Geography 316:  Biogeography     In progress 12/16/2003

  The Biogeography of Taxus Brevifolia, Pacific Yew
                                              by Heba Petursson, student in Geography 316  Fall 2003

Thank you for visiting our site. This web pages was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD.  All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.

                                                    Species Name:          Taxus Brevifolia

Description of Species

Taxus brevifolia, popularly known as the pacific or western yew, is a coniferous tree which inhabits the temperate rainforest of the Pacific Northwest, shown in Figure 1 below.  The trunk features either a straight or contorted shape and is comprised of a heavy and resistant wood.  The trunk sometimes resembles a series of trees fused together, this characteristic is known as fluting (Thomas 2000).  The pacific yew's bark is delicate and thin ranging from .5-.7 cm in width.  Its scaly bark has a reddish brown color whereas its outer bark has a purplish tone (Elias 1980).  The yew's slender branches droop creating a weeping appearance (Elias 1980).  The yew posseses relatively small evergreen leaves ranging from flat to slightly curved with pointed tips that are spirally arranged.  Needle size ranges from 1.2-2.6 cm in length and are approximately 1/8 inch in width (Thomas 2000).   The top side of the leaf is a darker green whereas the bottom is a paler yellow-green color. The yew retains its leaves for approximately 3-5 years and in rare cases can keep its leaves for ten years.  Its' wood is composed of dead empty tubes called tracheids which are linked together and make up 90-94% of the tree’s volume.  Tracheids provide an essential route for water passage and serve as a crutch to hold the tree in place (Thomas 2000).  The remaining 6-10% of the wood is comprised of rays, sheets of living cells that run along the radii of the wood (Thomas 2000).  The Pacific yew generally lives 200 to 300 years, although some specimens have lived in excess of 400 years.  The alkaloids in the yew are extremely poisonous and cause neurological damage resulting in convulsions, muscle fasciculation and paralysis (Thomas 2000).  The foliage, bark, and seeds of most Taxus species are toxic due to the presence of taxine, an alkaloid, however, taxine does not appear to be present in Taxus brevifolia.  

Distribution

(Figure 1) Taxus Brevifolia Distribution: US Geological Survey (USGS)    http://climchange.cr.usgs.gov/data/atlas/little/taxubrev.pdf

The Pacific Yew has a continuous distribution on the West coasts of the United States and Canada.  These trees inhabit Southeast Alaska, Montana, Oregon, Washington, Idaho and Northern California in the USA and are found in British Columbia and the Alberta Rockies in Canada.  They occur with greater frequency in the Coast Range in southern Oregon and northern California while isolated occurrences are found as far south as Marin and San Mateo County. The Yew extends through the Klamath Mountains, the western slopes of the Sierra Nevada and continues in the San Francisco Bay Area in California. It grows on the western slopes of the Rocky Mountains in British Columbia, western Montana and isolated pockets in eastern Washington and northeastern Oregon (Cunningham 1994). The Pacific yew exists in dense forests, along streams, moist flats, slopes, deep ravines and coves at an elevation between 0-2200 meters (Elias 1980).  The Pacific yew grows as an under-story dominant or co-dominant beneath a closed forest canopy in late-successional coniferous forests of the Pacific Northwest and northern Rocky Mountains.  Over-story dominants include grand fir (Abies grandis), white fir (Abies concolor), and western hemlock (Tsuga heterophylla).  Vine maple (Acer circinatum), queencup beadlily (Clintonia uniflora), and wild ginger (Asarum caudatum) are common co-dominants.   These trees provide the necessary shade for the yew’s growth.  Pacific yew is found in dry, sub humid rocky areas with an average annual precipitation as low as 470 mm, in this environment it is confined to streamside areas and the lower third of north-facing slopes.  It is also found within humid forests with precipitation of 1400 to 4000 mm, in this soil rich environment it can be found on all slopes, benches, and ridge tops.  In general the Pacific yew functions best in wetter forests of the coastal areas and the interior wet belt region.  Pacific yews are best suited to grow in deep, moist, rich, rocky soils such as ultisols, alfisols, and inceptisols. In dry interior forests, the species develops best along mountain streams, and in shady canyons, ravines or coves (Elias 1980). Within the moist maritime climate of the Pacific Northwest, it grows most abundantly in drier, warmer environments.

