Nootka Cypress is listed as Chamaecyparis nootkatensis (D. Don) Spach in all modern texts, placing it with 6–7 other
species, Ch. formosensis, Ch. henryae*, Ch. lawsoniana, Ch. obtusa, Ch. pisifera, Ch. taiwanensis
and Ch. thyoides (* syn. Ch. thyoides subsp. henryae).
This classification is based on three characters of Nootka Cypress in which it resembles these other Chamaecyparis species: small cones opening when ripe, 2–5 seeds per cone scale, and flat shoots. Cupressus species on the other hand usually have cones which remain closed for several years, 5–18 seeds per scale, and plumose shoots (branching in two planes, not one). But is this the best classification and are these three characters all fully valid?
The best test of classification lies in genetic similarity, measurable only in a multi-million-dollar DNA laboratory, but also reflected more visibly in reproductive phenology and morphology, chemical composition and hybridisation. What does Nootka Cypress show in these features? Does it resemble Chamaecyparis more than Cupressus here as well?
Phenology. All Cupressus species mature their seed in two years. All Chamaecyparis species mature their seed in one summer, in about 6–8 months. Except for Nootka Cypress: this matures its seed in two years.
Cone scales. Nootka Cypress cones have four (rarely six) widely spreading scales with a distinct open end around a columella (somewhat like Callitris); the scales also have a very prominent umbo. No other Chamaecyparis species shows these. Most Cupressus do not show these features either, but some, notably C. lusitanica, do at times. A prominent umbo is a common character of many Cupressus species. No other cypress (of either genus) has four scales as the commonest number.
Seed shedding. Chamaecyparis cones open and shed their seed as soon as ripe. So does Nootka Cypress. Most Cupressus do not but some (e.g., C. himalaica, C. funebris) do, and others show variability, some trees doing so and others not (e.g., C. lusitanica). Not shedding seeds is an adaptation to forest fires (see my article on fire-climax pines, CSA Newsletter 9:8, 1991) and of no useful significance for classification.
Cone Size. Nootka Cypress cones are smaller than most Cupressus species, but are not smaller than those of C. funebris or C. himalaica, and several other Cupressus can have cones as small though usually somewhat larger.
Seeds. Seeds of Cupressus species have a large scar where they were attached to the scale, and the corresponding scar on the scale is conspicuous. Seeds of Chamaecyparis species have a small scar and do not leave an obvious scar on the scale – except for Nootka Cypress, which has a large scar.
Resin analysis. Though full chemical analysis demands a large laboratory, the nose can provide a quick and simple but valuable resin analysis. When the shoots are crushed, all Chamaecyparis species have a fairly similar scent readily seen as allied, except one: Nootka Cypress. This has a very different smell to these, acrid and unpleasant, and quite similar to the smell of Cupressus bakeri.
Hybrids. Three documented hybrids involving Nootka Cypress exist. They are all with Cupressus species, C. macrocarpa, C. glabra, C. lusitanica; although no deliberate cross has yet succeeded, natural crossing is clearly not difficult as they are represented by at least 20 different clones. The first of the three hybrids is of course the infamous Leyland Cypress, that dreadful bane of British suburbia (see CSA Newsletter 10:11, 1991). No hybrid of Nootka Cypress with any other Chamaecyparis exists, nor has any other Chamaecyparis crossed with any Cupressus.
Note that a Nootka x Lawson’s cross cited by Dallimore and Jackson (Handbk. Coniferae 4th ed., 1966) is an error: see Hunt, J. Roy. Hort. Soc. 99:361, 1974. Krussmann (Man. cult. Conif., 1985) wrongly records the male parent of the Leyland clone ‘Stapehill’ as Lawson’s Cypress: resin analysis proved Nootka pollen parentage despite the female parent’s remoteness from any Nootka Cypresses (Mitchell, Conif. Brit. Isles, 1972).
Flat foliage. Several Cupressus also have flat shoots: C. torulosa, C. himalaica, C. funebris, as does one variety of C. lusitanica, var. benthamii. Note that these are mostly the same ones as had cones opening at maturity: why do they differ from other more typical Cupressus species? These all grow in areas with higher summer humidity and rainfall than most, suggesting that flat shoots are an adaptation to moist climates and need not indicate a genetic relationship. This can be confirmed from other Cupressaceae genera: flat shoots in moist climates, and plumose in drier climates. That C. lusitanica has varieties with plumose and flat shoots shows how unimportant this feature is: the two varieties are otherwise nearly indistinguishable, so there is no good case for using shoot form even as a specific discriminant, let alone a generic one.
This leaves the number of seeds per scale as the only remaining reason for placing Nootka Cypress in Chamaecyparis. The value of this character is unknown, and may be due simply to the difficulty of fitting the seeds into such a small cone. Conversely, the weight of evidence from phenology, cone scale morphology, scent and hybridisation all suggest it is better placed in Cupressus, leaving Chamaecyparis as a very well-defined genus easily separated by its distinct chemistry and single-season cone phenology. A full DNA analysis should be made to prove or disprove this. Transferral to Cupressus does not require a new combination; it was first described as Cupressus nootkatensis D. Don, before Chamaecyparis was separated as a new genus in 1842.
So, if Nootka Cypress is in Cupressus, to which species is it most closely related? This is more difficult. Seedlings of all the American Cupressus species have 3–5 acute cotyledons, while the Old World species have two blunt ones (Silba, Phytologia 52:349–61, 1983) as do also Chamaecyparis. Nootka Cypress has two blunt cotyledons, so it may be closer to the Old World species; in general aspect of the foliage of the whole tree, it perhaps most resembles C. himalaica. But its cones have prominent umbos, more a character of American species; they are most like those of the Mexican C. lusitanica, the cypress which next most commonly has few spreading scales and a columella. C. bakeri, the closest in foliage scent, is also American, as are all the species that Nootka Cypress has hybridised with. A possible phylogeny which could account for these points is shown below.