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Hesperocyparis Bartel & R.A.Price

Western-cypress

Synonyms
  • Neocupressus de Laub. [nom. illeg. superfl.]
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Treatment and Overview

Hesperocyparis, the western-cypress genus, comprises 18 accepted species-level lineages of evergreen trees and large shrubs in Cupressaceae. Its native range extends from southern Oregon and California through the southwestern United States, Baja California, Mexico, and Central America, with an insular lineage on Guadalupe Island. Tree TSAR also recognizes the provisional subspecies H. abramsiana ssp. butanoensis and the named cultivated hybrid H. × symbolica, neither of which is included in the 18-species count. The genus combines ancient evolutionary divergence with some of the narrowest conifer distributions in North America (Adams et al. 2009; Li et al. 2026).

Why Hesperocyparis is recognized

For much of botanical history, western-cypresses were included in a broad Cupressus with Old World cypresses. Their woody seed cones and scale foliage make that treatment superficially intuitive, but nuclear phylogenomics shows that the traditional boundary is not monophyletic if Juniperus remains outside Cupressus. The New World HCX lineage, Hesperocyparis + Callitropsis + Xanthocyparis, is recovered apart from a lineage containing Cupressus sensu stricto and Juniperus. Tree TSAR therefore treats Hesperocyparis as a correction of the generic boundary rather than as a preference for narrow genera (Mao et al. 2019; Liu et al. 2022).

Bartel, Adams, and Price established Hesperocyparis in 2009 for the Western Hemisphere cypresses, after earlier molecular work had shown that Old World and New World Cupressus did not form a simple natural unit. The competing name Neocupressus de Laub. is nomenclaturally superfluous and illegitimate. Chloroplast analyses subsequently supported Hesperocyparis monophyly, while nuclear transcriptomic and genome-scale work clarified the deeper HCX relationship and showed that cytonuclear disagreement reflects genuine reticulation and incomplete lineage sorting rather than a single analytical artifact (Adams et al. 2009; Terry et al. 2012; Mao et al. 2019; Li et al. 2026).

No single visible character separates every western-cypress from every Old World Cupressus. The distinction is expressed as a recurring morphological syndrome. Hesperocyparis commonly develops three to five cotyledons, whereas Cupressus sensu stricto usually has two, with exceptions in both groups. Western-cypress seeds are often more glaucous, and branch systems frequently develop a more three-dimensional architecture rather than the planar sprays common in many Old World cypresses. Leaf dimorphism, ultimate branchlet shape, and microscopic wood characters provide additional, imperfect signals (Adams et al. 2009). The lack of one absolute character helps explain the long history of misclassification within the group.

Evolution, reticulation, and species limits

Genome-scale analyses provide the strongest current internal framework. Li et al. (2026) analyzed 2,628 nuclear single-copy genes together with nearly complete plastomes and recovered Hesperocyparis as a coherent radiation with three principal living lineages. Their primary dating analysis placed the stem near 39.2 million years ago and the crown near 29.2 million years ago, with the Macrocarpa-Arizonica split near 24.5 million years ago. Much of the living diversification therefore extends back to the Oligocene and Miocene rather than representing only recent local differentiation.

That history is reticulate. Nuclear and plastid trees disagree at several nodes, gene trees contain substantial conflict, and network analyses recover historical gene flow. Li et al. (2026) favored a one-reticulation model within the Macrocarpa Clade and a two-reticulation model within the Arizonica Clade. These model results are not a literal census of all hybridization events. Their importance is that introgression and incomplete lineage sorting have been influential enough to leave genomic signatures throughout the radiation. A species can exchange genes with another lineage and remain diagnosable and evolutionarily independent; conversely, geographic isolation or an unusual phenotype does not by itself establish species rank.

Tree TSAR therefore evaluates species limits holistically. The Siskiyou population historically called Cupressus bakeri ssp. mathewsii remains within H. bakeri. Butano Ridge is recognized at subspecies rank within H. abramsiana. H. glabra is retained separately from H. arizonica, and H. forbesii remains distinct from the morphologically similar island species H. guadalupensis. H. benthamii, H. montana, and H. revealiana are provisionally retained at species rank pending denser population-genomic tests, while H. babaddoagae is a provisional Tree TSAR combination based on a coordinated inherited character suite that still requires broad genomic testing. These differences reflect different evolutionary models, not a mechanical score threshold.

