Hydrangea sousae
Chiapas sprawling hydrangea
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Introduction
Chiapas sprawling hydrangea is among the most imposing members of the Mexican radiation. It is a functionally dioecious woody root-climber capable of reaching about 60 m and, once it reaches the crown of a host tree, extending free-standing shrub-like branches above the canopy. This architecture immediately separates it from relatives such as true H. steyermarkii, which remains within its host's crown. Its coriaceous elliptic to lanceolate leaves have an unusually drawn-out acuminate tip and a revolute margin strengthened by a marginal vein; small glandular teeth may occur, particularly toward the leaf apex (Samain et al. 2019).
Its habitat is equally distinctive. Hydrangea sousae is confined to high, persistently humid cloud forest of the Sierra Madre de Chiapas and the northwestern slopes of Volcán Tacaná, principally from about 1,960 to 2,540 m. Known sites occupy steep north-facing slopes with heavy cloudiness, abundant epiphytes, and metamorphic or volcanic substrates. Some individuals even persist in secondary forest on Tacaná, demonstrating that an appropriate humid forest structure can develop after disturbance, although this should not be confused with tolerance of wholesale habitat conversion (Samain et al. 2019).
In the two-gene Cornidia phylogeny, H. sousae falls in the shallowly resolved portion of Mexican Clade I with H. tapalapensis and H. steyermarkii. The sampled taxa are not arranged as three clean, deeply diverged species branches. Yet their field morphology and architecture are strongly differentiated. In H. sousae, the combination of canopy-overtopping growth, leathery sharply acuminate leaves, revolute margins, small marginal glands, and typically two to three leaf pairs below an inflorescence is particularly useful (Samain et al. 2019; Granados Mendoza et al. 2021).
The species therefore reinforces a recurrent lesson from this entire batch: Clade I appears to represent a comparatively young Mexican radiation in which organismal differentiation can be considerably easier to see in the forest than in a genealogy constructed from two nuclear loci. Denser genome-scale sampling of H. sousae, H. tapalapensis, and H. steyermarkii would provide a much stronger test of their divergence and any historical gene flow.
Horticultural and Agricultural Uses
No established horticultural use has been documented for H. sousae. Its enormous eventual scale and dependence on extremely humid montane conditions would in any case make it primarily a plant for specialist collections rather than ordinary ornamental use.
Authenticated cultivated accessions could be scientifically valuable because the species' canopy-overtopping architecture is difficult to infer from herbarium specimens. Long-term observation would help document how attached juvenile shoots transition into free-standing crown growth, one of the architectural characters that proved essential to recognizing the Mexican Cornidia species.
Conservation Concerns
The specialist revision applied IUCN criteria and assessed H. sousae as Endangered. Its calculated extent of occurrence was only a little over 121 km², with severely fragmented and declining populations. Habitat destruction, human population pressure, and fires were identified as ongoing threats; because some populations occur on Volcán Tacaná, volcanic activity was also noted as a potential stochastic threat (Samain et al. 2019).
The high-elevation forest occupied by the species may add another long-term vulnerability. H. sousae depends on consistently cool, cloudy, moisture-rich microsites near mountain ridges. Protection of forest cover is therefore necessary but may not alone preserve those conditions if regional cloud regimes shift. Provenance-rich conservation collections would be particularly useful for retaining variation from the Sierra Madre and Tacaná populations.
Additional Information
- Plants of the World Online — Hydrangea sousae (opens in a new tab): Plants of the World Online accepts H. sousae as a Chiapas endemic.
- Trees and Shrubs Online — Section Cornidia (opens in a new tab): Trees and Shrubs Online emphasizes the revolute leaf margin and canopy-overtopping architecture that distinguish the species from true H. steyermarkii.
References and Further Reading
Granados Mendoza C, Martínez Salas EM, Goetghebeur P, Wanke S, Samain M-S (2021) Molecular phylogeny, character evolution, and biogeography of Hydrangea section Cornidia, Hydrangeaceae. Frontiers in Plant Science 12: 661522. https://doi.org/10.3389/fpls.2021.661522 (opens in a new tab)
Samain M-S, Hernández Najarro F, Martínez Salas EM (2019) The climbing hydrangeas (Hydrangeaceae) of Mexico, including description of six (critically) endangered new species. Acta Botanica Mexicana 126: e1463. https://doi.org/10.21829/abm126.2019.1463 (opens in a new tab)