Evolution

The earliest conifers, voltziales, are found in fossil records dating back to the late Carboniferous and Permian.  The taxads which include Taxaceae are believed to have originated from voltzian stock (Darrah 1960).  It was the Cenozoic that saw a radiation of conifers as a result of two processes. First, the early Cenozoic is marked by a gradual geographic radiation of genera and species.  Second, the late Cenozoic is marked by a gradual restriction, isolation and extinction.  Starting in the Eocene and into the Miocene a tendency towards cosmopolitanism developed, however, this trend was curbed by the development of mountains and arid regions in North America, South America, Africa and Asia (Darrah 1960).  The subsequent Pliocene cooling and Pleistocene glaciations reversed the process of cosmopolitanism.  During the Cenozoic Taxus became restricted to the northern hemisphere.   Conifers best illustrate two principles of paleontology: radiation resulting in their evolutionary diversification and spread followed by a gradual restriction or specialization from a common stock (Darrah 1960). Taxaceae dates back to the Jurassic along with four other families of conifers. The diversification of the conifers occurred during the Mesozic.  The oldest recognizable yew is the Triassic Paleotaxus rediviva, found in strata 200 million years old. The mid-Jurassic Taxus jurassica (140 ma old) is more recognizable as a member of Taxus, containing features characteristic of Taxus baccata, Taxus cuspidata, and Taxus brevifolia (Botanik 2003).  The species of Taxus are more geographically than morphologically separable; they are treated as subspecies of T. baccata. All species are poisonous; most contain the anti-cancer agent taxol; and a study of heartwood constituents of T. baccata, T. brevifolia, T. cuspidata and T. floridana found them to be chemically almost identical.  species are almost wholly allopatric and largely confined to the middle latitudes of the northern hemisphere, with some intrusion to the tropical highlands.   There are approximately 10 species of the yew tree, these include the Taxus baccata Linnaeus (Common yew), Taxus brevifolia Nuttall (Pacific yew), Taxus canadensis Marshall (Canadian yew), Taxus chinensis (Rehder Chinese yew), Taxus cuspidata (Siebold & Zuccarini Japanese yew), Taxus floridana Chapman (Florida yew), Taxus globosa Schlechtendal (Mexican yew), Taxus mairei (Maire yew), Taxus wallichiana Zuccarini (Himalayan yew) and the Taxus yunnanensis (Yunnan yew). The global distribution of Taxus brevifolia is shown in red in Figure 2 below.  The yew occurs in nature as a shrub or a tree. Three types have been recorded: cv erecta, a columnar form; cv nana, a dwarf form; and cv nutallii, a drooping form. Pacific yew was originally classified as a variety of T. baccata (European yew).  Different species growing in close proximity of one another produce interspecific hybrids lending support to the view that there is only one species. 

Figure 2 Global Taxus Distribution

 Habitat

The ideal biome for the yew is characterized by little seasonality and at least  200 cm/year of precipitation (Cunningham, 1994).   The biomes that house pacific yews have precipitation levels from 65-400 cm, the lower amounts occurring inland and the higher amounts along the coast.  Fog is an important component in the southern half of the biome. Morning and evening fog brings abundant moisture even during the drier months of the growing season. As a result, the forest floor is abundantly supplied with a multitude of ferns, shrubs and perennial wildflowers (Reveal).  The types of forests that host the Pacific Yew are the: Spruce-Cedar-Hemlock forest, the Cedar – Hemlock, Douglas fir, Silver fir - Douglas- fir forest, Mixed conifer forest, Redwood forest, Red fir forest, Douglas fir forest, Cedar - hemlock - pine forest, Grand fir, Douglas fir forest, Western spruce - fir forest and the California mixed evergreen forest (Botanik 2003).  Yew trees coexist and with populations of large carnivores, including wolves (Canis lupus), grizzly bears (Ursus arctos), wolverines (Gulo luscus), caribou (Rangifer tarandus), black bear (Ursus americanus), mountain goat (Oreamnos americanus), grouse (Dendragapus), waterfowl, black and white-tailed deer (Odocoileus hemionus and O. virginianus), and moose (Alces alces).  In the southeast, marten (Martes ameriana) and bobcat (Lynx rufus) occur, while in the northern part of the Southern Rocky Mountain Trench, coyote (Canis latrans) and cougar (Puma concolor) are found (Botanik).   Moose, deer, elk and caribou utilize the yew as an important food source.  Birds such as blackbirds, waxwings, and nuthatches as well as small rodents are usually in the vicinity of yews whose fruit these animals consume. 

