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A new name for macrocarpa, and for quite a few others?

Derrick Rooney, New Zealand Tree Grower November 2013.

Recent developments in molecular and DNA science have led to significant advances in medical research and in the understanding of many genetic disorders in humans. They have also resulted in similarly large advances in knowledge of the genetics and molecular structure of plants. This in turn has led to new understandings of the evolutionary inter-relationships of plant species.

Taxonomists in the past relied on morphological characteristics − the physical and reproductive ones rather than genetic factors when classifying plants in families, genera, and species. Until very recently there was no reliable way of factoring in genetics. As might be expected, the new knowledge has overturned some of the long-held conventions on the relationships between plants. An example is New Zealand beeches where it has been revealed that the New Zealand beeches are probably more closely related to the northern hemisphere birches than to the northern beeches.

Several economically important groups of plants have been affected by these advances in the understanding of genetics and evolution. One affected group of particular economic importance and interest to farm foresters is the cypresses.

The first new genus

In the last decade or so, as knowledge has evolved and previously unknown genetic relationships have been revealed, major taxonomic changes have occurred in the cypress family. The process could be said to have begun with the discovery in 1998 of a previously unknown conifer in northern Vietnam.

After detailed study at the Royal Botanic Gardens, Kew, it was placed in a new genus named Xanthocyparis, translated as golden cypress, in 2002. The Nootka cypress, which had been resting uneasily in the genus Chamaecyparis for many years before being transferred back to its original genus Cupressus at the end of the 20th century, was transferred again to the new genus. This resulted in Xanthocyparis becoming a genus with two species which were widely separated geographically.

Nootka cypress is one of the parents of the Leyland cypress. The Monterey cypress, then known as Cupressus macrocarpa, was the other parent. As a result of this discovery, Aljos Farjon, head of the temperate section of the herbarium at Kew, erected a new name for the Leyland cypress ×Cuprocyparis.

First past the post

The name of the new genus, Xanthocyparis, was appropriate, given the golden colour of the wood of the two species. However, two years later, researchers discovered that the name Callitropsis for the Nootka cypress had been published in Canada in 1865 by a Danish botanist, but for reasons now unknown had sunk into obscurity.

Under the first past the post rule in the international code regulating the naming of plants, Callitropsis therefore became the valid name of the two golden cypresses. In 2006, further research led American taxonomists to remove all the American cypresses from Cupressus and place them in Callitropsis. Farjon’s name for the Leyland cypress became redundant.

On the evidence published at the time, this seemed like a sound move. However, as research continued knowledge increased, and in 2009, mainly on the basis of molecular and DNA studies, taxonomists in the United States transferred all the New World cypresses to a completely new genus called Hesperocyparis. This name translates, perhaps appropriately, as western cypress.

All cypresses affected

The change affected all the familiar American cypresses and their interspecific hybrids grown for timber or shelter, including the macrocarpa or Monterey, Mexican, Arizona, and Guadalupe cypresses, and most of the species grown for amenity purposes. Only nine Old World species from the Mediterranean, the Himalaya, and China remained in the genus Cupressus. This now includes only C. atlantica, chengiana, duclouxiana, dupreziana, funebris, gigantea, sempervirens, tortulosa, and torulosa. The name C. tortulosa is the elegant weeping tree better, but wrongly, known in cultivation as C. cashmeriana.

Hesperocyparis includes Hh. arizonica, bakeri, benthamii, forbesii, glabra, goveniana, guadalupensis, lusitanica, macnabiana, macrocarpa, montana, nevadensis, sargentii and stephensonii. The Nootka and Vietnamese golden cypresses remain in Callitropsis and Xanthocyparis respectively. These are now both monotypic genera. A monotypic genus is one that contains only one species.

World division

Currently the botanical world appears to be divided over the latest name change. The name Hesperocyparis has been widely accepted in many countries, including Australia, but its status is still listed by the Royal Botanic Gardens as ‘unresolved’, and an international group of six taxonomists, including Aljos Farjon, has recommended that the names Cupressus, Callitropsis and Xanthocyparis be conserved until their evolutionary background has been resolved.

However, Hesperocyparis is now used for the New World cypresses in the United States Department of Agriculture database of plant names. Silviculturists, horticulturists, foresters, farmers, and eventually nurseries will probably have to get used to this name because it appears to be here to stay. As for the Leyland cypresses, they appear to be in limbo until someone comes up with an acceptable and pronounceable compound name for them.

Chronological sequence

This is the chronological sequence of name changes −

  • 1926 Intergeneric name ×Cupressocyparis, erected for the Leyland cypress by W. Dallimore, curator of the British National Pinetum at Bedgebury.
  • 2000 Nootka cypress, one of the parents, transferred to Cupressus, making the compound name unnecessary. Leyland cypress becomes Cupressus × leylandii.
  • 2002 New generic name Xanthocyparis erected for the Nootka cypress. New compound name, × Cuprocyparis, erected for the Leyland cypress and other hybrids between the Nootka cypress and American cypress species by A Farjon, RBG, Kew.
  • 2004 Older name Callitropsis, published in 1865, discovered for the Nootka cypress. The name Xanthocyparis is dropped.
  • 2006 All North American cypresses, including the macrocarpa, transferred from Cupressus to Callitropsis. Leyland cypress becomes Callitropsis × leylandii.
  • 2009 All North American cypresses except the Nootka cypress transferred from Callitropsis to a new genus, Hesperocyparis. Only the nine Old World species remain in the genus Cupressus. Nootka cypress reverts to Callitropsis, Vietnamese golden cypress to Xanthocyparis.
  • 2011 A group of six taxonomists from Finland, the United States, and Britain recommended that the names Cupressus, Callitropsis, and Xanthocyparis be conserved until the evolutionary history of the Cupressaceae has been resolved.
  • 2013 Scientific names for Leyland and other New World cypresses remain unresolved. The most recent validly published name for Leyland and other hybrids of Nootka cypress is still ×Cuprocyparis.

Major upheaval for all of us

Given the widespread use of cypresses, particularly those from North America, for shelter, ornamental and forestry planting this sequence of changes is a major upheaval affecting thousands of tree growers, foresters, horticulturists, and nurseries. Eventually all of them will probably have to get used to the new names. However, for the foreseeable future many will probably dispense with Latin names and use accepted common names for the cypresses. If nothing else, this might for once be less confusing.

It is important to remember that throughout these nominative disturbances the plants have not changed. The changes have occurred in our knowledge and understanding of their relationships and evolutionary background.

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