For decades, the Dioon populations scattered across the arid central valleys of Oaxaca were assigned to Dioon merolae — a catch-all name for all the “venetian blind” Dioon populations from Chiapas to central Oaxaca. But the plants of the upper Río Tehuantepec drainage — growing on rocky hillsides in the dry districts of Ocotlán, Yautepec, and Tlacolula — never quite looked right for Dioon merolae. Their leaflets were too flat, too stiff, too straight, and their reproductive structures too large. When Gutiérrez-Ortega et al. (2020) finally ran the genetics, the result was clear: these populations represented a distinct lineage, designated “West-A” in the molecular analyses, that warranted recognition as a separate species. The result was Dioon oaxacensis — the first of two new species carved from the old Dioon merolae concept (the second being Dioon salas-moralesiae in 2021). With its flat, stiff, straight leaflets obliquely inserted on a yellowish rachis, Dioon oaxacensis represents a lineage that evolved specifically in the arid habitats of eastern Oaxaca — co-occurring with cacti, agaves, and other xerophytes in a landscape that looks more like the Tehuacán Valley than the cloud forests of Chiapas where true Dioon merolae persists.
Quick Facts
| Scientific name | Dioon oaxacensis Gut.Ortega, Pérez-Farr. & Vovides |
| Family | Zamiaceae |
| Origin | Southern Mexico — central valleys of Oaxaca (Río Tehuantepec drainage) |
| Adult size | Trunk erect to decumbent, moderate size; crown of 33–62 leaves |
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 (estimated) |
| IUCN | Not yet formally assessed (likely VU or EN) |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 3/5 |
Taxonomy and Nomenclature
Dioon oaxacensis was described by Gutiérrez-Ortega, Pérez-Farrera, Vovides, Salas-Morales, and Chemnick in 2020, in Phytotaxa 474(1): 51–61. The holotype was collected by Pérez-Farrera and Gutiérrez-Ortega (no. 3467, male plant) on 9 November 2016 in the Sierra Sur of Oaxaca, and is deposited at HEM, with isotypes at MEXU.
The specific epithet oaxacensis refers to the state of Oaxaca, Mexico, where the species is endemic. A simple, geographic epithet — but one that anchors the species firmly in the extraordinary biodiversity hotspot that is Oaxaca, the Mexican state with the highest vascular plant diversity.
The triple split of Dioon merolae: the description of Dioon oaxacensis is part of a progressive dismemberment of the original Dioon merolae concept, which proceeded in three stages:
- Pre-2020: Dioon merolae was broadly circumscribed to include all populations from Chiapas to Oaxaca that shared the characteristic “venetian blind” leaflet arrangement.
- 2020: Gutiérrez-Ortega et al. described Dioon oaxacensis for the populations in the arid upper Río Tehuantepec drainage (the “West-A” lineage), based on morphological and genetic differentiation from true Dioon merolae.
- 2021: Gutiérrez-Ortega et al. described Dioon salas-moralesiae for the populations of southeastern Oaxaca (the “West-B/East” split), further narrowing Dioon merolae to its core range in Chiapas and the eastern side of the Isthmus of Tehuantepec.
Phylogenetic position: molecular analyses nest Dioon oaxacensis and Dioon merolae as sister species within the “Purpusii clade,” with Dioon purpusii and Dioon argenteum as the nearest relatives. Intriguingly, the protologue notes that the morphological similarities between Dioon oaxacensis and Dioon purpusii may represent shared ancestral traits or convergence — both species occur in arid Oaxacan habitats. However, the phylogenetic analyses confirm that Dioon oaxacensis is more closely related to Dioon merolae than to Dioon purpusii.
The protologue also identifies a potentially undescribed taxon: the “West-B” population from Tenango, Oaxaca, shows intermediate characteristics between Dioon oaxacensis and Dioon merolae, suggesting that further taxonomic work may yield additional species.
Common names: previously known in the trade as Dioon sp. “El Camarón” (rarepalmseeds.com); no formal common name established.
Morphological Description
Dioon oaxacensis is a medium-sized arborescent cycad. The trunk is cylindrical, erect when adult, becoming decumbent with age. Plants produce a dense crown of 33–62 leaves per trunk — a higher leaf count than many other Dioon species, contributing to a full, impressive crown.
Leaves: 103–148 cm long, forming an erect to ascending crown. The aspect is flat to moderately keeled. The rachis is notably yellowish when mature and glabrous — a field character that helps distinguish this species from Dioon merolae (which typically has a greener rachis).