Growth

Pacific yew tolerates shade and is usually found as an under-story tree in an undisturbed stand.  The Pacific yew grows slowly and the species is typically found as an occasional tree in stands of other tree species, therefore the volumes of the Pacific yew are low. Trees larger than 50 cm in diameter and 60 feet tall are rare within most of the species' range: they account for less than 2 percent of the yew trees .  A majority of the medium sized trees range from 20-60 feet in height (Jonas 1993).  The root system of Pacific yew are deep and wide spreading allowing its roots to infiltrate moist soils (Preston 1989) .  Yews are shade tolerant and require shade to grow and develop under heavy canopy layers in forests. The Pacific yew grows between elevations of 650-2500 meters (Elias 1980). Adaptations through morphological changes in the needles-length, cuticle thickness, and deflection from the horizontal-and development of epicormic (describes new growth borne on the old wood of trees) twigs accelerate its ability to grow.  The length of its growing season ranges from 60 to 300 days and the growth of the trees occurs very slowly (Elias  1980).

 (Figure 4) U.S. Department of Agriculture, Agriculture Handbook 386

Reproduction of Species: 

Pacific yew's reproductive nature is dioecious, meaning that the male and female trees are separate.  

Rarely, a pacific yew will be monoecious, carrying both the male and female sex on the same tree (Thomas 2000).  

Dioecity ensures genetic diversity by forcing the seed to travel in order to germinate.  The male trees display stalked, 

globe shaped pollen sacs which are pale yellow in color (Elias 1980).  They carry 6-14 staminate cones in a spherical-shaped 

cluster, each cone containing 5 to 9 anthers. The female seed is surrounded by an outgrowth of the seed stalk 

called an aril, a fleshy edible red shaped disk.  They are abundant on the underside of the branches and usually appear in 

May or June and survive for 3-4 months. Female seeds are solitary and greenish in color and there are fewer of them than 

their male counterparts. The aril partially covers a greenish-brown to bluish colored ovoid-oblong seed approximately 

6-8 mm long (Elias 1980).   The seed is broadest at the base and has a pointed tip protruding from the aril.

Seed Production and Dissemination

The yew produces large quantities of seeds on a regular basis, however, its seeds germinate slowly (Elias 1980).  The scarlet color of the aril attracts birds who eat the aril and in turn assist in the dispersal of the seed (Elias 1980).  Wind and rodents also play a crucial role in the dispersal of the yew's seeds.  Pollen is dispersed by the wind in the spring and by animals during the fall when the fruit has ripened.  Fruits ripen from August to October, the fruits either drop to the ground or are consumed by birds or rodents. Birds devour the fleshy arils and pass the seeds which remain viable. Chipmunks and squirrels often take only the seed.

Medicinal Properties

Considering 70% of pharmaceutical drugs are extracted from natural substances it is important that we understand and respect the potential medicinal values derived from plants (Gray 1993).  In the late 1960's, taxol, a complex compound extracted from yew bark, was identified as a possible anticancer agent. The National Cancer Institute (NCI) inadvertently found taxol, a promising remedy, when testing for anticancer properties in more than 120,000 plant compounds. Taxol quickly became the potential remedy for over 20,000 women diagnosed with ovarian cancer yearly, however, the demand for taxol could not be obtained from the number of existing pacific yews.  In order to produce 1lb of taxol, enough to treat 200-300 women, would require 7,000lbs. of dried bark or 2,000 trees.  One patient requires the taxol extracted from 3 trees. (Lanner 1995).   Fortunately, Robert Holton and his team of scientists recently developed a synthetic form of the precious and highly beneficial compound.   Synthetic taxol works both against and in favor of the pacific yew, once thrown away, burned or slashed as a weed tree, its medicinal properties elevated its status, will it be threatened again now that taxol is produced in a lab?  The drug has been used as an effective cancer fighter for more than 500,000 women who have used it in combating advanced breast cancer and ovarian cancer.   Taxol promises to be an effective treatment for lung, colon, prostate and pancreatic cancers, as well as fighting Alzheimer's disease and polycystic kidney diseases (Peck 1).

Anthropogenic Threats

The ideal environment for the pacific yew is the temperate rainforest, also known as the marine west coast in the pacific northwest. The biome was once dominated by old growth forests, however, logging has significantly reduced the amount of old growth forests throughout the United States, Canada and southern Alaska.  Clear cutting, the removal of all trees, is economically efficient, however, the long-term effects of soil erosion, downstream silting, and damaged natural regeneration makes the restoration of old growth forests near impossible (Reveal).  Logging, hard-rock mining, oil and gas development, and recreational-residential construction are all major anthropogenic threats to the region (Cunningham 1994). Converting land use for livestock grazing and the introduction of exotic species are affecting the components of the biome that cater to the Pacific yew. Fortunately, the pacific yew has an extraordinarily high resistance to urban air pollution which makes it an ideal tree to plant.

