Cypress (Eckenwalder 1993).
Cupressus has been a hot topic in conifer systematics through most of the 21st Century, with various proposals to subdivide it into multiple genera. I will not belabor the history of this controversy, but will summarize it as follows:
The relevant work of Mao et al. is shown in the cladogram at left, which is entirely based upon evidence from molecular data, i.e., evidence derived from sequencing portions of the DNA from each species represented. The cladogram shows degrees of similarity between existing taxa, but it can also be interpreted as a representation of the phylogeny of these taxa. Indeed, Mao et al. assign dates to a number of the important phylogenetic milestones in the species' evolution, based on variants of the "molecular clock" hypothesis that, subject to certain assumptions, changes in the genetic code accumulate at a relatively constant rate.
The molecular data used to generate the cladogram were processed in a very rigidly specified, unbiased manner to produce the cladogram. However, past experience with the Cupressaceae has shown that measured DNA data are only partial estimators of actual genetic relationships; consequently, interpretation of relationships between genera within the family, and relationships between species in each genus, has changed continuously over the years as new DNA data became available. Sometimes these interpretations have changed because different types of genetic material have been analyzed (for instance, chloroplast vs. nuclear ribosomal material); sometimes they have changed because different portions of the genome have been studied. The interested reader can see this by reviewing the studies by Gadek and Quinn (1993), Brunsfeld et al. (1994), Gadek et al. (2000), Little et al. (2004), Little (2006), and Mao et al. (2010); many other studies show a comparable progression of understanding based on analysis of genetic material in Juniperus.
Update: in late 2012, Terry et al. (2012) published a new molecular analysis covering much the same ground as Mao et al. (2010) but focusing on relationships between the New World taxa, with outgroup comparisons to Old World taxa and several species of Juniperus. Their cladograms are overall quite similar to those of Mao et al. (2010), but provide more detail to elucidate relationships within the New World taxa.
I have chosen here to broadly define Cupressus, and to reduce Hesperocyparis, Callitropsis and Xanthocyparis to synonymy, for two principal reasons. First, it isn't possible to distinguish Hesperocyparis from the old world Cupressus on the basis of morphology; the distinction is made by applying a complex analytical algorithm to a large molecular data set. I do not know of any other case where someone has tried to assert the existence of two separable genera on the basis of such an abstract, artificial process. Second, and perhaps even more important, it is not necessary to subdivide Cupressus. Although the species occupy very diverse ecological settings, ranging from dominant trees of the temperate rainforest to shrubby trees of the high mountains and subtropical deserts, yet still if you confine your gaze to their cones and foliage, then they all resemble each other much more than any of them resembles another genus in the Cupressaceae. Breaking apart Cupressus makes about as much sense as breaking apart Juniperus, or any other large conifer genus for that matter. It could be done, but it would be inconsistent with the existing norms for distinguishing between genera within the Cupressaceae.
Accordingly, Cupressus is here defined to have four subgenera (none of which have yet been formally described):
Subgenus Cupressus includes all but one (C. vietnamensis) of the species native to the Old World:
The analysis by Mao et al. (2010) indicates very high confidence that within this group, C. funebris is sister to a clade containing all of the other species.
Subgenus Hesperocyparis includes all but one (C. nootkatensis) of the species native to the New World:
The analyses by Mao et al. (2010) and Terry et al. (2012) indicates very high confidence that within this group, C. bakeri is sister to a clade containing all of the other species. Apart from C. bakeri, there are three well-supported clades: one contains only C. macnabiana; another contains C. goveniana, C. macrocarpa, C. nevadensis, and C. sargentii (northern species); and a third contains the remaining species named above (southern species) (Terry et al. 2012).
Subgenus Callitropsis is monotypic, containing only C. nootkatensis. (Callitropsis, when described as a genus by Oersted , was monotypic for nootkatensis.)
Subgenus Xanthocyparis is monotypic, containing only C. vietnamensis.
I now turn to the other debate, regarding the number of species in the genus.
Cupressus as described here contains 25 species, all of which have been described as species but many of which have been reduced by some authors to subspecies or varieties on debatable grounds. The large range of opinion occurs because cypresses generally occur in small isolated populations distinguished by small differences, interpreted by some as valid at species rank, but by others only at varietal or subspecific rank. Analyses performed in the 1990s using morphological and chemical data (e.g. Frankis 1992, Eckenwalder 1993, Farjon 2005) tended to "lump" taxa into a relatively small number of species (as few as 12). Conversely, the more narrow species concept was early advocated by Wolf (1948) in his monograph on the New World cypresses and was later supported by Rushforth (1987) and, for the California species, by Lanner (1999). A more recent analysis using morphological, chemical and multiple lines of molecular/genetic data (Little 2006) also judged that many of the isolated populations are sufficiently distinct to be regarded as discrete species.
