Yew
Yew
Economic and ecological importance of yews
Yews are various species of tree- or shrub-sized, woody plants that comprise the conifer family, Taxaceae. All yews are in the genus Taxus, and about seven species are known, occurring in moist, temperate forest habitats in North America, Europe, Asia, and Africa.
Biology of yews
Yews have dark-green colored, rather soft, evergreen, needle-like leaves. Yews usually have separate male and female plants, although sometimes these flowers will develop on separate branches of the same plant. The male flowers are small and inconspicuous, and develop as globular bodies in leaf axils. The female flowers of yews develop in the springtime, and they occur singly and naked in the axis of a leaf. The fruit is a relatively large, hard seed, encased in a bright-red or scarlet, fleshy, cup-like structure known as an aril. The arils are sought by birds as food, which eat and disperse the seeds. The seeds of yews are poisonous to people and other mammals, although the fleshy arils are not. The symptoms of poisoning by yew seeds or foliage are dilation of the pupils, pain, and vomiting, leading to coma and death if the dose is large enough.
The wood of yews is dense, strong, flexible, and resistant to decay. The bark is thin, fibrous, scaly, and dark colored. The young twigs of yews are generally greenish in color.
Yews are slow growing and highly tolerant of shading. As a result, yews can survive in the shade beneath a closed forest canopy. This can be especially true beneath angiosperm trees, which allow the yews to grow relatively freely during the spring and autumn when the trees are in a leafless condition. However, yews also do well beneath conifer trees, as long as occasional gaps in the canopy allow exposure to light as brief sunflecks during the day.
Yews have a dense foliage and so cast a rather deep shade. As a result of this shade, as well as a toxic quality of the yew’s leaf litter, few plants will grow beneath a closed canopy of yew tree or shrubs.
Species of yew
Three species of yew grow naturally in North America. The Pacific or western yew (Taxus brevifolia) is the only species that reaches the size of a small tree, typically 19-39 ft (6-12 m) tall, but as tall as 75 ft (23 m). The western yew is a species of the sub-canopy of conifer rain forests of the Pacific coast, ranging from central California to southern Alaska. The species also occurs on the relatively moist, western slopes of the Rocky Mountains in Washington and southern British Columbia. The western yew is relatively widespread in mature, conifer-dominated rainforests, but it is rarely abundant.
The Florida yew (T. floridana) is a rare, small tree that occurs in northern Florida. The Canadian yew (T. canadensis), also known as ground hemlock or poison hemlock, is relatively widely distributed in coniferous and mixed-wood forests of cool-temperate regions of eastern North America. The European or English yew (T. baccata) of Europe and Asia is a relatively tall species, which can grow to almost 65 ft (20 m) in height.
Economic and ecological importance of yews
Yew wood is very tough and elastic, a consequence of the structural qualities of its elongated, water-conducting cells known as tracheids. The wood of yews is of minor commercial importance. However, it is prized for certain uses in which strength and flexibility are required. During medieval times in western Europe, the wood of European yew trees was favored for the manufacture of bows. Today, bows are mostly made of synthetic materials, but some traditional archers still like to use bows made from yew. Canoe paddles are also sometimes carved from yew wood.
Yews are very popular shrubs in horticulture, probably being used more commonly than any other types of conifers. Various species are cultivated, but the European yew is the most common, and it is available in a number of cultivated varieties, or cultivars. It is quite easy to propagate desired cultivars of yews because these plants will root from stem cuttings, a relatively unusual trait for a conifer.
Yews are commonly used to accent the lines of buildings, or to create interesting entrances around doors. Very attractive hedges can also be developed using dense plantings of yews, which may be sheared to achieve a desired shape.
Sometimes, the dense foliage of taller yews is sheared in interesting ways to create a special visual effect. For example, yews can be trimmed to develop globular or cubed shapes, or to look like animals of various sorts. Yews are best sheared in the late summer.
other yews that are commonly used in horticulture are the Japanese yew (T. cuspidata) and the columnar-shaped T. hicksi and T. hilli, all originally from Asia. These and some other species of yews have been widely introduced into North America and elsewhere as attractive species in horticulture. However, there is no indication that any of these non-native yews have escaped from cultivation and become invasive pests of natural habitats.
