Douglas Fir. Stephen F. Arno
the genus Pseudotsuga. The physical expression of wide genetic diversity plays out in the tree’s uncanny ability to grow on vastly different sites—varying from six-hundred-year-old dwarfs (inland variety) occupying lava flows in the American Southwest to towering 300-foot-tall giants (coastal variety) anchored in the deep, wellwatered soils of the Pacific Northwest.
Though the coastal and inland varieties of Douglas-fir merge in southern British Columbia, in the western United States an area of semi-arid grassland and sagebrush east of the Cascades separates the two varieties. The coastal variety occupies the coastal mountains and lowlands south past San Francisco to the vicinity of Monterey, the Cascades and Sierra Nevada south to the moist canyons of Yosemite National Park, and the Lake Tahoe area of extreme western Nevada. Curiously, only the coastal variety inhabits California, while the inland form grows in all the western states except California.
Inland Douglas-fir is abundant in central and southern British Columbia but becomes much less common in the harsh, continental climate of Alberta—where it is confined to lower slopes of the Rocky Mountains and grows only as far north as Jasper National Park. In the United States, inland Douglas-fir is abundant in the mountains of eastern Washington, central and eastern Oregon, Idaho, Montana, and southward throughout the Rocky Mountain chain. It also occurs in the high-desert mountain ranges of Arizona, New Mexico, and extreme eastern Nevada, and in increasingly scattered groves all the way to the tropical mountains of southern Mexico, covering a north–south distance of more than 2500 miles—an exceptionally broad range for any American tree. Variation of the inland variety in isolated mountainous areas of Mexico spurred a proposal in 1949 to add four new species to the genus Pseudotsuga. However, genetic work indicates that Douglas-fir migrated southward in Mexico during the Pleistocene Ice Age; isolated populations moved along north–south mountain corridors and occasionally reconnected, but apparently left too little time for any to differentiate into additional species. While the proposal for adding new species was rejected, there is some consensus among taxonomists that the Mexican populations have enough in common, and are sufficiently different from the other two varieties of Douglas-fir, that they deserve recognition as a third variety.
Both coastal and inland varieties of Douglas-fir form tall trees in low and mid-elevation forests within their ranges. For example, coastal Douglas-firs grow from sea level to 3500 feet in northwestern Washington and the inland variety from 7000 to 9000 feet in southern Colorado, New Mexico, and Arizona. Inland Douglas-firs extend upward as shorter trees on south- and west-facing slopes and exposed ridges into the subalpine zone. Sometimes they morph into sheared, shrubby trees in wind-funneling passes along the crest of the Rocky Mountains, and occasionally Douglas-fir takes on the dense, low shrubby form called krummholz above the limit of even stunted, erect trees on high mountain peaks.
Because it adapts to a wide variety of habitats and appears in many forms, no simple description encompasses Douglas-fir. Young trees are the West’s most familiar wild Christmas trees. As they continue to grow, young Douglas-firs produce a broad cone-shaped canopy of upward-projecting limbs with abundant branchlets. When they mature, the trees develop an irregular canopy made up of spreading limbs with drooping branchlets that contrast with the more symmetrical branching habit of their true fir (genus Abies) associates, such as grand fir (A. grandis), white fir (A. concolor), red fir (A. magnifica), and subalpine fir (A. lasiocarpa).
Douglas-fir’s needlelike leaves are about an inch long and attached to all sides of the twigs or branchlets, and unlike those of spruce, they are not stiff or prickly to touch. The cones are very distinctive because of the three-pronged, pitchfork-shaped bracts that project from between the scales.
The cones are green at first but turn tan as they mature and grow up to 2.5 to 4 inches long in the coastal variety and somewhat shorter in the inland variety. They are often abundant, and found hanging among the branchlets or lying on the ground. Sometimes squirrels clip off the dense green cones, the size and shape of a small dill pickle, and then gather them up and cache them in a rotten log or underground burrow in order to have a seed supply as winter food. The brownish mature cones dry out on the tree, their scales flex open, and the papery-winged seeds are dispersed by the wind, sometimes several hundred feet. A sticky pitch may be present on both green and mature cones.
