Taxus (yew) description

Taxus

Common names

Yew.

Taxonomic notes

In this treatment, a genus of 13 species:

Taxus has always been one of the more "difficult" conifer genera in the sense that, to a first approximation, they all look the same. They were all treated by Pilger (1903) as subspecies of T. baccata. Other authors (e.g. Spjut [2007], who identifies 25 species and over 50 varieties) have erected new species on the basis of even very small morphological differences. Overall, efforts to subdivide the diversity of the genus have generally reached disparate conclusions except in those cases where taxa are clearly discrete on the basis of geography. Only in the 21st Century have molecular investigations finally started to reveal, with objective clarity, the heterogeneity of the genus.

Certain species have long been generally recognized on the basis of their geographic distribution. These include Taxus baccata in Europe and far western Asia, T. canadensis in the NE US and adjacent Canada, T. floridana in US: Florida, T. brevifolia in the far western US and adjacent Canada, T. globosa from Mexico to El Salvador, and T. cuspidata in Japan and adjacent mainland Asia. The remaining species in this treatment are found in the Himalaya, China and through SE Asia into Malesia, and though they have had a more complicated history in the taxonomic literature, they are now generally assigned to T. contorta in the western Himalaya, T. sumatrana in Malesia and tropical SE Asia, and a number of taxa that occur in the eastern Himalaya, subtropical SE Asia, and China, that have frequently overlapping distributions (though they often occur at differing elevations), and that are morphologically similar. It is this portion of the genus, here treated as T. calcicola, T. chinensis, T. florinii, T. mairei, and T. wallichiana, that has only recently been elucidated using a combination of morphological and molecular techniques.

The molecular studies include Hao et al. (2008) addressing the Taxaceae, and Möeller et al. (2013) specifically looking at the Chinese (and vicinity) species. Hao et al. (2008) considered multiple cpDNA markers and one nuclear (ITS) marker in a suite representing most of the widely-recognized taxa in the Taxaceae, in an analysis that confirmed the monophyly of Taxus and placed it sister to Pseudotaxus in a clade sister to Austrotaxus. This result was consistent across the various markers, and was largely consistent with a family-wide morphological analysis performed by Ghmire and Heo (2014) that differed in finding Austrotaxus sister to all the rest of the family. Within the genus, the analysis of Hao et al. (2008) was ambiguous between various molecular markers, but the weight of evidence placed T. baccata, T. brevifolia and T. globosa in a common clade (repeating a similar finding by Li et al. [2001]), and T. canadensis in a clade with T. cuspidata (plausible; the two species have a natural hybrid, T. x hunnewelliana; cf. Collins et al. [2003]). Results for the remaining east Asian/Malesian taxa were more complicated and unclear, partly due to inclusion of a number of hybrid taxa in the analysis, but one interesting point was inclusion of T. contorta sister to the baccata-brevifolia-globosa clade, a biogeographically interesting finding that corroborates findings by Hao et al. (2008) and suggests a common ancestor for T. baccata and T. contorta, which are now separated by the deserts and mountains of northern Iran and Afghanistan.

Möeller et al. (2013) used combined morphological and molecular (cpDNA) evidence to differentiate Taxus chinensis, T. mairei, and T. wallichiana in the eastern Himalaya, China, and Indochina; they also segregate T. florinii from T. chinensis and T. calcicola from T. wallichiana. These latter species are of limited distribution (T. florinii in SW Sichuan and NW Yunnan; T. calcicola on limestone karst near the Gulf of Tonkin); it is debatable whether these would more properly be treated at infraspecific rank, but currently POWO accepts them as species and I do likewise for the sake of simplicity. The analysis of Moeller et al. (2013) found 7 discrete haplotype lineages within this group of taxa, but the haplotypes are generally not exclusive to species, instead varying in frequency across taxa. Conversely, principal components analysis of the morphological data clearly distinguished 5 distinct groups; 4 of these groups corresponded to previously-described taxa, while the authors erected T. calcicola to accommodate the fifth. They also found a number of "atypical" specimens, primarily from areas around the Sichuan basin, that they interpret as providing evidence of past hybridization events between T. chinensis, T. mairei and T. wallichiana.

Finally, a note on nomenclature: in Latin, "Taxus" is a female noun, despite having the "-us" suffix typical of male nouns. Because of this, all botanical names based on Taxus (e.g. Austrotaxus, Cephalotaxus, etc.) are likewise female nouns, and the scientific names assigned to them must reflect that grammatical gender. E.g., the correct name is "Taxus baccata" rather than "Taxus baccatus".

