Global distribution of Tsuga; Fig. 1 from Feng et al. (2021) based on a map from Farjon and Filer (2013). Colored dots mark collections verified by Farjon.
Detailed drawing of Tsuga heterophylla by Matt Strieby (2018.03).
Tsuga mertensiana growing by a lake in Washington [C.J. Earle].
Tsuga dumosa growing on Erlang Shan near Luding, Sichuan [Daniel Winkler].
Tsuga
Hemlock (see Remarks), pruche [Canadian French] (Taylor 1993), ツガ属 (tsuga) [Japanese].
The eleven species in this treatment are:
|
|
Pinus L. sect. Tsuga Endlicher (1847) is often described as including two subgenera, subgen. Hesperopeuce (Lemmon) Ueno (syn. genus Hesperopeuce Lemmon; includes only T. mertensiana), and subgen. Tsuga (includes all other species). The subgenera are distinguished by both cone and foliage characters, but as noted below, are not supported by recent molecular studies. One species formerly described as an aberrant member of this genus, T. longibracteata W.C. Cheng, is now treated in a separate genus Nothotsuga, in some respects intermediate between Tsuga and Keteleeria.
The phlyogenetics of the genus have primarily been revealed in fossil evidence (LePage 2003) and through molecular studies using nuclear, chloroplast, and mitochondrial markers (Havill et al. 2008, Holman et al. 2017, Feng et al. 2021). The fossil record indicates that the genus arose in the Cretaceous and was distributed throughout North America and Eurasia from the Late Cretaceous to the Plio-Pleistocene; thus the extant species constitute a relict group. Molecular clock analysis indicates the extant species can be traced to a late Oligocene origin in North America, dispersing to East Asia via Beringia during the middle Miocene. This is a is remarkably common pattern in the Pinaceae, also being observed in Abies section Amabilis, Larix, Picea, Pinus subsection Strobus, and Pseudotsuga (Wei et al. 2010, Wen et al. 2016, Feng et al. 2021, Qiu et al. 2024). However, the fossil evidence suggests exchange between North America and Asia occurred throughout the Cenozoic via Beringia. Evidence of this exchange and the relict nature of the extant species is seen in molecular studies that consistently place T. caroliniana of the southeastern U.S. sister to one or more of the extant Asian species; the most recent and detailed analysis places it sister to all the Asian species (Feng et al. 2021).
The two western North American species, T. heterophylla and T. mertensiana, are sister to each other, and in turn sister to the eastern T. canadensis, in all molecular studies. The Asian species, however, show a "complex reticulate evolutionary pattern" (Feng et al. 2021) and there has been much debate about their phylogenetics. The most recent work (Feng et al. 2021) shows that T. chinensis as described by recent authorities is paraphyletic, and this has explained much of the disagreement. If the Taiwan populations of T. chinensis are assigned to T. formosana and the western and eastern Chinese populations are assigned to separate taxa, then all taxa have clearly discrete genomes, as shown by multiple samples in nearly all taxa (T. caroliniana and T. ulleungensis each having only one sample). T. forrestii and western populations of T. chinensis are sisters, and they are sister to eastern T. chinensis. T. formosana and T. sieboldii are sisters, in turn sister to the Chinese taxa. T. dumosa is sister to the above Asian species, while T. diversifolia and T. ulleungensis are basal to all other Asian taxa. The largest piece of the puzzle still missing is how to align the two molecular taxa of T. chinensis with the observed morphological variation and the existing nomenclature.