Leaflets: the key diagnostic features. There are 224–257 pairs (approximately 448–514 total leaflets per leaf) — a very high count. Leaflets are opposite to sub-opposite, linear to linear-lanceolate, flat, stiff, and straight — critically, they are not deflected and not falcate. They are inserted at 50–60° to the rachis, creating an oblique but not strongly angled pattern. Leaflets are slightly imbricate close to the rachis. Margins bear 1–3 prickles on the distal margin and 0–1 prickle on the proximal margin. The surface is slightly pruinose (faintly waxy), papyraceous when young, becoming rigid and coriaceous at maturity.
The combination of flat + stiff + straight + obliquely inserted leaflets is the diagnostic signature. In Dioon merolae, the leaflets are slightly deflected (bent downward) — a subtle but consistent difference confirmed by morphometric analysis.
Reproductive structures: notably large compared to Dioon merolae. Microstrobilus (male cone) solitary, erect, whitish and tomentulose when emergent, light brown at maturity, cylindrical. Female cones ovoid. The species is dioecious.
Seeds: ovoid, with cream/white sarcotesta.
Similar Species and Common Confusions
| Character | Dioon oaxacensis | Dioon merolae | Dioon purpusii |
|---|---|---|---|
| Leaflet posture | Flat, stiff, straight | Slightly deflected | Flat, glossy |
| Leaflet imbrication | Slightly imbricate | More strongly imbricate (“venetian blind”) | Closely spaced to overlapping |
| Rachis colour (mature) | Yellowish, glabrous | Greener | Green |
| Reproductive structures | Large | Smaller | Moderate |
| Leaflet surface | Slightly pruinose | Less pruinose | Glossy |
| Habitat | Arid valleys, rocky hillsides | Tropical deciduous forest, canyon slopes | Dry deciduous forest, canyons |
| Distribution | Central valleys of Oaxaca (W of Isthmus) | Chiapas + E of Isthmus | Tomellín Canyon, La Cañada |
The key to separating Dioon oaxacensis from Dioon merolae is: (1) leaflet posture — flat and straight in Dioon oaxacensis, slightly deflected in Dioon merolae; (2) rachis colour — yellowish in Dioon oaxacensis; (3) reproductive structure size — larger in Dioon oaxacensis; and (4) geography — central valleys of Oaxaca (west of the Isthmus of Tehuantepec) for Dioon oaxacensis, Chiapas and eastern Oaxaca for Dioon merolae.
The similarity with Dioon purpusii is noted in the protologue as potentially convergent: both species occur in arid Oaxacan habitats and share some leaflet characteristics, but phylogenetic analyses confirm they belong to different subgroups within the Purpusii clade.
Distribution and Natural Habitat
Dioon oaxacensis is endemic to the arid central valleys of Oaxaca, specifically the upper portions of the Río Tehuantepec drainage in the districts of Ocotlán, Yautepec, and Tlacolula. This is one of the driest regions of southern Mexico — a rain shadow valley system between the Sierra Norte and Sierra Sur of Oaxaca.
The species grows on rocky hillsides, typically in agglomerated groups on steep, well-drained slopes. The vegetation is arid tropical scrub, with associated species characteristic of extreme aridity: cacti (Stenocereus, Myrtillocactus, Opuntia), agaves, Bursera, Ceiba, and other xerophytes. The protologue describes plants “co-habiting with plants well recognized to be adapted to aridity” — an explicit ecological indicator of the species’ drought-adapted nature.
Geographically, Dioon oaxacensis occurs on the other side of the Sierra Norte from Dioon argenteum (which inhabits the Sierra Norte’s canyons at 1,100–1,600 m in the transition zone between oak-pine forest and tropical deciduous forest). The two species are thus neighbours separated by a mountain range, with Dioon argenteum in the wetter, higher habitat and Dioon oaxacensis in the dry valleys below.
Climate in the native range:
| Parameter | Estimated range (Central valleys, 800–1,500 m) |
|---|---|
| Mean annual temperature | 18–24 °C |
| Mean January minimum | 6–10 °C |
| Historical minimum | 0 to −3 °C (occasional frost at higher elevations) |
| Mean summer maximum | 30–35 °C |
| Annual rainfall | 500–800 mm (strongly summer-dominant) |
| Dry season | November–May (5–6 months) |
The central valleys of Oaxaca are significantly drier than the habitat of Dioon merolae in Chiapas, but less extreme than the Sonoran Desert habitat of Dioon sonorense. The arid conditions are mitigated by the relatively high elevation (800–1,500 m), which moderates summer temperatures.
Conservation
Dioon oaxacensis has not yet received a formal IUCN assessment as a separate species (described in 2020). Prior to its separation, the populations were included under the umbrella of Dioon merolae, which is listed as Vulnerable (VU) under Mexican federal law (NOM-059-SEMARNAT-2010, category “P” — threatened with extinction). Given the restricted range within the central valleys of Oaxaca, the arid habitat vulnerability, and ongoing threats, a status of Vulnerable (VU) or Endangered (EN) is likely when assessed.
