University of Washington Publications in Geology The Geology of the San Juan Islands

CLIMATE AND VEGETATION

The San Juan Islands are located within a "dry belt." Along the west coast of Vancouver Island the annual rainfall is about 150 inches. On the east side of Vancouver Island in the vicinity of Victoria, however, the yearly rainfall is only 22 inches. At Friday Harbor the rainfall is about 25 inches, and farther eastward the amount increases until a maximum is reached on the western slopes of the Cascade Mountains. The annual rainfall occurring on Mount Constitution Range and on other mountain ranges of the region, is considerably greater than that on the areas of low relief. The rainfall on the San Juan Islands is largely confined to the winter months, though occasional rains do occur at all times of the year. During the winter months the region is swept by strong wind-storms which usually come from the southward.

The residents of the San Juan Islands enjoy remarkably beautiful sunsets, due to the fact that the sun appears to set in the rainy district on the west side of Vancouver Island.

The vegetation on some of the islands is dense and varied while on others it is wholly lacking. Forests of conifers occur on many of the larger islands. The wood contains an unusually large amount of pitch, and while this condition injures its value for lumber, it greatly adds to its value as fuel.

The following is a list of the trees and shrubs occurring on the San Juan Islands,—

(Taken from an unpublished manuscript called the "Key-Trees and Shrubs of San Juan Islands," compiled by Miss Leona Sundquist of the University of Washington.)

Taxus brevifolia (Western Yew)
Juniperous scopularum (Rocky Mountain Juniper)
Thuja plicata (Giant Cedar)
Abies grandis (White Pine)
Picea sitchensis (Spruce)
Pseudotiuga taxifolia (Douglas Fir)
Tsuga heterophylla (Hemlock)
Pinus contoria (Twisted Cone Pine)
Pinus monticola (Western Whiie Pine or Mountain Pine)
Fatsia horrida (Devil's Club)
Hedera helix (Ivy)
Oxycoccus oxycoccus (Cranberry)
Vaccinium parvifolium (Red Huckleberry)
Ledum greenlandicum (Laborador Tea)
Arbutus menziesii (Madrona)
Gaultheria shallon (Salal)
Kalmia polifolia (Swamp Laurel)
Pachistima myrsinites
Chimaphila umbellata
Berberis nervosa (Dull Oregon Grape)
Berberis aquifolium (Shining Oregon Grape)
Ulex europeus (Gorse)
Cytisus scoparius (Scotch Broom)
Corylus california (Hazel)
Betula occidentalis (Western Birch)
Alnus oregona (Red Alder)
Populus tremuloides (Aspen)
Populus trichocarpa (Cottonwood)
Salix lasiandra (Gland Willow)
Salix scouleriana (Scouler Willow)
Salix hookeriana (Hooker Willow)
Quercus garryana (White Oak)
Crataegus rivularis (Hawthorn)
Amelanchier florida (Service Berry)
Osmaronia cerasiformis (Indian Plum)
Prunus emarginata (Wild Cherry)
Holodiscus discolor (Ocean Spray)
Spiraea douglasii (Hardhack)
Rubus parviflorus (Thimbleberry)
Rubus ursinus (Trailing Blackberry)
Rubus laciniatus (Evergreen Blackberry)
Rubus spectabilis (Salmonberry)
Rubus leucodermis
Rosa gymnocarpa (Naked Rose)
Rosa nutkana (Nutka Rose)
Rosa pisocarpa (Bunched Rose)
Shepherdia canadensis (Soap Olallee)
Cornus nuttallii (Flowering Dogwood)
Cornus occdentalis (Western Dogwood)
Linnea borealis (Twin Flower)
Symphoricarpos racemosus (Snowberry)
Sambucus callicarpa (Red Elder)
Lonicera involucrata (Black Twin Berry)
Lonicera ciliosa (Orange Honey Suckle)
Lonicera hispidula (Pink Honey Suckle)
Ribes sanquineum (Red-flowered Currant)
Ribes divaricatum (Gooseberry)
Ribes lacustre (Swamp Gooseberry)
Acer macrophyllum (Large Leaved Maple)
Acer glabrum (Rocky Mountain Maple)
Acer circinatum (Vine Maple)
Philadelphus gordonianus (Syringa)
Fraxinus oregona (Oregon Ash)

On Cypress Island, and on other islands composed of the Fidalgo formation, it is noticeable that very little underbrush or surface vegetation exists. This condition is probably brought about by the oxidation of chromite to form chromic salts. (Dr. T. C. Frye, Professor of Botany at the University of Washington, called the author's attention to the fact that chromic salts are poisonous to plant life.)

In places where the rocks of the Fidalgo formation are covered with a few inches of glacial drift, the surface vegetation is usually dense.

Sphagnum moss-bogs occur on Mount Constitution Range at an elevation of about 2000 feet. They are also found at many other localities in the San Juan Island map-area.

Some of the small barren islands have a species of prickly cactus growing on them.