Monoecious evergreen trees with a conic to irregularly ovoid (in some Asian species) crown of horizontal branches, often arranged in flattened 'sprays' and arched downward; leading shoot usually drooping. Bark gray to brown, scaly, often deeply furrowed. Twigs have moderately developed short (spur) shoots; young twigs are flexuous and pendent, roughened by peglike projections persisting after the leaves fall. Leaves are borne singly, persisting several years, ±2-ranked or radiating in all directions, flattened to somewhat angular; abruptly narrowed to a petiolelike base, set on peglike projections, these angled, projected forward, sheath absent; apex acute, rounded or notched; stomata in two bands below; upper surface free of stomata except in T. mertensiana; 1 resin canal. Buds mostly rounded at apex, not resinous. Cotyledons 4-6. Pollen cones <8 mm long, solitary, globose, brown, borne on year-old twigs. Seed cones also borne on year-old twigs, maturing in 5-7 months, shedding seeds and falling soon thereafter or persisting for several years; pendent, ovoid or oblong (long-ellipsoid in T. mertensiana), sessile or nearly so. Cone scales persistent, shape various, thin, leathery, glabrous (pubescent in T. mertensiana), lacking apophysis and umbo; bracts small, included. Seeds ca. 3-5 × 2-3 mm, with numerous small resin vesicles; winged, with wing thin, 5-10 mm. x=12 (Taylor 1993, M. P. Frankis e-mail 1999.02.07).
North America and E Asia, occupying relatively moist climates where water stresses are typically low. Most are conspicuous, if not dominant, members of the communities in which they occur (Taylor 1993).
The largest extant trees are probably in Tsuga heterophylla. The oldest trees are probably T. mertensiana. Historical records also document great size and age in T. dumosa, but such trees have not been reported for a long time. The largest and oldest extant Asian trees are probably in T. sieboldii.
"Hemlock wood is moderately strong and pliable and lacks resin ducts. With the decline of associated species considered superior in commercial value, hemlocks have become important in the timber industry, especially for pulp. Hemlocks are also widely used for horticultural purposes; numerous cultivars have been developed" (Taylor 1993).
The Latin name is from the Japanese name for T. sieboldii. T. canadensis and T. heterophylla were discovered early (before 1800) and until 1855 were simply treated as species of Pinus, but in 1842 T. sieboldii, the southern Japanese hemlock, was described as Abies tsugae. In 1847 Endlicher transferred it into Pinus sect. Tsugae, and then in 1855 Carriere gave us the modern name for the genus.
My friend Michael Frankis (M.P. Frankis, e-mail 1999.02.07) has advised me that the common English name "hemlock" refers to the percieved similarity in the smell of the crushed foliage of T. canadensis to that of the poisonous umbelliferous herb, water hemlock Conium maculatum. The two plants are of course totally unrelated, and Tsuga is not poisonous. I have tested this supposition in my back yard, where I have growing both T. heterophylla and Conium maculatum. My plants do indeed resemble each other in odor, apart from a distinct resinous element in the scent of the Tsuga. I don't know for certain that this resemblance, observed in two plants native to western Washington, remains true in eastern North America or in Britain, but it seems plausible. The Conium leaf, by the way, resembles that of certain Cupressaceae in form (especially Thujopsis), a fact noted by Charles F. Partington in The British Cyclopedia of 1836, who says that the "hemlock spruce" is "so called from its branches in tenuity and position resembling the foliage of the common hemlock." André (1964) also asserted a resemblance between the foliage of Conium and that of the conifer, and that this was the reason the genus name name Conium was applied to poison hemlock. So there are several convergences between conifers and this flowering plant.
We now turn to Roman times, when the plant we now call Picea abies was known simply as Picea, because it was such a common tree in the land of Picea, which we now call Prussia. The common name "spruce" is also derived from the word "Prussia", as is the French name of hemlock, "pruche." But when did they start to give that name to the hemlock?
