San Francisco State University
Douglas-fir (Pseudotsuga menziesii)
Jordin A. Williams
Student in Geography 316, Fall 2000
|Old Growth Douglas-fir near Mendocino, California, October, 2000. Estimated to be between 300-400 years old. ( Photo by author)||Growth Douglas-fir near Mendocino,California.
October, 2000. Diameter at breast height 5.67 feet and approximately 180-210 feet tall. (Photo by author. )
Species: Pseudotsuga menziesii
(From Integrated Taxonomic Information System, 2000)
Douglas-fir was originally named after the Scottish botanist David Douglas who was sent by the Royal Horticultural Society to study the tree in the late 1700s (Hebda 1995). In 1867, Douglas-fir was given its own genus Pseudotsuga, which means false (pseudo) hemlock (tsuga), and is considered not to be a true fir. The genus Pseudotsuga consists of six tree species, two native to North America and four native to eastern Asia (Bailey 1955). This paper focuses on the species Pseudotsuga menziesii, which is native to North America, and consists of two distinct varieties: P. menziesii var. menziesii, called coast Douglas-fir, and P. menziesii var. glauca, called Rocky Mountain or blue Douglas-fir (Hermann 1982).
Description of Species:
Douglas-fir (Pseudotsuga menziesii) is a clear, straight-trunked tree, with a spire-like crown, and the capacity to grow over 300 feet tall. The upper branches of the tree point up and the lower branches droop down and recurve (Bailey 1955). Its bark is characterized as gray to reddish brown with deep, thick grooves, which form by the time it reaches maturity, and serve as a defense mechanism against wildfires (Allen and Owens 1972). Spreading needle covered branches consist of 1 inch needles, which are slightly pointed at the tip. Coast Douglas-fir (P. menziesii var. menziesii), have yellowish green needles, while Rocky Mountain or blue Douglas-fir (P. menziesii var. glauca), have bluish green needles both varieties having two white bands on the underside of the needle (Hebda 1995). Male flowers of Douglas-fir (Pseudotsuga menziesii) are bright red, female flowers are green and have prominent bracts. Mature seed cones are approximately 3 to 4 inches long, reddish brown in color, and have pitchfork-shaped bracts. The entire cone is shed in the fall, and the seed is a light brown color, approximately 5-6 mm long (Peattie 1991).
Douglas-fir (Pseudotsuga menziesii) ranks as the second tallest tree species in the world behind coastal redwood (Sequoia sempervirens), and contains the largest trees in the entire Pinaceae family. The tallest known Douglas-fir (Pseudotsuga menziesii) is the Brummit Fir (located in Coos County, Oregon), which reaches the height 328 feet. In terms of thickness the Queets Fir, located in Olympic National Park, has a diameter at breast height of 14.3 feet. Most old growth Douglas-fir (Pseudotsuga menziesii) range in height from 200 to 250 feet, and have a diameter of 5 to 8 feet. The oldest known Douglas-fir (Pseudotsuga menziesii) can be found on Vancouver Island in British Columbia, and is estimated to be between 1300 to 1400 years old (Earle 1999).
Second-Growth Douglas-fir. Photo by author.
Douglas-fir (Pseudotsuga menziesii) can grow under a wide variety of climatic conditions from the maritime climate of the Pacific Northwest to the mild continental climates of the Rocky Mountains (Earle 1999). Rocky Mountain or blue Douglas-fir (P. menziesii var. glauca), grows in the harshest of all environments marked by a relatively few frost-free days, high annual snow fall, and low annual precipitation. In the northern Rocky Mountains, the climate has a marked maritime influence, with an annual snowfall ranging between 16 to 320 inches and an average January temperature of 19 to 28 F. Frost may occur during any month of the year even though it has an annual frost-free period of 60 to 120 days. In the central Rocky Mountains, the climate is continental, with a relatively low annual precipitation range of 14 to 24 inches and a frigid average January temperature of 16 to 22 F. Winters in this region are often long and severe; summers in contrast are hot and extremely dry. The southern Rocky Mountains has a narrow fluctuation in temperature ranging from a low of 32 F in winter to a high of 52 F in summer (Hermann and Lavender 1999).