Response to Disturbance

Fire suppression since the early 1900's in temperate rain forests resulted in a massive accumulation of debris on the forest floor. Rain, fog and boggy soils often means that fires, while common, burned slowly, rarely killing mature trees.  Today, when fires occur, the resulting loss is often far greater than otherwise would occur were fire permitted to be a natural part of the ecosystem.  The Pacific Yew is extremely sensitive to fire and usually dies as a result of prolonged exposure (Stuart 2001).  The Pacific yew is sometimes damaged by excessive heat, frost, and wind, especially after over-story trees have been removed.  The yews ability to sprout from cut stumps is as impressive as is its ability to layer.  Layering occurs when the tree's branches have been pressed to the ground for a prolonged period, it can begin to sprout upon contact with the moist soil of the forest floor and strike roots.  This process can lead to the growth of a new tree separated from its parent tree (Lanner 1995).   Overstory removal weakens the pacific yew by exposing it to the sun, wind, and cold.   It resists urban pollution damage from sulfur dioxide and was the least sensitive of 12 coniferous species to smelter fumes at Trail, British Columbia.  The Pacific yew can become damaged by heart-rot fungi such as Phellinus nigrolimitatus, P. pini, P. robustus, and Fomitopsis rosea (Hepting 1971).

Importance of Native Plants

Native plants, having evolved in their geographic region are best suited to for the environment. They provide food and shelter for native animals.  To ensure the maximum diversity in animal populations we must require the maximum diversity of plants.  Biological diversity is vital to humans because we rely on the land and its resources.  Native plants continue to play a crucial role in the development of food, medicine and industrial products.  Native plants are also a major element in the beauty of the earth's landscape.  Non-native plants take over natural areas and out-compete native plants. They are able to do so because pests, diseases or weather conditions which kept the plants in check in their native regions do not exist in their imported area. Exotic plants deprive native animals of food and shelter.  They also  damage waterways and dry land, clogging water flow and kill aquatic plants essential to wildlife (CNPS 2003).

                                                                  www.aphios.com/company/ taxoids.htm

Bibliography:

Cunningham, William, Ball Terence, Cooper Terence, Guille Gohan, Hepworth Malcolm. Environmental 

               Encyclopedia, First Edition. Gale Research. Detroit 1994.

Darrah, C. William. Principles of Paleobotany. Second Edition.  Ronald Press Company. New York. 1960.

Elias, Thomas.  The Complete Trees of North America: Field Guide and Natural History.  

            Van Nostrand.   NY. 1980.

Gray, Robert.  The Tree. Stockpole Books. Pennslyvania. 1993.

Hepting, George H. Diseases of forest and shade trees of the United States. U.S. Department of Agriculture, Agriculture Handbook 386. Washington, DC. 1971.

Jonas, Gerald. The Living Earth Book of North American Trees. Readers Digest Publishing.        

          NY. 1993.

Lanner, Ronald. Conifers of California. Cachuma Press.  Los Olivios, California. 1999. 

Preston, Richard. North American Trees. Fourth Edition. Iowa State University Press. Iowa. 

          1989.

Peck, Dana.  Florida State Times Magazine. Accessed October 14, 2003

      http://www.fsu.edu/~fstime/FS-Times/Volume5/april00web/8april00.html

Stuart, John, Sawyer John. Trees and Shrubs of California. University of California Press. 

           Berkeley. 2001.

Reveal, James L. Biomes of North America. College Park, Maryland. Norton-Brown 

        Herbarium. University of Maryland College Park.

Thomas, Peter.  Trees: Their Natural History. Cambridge Press. NY. 2000.

References

 Botanik. website address: http://www.botanik.uni-bonn.de/conifers/ta/ta/index.htm 

	This page is from the Gymnosperm Database, URL: http://www.conifers.org/ta/ta/index.htm 

	[On-Line] accessed November 3, 2003.

California Native Plant Society. 2707 K Street, Suite 1, Sacramento, CA 95816-5113(916) 447-2677 

	website address: cnps@cnps.org and http://www.cnps.org/activities/natives.htm [On-Line] 

	accessed November 3, 2003.

ForestWorld http://www.forestworld.com/public/silvics/conifers/taxus/brevifolia_f.htm
         211 Maple Street, Suite 27B, PO Box 491, Middlebury, VT 05753. Phone number: 802-382-        8888

Pacific Northwest Research Station (PNW). U.S.Department of Agriculture, Forest Service.

	website address: http://www.fs.fed.us/pnw/ [On-Line] accessed November 20, 2003.

         website address: info@ForestWorld.com [On-Line] accessed October 14, 2003.

USDA Forest Service: http://www.na.fs.fed.us/spfo/pubs/silvics_manual/Volume_1/taxus/brevifolia.htm

 	Page last modified November 14, 2003.[On-Line] accessed November 17, 2003.

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