I have chosen, on the basis of these findings, to recognize certain taxa as good species, while retaining others as subspecies or varieties. The close relationship between the varieties of C. goveniana appears to be well supported, and is here retained. However, taxa that have been treated as varieties of C. arizonica in recent treatments (Eckenwalder 1993, Farjon 2005) all may warrant species rank, or more particularly, may be more closely related to other species (such as C. lusitanica and C. guadalupensis) but have not yet been so described. A similar problem applies to C. forbesii, formerly treated as a variety of C. guadalupensis but now seeming to be just as closely related to C. stephensonii. Indeed, these taxa represent a significant, persistent taxonomic problem that dogs the taxa here treated as C. arizonica, C. benthamii, C. forbesii, C. guadalupensis, C. lusitanica, C. montana, C. nevadensis and C. stephensonii: namely, that all of these taxa appear to represent a species complex rooted in northwest Mexico, an area which has not been adequately studied. Fully understanding how these taxa are related, and whether any of them can be meaningfully distinguished as separate species, awaits comprehensive study of the many geographically distinct Cupressus populations in this vast area.
Trees or large shrubs, evergreen. Branchlets terete or quadrangular, in decussate arrays in most species; flattened (comblike), superficially resembling Chamaecyparis or Thuja in others. Leaves opposite decussate in 4 ranks, rarely in alternating whorls of 3 in 6 ranks. Adult leaves appressed to divergent, scalelike, rhomboid, free portion of long-shoot leaves to 4 mm; abaxial gland present or absent. Pollen cones with 4-10 pairs of sporophylls, each sporophyll with 3-10 pollen sacs. Seed cones maturing in (?1-)2 years, exact maturation period poorly researched, mostly varying from about 16-25 months after pollination; generally persisting closed many years or until opened by fire, but opening on maturity, and falling soon after seed release, in a few species; globose or oblong, 8-43 mm; scales persistent, (2)3-6(7) opposite decussate pairs, valvate, peltate, thick and woody, the terminal pair either fused or open with a small central columella (varying from cone to cone on a single tree). Seeds (3)5-20 per scale, lenticular or faceted, narrowly 2-winged; cotyledons 2-5. x= 11 (Eckenwalder 1993, Frankis 1999).
Cool north temperate regions to the northern subtropics: Canada, USA, Mexico and adjoining Central America, NW Africa, Middle East and eastward along the Himalaya to SW & Central China and Vietnam.
There are few age data on Cupressus in general. Based on habitat (many conifers native to desert mountains live a very long time), C. dupreziana may attain great ages (Frankis 1999).
Several species have proven useful in climate reconstruction, archeological dating, and ecological studies. The genus appears to be less intractable than most of the Cupressaceae with regard to such problems as poor ring boundaries, false rings, and poor circuit uniformity. See the individual species for details.
Several species are of horticultural importance; fastigiate forms of C. sempervirens, C. duclouxiana and C. funebris have been cultivated for ornament for several thousand years in the Mediterranean region and S China respectively, and the highly decorative weeping C. tortulosa has been tended for many centuries around Buddhist temples in Sikkim, Bhutan, Assam and nearby areas of Tibet and India.
The wood is valued for its sweet scent and resistance to decay. Famous uses of the wood (of C. sempervirens) include Noah's Ark (The Bible, Genesis 6:14), and the doors to St. Peter's, Vatican City, Rome, which were still sound after 1,100 years' use (Loudon, 1838; reference misplaced).
See the species accounts. Many temperate zone arboreta contain good collections, too.
The genus name is from the Roman name for C. sempervirens, itself a loan word via Greek Kuparissos from Hebrew Gopher (Frankis 1992).
Frankis, M.P. 1992. Cupressus. In: Griffiths et al. (eds) The New RHS Dictionary of Gardening 1: 781-783.
Frankis, M.P. 1999. Contributions based on personal experience, via e-mail, 1999.02.03.
Ørsted, A.S. 1864. Frilands-Trøvøxten i Danmark, vol. 1, pp. 17-18. Copenhagen. This is an incredibly obscure publication; the relevant text has been quoted by Maerki and Frankis (2012).
Terry, R.G., J.A. Bartel, and R.P. Adams. 2012. Phylogenetic relationships among the New World cypresses (Hesperocyparis; Cupressaceae): evidence from noncoding chloroplast DNA sequences. Plant Systematics and Evolution DOI: 10.1007/s00606-012-0696-3.
This page co-edited with M.P. Frankis, 1999.02.
The Cupressus Conservation Project provides a wealth of information on old and new world Cupressus, including a taxonomic review, historical accounts, cone photographs, and various other pertinent information.
Debreczy, Z., K. Musial, R.A. Price, and I. Racz. 2009. Relationships and nomenclatural status of the Nootka cypress (Callitropsis nootkatensis, Cupressaceae). Phytologia 91(1):140-159.
Farjon (2005) provides a detailed account.
Farjon, A. 2009. Do we have to chop up the cypresses? Conifer Quarterly 26(4):12-17.
Goggans, J.F. and C.E. Posey. 1968. Variation in seeds and ovulate cones of some species and varieties of Cupressus. Circ. Agric. Exp. Sta., Alabama 160:1-23.
Masters, M.T. 1896. A general view of the genus Cupressus. Journal of the Linnaean Society, Botany 31: 312-363. http://www.cupressus.net/Masters.html, courtesy of the Cupressus Conservation Project website.
Silba, J. 1998. A monograph of the genus Cupressus L. Journal of the International Conifer Preservation Society 5(2):1-98.
Last Modified 2012-11-23