Medicinal uses of yews
Yews have widely been recognized as toxic to livestock and humans. In smaller doses, yew has been used as a minor folk medicine in some parts of its range. Example of the medicinal uses of yews include the induction of menstruation, and the treatment of arthritis, kidney disease, scurvy, tuberculosis, and other ailments. However, in recent years, yews have become famous for their use in the treatment of several deadly cancers.
In particular, the dark-brown or purple bark of western yew has been found to contain relatively large concentrations of an alkaloid known as taxol. Taxol has been demonstrated as being an effective treatment against advanced cases of ovarian and breast cancer, two deadly diseases. The use of taxol for these purposes is now approved by the U.S. Food and Drug Administration and by regulatory agencies in other countries. This recently developed use of western yew has led to an essentially insatiable demand for its bark.
The concentrations of taxol in bark are variable, ranging from only one part per million (ppm) to 690 ppm. The concentration of taxol in foliage of western yew is generally smaller, ranging from 12 to 80 ppm. At the present time, taxol is most efficiently extracted from yew bark. Unfortunately, it takes about 14 kg of yew bark to yield only 1 g of taxol, equivalent to three to 12 yew trees per cancer patient treated. Clearly, very large amounts of yew bark must be collected in order to have sufficient material to satisfy the medical and research demands.
Yew is not a commercial species in forestry, and up until about 1989 the species was routinely trashed and usually burned after logging. Today, however, the bark of western yew is routinely stripped from the survivors of the logging aftermath. In addition, the bark is now widely stripped from living western yews in unlogged forests, including old-growth rainforests of the Pacific coast, where this species tends to be most abundant. Sometimes, this process is rather inefficient, and in some cases of poaching, the bark has only been taken from the lower part of taller yew trees, where it can be easily reached. Unfortunately, the removal of its bark usually kills the yews by destroying its vascular system, so bark stripping is a rather wasteful use of the plant. A system has been designed and implemented to regulate the harvest of yew bark, but it is difficult to enforce over extensive areas, and a great deal of poaching occurs.
The enthusiastic collection of western yew bark, while contributing to the successful treatment of some cancer cases, threatens to exhaust the very resource upon which this treatment depends, as it is extremely wasteful and does not aim to conserve the species for future use. This dilemma may be resolved if
KEY TERMS
Aril —A fleshy, often brightly colored covering that partially encases a seed. The aril is edible and is intended to encourage an animal to eat the fruit and thereby disperse the seed.
Cultivar —A distinct variety of a plant that has been bred for particular, agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered to be subspecies.
Part per million (ppm) —A unit of concentration, equivalent to 0.0001%, or one milligram in a kilogram, or one milliliter in a liter.
Sunfleck —A transient patch of sunlight that travels over the forest floor as the Sun arcs overhead during the day.
Taxol —An alkaloid chemical that can be extracted from the bark and other tissues of yews and is active in the treatment of human ovarian and breast cancers.
pharmaceutical biochemists develop an economical method of synthesizing taxol in the laboratory, making the extraction of the alkaloid from western yew bark unnecessary. Alternatively, means could be found to economically extract the taxol found in relatively small concentrations in other parts of the yew, especially the foliage. Another possibility is to establish managed plantations of western yew for the specific purpose of obtaining taxol. This is actually being done, but the western yew is slow growing, and it will take some years before the plantations can be economically harvested.
So far, these relatively sustainable approaches have not been developed to the point where pressure on wild western yews can be relieved, and this plant is being rapidly mined from its natural habitats. Because the western yew is not naturally abundant, its populations will rapidly become depleted, and the species will become endangered in the wild.
Resources
BOOKS
Mitchell, Alan. Trees of North America. California: Thunder Bay Press, 2003.
Bill Freedman
Yew
Yew
Yews are various species of tree- or shrub-sized, woody plants that comprise the conifer family, Taxaceae. All yews are in the genus Taxus, and about seven species are known, occurring in moist, temperate forest habitats in North America , Europe , Asia , and Africa .
