Callitropsis Oerst.

The true and yellow cypresses – a legacy of taxonomic chaos:

The Vietnamese Yellow Cypress, Xanthocyparis vietnamensis, was discovered in northern Vietnam in 1999. Since its discovery conifer taxonomists have hotly debated how best to interpret its near relatives in light of the new discovery, namely the true cypresses (Cupressus spp., distributed in the New and Old Worlds) and the Nootka Cypress, distributed in the Pacific Northwest of North America. This debate has been fuelled by numerous molecular studies that were undertaken through the first 20 years of the 21st century, often with rather reactive publication of new names as a result. The principal twists and turns of this period are summarised here:

• 2002 – Farjon et al. (2002) publish a new genus, Xanthocyparis Farjon & T.H.Nguyên, for the Vietnamese Yellow Cypress (Xanthocyparis vietnamensis Farjon & T.H.Nguyên) discovered in northern Vietnam in 1999. The Nootka Cypress, generally known up to this point as Chamaecyparis nootkatensis, is assigned to the new genus as Xanthocyparis nootkatensis.

• 2004 – Little et al. (2004) point out that if Nootka Cypress and Vietnamese Yellow Cypress are to be treated in the same genus then Callitropsis Oerst. (an old name once used for Nootka Cypress) should take precedence over Xanthocyparis. This makes Xanthocyparis a superfluous and invalid name; the authors of Xanthocyparis later concede the point but successfully appeal for that name to be conserved over Callitropsis in 2006–07 (Yang et al. 2022). This position was adopted in New Trees (Grimshaw & Bayton 2009).

• 2006 – Little (2006) publishes the results of detailed molecular analyses of Cupressus and concludes that there is sufficient evidence to support splitting the genus along approximately Old World / New World lines. He assigns all North American Cupressus taxa to Callitropsis, together with the Nootka and Vietnamese Cypresses (i.e. Callitropsis nootkatensis and C. vietnamensis).

• 2009 – Debreczy et al. (2009) resolve some long-standing nomenclatural confusion around Nootka Cypress and in so doing clarify the proper naming of related groups. Principle among these implications is that Little’s 2006 use of the name Callitropsis for North American true cypresses is invalid, because according to the rules of botanical nomenclature this name must be applied to the Nootka Cypress, and the phylogenetic work of recent years has already demonstrated that this does not nest within the other North American cypress lineage.

• Also in 2009 Adams, Bartel & Price (2009) perform further molecular analyses which support Little’s 2006 conclusion that the New World cypresses form a strong clade, separate to Old World Cupressus but closely allied to (though still well differentiated from) the Vietnamese and Nootka Cypresses, hence these authors favour treating these latter two entities as two distinct monospecific genera. They therefore recognise Xanthocyparis vietnamensis and Callitropsis nootkatensis as monospecific genera and assign all other North American cypresses to a new genus, Hesperocyparis Bartel & R.A.Price, with all the Old World cypresses (minus the Vietnamese Yellow Cypress) remaining in a reduced Cupressus.

• 2011 – Christenhusz et al. (2011) counter with a more conservative treatment as part of their new classification of extant gymnosperms, which proposes placing all the true cypresses (i.e. Old World Cupressus and New World Hesperocyparis) together with Xanthocyparis and Callitropsis into a single large Cupressus (Nootka Cypress becoming Cupressus nootkatensis, and Vietnamese Cypress becoming Cupressus vietnamensis).

• 2010–2019 – multiple studies, including Mao et al. (2010), Terry, Bartel & Adams (2012), Zhu et al. (2018) and Mao et al. (2019) undertake probably the most detailed series of molecular analyses to date including study of nearly all known species of Juniperus, Cupressus, Chamaecyparis and several related monospecific genera including Nootka and Vietnamese Cypresses. They demonstrate that treating Cupressus as a single large genus incorporating Old and New World cypresses together with Nootka and Vietnamese cypresses would render it paraphyletic with respect to Juniperus. The only tenable way forward (at least without a significant and inevitably chaotic reorganisation of Juniperus) is to treat the four groups of cypresses in four genera, viz. Cupressus sensu stricto (Old World cypresses), Hesperocyparis (New World cypresses), Callitropsis (Nootka Cypress) and Xanthocyparis (Vietnamese Yellow Cypress).

This ‘four genera’ approach has since been adopted by major databases including Plants of the World Online, World Flora Online, and by major collections including the Royal Botanic Garden Edinburgh, an acknowledged centre of coniferous expertise. Christenhusz et al’s 2011 counter-treatment, sinking Nootka and Vietnamese Cypresses into a broad Cupressus, gave rise to a persistent ‘four genera or one’ narrative which some still use to argue for a single large Cupressus (the basic tenet being that one can either have Cupressus, Hesperocyparis, Xanthocyparis and Callitropsis, or else just Cupressus with the latter three lumped into it, i.e. four genera or one). The persistence of this ‘or one’ option owes a lot to its attractiveness: the evidence-base for the separation of Old and New World Cupressus lies principally in consistent differences at the molecular level; there are few consistent differences in gross morphology beyond the seedling stage and this makes such a taxonomy rather unhelpful to most end users. One large Cupressus would also simplify the taxonomy concerning intergeneric crosses between Nootka Cypress and North American true cypresses, including the horticulturally significant Leyland Cypress.

Unfortunately this cannot be; we must learn to live with the ‘four genera’ solution as recent studies have shown one large Cupressus to be untenable without significant and inevitably chaotic reorganisation of Juniperus, an exercise which would render the splitting of Cupressus seem moderate.

The ‘four genera’ approach outlined above is gradually becoming more widely accepted and implemented. In order to further this, and to future-proof the article structure on Trees and Shrubs Online, we are implementing these nomenclatural changes in October 2024: the species article texts will remain unchanged from the accounts in Trees and Shrubs Hardy in the British Isles and New Trees until such time as we receive sponsorship to update them, but they will now appear under the correct modern name with appropriate synonymy. If you would like to support the revision of these articles please contact the editors.