Hesperocyparis glabra (Sudw.) Bartel — Smooth Arizona western-cypress

Introduction

The smooth Arizona western-cypress, Hesperocyparis glabra, is an evergreen tree centered in central Arizona and distinguished most visibly by its thin, smooth reddish bark, which exfoliates in plates or strips rather than becoming persistently fibrous. The foliage is commonly glaucous and bears active resin glands. These characters make the species readily teachable in the field, but Tree TSAR’s recognition does not rest on bark alone.

H. glabra was long treated as a variety or subspecies of H. arizonica. Comparative morphology, RAPD data, leaf-essential-oil profiles, geography, and genome-scale sampling now support a coherent lineage distinct from true H. arizonica (Bartel et al. 2003; Adams et al. 2010; Li et al. 2026). Li et al. sampled four H. glabra accessions and recovered them as a coherent part of the Arizonica radiation, while the single sampled H. arizonica accession fell elsewhere within that clade.

Horticultural and Agricultural Uses

Smooth Arizona western-cypress is horticulturally attractive for its blue-green to glaucous foliage and smooth red-brown bark. Plants have circulated historically under broad “Arizona cypress” names, and glaucous selections can be mislabeled or confused with forms of H. arizonica. Accurate botanical identity and provenance therefore matter in collections and in cultivar documentation.

Like its relatives, the species is useful for dry-climate screening, specimen planting, and evergreen structure where soils are well drained and summer moisture is not excessive. Its central-Arizona origin suggests tolerance of heat and seasonal drought, but cultivated material should not be assumed to represent the full genetic breadth of wild populations.

Conservation Concerns

H. glabra has a restricted central-Arizona distribution but is locally common in some canyon and mountain systems. It is less globally imperiled than the narrowest California and Baja endemics, yet its confinement to a limited geographic region leaves it exposed to severe drought, high-intensity wildfire, and climate-driven changes in montane woodland conditions.

Taxonomic clarity has a practical conservation consequence. Older records and assessments may refer to Cupressus arizonica var. glabra rather than H. glabra, so managers and collection curators should reconcile historical names before evaluating distribution, collecting provenance, or ex situ representation. The species should be conserved as its own lineage rather than as a color or bark form of H. arizonica.

Additional Information

Plants of the World Online — Hesperocyparis glabra (opens in a new tab): Current Kew taxonomic and distributional treatment.
Trees and Shrubs Online — Hesperocyparis glabra (opens in a new tab): Horticultural and historical account under the modern species name.
Adams et al. 2010 — essential-oil variation (opens in a new tab): Comparative chemical evidence for H. glabra and H. arizonica.

References and Further Reading

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 (opens in a new tab).

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 (opens in a new tab).

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 (opens in a new tab).

Royal Botanic Gardens, Kew (2026) Hesperocyparis glabra (Sudw.) Bartel. Plants of the World Online. https://powo.science.kew.org/taxon/urn%3Alsid%3Aipni.org%3Anames%3A60451551-2 (opens in a new tab).