Pacific Yew (Taxus brevifolia)

Pacific Yew (Taxus brevifolia)

by  Michael Wood 

Greetings from the southern Black Forest of Germany, my wife’s Heimat. This report comes to you from the farmhouse of a friend, built in 1599, in the tiny Dorf of Adelhausen, about 15 km north of the Rhine River and the border of Switzerland, elevation 1,500 feet. We are one month into our new nomadic life.

On the first stop of our travels, we visited southwestern Ireland where we explored a rare English yew woodland (Taxus baccata) in Killarney National Park. This semi-natural stand is one of the oldest and most intact yew forests left in Europe. A quick cross-reference with our chapter’s SF checklist and I see that we have a single location for our own native yew species, T. brevifolia. Thus, the subject for this edition of Focus on Rarities was conceived (actually, ill-conceived, as you will soon learn!)

The yew family (Taxaceae) is an interesting and curious group of gymnosperms (plants with “naked seeds” not enclosed in an ovary, in contrast to angiosperms, the so-called flowering plants.) Other gymnosperms include Ephedra, Ginkgo, Welwitschia of Namibia, the cycads, and the conifers. The family is believed to have arisen in Asia in the Eocene around 44 million years ago. Members of the yew family are primarily dioecious, meaning male and female reproductive parts (in this case, cones) are produced on separate plants. The male cones are very tiny (just 2–5 mm long), and shed pollen in the early spring. What makes the yews so unique are the female cones, which don’t look like cones at all. They are, in fact. highly reduced and modified, being  comprised of a single scale and one seed. As the seed matures, the scale develops into a red, fleshy cup-shaped organ called an aril, which partially encloses the seed. Appearing between August and October, the mature arils are bright, soft, juicy and sweet, looking more like berries than cones. The seeds, which are toxic to humans, are readily disseminated by birds and rodents.

The Taxaceae includes six genera, including Austrotaxus (1 species endemic to New Caledonia); Pseudotaxus (1 species endemic to southern China); Amentotaxus (5 species endemic to subtropical Southeast Asia); Cephalotaxus (11 species endemic to eastern Asia); Torreya (four species native to eastern Asia and two native to North America); and Taxus. The genus Taxus is comprised of 6-10 species of shrubs and small trees found in temperate North America and Eurasia, and at high elevations of tropical southeast Asia and Central America. To look at a map of the distribution of this family is fascinating, from an evolutionary perspective.

Image: courtesy Wikipedia

Members of the genus Taxus are evergreen and highly shade tolerant, occurring as scattered individuals in the understory of forests with a closed canopy. They rarely become a dominant tree in any given woodland. They have attractive, scaly, reddish brown bark, flattened, flexible leaves that appear two-ranked (attached in pairs opposite one another on the stem). Four species are native to North America. These include T. globosa, a rare shrub found in a small number of locations in eastern Mexico, Guatemala, El Salvador and Honduras; T. floridana, a rare endemic shrub or tree reaching 10 meters tall (33’) and found only along the Appalachicola River in northwestern Florida; T. canadensis, a 2 meter tall shrub (6.6’) found throughout eastern North America from Manitoba to Newfoundland, south to Missouri, Kentucky, and Virginia; and T. brevifolia, found in western North America from Alaska southward to northern California.

Pacific yew, also called western yew, California yew, Oregon yew, and American yew, is a shrub to small tree. Although specimens are most commonly 5-15 meters tall (16-50’), it can grow up to 25 meters tall (82’) with trunks as much as 1 meter across (3.3’). The Pacific yew is slow-growing and a tree 10 meters tall (33’) may be over 100 years old.

Pacific yew occurs In open to dense forests, along streams, moist flats, slopes, deep ravines, and coves, from sea level to 2200 meters in elevation (7200’). Throughout most of its range, it occurs in the understory of closed canopies in late-successional forests dominated by large conifers such as Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). In the  southern parts of its range, Pacific yew can be found growing along seasonal streams beneath stands of coast redwood (Sequoia sempervirens), madrone (Arbutus menziesii), tan oak (Lithocarpus densiflorus), bay laurel (Umbellularia californica), and buckeye (Aesculus californica).

In both the New World and Old World, yew wood has been valued by indigenous peoples for its strength and durability, making it an excellent source material for bows, spears, harpoons, clubs, fish hooks, spoons, spring poles for animal traps, fish net frames, combs, paddles and ceremonial objects. Since colonization of the Pacific Northwest, however, yew trees were not considered to have any value as a lumber source and they were cut indiscriminately and cast aside in the course of commercial logging operations.

But then something interesting happened. Pacific yew has long been thought to have medicinal properties. As early as 1962, naturally occurring compounds found in the bark of Pacific yew became the focus of research into its potential for treating various cancers. And in 1977 the anti-tumor properties of yew bark were confirmed in the laboratory. This occurred at a time when the term biodiversity was entering the popular vernacular, and this discovery was commonly cited when arguing the case for the preservation of the Earth’s temperate and tropical rain forests and the plant and animal species yet to be discovered there. In 1992, the FDA approved the use of Taxol for the treatment of ovarian cancer and in 1994 for the treatment of breast cancer. In 2000, annual sales of Taxol peaked at $1.6 billion, making it the best-selling cancer drug ever manufactured. The downside (and there is always a downside) is that it took large quantities of Pacific yew bark to produce the drug, posing a risk to natural stands of the tree. Fortunately, manufacturers have figured out how to synthesize the drug from the leaves of English yew, a species that is easily and widely cultivated.

In California, Pacific yew is found from Marin and Lake counties northward along the coast and from Mariposa County northward in the Sierra Nevada. It is not known to occur in the Santa Cruz Mountains. Although collections have been made in the San Francisco Bay Area from Alameda, Santa Clara, San Mateo and San Francisco counties, these are suspected to have come from planted specimens. Griffin and Critchfield (1972) concluded that Marin County is likely its southernmost limit. Pacific yew was not included in Howell, et al. (1958). So, because you won’t be coming across Pacific yew in San Francisco, you could reasonably ask why I chose it as the subject of this column. Well, I just thought it was interesting. And I learned a bunch of things in the process. Maybe you did, too.

 

Literature Cited

Griffin and Critchfield. 1972. The Distribution of Forest Trees in California.USDA Forest Servive Research Paper PSW- 82 /1972. Available online at https://www.fs.fed.us/psw/publications/documents/psw_rp082/psw_rp082.pdf

Howell, J.T., P.H. Raven, and P. Rubtzoff. 1958. A Flora of San Francisco, California. Univ. of San Francisco. 157 pp.

Posted in Focus on Rarities.

Michael Wood

President, Wood Biological Consulting, Inc.
Wood Biological Consulting, Inc. San Francisco State University

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