The main threats are habitat degradation (agriculture, grazing, road construction) and overcollection. The central valleys of Oaxaca are densely populated compared to the remote canyons of the Sierra Norte, increasing anthropogenic pressure on wild populations.
All Dioon species are listed on CITES Appendix II.
Cultivation
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 (estimated) |
| Light | Full sun to light partial shade |
| Soil | Well-drained, mineral-dominant; rocky substrates ideal |
| Watering | Moderate in summer, dry in winter; less water is better |
| Adult size | Medium; crown of 33–62 leaves |
| Growth rate | Slow |
| Difficulty | 3/5 |
No specific cultivation data exists for Dioon oaxacensis as a named species. However, plants from the central valleys of Oaxaca have been in specialist collections for decades under the informal designation Dioon sp. “El Camarón” — rarepalmseeds.com has listed this form, describing it as “recently scientifically described” and “found only in a small area.” Cultivation recommendations are extrapolated from the species’ ecology and from experience with the closely related Dioon merolae and Dioon purpusii.
Light: full sun to light partial shade. The arid, open hillside habitat suggests a preference for high light levels. Full sun is likely to produce the best foliage quality.
Soil and drainage: excellent drainage is essential. The rocky hillside habitat provides rapid water shedding. In cultivation, use a gritty, mineral-dominant mix with modest organic matter.
Watering: less water is better. The 500–800 mm annual rainfall concentrated in a summer monsoon dictates seasonal watering: moderate during the warm growing season, reduced to minimal in winter. The species is adapted to 5–6 months of drought per year.
Cold hardiness: moderate. The central valley habitat at 800–1,500 m experiences occasional frost, suggesting tolerance of −2 to −4 °C. USDA zone 9b and above.
Container culture: well suited. Use a well-draining mineral mix in a terracotta pot. Full sun, seasonal watering.
Fertilization: light applications of slow-release fertilizer during the growing season.
Buying Advice
Availability: Dioon oaxacensis has been available in the specialist trade under the name Dioon sp. “El Camarón” before its formal description. Rarepalmseeds.com has listed seeds. Expect moderate prices and limited availability.
Identification: flat, stiff, straight leaflets inserted obliquely on a yellowish rachis; slightly pruinose surface; arid-habitat provenance from the central valleys of Oaxaca. If the leaflets are deflected (bent downward) and the rachis is green, the plant is more likely Dioon merolae.
Propagation
Seed: standard Dioon protocol. Remove sarcotesta, soak 24–48 hours, sow horizontally in a well-draining mineral mix at 25–30 °C. Germination is cryptocotylar.
Offsets: no specific data. Not typically a prolific suckering species.
Pests and Diseases
Root rot: the primary cultivation risk. Prevention through substrate choice and seasonal watering discipline.
Cycad aulacaspis scale (Aulacaspis yasumatsui): likely susceptible.
Landscape Use
Dioon oaxacensis is an excellent candidate for warm-climate xeriscape plantings and Mediterranean gardens. The dense crown of 33–62 leaves, the subtle pruinose sheen, and the arid-habitat provenance position it as a drought-tolerant cycad for rocky slopes, raised beds, and succulent gardens. It associates naturally with Oaxacan xerophytes: columnar cacti, agaves, Beaucarnea, and Bursera. The species also has botanical prestige value as a recently described taxon (2020) from one of the world’s most biodiverse states. For collectors interested in the Oaxacan canyon system radiation — which has produced Dioon purpusii, Dioon califanoi, Dioon argenteum, Dioon caputoi, Dioon rzedowskii, Dioon spinulosum, and now Dioon oaxacensis — this species completes the arid-side component of that extraordinary evolutionary story.
Frequently Asked Questions
What is the difference between Dioon oaxacensis and Dioon merolae?
The leaflets of Dioon oaxacensis are flat, stiff, and straight; those of Dioon merolae are slightly deflected (bent downward). The rachis is yellowish and glabrous in Dioon oaxacensis, greener in Dioon merolae. The reproductive structures are larger in Dioon oaxacensis. Geographically, Dioon oaxacensis occurs in the arid central valleys of Oaxaca (west of the Isthmus of Tehuantepec); Dioon merolae occurs in Chiapas and the eastern side of the Isthmus.
Is Dioon oaxacensis the same as Dioon sp. “El Camarón”?
Yes. Before its formal description in 2020, plants from the central valleys of Oaxaca were known in the specialist trade as Dioon sp. “El Camarón.” The formal scientific name is now Dioon oaxacensis.
