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Taxonomic treatments of Cupressus vary widely in the number of species they recognise: Silba (1981a, 1983, 1998) included 25 species while Farjon (2001, 2005c) recognised only 16. DNA-based studies suggest that further realignment of the species may be necessary (Bartel et al. 2003, Little et al. 2004), although this is strongly refuted by others (Farjon 2007a). Cupressus species usually occur in small, disjunct populations, and are distributed in the Mediterranean Basin and North Africa, southwestern North America and from the Himalaya to western China. True cypresses are evergreen trees or large shrubs with somewhat pendulous, terete or quadrangular branchlets. The branchlets are typically decussate, though in C. macnabiana they are flattened and comb-like. As in many Cupressaceae, there are two leaf forms: the juvenile leaves are needle-like and the mature leaves are scale-like. Adult leaves are decussate, appressed to divergent and with an abaxial resin gland in some species; the lateral and facial pairs are largely similar. The male strobili are terminal and solitary on short, lateral branchlets, with 6–16 decussate microsporophylls. The female cones are terminal, solitary, roughly globose, valvate, and mature in one or two years; the cones are dehiscent or shed their seeds after being burnt. The seed scales are arranged in three to six basally fused, decussate pairs; they are peltate, thick and woody with a mucronate apex; the umbo is small and spiny. There are 5–20 seeds per scale and they are lenticular with two narrow wings.
Relationships within the Cupressaceae have been debated long and hard for many years, and the discussion continues at full force today. An enduring dilemma has been whether Cupressus should be combined with Chamaecyparis, but there are ample morphological and genetic differences to support their distinction (Wolf & Wagener 1948, Rushforth 1987a, Watson & Eckenwalder 1993, Silba 1998, Fu et al. 1999e). More recently, phylogenetic studies using DNA and morphological data suggest that Cupressus is more closely related to Juniperus and Xanthocyparis than to Chamaecyparis (Gadek et al. 2000, Little et al. 2004). Some researchers (Little et al. 2004, Xiang & Li 2005) suggest that Cupressus is not monophyletic, and propose various options for recognising this, including combining into one genus all members of Cupressus, Juniperus and Xanthocyparis, or retaining Juniperus and splitting Old and New World Cupressus into two genera. Little (2006) proceeded to establish a segregate genus for the New World cypresses, including the two species of Xanthocyparis (X. nootkatensis and X. vietnamensis), unfortunately using the disputed name Callitropsis (see the account of Xanthocyparis for a full exegesis), and making new combinations in Callitropsis for all the species involved. The name Xanthocyparis has now, however, been conserved against Callitropsis (Brummitt 2007), making use of the latter name illegitimate. If segregation of the New World cypresses is to be accepted, it will therefore have to be under the genus Xanthocyparis (in which no combinations have as yet been published). Farjon (2007a) rejects such an approach, claiming that the genetic differentiation found by Little et al. (2004) is insignificant, and that no justification for splitting the genus can be found in anatomical and morphological characters – a view that is followed here. It should also be noted that the option of including the two Xanthocyparis in Cupressus is favoured by some (Rushforth 2007), but that, as yet, a grand lumping together of Cupressus, Juniperus and Xanthocyparis has not been seriously proposed.
Gardeners are faced by the presence in horticulture of several poorly defined species, and a plethora of names, over which botanists continue to wrangle. Most are from the mountains of the Sinohimalaya and their extensions into southeastern Asia, often with very narrow ranges and sometimes with natural distributions obscured by transplantations into cultivation. The apparently geographically and morphologically distinct Cupressus austrotibetica is a case in point (see below).
Whether as the narrow columns of the Italian Cypress Cupressus sempervirens, or the craggy wide-stretched limbs of C. macrocarpa on the cliffs of California or coastal Europe, Cupressus has a notable part in the landscape of the more temperate parts of our region, especially where Mediterranean or near-Mediterranean climatic conditions prevail. The genus seems not to be highly regarded in the eastern United States, and neither Dirr (1998) nor Sternberg (2004) give it much space, though it is clear that some species will perform well in parts of the southeastern United States despite the high summer temperatures and humidity. Most cypresses seem to grow satisfactorily in any well-drained soil, including shallow chalk (Hillier & Coombes 2002), although they should be given a site in full sun. Many are very well adapted to dry conditions, but the distribution of British and Irish champions suggests that they do better in damper conditions (Johnson 2003). Cypress canker (Seiridium spp.) can be a problem, affecting the cambium layer and effectively cutting off the circulation to shoots beyond the point of infection, causing dieback. It seems to be most prevalent on Cupressus sempervirens in Europe, and C. macrocarpa and its derivative ×Cuprocyparis leylandii elsewhere, but it can affect other species in the genus and the wider Cupressaceae (Barnard & Leahy 2004). Smaller specimens can be treated by fungicide but treatment of larger specimens is difficult.
A small genus of evergreen coniferous trees, natives of western N. America, Mexico, Guatemala, the Mediterranean region, the Himalaya and China. The leaves are always minute, scale-like and flattened to the branchlet, being superposed in four rows. The ultimate divisions of the branchlet are usually arranged irregularly, not in flattened sprays as in Chamaecyparis and most thujas. Flowers unisexual, both sexes on the same tree but in different catkins. Males composed of numerous short-stalked stamens; fruit a globose or elliptical cone composed of mushroom-shaped (peltate) scales with a ‘boss’ or enlargement in the centre; scales with numerous seeds (usually five or less in Chamaecyparis).
The hardiest of the cypresses are now considered to belong to the separate genus Chamaecyparis. Those that remain in Cupressus are nearly all somewhat tender in the average climate of Britain, the hardiest being arizonica and the allied glabra, macnabiana and macrocarpa, though bakeri, little known here at present, may prove to be as hardy as these or hardier. C. lusitanica is on the borderline, but is hardy as far east as Sussex and Surrey. They thrive in either loamy or peaty soil, well drained; and should be given a sheltered place, as they are subject to injury by wind, especially where they grow fast. Some species, notably macrocarpa and sempervirens, show two curiously diverse types of habit, viz., the horizontal-branched and the fastigiate, but most of them are, when young, of columnar or pyramidal form.
Most of the cypresses can be increased by means of cuttings, which, although probably not so good as seeds, still make good trees. All, if growing in poor soil, are benefited by applications of manure water or by top-dressings of manure. They are subject, especially in poor soils, and during a succession of dry seasons, to attacks by white scale insects. The best remedy is spraying with Malathion in March and April, when the young hatch out.
The North American species of Cupressus are studied by C. B. Wolf in El Aliso, Vol. 1, pp. 1–250 (1948).
The genus is reviewed by John Silba in Phytologia, Vol. 49, pp. 390–99 and Vol. 52, pp. 349–61 (1983).