Description

Evergreen trees or shrubs, dioecious (T. canadensis is monoecious). Seed arils resinous. Bark thin, red- or purple-brown, smooth, becoming scaly, fibrous, or corky. Twigs alternate, glabrous, green for the first year or two. Leaves helically inserted, twisted at base so as to appear 2-ranked or distichous; linear or falcate, flattened, flexible, slightly leathery, with a prominent midrib on both upper and lower surfaces, and two pale stomatal bands on the lower side; resin canals absent. Pollen cones axillary, solitary, usually forming double rows along the twig, globose; pollen spherical, small (20-40 µm). Seed cones highly modified, a single ovule producing a solitary seed enclosed within an aril, open at the end; borne axially. Seeds hard; aril at first green, fleshy, ripening to red (sometimes orange or yellow), glutinous. Seedlings with 2-3 cotyledons. Chromosomes x=12 (Eckenwalder 2009, Farjon 2010). The foliage, bark, and seeds—but not the aril—of most Taxus species are toxic due to the presence of taxine (Kingsbury 1964, Cooper et al. 1984); this alkaloid, however, is not found in T. brevifolia (Jones and Lynn 1933).

Most species contain the anti-cancer agent taxol. A study of heartwood constituents of T. baccata, T. brevifolia, T. cuspidata and T. floridana found them to be chemically almost identical (Hartzell 1991).

Distribution and Ecology

Europe: Britain to N Iran. Asia: Afghanistan, Pakistan, India, Nepal, Sikkim, Bhutan, Russia, North Korea, South Korea, Japan, China, Taiwan, Burma, Vietnam, Laos, Philippines, Malaysia, Indonesia. Americas: SE Alaska to California, SE Canada to NE USA, Florida, Mexico, Guatemala, El Salvador. The genus is largely confined to the temperate latitudes of the northern hemisphere, with some intrusion to tropical highlands, the northernmost occurrence being in Norway and the southernmost below the equator in the S Celebes. Plants are found in the understory or canopy of moist temperate or tropical mountain forests. Their elevation ranges from near sea level in northern stations to 3000 m in tropical forests (de Laubenfels 1988).

Distribution data for all species from Conifers of the World, downloaded on 2018.01.26. T. wallichiana shown here includes T. calcicola and T. florinii, because only a few herbarium collections are currently assigned to those species; the majority have instead been assigned to T. wallichiana and detailed review of the herbarium material will be needed to rectify this situation.

Remarkable Specimens

The largest trees are in Taxus baccata. There are few data for most species, but T. brevifolia and T. sumatrana also reach large sizes. Many trees of T. baccata and T. contorta are located at sacred sites and have been preserved for centuries due to their spiritual importance. The oldest specimens are also thought to be in Taxus baccata.

Ethnobotany

Although yew has had many interesting uses in both traditional and modern culture, it is most noteworthy for being a tree of profound spiritual significance. This is best documented for Taxus baccata, but has also been shown for T. brevifolia and T. contorta, and is likely true of some other species as well. The reasons for this likely vary between species, but in general, authors have pointed to the evergreen character that gives the plant the symbolism of eternal life, juxtaposed against the high toxicity of the foliage and bark, which make the plant symbolic of death itself. The latter effect, toxicity, is heightened by the speed of death (yew extracts have been used as suicide decoctions by some cultures, including modern Euroamerican culture) and by the fact that some ungulates, notably deer, can apparently consume yew without ill effects: the poison appears specific to humans and their domestic animals (I say "appears" because it has not been widely tested). The spiritual significance of yew has been nicely explored by Laqueur (2015) as well as by other authors (books shown at right).

In modern times, the yew is justly famous for being able to cure cancer (Bryan 2011). The drug taxol, containing the compound paclitaxel, is found in the vegetative parts of all yews. It provides an extremely effective treatment for breast and ovarian cancer, and is useful in treatment of certain other cancers; it works by blocking mitosis, and thus, tumor growth. This discovery in the 1990s led quickly to global overexploitation of yews to extract paclitaxel from the bark and foliage, pushing a number of species toward "endangered" status. Now paclitaxel is largely produced from plantation-grown yews, especially T. brevifolia, which gives a relatively large yield of the drug; there is also increased interest in possible medical use of other yew extractives (Rao et al. 1995, Sun et al. 2015). On a related front, yews also contain an endophytic fungus, the basidiomycete Ceriporiopsis andreanae (formerly known as Taxomyces andreanae), which also produces paclitaxel; this microbe has been experimentally used to synthesize paclitaxel, but has not yet been used in commercial-scale production (Stierle et al. 1993, Strobel et al. 1993, both cited by Navis 2013; see also Meštrović 2021).

Yew has an extraordinarily high resistance to urban air pollution. Several of the species are used as ornamentals, and there are hundreds of yew cultivars (Hartzell 1991).

"Taxus baccata (English yew) and T. cuspidata (Japanese yew) are best known and documented for toxicity. Cattle have been poisoned by T. canadensis planted in British Columbia, but toxicity of T. brevifolia has not been conclusively recorded (Kingsbury 1964). Although horses, cattle, and humans have been poisoned by ingesting yew leaves and seeds, the fresh foliage of T. canadensis is browsed by deer, and that of T. brevifolia by moose" (Hils 1993).

Aboriginal peoples used the hard, decay-resistant wood of various Taxus species for construction, and for long-lasting tools such as arrow shafts, bows, tool handles, fishhooks, etc. See the species accounts for details.

Observations

None of the species are particularly rare, and most can be found in botanic gardens. Taxus baccata is, within the temperate zone, among the commonest ornamental conifers.

