Hydrangea involucrata var. tokarensis M.Hotta & Shiuchi — Nansei bracted hydrangea

Introduction

Nansei bracted hydrangea (Hydrangea involucrata var. tokarensis M.Hotta & Shiuchi) is an extremely localized southern island expression of H. involucrata, native to the Tokara Islands of the northern Ryukyu chain. It was described in 1996 from Kuchinoshima and remains recognized at variety rank by current Japanese and international taxonomic treatments. Tree TSAR likewise retains it provisionally as a variety rather than treating it either as a separate species or as undifferentiated H. involucrata.

Hotta & Shiuchi distinguished the Tokara plants principally by their reproductive morphology. The pistils are usually composed of three or four carpels, and the fruits are conspicuously larger than in typical H. involucrata. Japan's current conservation account additionally describes leaves 15–20 cm long that are relatively thick, somewhat leathery, and conspicuously hairy, together with capsules about 5 mm across.

Tree TSAR deliberately treats the variety differently from the former var. idzuensis. Izu/Oshima material was directly represented in De Smet's molecular investigation and failed to form an independently supported lineage. No equivalent population-genomic test of the Tokara populations has been located. The absence of genomic evidence therefore cannot be treated as negative evidence against var. tokarensis. Its strong island isolation and consistent morphological differentiation support preserving the variety while explicitly marking its rank as provisional.

Species rank would currently go too far. The Tokara plants retain the fundamental morphology of H. involucrata, and available cytological evidence does not establish a deeper chromosomal separation. What remains unknown is whether prolonged island isolation has produced genomic differentiation corresponding to the morphology. Population-genomic comparison among Kuchinoshima, Suwanosejima, Honshu, and the Izu islands is the clearest review trigger.

Horticultural and Agricultural Uses

No substantial commercial or historical cultivar tradition specific to var. tokarensis was located. Given its extreme rarity, conservation rather than ornamental dissemination should be the central consideration for any living material of documented wild origin.

Ex situ collections could nevertheless be exceptionally valuable if they are genetically representative, legally and ethically sourced, and maintained with precise provenance. Because only very small wild populations are known, horticultural propagation may eventually serve as a safeguard against complete loss, but cultivated survival cannot substitute for protection of the natural populations and their island environments.

Conservation Concerns

Nansei bracted hydrangea is among the most urgent conservation cases encountered so far in Hydrangea. Japan's fifth national Red List and Red Data Book classify H. involucrata var. tokarensis as Critically Endangered (CR) under criterion E. The assessment reports a quantitative probability of extinction in the wild of at least 50% within ten years.

The currently documented distribution is limited to Kuchinoshima and Suwanosejima in the Tokara Islands, where it occupies moist forest margins. A 2018 assessment estimated only several surviving individuals within a single survey mesh. The Japanese account does not list a specific statutory conservation designation protecting the variety.

These facts make taxonomic and conservation work inseparable. Genomic sampling would be scientifically valuable, but collecting must be designed around the precarious demographic condition of the remaining plants. Destructive or unnecessarily intensive sampling would be inappropriate for such a small population.

Whether later genomic evidence supports variety rank, species rank, or inclusion within undivided H. involucrata, the Tokara populations represent irreplaceable geographic and genetic diversity. A future rank change would therefore alter the label, not eliminate the conservation priority.

Additional Information

Plants of the World Online — Hydrangea involucrata var. tokarensis (opens in a new tab): Plants of the World Online: Accepted varietal name, authorship, distribution, and the homotypic Platycrater combination.

Japan Red Data Book — Tokara hydrangea (opens in a new tab): Japan Red List / Red Data Book: Current official CR assessment, distribution, habitat, morphology, and population information.

References and Further Reading

Hotta M, Shiuchi T (1996) Notes on the flora of the Ryukyu Islands 1. Two new varieties from the Tokara Islands, Peucedanum japonicum Thunb. var. latifolium (Umbelliferae) and Hydrangea involucrata Sieb. var. tokarensis (Hydrangeaceae). Journal of Japanese Botany 71(4): 183–187. https://doi.org/10.51033/jjapbot.71_4_9076 (opens in a new tab)

Ministry of the Environment, Japan (2025) Hydrangea involucrata var. tokarensis. Fifth Red List: Vascular plants. https://www.env.go.jp/nature/kisho/5th-rl-2025-book/01-5thredlist-vascularplant.pdf (opens in a new tab)

Ohba H, Akiyama S (2014) A revision of the species of Hydrangea (Hydrangeaceae) described by Siebold and Zuccarini, part 2. Bulletin of the National Museum of Nature and Science, Series B (Botany) 40(1): 29–45. https://www.kahaku.go.jp/albums/abm.php?d=4703&f=abm00001812.pdf&n=BNMNS_B40-1_29-45.pdf (opens in a new tab)