Biogeographic reconstruction places the early history of the genus in northwestern North America, followed by southward and eastward expansion as western North American climates became cooler, more seasonal, and increasingly arid. Modern western-cypresses persist in a mosaic of coastal terraces, serpentine and ultramafic soils, desert-edge mountains, gabbro-derived soils, maritime islands, and Mesoamerican montane forests. Fire is a recurring ecological force, especially in California, but it is not uniform across the genus. Many species rely on serotinous cones and stand-replacing fire, while high-elevation H. montana releases seed without fire and the island species H. guadalupensis can open detached cones under dry conditions without extreme heat (Garcillán 2010; Milich et al. 2012; Li et al. 2026).

Horticultural and Agricultural Uses

Western-cypresses range from important forestry and landscape trees to species known mainly from specialist collections. Monterey western-cypress, H. macrocarpa, is the clearest example of horticultural abundance contrasting with wild rarity. It is planted widely in maritime climates and has contributed to numerous cultivated forms. Its hybridization with Nootka cypress, Callitropsis nootkatensis, produced the Leyland-cypress lineage placed by Tree TSAR in ×Hesperotropsis (Garland & Moore 2012; Royal Botanic Gardens, Kew 2026a).

Mexican western-cypress, H. lusitanica, has a long history in forestry and ornamental planting well outside Mesoamerica. Its cultivation in Portugal was so influential that it obscured the species’ American origin and contributed to a complicated nomenclatural history. Arizona western-cypress, H. arizonica, is also cultivated for dry-climate landscaping, environmental uses, and shelter, while H. bakeri is valued in specialist collections for cold hardiness relative to many western-cypresses (Royal Botanic Gardens, Kew 2026a; Trees and Shrubs Online 2026a, 2026b).

Cultivation requirements differ sharply among species. Coastal H. macrocarpa performs best in cool maritime conditions and is prone to cypress canker when planted inland in warmer, drier situations. Montane species such as H. bakeri tolerate substantially colder winters, while Mexican and southwestern species vary in their tolerance of summer heat, seasonal drought, and winter cold. For conservation collections, provenance is especially important because many accepted species contain strongly differentiated populations even when those populations are not recognized taxonomically.

Several western-cypresses are uncommon in general horticulture because wild seed is scarce, conservation restrictions are appropriate, or their ecological requirements are specialized. These species are most valuable in documented botanic-garden and research collections, where accessions can preserve geographic variation and support comparative studies of fire response, cone serotiny, chemistry, and climate tolerance.

Conservation Issues

Conservation risk is concentrated in the genus because many western-cypresses occupy tiny, fragmented ranges. Santa Cruz western-cypress is federally listed as Threatened in the United States, while Cuyamaca western-cypress, Paiute western-cypress, Mendocino western-cypress, Gowen western-cypress, and the two naturally occurring Monterey western-cypress stands represent additional examples of highly localized lineages or conservation units (U.S. Fish & Wildlife Service 2016; NatureServe 2026).

Fire creates the most important recurring management paradox. Many Californian species are obligate seeders: adults are commonly killed by crown fire, while heat opens serotinous cones and creates exposed seedbeds. Fire exclusion can suppress regeneration and allow competitors to overtop aging stands, but short-interval reburning can destroy a new cohort before it has accumulated a replacement canopy seed bank. Experimental and demographic studies show that the outcome depends on stand age, fire severity, cone heating, seed viability, and postfire climate rather than simply on whether a fire occurs (Milich et al. 2012; Brennan & Keeley 2019; Merriam & Bovee 2026).

Other threats are strongly local. Coastal development, recreation, logging, and road construction affect some Californian endemics. Introduced goats caused severe regeneration failure in H. guadalupensis on Guadalupe Island, but recruitment increased dramatically after goat eradication, demonstrating the value of removing chronic herbivore pressure (Garcillán et al. 2009; Rosas-Escobar et al. 2011). Climate change can interact with altered fire regimes by changing drought stress, fuel conditions, and the time required for young stands to reach reproductive maturity.

Ex situ conservation should therefore be population-aware. A collection represented by a few cultivated clones may preserve a name without preserving much of the evolutionary diversity within that taxon. For narrowly endemic or highly structured western-cypresses, documented wild provenance and sampling across distinct populations are especially important.