Remarks

Named from the Greek TOXUS, reflective of TOXON (bow) and TOXIKON (poison); a yew extract was used as an arrow poison (Hartzell 1991).

Paleobotany: The oldest recognizable yew is the Triassic Paleotaxus rediviva, found in strata 200 ma old. The mid-Jurassic Taxus jurassica (140 ma old) is more recognizable as a member of Taxus, containing features characteristic of T. baccata, T. cuspidata, and T. brevifolia. A Quaternary yew, Taxus grandis, is probably simply T. baccata (Hartzell 1991).

Pollination is by wind dispersal. Seed dispersal is primarily by birds, which eat the seeds in the aril and subsequently excrete viable seed (de Laubenfels 1988).

Citations

Bryan, J. 2011. How bark from the Pacific yew tree improved the treatment of breast cancer. Pharmaceutical Journal. Available https://www.pharmaceutical-journal.com/news-and-analysis/news/how-bark-from-the-pacific-yew-tree-improved-the-treatment-of-breast-cancer/11084729.article, accessed 2018.01.28.

Collins, D., R. R. Mill, and M. Möller. 2003. Species separation of Taxus baccata, T. canadensis, and T. cuspidata (Taxaceae) and origins of their reputed hybrids inferred from RAPD and cpDNA data. American Journal of Botany 90:175–182.

Cooper, M. R. and A. W. Johnson. 1984. Poisonous plants in Britain and their effects on animals and man. London. [Minist. Agric., Fisheries & Food Ref. Book 161.]

Ghmire, B. and K. Heo 2014. Cladistic analysis of Taxaceae s.l. Plant Systematics and Evolution 300:217-223, doi:10.1007/s00606-013-0874-y

Hao, Da Cheng, Pei Gen Xiao, BeiLi Huang, Guang Bo Ge, and Ling Yang. 2008. Interspecific relationships and origins of Taxaceae and Cephalotaxaceae revealed by partitioned Bayesian analyses of chloroplast and nuclear DNA sequences. Plant Systematics and Evolution 276:89–104. DOI 10.1007/s00606-008-0069-0

Hao da, C., B. Huang, and L. Yang. 2008. Phylogenetic relationships of the genus Taxus inferred from chloroplast intergenic spacer and nuclear coding DNA. Biol Pharm Bull 31:260–265.

Jones, I. and E. V. Lynn. 1933. Differences in species of Taxus. Journal of the American Pharmaceutical Association 22:528-531.

Kingsbury, J. M. 1964. Poisonous plants of the United States and Canada. Englewood Cliffs.

Laqueur, Thomas W. 2015. Beneath the yew tree's shade. Paris Review, October 31. Available https://www.theparisreview.org/blog/2015/10/31/beneath-the-yew-trees-shade/, accessed 2018.01.28.

Li, J., C. C. Davis, P. Del Tredici and M. J. Donoghue. 2001. Phylogeny and biogeography of Taxus (Taxaceae) inferred from sequences of the internal transcribed spacer region of nuclear ribosomal DNA. Harvard Papers in Botany 6: 267-274.

Meštrović, Tomislav. 2021.04.22. Paclitaxel Production. https://www.news-medical.net/health/Paclitaxel-Production.aspx, accessed 2024.01.17.

Möller, Michael, Lian-Ming Gao, Robert R. Mill, Jie Liu, De-Quan Zhang, Ram C. Poudel, and De-Zhu Li. 2013. A multidisciplinary approach reveals hidden taxonomic diversity in the morphologically challenging Taxus wallichiana complex. Taxon 62(6):1161-1177.

Navis, Kelsey. 2013.07.21. Taxomyces andreanae. https://microbewiki.kenyon.edu/index.php/Taxomyces_andreanae, accessed 2024.01.17.

Rao, K. V., J. B. Hanuman, C. Alvarez, M. Stoy, J. Juchum, R. M. Davies, and R.Baxley. 1995. A New Large-Scale Process for Taxol and Related Taxanes from Taxus brevifolia. Pharmaceutical Research 12(7):1003-1010.

Spjut, R. W. 2007. Taxonomy and nomenclature of Taxus. Journal of the Botanical Research Institute of Texas 23:203–289.

Stierle, A., G. Strobel, and D. Stierle. 1993. Taxol and Taxane production by Taxomyces andreanae, an endophytic fungus of Pacific yew. Science 260(5105):214–216.

Strobel, G., A. Stierle, D. Stierle, and W. M. Hess. 1993. Taxomyces andreanae, a proposed new taxon for a bulbilliferous hyphomycete associated with Pacific yew (Taxus brevifolia). Mycotaxon 47:71–80.

Sun, Zhang-Hua, Yu Chen, Yan-Qiong Guo, Jie Qiu, Cui-Ge Zhu, Jing Jin, Gui-Hua Tang, Xian-Zhang Bu, and Sheng Yin. 2015. Isolation and cytotoxicity evaluation of taxanes from the barks of Taxus wallichiana var. mairei. Bioorganic & Medicinal Chemistry Letters 25(6):1240-1243.