|Taxus brevifolia (Pacific Yew) foliage and fruit|
Taxus brevifolia (Pacific yew or western yew) is a conifer native to the Pacific Northwest of North America. It ranges from southernmost Alaska south to central California, mostly in the Pacific Coast Ranges, but with isolated disjunct populations in southeast British Columbia (most notably occurring on Zuckerberg Island near Castlegar) and in north to central Idaho.
The Pacific yew is a small to medium-sized evergreen tree, growing 10–15 m tall and with a trunk up to 50 cm diameter, rarely more. In some instances, trees with heights in excess of 20 m occur in parks and other protected areas, quite often in gullies. The tree is extremely slow growing, and has a habit of rotting from the inside, creating hollow forms. This makes it difficult and sometimes impossible to make accurate rings counts to determine a specimen's true age. Often damaged by succession of the forest, it usually ends up in a squat, multiple leader form.
It has thin scaly brown bark, covering a thin layer of off-white sap wood with a darker heartwood that varies in color from brown to a magenta/purplish hue. The leaves are lanceolate, flat, dark green, 1–3 cm long and 2–3 mm broad, arranged spirally on the stem, but with the leaf bases twisted to align the leaves in two flat rows either side of the stem except on erect leading shoots where the spiral arrangement is more obvious.
The seed cones are highly modified, each cone containing a single seed 4–7 mm long partly surrounded by a modified scale which develops into a soft, bright red berry-like structure called an aril, 8–15 mm long and wide and open at the end. The arils are mature 6–9 months after pollination. The seeds contained in the arils are eaten by thrushes and other birds, which disperse the hard seeds undamaged in their droppings; maturation of the arils is spread over 2–3 months, increasing the chances of successful seed dispersal. The male cones are globose, 3–6 mm diameter, and shed their pollen in early spring. It is mostly dioecious, but occasional individuals can be variably monoecious, or change sex with time.
Pacific yew grows in varying types of environments; however, in drier environments it is mostly limited to stream side habitats, whereas in moist environments it will grow up onto slopes and ridgetops. Pacific yew is shade tolerant; however it can also grow in sun. The tree's shade tolerance allows it to form an understory, which means that it can grow along streams providing shade to maintain water temperature.
Varieties of Pacific yew (Taxus brevifolia)
Taxus brevifolia var. reptaneta. Although T. brevifolia is typically a tree as described above, T. brevifolia var. reptaneta is a shrub variety that generally occurs in the mid to upper elevation range of the typical variety, 3,500 to 4,000 ft (1000–1219 m) at its southernmost occurrence in the Klamath Mountains region, and at lower elevations further north. It is distinguished from young trees of the typical variety (var. brevifolia) by its stems initially creeping along the ground for a short distance before ascending (curving) upwards and by the branches growing off to one side of the stem, usually the upper side. The epithet reptaneta is from the Latin reptans which means “creeping, prostrate, and rooting,” which is exactly what this variety does; in rooting it forms yew thickets; hence, the epithet reptaneta (etum means collective place of growth) and hence the common name, thicket yew. Unlike the typical variety, thicket yew grows in abundance on open sunny avalanche shoots or ravines as well as in the forest understory. It also occurs along forest margins. In northwestern Montana, a variant of the thicket yew does not ascend upwards; rather, it remains along the ground. This is probably the ancestral form; the upright form with branches along the upper side would be the expected growth pattern that might evolve from one with stems that strictly creep along the ground since branches can only arise from the upper surface.
Taxus brevifolia var. reptaneta has been arbitrarily indicated synonymous with typical yew, T. brevifolia (var. brevifolia); there are no studies to support this view. Even though the two varieties may be genetically distinct, some botanists recognize species or varieties only if they have different geographically ranges. For example, T. mairei var. speciosa, which occurs with the typical variety in southern China in 10 of 13 provinces, was rejected because “there is no geographic reason” for recognizing it even though it appears genetically distinct.
Taxus brevifolia var. reptaneta has also been proposed to be elevated to subspecies status without justification or explanation. Such a change would likely cause considerable confusion in view of the subspecies rank having already been used in the genus Taxus for defining geographically separated subspecies of a single species (T. baccata). Further, it has been recommended that taxonomists be strongly discouraged from “elevating a ‘variety’ to a ‘subspecies’ unless there is sufficient scientific evidence to warrant such an elevation,” and that “it is crucial to provide continuity.”
Taxus brevifolia var. polychaeta. Typical Taxus brevifolia, like most species in the genus, usually produces a single ovule on a complex scaly shoot, composed of a primary shoot and a secondary short shoot. To the casual observer they appear as one funnelform shoot with an ovule at the apex. However, Taxus brevifolia var. polychaeta differs from var. brevifolia in producing a relatively longer primary shoot with as many five secondary shoots. The epithet, polychaeta, is in reference to the primary shoot resembling a polychaete worm; hence, its common name “worm cone yew.” Variety polychaeta appears to be relatively rare. It may have been extirpated from the type locality—around Mud Bay near Olympia, Washington—as a result of urban expansion. It is also known from northern Idaho and Sonoma County, California.
As in the case with thicket yew, worm yew has been indicated to be the same as the typical variety, but again there are no specific studies to support this conclusion. The authority of thicket yew and worm cone yew has been involved in the study of the genus Taxus for 25 years at the time the varieties were described.
Traditionally, the wood was used by native Americans to make bows and paddles for canoes, in addition many other items from daily life. The Japanese have also used the wood for decorative purposes.
Discovery of taxol
The chemotherapy drug paclitaxel (taxol), used in breast, ovarian, and lung cancer treatment, is derived from Taxus brevifolia. As it was already becoming scarce when its chemotherapeutic potential was realized, the Pacific yew was never commercially harvested from its habitat at a large scale; the widespread use of the paclitaxel (taxol) was enabled when a semi-synthetic pathway was developed from extracts of cultivated yews of other species. Unlicensed pharmaceutical production use of closely related wild yew species in India and China may be threatening some of those species.
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Media related to Taxus brevifolia at Wikimedia Commons
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