Taxus canadensis (Canada yew) description

Taxus canadensis

Marshall 1785

Common names

Canada yew, American yew, ground-hemlock, li du Canada, sapin trainard (Hils 1993).

Taxonomic notes

Syn: Taxus baccata L. subsp. canadensis (Marshall) Pilger 1903; T. baccata L. var. minor Michaux 1803; T. minor (Michaux) Britton ex Small et Vail 1893; T. baccata L. var. procumbens Loudon 1842 (Farjon 1998).

Description

"Shrubs to 2 m, usually monoecious, low, diffusely branched, straggling, spreading to prostrate. Bark reddish, very thin. Branches spreading and ascending. Leaves 1-2.5 cm ´ 1-2.4 mm, pale green abaxially, mostly without cuticular papillae along stomatal bands, dark green to yellow-green adaxially, epidermal cells as viewed in cross section of leaf wider than tall or ± isodiametric. Seed somewhat flattened, 4-5 mm. 2n = 24. Seeds maturing late summer - early fall" (Hils 1993).

Although long-distance dispersal is commonly accomplished by birds, Canada yew commonly reproduces by layering, forming a continuous population of clones. The connections between ramets usually rot (Sullivan 1993, citing Allison 1992).

Distribution and Ecology

Canada: extreme SE Manitoba, Ontario, Québec, Prince Edward Island, New Brunswick, Nova Scotia, Newfoundland; France: St. Pierre and Miquelon; USA: Connecticut, Illinois, Indiana, Iowa, Kentucky, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, Ohio, Pennsylvania, Rhode Island, Tennessee, Vermont, Virginia, Wisconsin, and West Virginia; at 0 - 1500 m elevation as an understory shrub in rich forests (deciduous, mixed, or coniferous), or in bogs, swamps, gorges, ravine slopes, and rocky banks (Hils 1993).

Distribution data for all species native to the Americas, from Conifers of the World, downloaded on 2018.01.26.

Hardy to Zone 4 (cold hardiness limit between -34.3°C and -28.9°C) (Bannister and Neuner 2001). Climate in the range of Canada yew is humid and continental. Soils are podzols, optimally well-drained slightly acidic silt loams, but the species also occurs near bogs (Sullivan 1993).

Ecologically, Canada yew can be described as an understory shrub of late successional forests. This is also the principal habitat of western yew, Taxus brevifolia, and is a common habitat for all species in the genus. Like other yews, it is a very shade-tolerant species. Within this habitat it primarily reproduces asexually, by layering, thus stands of it normally form a fairly continuous, sparse to dense ground cover, with most plants normally less than a meter tall (photos at right). The species does not occur in early seral communities and when found in mature forests, usually consists of individual plants in relatively shady microsites. This suggests that regeneration by seed is primarily from the droppings of birds that perch on trees in forests that are beginning to enter a late successional condition. Sullivan (1993) states that "Canada yew populations migrate; they increase in size by layering, and die back in older portions of the genet, which then allows other plants to come in;" this suggests that individual ramets do not attain great ages, although a genet might persist until a sufficiently severe disturbance eliminates its late successional forest environment.

In view of its late successional forest dependency, Canada yew is highly intolerant of disturbance, being readily extirpated by logging or fire (Sullivan 1993).

Sullivan (1993) lists the species as an understory shrub in the following forest cover types:

Pinus banksiana (jack pine)
Abies balsamea (balsam fir)
Picea mariana (black spruce)
Picea mariana—Larix laricina (black spruce - tamarack)
Prunus pensylvanica (pin cherry)
Betula populifolia—Acer rubrum (gray birch—red maple)
Pinus strobus—Quercus rubra—Acer rubrum (white pine—northern red oak—red maple)
Pinus strobus (eastern white pine)
Pinus strobus—Tsuga canadensis (white pine - hemlock)
Tsuga canadensis (eastern hemlock)
Tsuga canadensis—Betula alleghaniensis (hemlock—yellow birch)
Acer saccharum—Fagus grandifolia—Betula alleghaniensis (sugar maple - beech - yellow birch)
Acer saccharum—Tilia americana (sugar maple—basswood)
Acer saccharum (sugar maple)
Prunus serotina—Acer spp. (black cherry—maple)
Picea rubens—Betula alleghaniensis (red spruce - yellow birch)
Acer saccharum - Fagus grandifolia (red spruce—sugar maple—beech)
Picea rubens (red spruce)
Picea rubens—Abies balsamea (red spruce—balsam fir)
Picea rubens—Abies fraseri (red spruce—Fraser fir)
Betula papyrifera—Picea rubens—Abies balsamea (paper birch—red spruce—balsam fir)
Chamaecyparis thyoides (northern white-cedar)
Larix laricina (tamarack)
Fagus grandifolia—Acer saccharum (beech—sugar maple)
Acer rubrum (red maple)