When cones aren’t available for identification, not even old cones on the ground, the tree’s buds are another distinctive feature. They are a rich chestnut-brown color, oval and sharp-pointed, and covered with overlapping papery scales. After the buds burst open in late spring, new light green twigs start to emerge. The bud scales bend backward during this period but remain attached to the previous year’s woody twigs. In contrast, buds of true firs and many other conifers are less conspicuous, blunt, light colored, and covered with wax.
The bark of young Douglas-fir trees is smooth, gray, and spotted with pea-size blisters filled with sticky resin. With age the bark becomes rough, and after a century or longer develops into a dark gray-brown corky substance with deep vertical furrows and no resin blisters. Cutting into the bark of a maturing Douglas-fir with a knife exposes wavy bands of contrasting dark brown and light tan. Mature Douglas-firs, such as one-hundred-year-old “second-growth” that sprang up after historical logging, often have a vertical strip of dried pitch on their bark. The bark is also commonly covered with lichens, some a dull dark color, while others may be bright yellow or a beautiful pastel blue-green. Near the base of big old trees, particularly the coastal variety, bark often thickens to at least 6 inches and protects the tree’s sap-filled growing tissue from lethal heating by fire, which occurs at about 145° Fahrenheit.
One striking attribute of Douglas-fir is exceptional height. Oldgrowth coastal Douglas-firs in sheltered valleys commonly reach 250 feet tall and attain diameters of 5 to 8 feet. Occasionally these monarchs tower 300 feet or slightly higher, putting them among a handful of the tallest tree species in the world, surpassed only by the California coastal redwoods (Sequoia sempervirens), whose record is a tree named Hyperion, measuring 381 feet. Inland Douglas-fir grows more slowly than its coastal kin due to frigid winters and drought-plagued summers exacerbated by daytime relative humidity of 15 percent or less. Still, in narrow valleys and canyons they can attain heights of 150 to 200 feet.
Centuries-old trees are another noteworthy feature of coastal Douglas-fir forests. Trees greater than five hundred years of age are fairly common, and occasionally trees a thousand years and older have been reported for both Douglas-fir and a number of its associates. The oldest coastal Douglas-fir was believed to be a more than 1000-year-old monarch in Lynn Valley in British Columbia. That trees and forests of this species and its associates have survived for centuries may give the impression that they have been a centerpiece of the Pacific Northwest landscape extending back into antiquity. Massive trees, coupled with giant moss-covered logs slowly decaying into the forest floor, seemingly provide visual evidence of sustainable “ancientness.” But tree pollen found in sediment layers underlying small ponds across the coastal Northwest paints a different picture. Today’s Douglas-fir-dominated old-growth forests first appeared in similar form only about six thousand years ago—a mere blip in the calendar of geologic time. Prior to that time, forest vegetation in the Northwest varied greatly depending on rapidly changing temperatures that occurred 10,000 to 6000 years ago.
Douglas-fir’s longevity plays a key role in current efforts to determine climatic patterns (particularly precipitation) over the past centuries and even millennia, providing a baseline for assessing modern-day climate trends. Here it is not the coastal giants but dwarfed, bonsai-shaped inland Douglas-fir trees that have special value.
Growing in the cracks of lava flows at El Malpais National Monument in New Mexico, tiny water-stressed trees survive to amazing ages and their year-to-year growth is highly sensitive to precipitation. Tree rings counted on increment cores show that the oldest known living Douglas-fir at El Malpais dates back 1280 years; another old tree on the flow began life about 960 years ago. These and other centuries-old trees serve as nature’s version of solar-powered data recorders, archiving tree-ring widths through good times and bad. Scientists have used the tree-ring patterns (chronologies) to quantify relationships between ring widths and precipitation over the period for which weather records are available.
After establishing such relationships, scientists use