The coastal regions of the Pacific Northwest and Northern California, which contain coast Douglas-fir (P. menziesii var. menziesii), has a maritime climate characterized by mild, wet winters and cool, relatively dry summers, a long frost-free season, and narrow diurnal fluctuations of temperature from 43 to 46 F (Hermann and Lavender 1999). This region is also characterized by extremely high annual precipitation of 34 to 134 inches and very low annual snowfall of 0 to 24 inches. Fluctuation in temperature ranges from a low of 28 F in winter to a high of 81 F in summer. In the Cascades and Sierra Nevada, the climate is characterized by colder winters, shorter frost-free seasons, and an even broader range of seasonal temperature from a low of 15 F in winter to a high of 86 F in summer (Allen & Owens 1972).
Douglas-fir (Pseudotsuga menziesii) reaches its best growth on well-drained, moist, deep, loamy soils with a pH range from 5 to 6. Coast Douglas-fir (P. menziesii var. menziesii) thrives in soil that originated from maritime sandstone and shale, which was weathered into fine textured, well-drained soils by the mild, humid climate of the Pacific Northwest. These soil groups include Haplohumults (Reddish Brown Lateritics), Dystrochrepts (Brown Lateritics), Haplumbrepts (Sols Bruns Acides), Haplorthods (Western Brown Forest soils), Xerumhrepts (Brown Podzolic soils) and Vitrandepts (Regosols) (Earle 1999). Rocky Mountain or blue Douglas-fir (P. menziesii var. glauca), grows in soils that consist of a considerable array of parent material. Throughout the Rocky Mountains the major soil types were developed primarily from glacial deposits, crystalline granitic rocks, conglomerates, sandstones, and limestone (Hermann and Lavender 1999).
The seasonal growth of Douglas-fir (Pseudotsuga menziesii) begins during the spring (usually April) with the onset of vegetative bud growth. During the months of May and June the vegetative buds begin to burst forth triggering the enlargement of the lateral bud primordia (Hermann and Lavender 1999). From July to November the lateral bud primordia becomes determined and we begin to see the initiation of the leaf, bract, and microsporophyll. Meiosis and pollen development begin to occur around March and by April the cone buds burst or flower and pollination of the seed cones begins. In May and June fertilization is underway and the seed cones begin to enlarge at a rapid pace. From August to September embryo and seed development is in full swing and by late September cones have matured and begin to shed their seeds. The entire reproductive cycle of Douglas-fir (Pseudotsuga menziesii) extends over a 17 month period from early April to late September of the following year (Allen and Owens 1972).
Reproductive Cycle of Douglas-fir.
(From Allen and Owens, 1972)
The fossil record of Pseudotsuga like forms in North America begins in the Early Tertiary period about 50 million years ago. The cones, seeds, and needles of the modern genus Pseudotsuga can scarcely be distinguished from those of its ancestors. Unfortunately, no concrete record of Pseudotsuga exists in the pre-Cenozoic era, so we are forced to work with only a small portion of the evolutionary history of the genus Pseudotsuga. The earliest known occurrence of fossil cones, seeds, and leaves of Pseudotsuga in North America was found in the Eocene Copper Basin flora of northeastern Nevada. Although little is known about the range of Pseudotsuga during the Early to Middle Tertiary periods the fossil records indicate that its range was rather restricted. However, by the Late Tertiary period its range had expanded considerably to as far north as Alaska, which lasted well into the Middle to Late Miocene about 20 million years ago (Hermann 1985).