Common understory associates in many forest types include mountain maple (Acer spicatum), striped maple (A. pensylvanicum), beaked hazelnut (Corylus cornuta), leatherwood (Dirca palustris), prickly gooseberry (Ribes cynosbati), prickly currant (R. lacustre), red currant (R. triste), red raspberry (Rubus idaeus), serviceberries (Amelanchier spp.), American fly honeysuckle (Lonicera canadensis), partidgeberry (Mitchella repens), and scarlet elder (Sambucus pubens) (Sullivan 1993, citing Dansereau 1959, Darby 1973 and Stearns 1951). In addition to the above-mentioned species, shrub layer associates in late successional eastern hemlock forests include alternate-leaved dogwood (Cornus alternifolia) and hobblebush (Viburnum alnifolium) (Sullivan 1993, citing Nichols 1935).

Canada yew engages in some interesting ecological interactions. Where it forms dense ground cover, it apparently prevents establishment of Abies balsamea (Sullivan 1993, citing several authors). It is highly preferred as browse for moose (Alces alces) and white-tailed deer (Odocoileus virginianus) (Sullivan 1993, citing Krefting 1974), but is intolerant of such browsing and can thereby be extirpated (Sullivan 1993, citing several authors). Since moose were introduced to Isle Royale, Michigan, Canada yew has nearly been extirpated there (Sullivan 1993, citing several authors), and its widespread decline in Wisconsin has been attributed to excessive deer browsing (Fewless [nd]). The fleshy aril of Canada yew is eaten by many birds, including ruffed grouse (Bonasa umbellus), cedar waxwing (Bombycilla cedrorum), and robin (Turdus migratorius) (Sullivan 1993, citing several authors). These birds constitute the primary agent of dispersal for the seeds.

Remarkable Specimens

Some wild plants are said to reach 3.7 m tall and have an arboreal growth form (Hartzell 1991). There are no age data. The plant rarely has a stem of much substance, and as noted above, older plants tend to die and be replaced by younger.

Ethnobotany

Although all parts of the plant except the aril are at least mildly poisonous, it was widely used by native Americans: "Abenaki infused the leaves for rheumatism. Algonquin boiled the needles with wild cherry for rheumatism, taking the tea after childbirth. Chippewa employed the twig decoction, externally or internally, for rheumatism. Malecite employed the plant to bring out clots and alleviate pain following childbirth. Canada's Maritime employed yew for afterbirth, clots, fever, pain, and scurvy. Menominee steamed the plant in herbal sweat baths for numbness, paralysis, and rheumatism. Micmac used it for bowel ailments, fever, and scurvy. Montagnai use it with Lycopodium for debility and fever. Ojibwa used leaf decoction for arthritis. Penobscot steeped the leaves for colds. Potawatomi used the leaf decoction as a diuretic, for gonorrhea. Tete-de-Boule infused the twigs with ash for dysmenorrhea and stomachache" (Duke 1986, cited in [Hartzell 1991]).

The Ontario Ministry of Natural Resources (2009) reports:

In 1960, the United States Department of Agriculture and the National Cancer Institute started testing plants for chemicals that have the potential to fight cancer. Paclitaxel was identified as the active anti-cancer chemical in the bark of Pacific yew [Taxus brevifolia] but the demand for paclitaxel was greater than the supply of Pacific yew. Paclitaxel continues to be difficult to produce by synthetic means.

To help overcome this shortage, all yew species were investigated as potential paclitaxel sources. This led to the discovery that Canada yew was a good source of paclitaxel as well as two other taxanes that can be used to synthesize paclitaxel. Given the high levels of taxanes in Canada yew, this species could become one of the most valuable natural sources of taxanes for the pharmaceutical industry.

