Study reveals iconic alpine "glasshouse" plant Rheum nobile is not one but four distinct species

Since 1855 when the renowned British botanist J.D. Hooker first formally described Rheum nobile from Sikkim — calling it "certainly the most striking of the many fine alpine plants of Sikkim" — this towering herb has been regarded as a single, widely distributed species. Growing at elevations between 4,000 and 4,800 meters in the alpine-subnival belt, R. nobile is famed for its spectacular translucent bracts that form a "glasshouse" structure up to 2 meters tall, protecting its inflorescence from UV radiation, cold, and rain. As a monocarpic perennial, the plant takes an average of 33.5 years to reach reproductive size before flowering once and dying.

Now, a study published in Plant Diversity overturns 170 years of taxonomic consensus by revealing that this iconic species is actually four.

The research team sampled 18 natural populations across the entire distribution range of R. nobile, covering nearly all known localities in China, as well as two populations from Nepal. By integrating plastid genomes, nuclear ribosomal ITS sequences, and 89 low-copy nuclear genes, all phylogenetic analyses consistently resolved R. nobile into four well-supported monophyletic clades, each corresponding perfectly to a geographic region: the Central Himalayas (CHM), the Eastern Himalayas (EHM), the Hengduan Mountains (HDM), and the Gaoligong Mountains (GLG). Bayesian species delimitation using the program BPP provided overwhelming support for a four-species model (posterior probability = 0.977), while alternative models received negligible support.

Morphometric analyses of 32 traits further confirmed clear differentiation among the four lineages. Each exhibits diagnostic features distinguishable in the field:

• Rheum nobile s. str. (Central Himalayas) — retains the original name, as the type locality (Sikkim) falls within this region. It is characterized by small fruits (4–6 mm long, 2.5–4.5 mm wide), medium plant height (80–150 cm), and cordate-ovate to round bracts.
• Rheum zhengyii (Eastern Himalayas) — the largest of the four, reaching over 2 meters in height with inflorescences exceeding 2 meters. Named in honor of the eminent Chinese plant taxonomist Academician Wu Zhengyi. It features broad cordate-ovate bracts with stalks typically longer than 1 cm and large fruits (6–9 × 4.5–7.5 mm).
• Rheum hengduanense (Hengduan Mountains) — a medium-sized species (85–130 cm) distinguished by elliptical to ovate bracts with stalks typically shorter than 1 cm, and large fruits similar to R. zhengyii.
• Rheum lushuiense (Gaoligong Mountains) — the smallest member, flowering at only 45–80 cm with inflorescences of 20–55 cm. Uniquely, the lower stem produces no floral structures during anthesis. This species occurs at lower elevations (3800–4000 m) in meadow habitats, unlike the other three species found above 4000 m on scree slopes.

The researchers attributed the speciation of these four lineages primarily to the "sky island" effect — the geographic isolation of high-elevation ridges (exceeding 4,000 m) separated by intervening lowland forests across the Qinghai-Xizang Plateau, the Himalayas, and the Hengduan Mountains. “This pronounced geographic isolation, coupled with genetic drift, has driven independent evolutionary trajectories,” shares co-corresponding author Hang Sun. “The concordance between geographic distribution, molecular phylogeny, and morphological differentiation supports vicariance driven by topographic fragmentation as the primary mechanism of diversification.”

Additionally, coalescent simulations indicate that historical gene flow — including hybridization and introgression — has shaped the distinct evolutionary paths of the four species, explaining the cytonuclear discordance observed between plastid and nuclear phylogenies.

“The unusually small stature of R. lushuiense may reflect ecological pressures unique to its meadow habitat,” adds Sun. “Unlike the other three species, which grow on scree slopes with minimal herbivory, R. lushuiense faces more intense interspecific competition and mammalian herbivory in meadows.”

Notably, as a monocarpic perennial requiring decades to reach flowering size, shorter individuals in the Gaoligong populations that flower at a younger age may have a selective advantage, leading to reduced stature over evolutionary time.

“Our findings carry implications for biodiversity estimation across the Qinghai-Xizang Plateau,” says Sun. “If a morphologically exceptional and well-studied flagship species like Rheum nobile has concealed three unrecognized species for 170 years, other widely distributed but less conspicuous alpine plants with similar sky-island distributions may harbor even greater cryptic diversity.”

Consequently, the researchers call for systematic integrative taxonomic surveys of broadly distributed alpine herbaceous plants to provide a more reliable scientific basis for conservation prioritization in this global biodiversity hotspot.

Contact the authors:

Hang Sun, sunhang@mail.kib.ac.cn; Bo Song, songbo@mail.kib.ac.cn

Funder:

This research was supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (2024QZKK0200), the Science and Technology Major Project of Xizang (XZ202501ZY0151), the Key Projects of the Joint Fund of the National Natural Science Foundation of China (U23A20149), the National Natural Science Foundation of China (32371702), the CAS "Light of West China" Program (xbzg-zdsys-202319), the Key Projects of Natural Science Foundation of Yunnan Province (202501AS070094), the Biological Resources Programme, Chinese Academy of Sciences (CAS-TAX-24-061), and the Ten-thousand Talents Program of Yunnan Province (YNWR-QNBJ-2020-292).

Conflict of interest:

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors Hang Sun and Bo Song are Editorial Board Members for Plant Diversity and were not involved in the editorial review or the decision to publish this article.

See the article:

Journal: Plant Diversity

DOI: https://doi.org/10.1016/j.pld.2026.05.002

Article title: From monotypic to multispecies: Integrative data redefine Rheum nobile complex (Polygonaceae) in sky island-alpine habitats