During the Tertiary period, fossil records indicate that Pseudotsuga was a minor component of the forests of western North America. In the Pleistocene epoch Pseudotsuga began to develop into a vegetational unit of the forests located at medium latitudes and medium altitudes in the northwestern region of North America. Fossil pollen found in Quaternary deposits indicate that during the interglacial intervals Douglas-fir (Pseudotsuga menziesii) became an important part of northwestern forests in North America. It's theorized that during glacial periods the repeated growth and waning of ice sheets caused large fluctuation in the range of Douglas-fir (Pseudotsuga menziesii). During the post-glacial period Douglas-fir (Pseudotsuga menziesii) emerged as a dominant element in the coniferous forests of western North America (Hermann 1985). Beginning about 10,000 years ago, Douglas-fir (Pseudotsuga menziesii) began to migrate from glacial refugia into the range it occupies today (Bartlein, K. Anderson & P. Anderson 1998).
Geographic location of finds of Pseudotsuga megafossils from the
Tertiary and the Quaternary, and Tertiary microfossils, in North America.
(From Hermann, 1985)
The native ranges of both varieties of Douglas-fir (menziesii and glauca) resembles an inverted V with uneven sides, with the left arm stretching for over 1,367 miles and the right arm stretching for over 2,796 miles. The left arm represents the native range of coast Douglas-fir (P. menziesii var. menziesii), which extends from central British Columbia (55 N) south along the Pacific Coast Ranges into central California to a latitude of 34 44 N. Within California, coast Douglas-fir (P. menziesii var. menziesii) is predominately found on the western side of the Sierra Nevada and covering a good portion of Northwestern corner of the state. Conversely, the native range of Rocky Mountain or blue Douglas-fir (P. menziesii var. glauca), extends from central British Columbia (55 N) south alone the Rocky Mountains and into the mountains of central Mexico to a latitude of 19 N (Hermann and Lavender 1999).
Coast Douglas-fir (P. menziesii var. menziesii) typically grows in elevations that range from sea level to 5,500 feet above sea level along the coastal ranges and west of the mountain regions in the Pacific Northwest. In the Sierra Nevada and Cascades its altitudinal range is between 2,000 and 7,500 feet. While its counterpart Rocky Mountain or blue Douglas-fir (P. menziesii var. glauca), can grow anywhere from 1,800 to 9,500 feet in the Rocky Mountain range. Reaching its highest point at 10,700 feet atop Mount Graham in Southern Arizona, so we can conclude from this information that the altitudinal distribution of both varieties of Douglas-fir increases from north to south (Hermann & Lavender 1999).
Limits on Distribution
The distribution of Douglas-fir (Pseudotsuga menziesii) is limited by factors such as unfavorable climate conditions like high seasonal winds and heavy wet snows (Project Bio: Iowa State University 1998). In the northern part of its range temperature is a major limiting factor, which is why Douglas-fir (Pseudotsuga menziesii) is mainly found on southern slopes in this region. In the southern part of its range moisture is a major limiting factor, which is why we find Douglas-fir (Pseudotsuga menziesii) growing on northerly exposures. This tree also does badly in regions with dry, poorly drained, or compacted soils. All of these factors adversely effect the growth of the tree, and limit its range to Western North America (Earle 1999).
Distribution of Douglas-fir (Pseudotsuga menziesii)
(From Project Bio, Iowa State University, 1998)
Other interesting issues:
Fire has been a major component of Douglas-fir (Pseudotsuga menziesii) forests for thousands of years, and has helped to create almost pure stands of Douglas-fir (Pseudotsuga menziesii) throughout the Pacific Northwest. Logging, on the other-hand, has eliminated much of the original old-growth forests in this region, and massive clear-cutting has severely fragmented the environment, especially for wildlife (Rochelle 1999). Timber companies often employ a method of burning slash after a clear-cut, which helps Douglas-fir (Pseudotsuga menziesii) to regenerate faster, but by this time the overall quality of the forest ecosystem has been significantly damaged.
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