deep and lasting. Summer temperatures are high and humidity and precipitation low. Coniferous forests of the type of arid regions form the principal tree cover. Typical plant species include the white fir (Abies grandis), alpine fir (Abies lasiocarpa), larch (Larix occidentalis), spruce (Picea columbiana), and such shrubs as fool huckleberry (Menziesia ferruginea), Pachystima myrsinites, dogwood (Cornus canadensis), wild current (Ribes petiolare), mountain mahogany (Cercocarpus ledifolius), spirea (Spiraea sp.), lupines (Lupinus) of several species, maple (Acer douglasii), buckbrush (Ceanothus sanguineus), sticky brush (Ceanothus velutinus), and huckleberry (Vaccinium membranaceum).

The Columbian Plateau and southeastern Washington present desert conditions. At Odessa, 1590 feet, Lincoln County, the average annual precipitation is only 9.38 inches, and only in the winter may more than one inch of precipitation per month be expected. The average temperature is 48.5°. In the winter the average is below freezing but in July it is 71.3°. The average maximum for July is 90° and an extreme of 111° is recorded. Walla Walla, 991 feet, has a higher annual precipitation (16.66 inches) but higher temperature (yearly average 53.5°, July average 75.0°, average July maximum 88.6°, extreme 113°). Winter temperatures on the Columbian Plateau are low. The January average at Odessa is 25.3° and at Walla Walla 32.4° The average yearly snowfall at Odessa is 19.4 inches and at Walla Walla 23.5 inches. Vegetation of the Columbian Plateau and southeastern Washington is of the desert type. A few pines and junipers grow in favored places. Along streams the cottonwood (Populus hastata) and willow (Salix) of several species are common. Most typical are grasses and shrubs such as the bunch grass (Agropyron inerme, Agropyron spictatum), foxtail (Alopecurus aequalis), cheat grass (Bromus tectorum), saltbrush (Atriplex truncata), greasewood (Sarcobatus vermiculatus) mustard (Arabis sp., Brassica sp.), sagebrush (Artemisia rigida, Artemisia tridentata), rabbit brush (Chrysothamnus nauseosus, Chrysothamnus viscidiflorus) and cactus (Opuntia polyacantha). The arid climate of the Columbian Plateau affects, to some extent, surrounding areas. Thus the Yakima Valley Area, the Columbia Valley, where it borders the Plateau, and the Okanogan Valley possess vegetation typical of the Columbia Plateau.