Hydrangea tapalapensis Samain, Najarro & E.Martínez — Tapalapa sprawling hydrangea

Introduction

Tapalapa sprawling hydrangea is a spectacular functionally dioecious woody climber endemic to the mountains around Tapalapa in Chiapas. It can rise approximately 45 m through a host tree and then produce free-standing, shrub-like growth above the canopy. Its often broad elliptic-obovoid leaves can become nearly orbicular, with slightly undulate, lobed, or toothed glandular margins. The flowers are also unusual among its immediate Mexican relatives: their petals range from white with reddish lines to distinctly pinkish (Samain et al. 2019).

It is a cloud-forest specialist of exceptionally wet, cloudy sites at about 1,990–2,180 m, principally on very steep north-facing slopes exposed directly to humid winds. The species occurs on metamorphic and volcanic terrain in and around the municipal reserve at Tapalapa and Cerro La Bandera. One historical locality in neighboring Pueblo Nuevo Solistahuacán had already been destroyed by the time of the specialist revision (Samain et al. 2019).

Hydrangea tapalapensis is particularly useful for understanding why morphology and molecular genealogy need to be interpreted together in Cornidia. Three accessions were included in the two-gene nuclear analysis. One was sister to a weakly supported group containing the other two H. tapalapensis together with H. sousae and H. steyermarkii. It was therefore not one of the strongly monophyletic species recovered in that study. Nevertheless, its canopy architecture, broad glandular-margined leaves, flower color, and ecological setting distinguish it consistently from its relatives (Granados Mendoza et al. 2021; Samain et al. 2019).

The contrast with H. breedlovei is especially informative. Hydrangea tapalapensis can overtop the canopy, usually has broader leaves with toothed or undulate margins, produces reddish-lined or pinkish petals, and occurs on volcanic substrates. Hydrangea breedlovei remains within the canopy, has narrower entire leaves, white petals, and is associated with limestone terrain. The male staminal filaments also differ markedly in length. These are coherent suites of organismal and ecological differences, not a single arbitrary leaf character (Samain et al. 2019).

There is also direct evidence that species boundaries in this radiation can remain permeable. Samain et al. collected one functionally male individual with characters intermediate between H. tapalapensis and the more widespread H. albostellata. They interpreted it as a putative hybrid: most features matched H. tapalapensis, while the indument approached H. albostellata. It remains an individual hybrid hypothesis rather than a named nothotaxon, but it provides an important reminder that introgression is biologically plausible in Cornidia (Samain et al. 2019).

Horticultural and Agricultural Uses

No established horticultural or agricultural use has been documented for H. tapalapensis. Its combination of immense climbing architecture, pink-tinged flowers, and cloud-forest ecology could make it scientifically interesting in specialist collections, but its extreme rarity gives conservation provenance greater importance than ornamental selection.

One field observation may also help explain performance in cultivation: at Cerro La Bandera, flowering intensity increased with local humidity. That does not yet amount to a cultivation trial, but it reinforces the degree to which this species' reproductive performance is tied to the exceptionally humid environment of its native cloud forest (Samain et al. 2019).

Conservation Concerns

Samain et al. (2019) applied IUCN criteria and assessed H. tapalapensis as Critically Endangered, with an extent of occurrence just under 39 km². Populations were described as severely fragmented and declining, with continued logging identified as a principal threat.

There is nevertheless an unusually encouraging human component to its conservation story. The species was named for the community of Tapalapa, whose decision to conserve the municipal reserve protects important surviving habitat. This local action is especially consequential because some historically occupied habitat outside the protected area has already disappeared.

Any future genetic work should include both the major Tapalapa populations and the apparent H. albostellata × H. tapalapensis individual or comparable plants. Such data could simultaneously clarify shallow relationships within the tapalapensis–sousae–steyermarkii group and test whether interspecific gene flow contributes to the incomplete sorting seen elsewhere in Cornidia.

Additional Information

Trees and Shrubs Online — Section Cornidia (opens in a new tab): Trees and Shrubs Online summarizes the modern diagnosis and contrasts the canopy-overtopping habit with true H. steyermarkii.
Samain et al. 2019 — Mexican climbing hydrangeas (opens in a new tab): The 2019 revision contains the full description, photographs, habitat data, conservation treatment, and record of the putative H. albostellata × H. tapalapensis individual.

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)