Infrageneric Groups

Bakeri Clade. This earliest-diverging living lineage contains H. bakeri. Genome-scale analyses place it sister to the combined Macrocarpa and Arizonica radiations. Its isolated northern distribution in southern Oregon and northern California gives the clade both phylogenetic and biogeographic distinctiveness (Li et al. 2026).

Macrocarpa Clade. Tree TSAR places H. abramsiana, H. goveniana, H. guadalupensis, H. macnabiana, H. macrocarpa, H. nevadensis, H. pygmaea, H. sargentii, and H. stephensonii here. Li et al. (2026) sampled all of these except H. pygmaea. Particularly informative results include the close nuclear association of H. guadalupensis with H. stephensonii and the placement of H. macnabiana with H. nevadensis, relationships that cut across several historical morphology-based classifications.

Arizonica Clade. H. arizonica, H. benthamii, H. forbesii, H. glabra, H. lusitanica, and H. montana were directly sampled in the genome-scale study. Tree TSAR additionally places the unsampled H. revealiana and provisionally accepted H. babaddoagae here. Several of the genus’s most difficult species boundaries occur in this clade, including arizonica-glabra, benthamii-lusitanica, and montana-revealiana (Adams et al. 1997, 2010, 2014; Li et al. 2026).

Accepted Taxa
Hesperocyparis abramsiana (C.B.Wolf) Bartel Santa Cruz western-cypress
Hesperocyparis abramsiana ssp. butanoensis Silba [TSAR]* Butano Ridge western-cypress
Hesperocyparis arizonica (Greene) Bartel Arizona western-cypress
Hesperocyparis babaddoagae Maerki [TSAR]* Babad Do'ag western-cypress
Hesperocyparis bakeri (Jeps.) Bartel Modoc western-cypress
Hesperocyparis benthamii (Endl.) Bartel Bentham western-cypress
Hesperocyparis forbesii (Jeps.) Bartel Tecate western-cypress
Hesperocyparis glabra (Sudw.) Bartel Smooth Arizona western-cypress
Hesperocyparis goveniana (Gordon) Bartel Gowen western-cypress
Hesperocyparis guadalupensis (S.Watson) Bartel Guadelupe Island western-cypress
Hesperocyparis lusitanica (Mill.) Bartel Mexican western-cypress
Hesperocyparis macnabiana (A.Murray bis) Bartel McNab western-cypress
Hesperocyparis macrocarpa (Hartw.) Bartel Monterey western-cypress
Hesperocyparis montana (Wiggins) Bartel San Pedro Martir western-cypress
Hesperocyparis nevadensis (Abrams) Bartel Paiute western-cypress
Hesperocyparis pygmaea (Lemmon) Bartel Mendocino western-cypress
Hesperocyparis revealiana (Silba) Silba El Rincon western-cypress
Hesperocyparis sargentii (Jeps.) Bartel Sargent western-cypress
Hesperocyparis stephensonii (C.B.Wolf) Bartel Cuyamaca western-cypress
Hesperocyparis × symbolica (Maerki) Z.H.Feng Roundcone western-cypress

Additional Information

References and Further Reading

Adams RP, Bartel JA, Price RA (2009) A new genus, Hesperocyparis, for the cypresses of the western hemisphere (Cupressaceae). Phytologia 91(1): 160–185. https://www.biodiversitylibrary.org/part/220373.

Adams RP, Bartel JA, Thornburg DC, Allgood A (2010) Geographic variation in the leaf essential oils of Hesperocyparis arizonica and H. glabra. Phytologia 92(3): 366–387. https://www.biodiversitylibrary.org/part/220432.

Adams RP, Bartel JA, Terry RG, Callahan F, Bisbee JR (2014) Taxonomy of Hesperocyparis montana, H. revealiana and H. stephensonii: evidence from leaf essential oils analyses and DNA sequences. Phytologia 96(2): 71–83. https://www.biodiversitylibrary.org/part/184358.

Adams RP, Zanoni TA, Lara A, Barrero AF, Cool LG (1997) Comparisons among Cupressus arizonica, C. benthamii, C. lindleyi and C. lusitanica using leaf essential oils and DNA fingerprinting. Journal of Essential Oil Research 9(3): 303–309. https://doi.org/10.1080/10412905.1997.10554249.