Biology of yews
Yews have dark-green colored, rather soft, evergreen, needle-like leaves. Yews usually have separate male and female plants, although sometimes these flowers will develop on separate branches of the same plant . The male flowers are small and inconspicuous, and develop as globular bodies in leaf axils. The female flowers of yews develop in the springtime, and they occur singly and naked in the axis of a leaf. The fruit is a relatively large, hard seed, encased in a bright-red or scarlet, fleshy, cup-like structure known as an aril. The arils are sought by birds as food, which eat and disperse the seeds . The seeds of yews are poisonous to people and other mammals , although the fleshy arils are not. The symptoms of poisoning by yew seeds or foliage are dilation of the pupils, pain , and vomiting, leading to coma and death if the dose is large enough.
The wood of yews is dense, strong, flexible, and resistant to decay. The bark is thin, fibrous, scaly, and dark colored. The young twigs of yews are generally greenish in color .
Yews are slow growing and highly tolerant of shading. As a result, yews can survive in the shade beneath a closed forest canopy. This can be especially true beneath angiosperm trees, which allow the yews to grow relatively freely during the spring and autumn when the trees are in a leafless condition. However, yews also do well beneath conifer trees, as long as occasional gaps in the canopy allow exposure to light as brief sunflecks during the day.
Yews have a dense foliage and so cast a rather deep shade. As a result of this shade, as well as a toxic quality of the yew's leaf litter, few plants will grow beneath a closed canopy of yew tree or shrubs.
Species of yew
Three species of yew grow naturally in North America. The Pacific or western yew (Taxus brevifolia) is the only species that reaches the size of a small tree, typically 19-39 ft (6-12 m) tall, but as tall as 75 ft (23 m). The western yew is a species of the sub-canopy of conifer rain forests of the Pacific coast, ranging from central California to southern Alaska. The species also occurs on the relatively moist, western slopes of the Rocky Mountains in Washington and southern British Columbia. The western yew is relatively widespread in mature, conifer-dominated rainforests, but it is rarely abundant.
The Florida yew (T. floridana) is a rare, small tree that occurs in northern Florida. The Canadian yew (T. canadensis), also known as ground hemlock or poison hemlock, is relatively widely distributed in coniferous and mixed-wood forests of cool-temperate regions of eastern North America. The European or English yew (T. baccata) of Europe and Asia is a relatively tall species, which can grow to almost 65 ft (20 m) in height.
Economic and ecological importance of yews
Yew wood is very tough and elastic, a consequence of the structural qualities of its elongated, water-conducting cells known as tracheids. The wood of yews is of minor commercial importance. However, it is prized for certain uses in which strength and flexibility are required. During medieval times in western Europe, the wood of European yew trees was favored for the manufacture of bows. Today, bows are mostly made of synthetic materials, but some traditional archers still like to use bows made from yew. Canoe paddles are also sometimes carved from yew wood.
Yews are very popular shrubs in horticulture , probably being used more commonly than any other types of conifers. Various species are cultivated, but the European yew is the most common, and it is available in a number of cultivated varieties, or cultivars. It is quite easy to propagate desired cultivars of yews because these plants will root from stem cuttings, a relatively unusual trait for a conifer.
Yews are commonly used to accent the lines of buildings, or to create interesting entrances around doors. Very attractive hedges can also be developed using dense plantings of yews, which may be sheared to achieve a desired shape.
Sometimes, the dense foliage of taller yews is sheared in interesting ways to create a special visual effect. For example, yews can be trimmed to develop globular or cubed shapes, or to look like animals of various sorts. Yews are best sheared in the late summer.
Other yews that are commonly used in horticulture are the Japanese yew (T. cuspidata) and the columnar-shaped T. hicksi and T. hilli, all originally from Asia. These and some other species of yews have been widely introduced into North America and elsewhere as attractive species in horticulture. However, there is no indication that any of these non-native yews have escaped from cultivation and become invasive pests of natural habitats.