In response, commercial yew harvest began in Ontario in 2003 with a harvest of 5,000 kg and by 2005 had increased to a harvest of 400,000 kg. Both the European Union and the United States Food and Drug Agency regulations require that plant material used for drug production be harvested in a sustainable manner. An audit of some harvesters in Ontario concluded that they were using sustainable harvest practices, but the regulations are difficult to enforce. Some yew supply companies operating in Ontario are voluntarily seeking an independent certification which confirms that their yew harvest practices are sustainable. Supply companies that are not certified may find themselves at a disadvantage in the international market (Ontario Ministry of Natural Resources 2009). It remains possible that wild Canada yew populations may be being depleted.

Prior to the discovery of paclitaxel, Euroamerican use of Canada yew was very limited. All parts of Canada yew, except for the aril, are poisonous to horses and cattle (Sullivan 1993, citing Chapman and Bessette 1990). It is planted as an ornamental but is more often used as parental stock for the formation of new hybrids. It is not as versatile as other species of yew for ornamental purposes. Numerous horticultural varieties are available (Sullivan 1993). Hardy in Zones 2 to 6.

Canada yew has not been used in dendrochronology.

Observations

The late successional forest habitats that are optimal for this species are getting hard to find. Reproducing populations have been documented at Acadia, Isle Royale, Shenandoah, and Voyageurs National Parks in the U.S., and photos here show the extensive stand near the Miner's Falls trailhead at Pictured Rocks National Lakeshore in Michigan. One of the best and most easily-encountered populations is on South Manitou Island, a unit of the Sleeping Bear Dunes National Lakeshore easily reached by ferry. Mark Hedden (email 2017.07.10) reports that "no cervids have yet managed to swim across to this island (although white-tails were introduced to its larger neighbor, North Manitou Island, in the 1920s), and so the yews grow in profusion and up to 10 feet high, and can easily be seen right off the trail less than two miles from the ferry dock." The island also has excellent old-growth stands of large Thuja occidentalis, with some trees over a meter in diameter.

Remarks

The epithet refers to Canada, and Marshall (1785) gives the common name as "Canadian yew".

Citations

Allison, Taber D. 1992. The influence of deer browsing on the reproductive biology of Canada yew (Taxus canadensis Marsh.). Oecologia 89(2):223-228.

Chapman, William K., and Alan E. Bessette. 1990. Trees and shrubs of the Adirondacks. Utica, NY: North Country Books, Inc. 131 p.

Dansereau, Pierre. 1959. The principal plant associations of the Saint Lawrence Valley. No. 75. Montreal, Canada: Contrib. Inst. Bot. Univ. Montreal. 147 p.

Fewless, Gary. [no date]. Trees of Wisconsin: Taxus canadensis, American yew. http://www.uwgb.edu/BIODIVERSITY/herbarium/gymnosperms/taxcan01.htm, accessed 2009.03.28.

Krefting, Laurtis W. 1974. The ecology of the Isle Royale Moose with special reference to the habitat. Tech. Bull. 297, Forestry Series 15. Minneapolis, MN: University of Minnesota, Agricultural Experiment Station. 75 p.

Leak, W. B. 1973. Species and structure of a virgin northern hardwood stand in New Hampshire. Res. Pap. NE-181. Upper Darby, PA: USDA Forest Service, Northeastern forest Experiment Station. 4 p.

Marshall 1785. Arbustrum Americanum.... Philadelphia: Crukshank. P. 151.

Nichols, G.E. 1935. The hemlock-white pine-northern hardwood region of eastern North America. Ecology. 16(3): 403-422.

Ontario Ministry of Natural Resources. 2009. Canada Yew Status. http://www.mnr.gov.on.ca/en/Business/SORR/2ColumnSubPage/STEL02_165820.html, accessed 2009.03.28, now defunct.

Stearns, Forest. 1951. The composition of the sugar maple-hemlock-yellow birch association in northern Wisconsin. Ecology. 32(2): 245-265.

Sullivan, Janet. 1993. Taxus canadensis. In: Fire Effects Information System, USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. https://www.fs.fed.us/database/feis, accessed 2019.03.01.