Approximately six species comprise the genus Pseudotsuga, colloquially known as the Douglas-firs. They are some of the most iconic trees of the Pacific Northwest, and they often grow as pure or nearly pure stands. The trees are very important commercially, which has caused them to become interwoven with local cultures. For example, Douglas-firs are Oregon’s official state trees.
Douglas-firs are large, evergreen trees that are an early successional species. They grow very quickly when bathed in direct sunlight, such as occurs in areas cleared by fire or along the edges of established forests, but they languish in the dim light below established canopies.
As young trees, Douglas-firs have smooth, gray bark, but as they age, the bark becomes thicker and fissured. Like other conifers, they lack true flowers; instead, they produce, store and release pollen from small, red-yellow cones. Their seed cones, which broadly resemble pinecones, grow on 2-year-old twigs, and feature long, feathery bracts. Douglas-fir seeds represent a very important food source for most small herbivores and omnivores that share their range, including a variety of songbirds, squirrels and mice.
Classification and Diversity
Although the term “Douglas-fir” may refer to the entire Pseudotsuga genus, most people use this term to refer to a single species – Pseudotsuga menziesii. This species has a scattered distribution pattern, ranging across much of the western United States, as well as portions of Canada and Mexico. Most botanists recognize two or three varieties (or subspecies) of this species.
Coastal Douglas-firs (Pseudotsuga menziesii var. menziesii) grow along the western boundary of the species’ range, while Rocky Mountain Douglas-firs (Pseudotsuga menziesii var. glauca) grow, as their name implies, farther inland. Some researchers consider specimens growing in Mexico to represent an additional variety (Pseudotsuga menziesii var. lindleyana), while others consider these trees as part of the Rocky Mountain variety. The only other U.S. species is the bigcone Douglas-fir (Pseudotsuga macrocarpa), which grows throughout southern California.
Few of the other members of the genus are commercially important, and most of them inhabit rather restricted ranges in Asia. At least three species in the genus – Pseudotsuga sinensis, Pseudotsuga japonica and Pseudotsuga brevifolia – are classified as “Vulnerable” in their native ranges.
Size and Age
Douglas-firs often become massive trees. A few of the largest living specimens reach more than 300 feet, but historical accounts detail a few trees approaching 400 feet in height. Currently, a specimen known as the Brummit Fir stands approximately 325 feet high, thereby making it the tallest living Douglas-fir, the tallest living member of the family Pinaceae and the third tallest living tree in the world. Some of the oldest and largest specimens have massive trunks of up to 20 feet in diameter.
Although they are fast-growing trees, it takes the tallest specimens a very long time to reach such heights. Most of the tallest examples are found amid the few remaining old-growth forests. Many live for 600 or more years, and several have been alive for the last 800 years. The maximum documented age is 1350 years, but this age was determined from a stump, not a living tree. Thanks to their long life spans and tendency to produce regular growth rings, Douglas-firs are often used to study climate change and the impact of air pollution on tree growth.
Douglas-firs are some of the most important timber trees in the world, and they have been harvested extensively over the last 200 years. They are used in a variety of construction contexts, and are made into everything from dimensional lumber to plywood.
Douglas-firs are commonly used as landscape trees, particularly in screening projects. The trees are also commonly grown as Christmas trees.