The first hemlock known to western science was T. canadensis, which would have been encountered by European immigrants to what is now maritime Canada and New England, places where this tree grows near the sea. It is thus likely that doctors or apothecaries, who would have been interested in the medicinal uses of the foliage of both plants, here first noted the resemblance in odor between Tsuga and Conium, and gave the plant its common name. Soon, though, it came to the attention of botanists. Linnaeus in 1763 acknowledged the existence of the species with the name Pinus canadensis, but it is likely that it was known to the botanical world much earlier (by the mid-1600s its presence in the New World would have been noted) and began to be known as "hemlock spruce", a fine example of an English binomial: the adjective (today we might say "epithet") "hemlock" modifying the generic noun "spruce." This gives a clue to the origin of both its French and English names. Speaking only of conifers (not angiosperms), the French domains in the New World were dominated by conifers of the genus Picea, with abundant P. rubens, P. glauca and P. mariana across the landscape. The English domains, on the other hand, were dominated by various species of Pinus. So the common name "hemlock spruce" suggests a northern origin for the term "hemlock", perhaps French, migrating thence to the English. At any rate, the name "hemlock spruce" was in wide use by the time Menzies used it to describe the new western hemlock (T. heterophylla) he found in 1792, although the genus Picea would not be formally described for another 32 years, and the segregation of Tsuga would not occur for an additional 31 years.
André, Jacques. 1964. La résine et la poix dans l'antiquité, technique et terminologie. L'antiquité classique 33(1):86-97.
Farjon, Aljos and Denis Filer. 2013. An Atlas of the World's Conifers. Leiden, Netherlands: Brill Academic Publishers.
Feng, Yuan-Yuan, Ting-Ting Shen, Cheng-Cheng Shao, Hong Du, Jin-Hua Ran, and Xiao-Quan Wang. 2021. Phylotranscriptomics reveals the complex evolutionary and biogeographic history of the genus Tsuga with an east Asian-North American disjunct distribution. Molecular Phylogenetics and Evolution 157:107066. https://doi.org/10.1016/j.ympev.2020.107066.
Gernandt, David S., Susana Magallón, Gretel Geada López, Omar Zerón Flores, Ann Willyard, and Aaron Liston. 2008. Use of simultaneous analyses to guide fossil-based calibrations of Pinaceae phylogeny. International Journal of Plant Sciences 69(8):1086–1099.
Havill, Nathan P., Christopher S. Campbell, Thomas F. Vining, Ben LePage, Randall J. Bayer, and Michael J. Donoghue. 2008. Phylogeny and biogeography of Tsuga (Pinaceae) inferred from nuclear ribosomal ITS and chloroplast DNA sequence data. Systematic Botany 33(3):478–489.
Holman, G., P. Del Tredici, N. Havill, N. S. Lee, R. Cronn, K. Cushman, S. Mathews, L. Raubeson, and C. S. Campbell. 2017. A new species and introgression in eastern Asian hemlocks (Pinaceae: Tsuga). Systematic Botany. 42(4):733–746.
LePage, B. A. 2003. A new species of Tsuga (Pinaceae) from the middle Eocene of Axel Heiberg Island, Canada, and an assessment of the evolution and biogeographical history of the genus. Botanical Journal of the Linnean Society 141:257–296.
Qiu, Xiu-Fei, Yan-Yan Liu, Ge Wu, Cong-Hui Xu, Xin-Quan Liu, Xiao-Yan Xiang, Xiao-Xin Wei, and Xiao-Quan Wang. 2024. Phylogenomic analyses shed new light on the spatiotemporal evolution of global larches: implications for the dynamics of boreal forests. Molecular Phylogenetics and Evolution 202:108240. doi.org/10.1016/j.ympev.2024.108240.
Wei, X.-X., Z.-Y. Yang, Y. Li, and X.-Q. Wang. 2010. Molecular phylogeny and biogeography of Pseudotsuga (Pinaceae): insights into the floristic relationship between Taiwan and its adjacent areas. Molecular Phylogenetics and Evolution 55:776–785.
Wen, J., Z.-L. Nie, and S. M. Ickert‐Bond. 2016. Intercontinental disjunctions between eastern Asia and western North America in vascular plants highlight the biogeographic importance of the Bering land bridge from late Cretaceous to Neogene. Journal of Systematics and Evolution 54:469–490.
Last Modified 2025-02-19
Back to top