Bartel JA, Adams RP, James SA, Mumba LE, Pandey RN (2003) Variation among Cupressus species from the western hemisphere based on random amplified polymorphic DNAs. Biochemical Systematics and Ecology 31(7): 693–702. https://doi.org/10.1016/S0305-1978(02)00229-6.

Brennan TJ, Keeley JE (2019) Postfire population dynamics of a fire-dependent cypress. Plant Ecology. https://doi.org/10.1007/s11258-019-00939-8.

Garcillán PP (2010) Seed release without fire in Callitropsis guadalupensis, a serotinous cypress of a Mediterranean-climate oceanic island. Journal of Arid Environments 74(4): 512–515. https://doi.org/10.1016/j.jaridenv.2009.09.017.

Garland MA, Moore G (2012) ×Hesperotropsis, a new nothogenus for hybrids between Callitropsis and Hesperocyparis (Cupressaceae). Taxon 61(3): 667–670. https://doi.org/10.1002/tax.613015.

Li J, Jia S, Wu D, Tao W, Ruhsam M, Mao K (2026) Phylogenomics reveals the reticulate evolution and biogeographic history of New World cypresses, Hesperocyparis (Cupressaceae). Cladistics 42(2): 109–127. https://doi.org/10.1111/cla.70021.

Liu XQ, Xia XM, Chen L, Wang XQ (2022) Phylogeny and evolution of Cupressaceae: updates on intergeneric relationships and new insights on ancient intergeneric hybridization. Molecular Phylogenetics and Evolution 177: 107606. https://doi.org/10.1016/j.ympev.2022.107606.

Mao KS, Ruhsam M, Ma YZ, Graham SW, Liu JQ, Thomas P, Milne RI, Hollingsworth PM (2019) A transcriptome-based resolution for a key taxonomic controversy in Cupressaceae. Annals of Botany 123(1): 153–167. https://doi.org/10.1093/aob/mcy152.

Milich KL, Stuart JD, Varner JM III, Merriam KE (2012) Seed viability and fire-related temperature treatments in serotinous California native Hesperocyparis species. Fire Ecology 8(2): 107–124. https://doi.org/10.4996/fireecology.0802107.

Rosas-Escobar P, Gernandt DS, Piñero D, Garcillán PP (2011) Plastid DNA diversity is higher in the island endemic Guadalupe cypress than in the continental Tecate cypress. PLOS ONE 6(1): e16133. https://doi.org/10.1371/journal.pone.0016133.

Terry RG, Bartel JA, Adams RP (2012) Phylogenetic relationships among the New World cypresses (Hesperocyparis; Cupressaceae): evidence from noncoding chloroplast DNA sequences. Plant Systematics and Evolution 298(10): 1987–2000. https://doi.org/10.1007/s00606-012-0696-3.

Garcillán PP, Vega E, López-Reyes E (2009) Recruitment response of Guadalupe cypress (Callitropsis guadalupensis) three years after goat eradication on Guadalupe Island. In: Damiani CC, Garcelon DK (eds), Proceedings of the Seventh California Islands Symposium. Institute for Wildlife Studies, Arcata, California, pp 177–183.

Merriam KE, Bovee KM (2026) Complex effects of fire severity on a serotinous conifer. Fire Ecology 22: 81. https://doi.org/10.1186/s42408-026-00489-1.

NatureServe (2026) Hesperocyparis stephensonii. NatureServe Explorer. https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.135758/Hesperocyparis_stephensonii.

Royal Botanic Gardens, Kew (2026a) Hesperocyparis. Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:60451544-2.

Trees and Shrubs Online (2026a) Hesperocyparis. https://www.treesandshrubsonline.org/articles/hesperocyparis/.

Trees and Shrubs Online (2026b) Hesperocyparis bakeri. https://www.treesandshrubsonline.org/articles/hesperocyparis/hesperocyparis-bakeri/.

U.S. Fish & Wildlife Service (2016) Final rule: reclassifying Hesperocyparis abramsiana (= Cupressus abramsiana) as threatened. Federal Register 81: 8408. https://www.fws.gov/species-publication-action/final-rule-reclassifying-hesperocyparis-abramsiana-cupressus-abramsiana.