How does Dioon oaxacensis relate to the triple split of Dioon merolae?
The old Dioon merolae concept included populations from Chiapas to central Oaxaca. Genetic analyses revealed three distinct lineages: (1) “West-A” = Dioon oaxacensis (central valleys of Oaxaca, described 2020); (2) “East” = Dioon merolae sensu stricto (Chiapas); and (3) the southeastern Oaxaca populations = Dioon salas-moralesiae (described 2021). A possible fourth lineage (“West-B” from Tenango) shows intermediate characters and may represent yet another undescribed species.
How many Dioon species are now known from Oaxaca?
Seven: Dioon purpusii, Dioon califanoi, Dioon argenteum, Dioon caputoi, Dioon rzedowskii, Dioon planifolium, and Dioon oaxacensis — plus Dioon spinulosum downstream in Veracruz but with its range extending into the Oaxacan canyon system. Oaxaca is the global centre of diversity for the genus.
Is Dioon oaxacensis toxic?
Yes. Like all cycads, all parts contain cycasin and other toxic glycosides. Seeds, leaves, and roots are poisonous to dogs, cats, livestock, and humans.
Authority Websites and Databases
POWO — Plants of the World Online (Kew)
https://powo.science.kew.org/…
The accepted nomenclatural record. Native range: Mexico (Oaxaca). Seasonally dry tropical biome.
World List of Cycads — cycadlist.org
https://cycadlist.org/scientific_name/893
Nomenclatural record: first published in Phytotaxa 474(1): 51–61 (2020). Etymology (Haynes 2022): referring to Oaxaca.
Gutiérrez-Ortega et al. (2020) — protologue
https://doi.org/10.11646/phytotaxa.474.1.5
The original species description in Phytotaxa 474(1): 51–61. Full Latin diagnosis, morphological description with measurements (33–62 leaves per trunk, 103–148 cm long, 224–257 pairs of flat, stiff, straight leaflets at 50–60° to the rachis, yellowish rachis), habitat photographs showing agglomerated plants on rocky hillsides, comparison table with Dioon merolae, and discussion of the “West-B” (Tenango) populations as potentially distinct. Essential reference.
Gutiérrez-Ortega et al. (2021) — Dioon salas-moralesiae description
https://doi.org/10.11646/phytotaxa.528.2.3
The companion paper completing the triple split of Dioon merolae. Includes morphometric analyses comparing all three taxa and discusses the criteria for species delimitation within the “merolae” complex.
José Said Gutiérrez-Ortega — blog post
https://josesgo.wordpress.com/…
Summary by the first author: “Dioon oaxacensis represents a lineage of cycads that evolved in arid habitats of eastern Oaxaca.”
rarepalmseeds.com — Dioon oaxacensis
https://www.rarepalmseeds.com/dioon-oaxacensis
Specialist seed dealer. Previously listed as Dioon sp. “El Camarón.” Describes the species as recently scientifically described and found only in a small area.
Dorsey et al. (2018) — Pleistocene diversification in Dioon
https://doi.org/10.1002/ajb2.1149
Genus-wide molecular phylogeny providing the broader context for the position of the “merolae” group within the Purpusii clade.
Bibliography
Chemnick, J., Gregory, T. J., & Salas-Morales, S. (1997). A revision of Dioon tomasellii (Zamiaceae) from western Mexico, a range extension of D. merolae, and clarification of D. purpusii. Phytologia, 83(1), 1–6.
Dorsey, B. L., Gregory, T. J., Sass, C., & Specht, C. D. (2018). Pleistocene diversification in an ancient lineage: a role for glacial cycles in the evolutionary history of Dioon Lindl. (Zamiaceae). American Journal of Botany, 105(9), 1512–1530.
Gutiérrez-Ortega, J. S., Pérez-Farrera, M. A., Vovides, A. P., Salas-Morales, S. H., & Chemnick, J. (2020). Dioon oaxacensis (Zamiaceae): a new cycad species from the arid central valleys of Oaxaca (Mexico). Phytotaxa, 474(1), 51–61.
Gutiérrez-Ortega, J. S., Pérez-Farrera, M. A., Chemnick, J., & Gregory, T. J. (2021). A reassessment of Dioon merolae (Zamiaceae) leads to the description of Dioon salas-moralesae, a new cycad species from Southeastern Oaxaca, Mexico. Phytotaxa, 528(2), 101–120.
Haynes, J. L. (2022). Etymological compendium of cycad names. Phytotaxa, 550(1), 1–31.
Jones, D. L. (1993). Cycads of the World. Reed, Chatswood, NSW.
Norstog, K. J., & Nicholls, T. J. (1997). The Biology of the Cycads. Cornell University Press, Ithaca.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.