Medicinal uses of yews
Yews have widely been recognized as toxic to livestock and humans. In smaller doses, yew has been used as a minor folk medicine in some parts of its range. Example of the medicinal uses of yews include the induction of menstruation, and the treatment of arthritis , kidney disease , scurvy, tuberculosis , and other ailments. However, in recent years, yews have become famous for their use in the treatment of several deadly cancers.
In particular, the dark-brown or purple bark of western yew has been found to contain relatively large concentrations of an alkaloid known as taxol. Taxol has been demonstrated as being an effective treatment against advanced cases of ovarian and breast cancer , two deadly diseases. The use of taxol for these purposes is now approved by the U.S. Food and Drug Administration and by regulatory agencies in other countries. This recently developed use of western yew has led to an essentially insatiable demand for its bark.
The concentrations of taxol in bark are variable, ranging from only one part per million (ppm) to 690 ppm. The concentration of taxol in foliage of western yew is generally smaller, ranging from 12 to 80 ppm. At the present time, taxol is most efficiently extracted from yew bark. Unfortunately, it takes about 14 kg of yew bark to yield only 1 g of taxol, equivalent to three to 12 yew trees per cancer patient treated. Clearly, very large amounts of yew bark must be collected in order to have sufficient material to satisfy the medical and research demands.
Yew is not a commercial species in forestry , and up until about 1989 the species was routinely trashed and usually burned after logging. Today, however, the bark of western yew is routinely stripped from the survivors of the logging aftermath. In addition, the bark is now widely stripped from living western yews in unlogged forests, including old-growth rainforests of the Pacific coast, where this species tends to be most abundant. Sometimes, this process is rather inefficient, and in some cases of poaching, the bark has only been taken from the lower part of taller yew trees, where it can be easily reached. Unfortunately, the removal of its bark usually kills the yews by destroying its vascular system, so bark stripping is a rather wasteful use of the plant. A system has been designed and implemented to regulate the harvest of yew bark, but it is difficult to enforce over extensive areas, and a great deal of poaching occurs.
The enthusiastic collection of western yew bark, while contributing to the successful treatment of some cancer cases, threatens to exhaust the very resource upon which this treatment depends, as it is extremely wasteful and does not aim to conserve the species for future use. This dilemma may be resolved if pharmaceutical biochemists develop an economical method of synthesizing taxol in the laboratory, making the extraction of the alkaloid from western yew bark unnecessary. Alternatively, means could be found to economically extract the taxol found in relatively small concentrations in other parts of the yew, especially the foliage. Another possibility is to establish managed plantations of western yew for the specific purpose of obtaining taxol. This is actually being done, but the western yew is slow growing, and it will take some years before the plantations can be economically harvested.
So far, these relatively sustainable approaches have not been developed to the point where pressure on wild western yews can be relieved, and this plant is being rapidly mined from its natural habitats. Because the western yew is not naturally abundant, its populations will rapidly become depleted, and the species will become endangered in the wild.
Resources
books
Brockman, C.F. Trees of North America. Golden Press, New York: 1968.
periodicals
Daly, D. "Tree of Life." Audubon 70 (1992): 76-85.
Joyce, C. "Taxol: Search for a Cancer Drug." BioScience 43 (1993): 133-36.
Bill Freedman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Aril
—A fleshy, often brightly colored covering that partially encases a seed. The aril is edible and is intended to encourage an animal to eat the fruit and thereby disperse the seed.
- Cultivar
—A distinct variety of a plant that has been bred for particular, agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered to be subspecies.
- Part per million (ppm)
—A unit of concentration, equivalent to 0.0001%, or one milligram in a kilogram, or one milliliter in a liter.
- Sunfleck
—A transient patch of sunlight that travels over the forest floor as the Sun arcs overhead during the day.
- Taxol
—An alkaloid chemical that can be extracted from the bark and other tissues of yews and is active in the treatment of human ovarian and breast cancers.
yew
yew / yoō/ • n. (also yew tree) a coniferous tree (genus Taxus, family Taxaceae) that has red berrylike fruits, and most parts of which are highly poisonous. Its species include the American yew (T. canadensis) and the English (or European) yew (T. baccata).