TWO NEW TAXA OF RHODODENDRON (ERICACEAE) FROM VIETNAM

Two new taxa from Vietnam are described and illustrated: Rhododendron tephropeploides, in subsection Tephropepla, and R. chunii subsp. vietnamense, in subsection Tsutsusi.


Introduction
In recent years, several new species of Rhododendron have been described from Vietnam. These include Rhododendron suoilenhense D.F.Chamb., N.T.T.Huong & Rushforth and R. rushforthii Argent & D.F.Chamb.
In collaboration with researchers from the Vietnamese Institute of Ecology and Biological Resources (IEBR), staff from the Royal Botanic Garden Edinburgh, Royal Botanic Gardens, Kew, Longwood Gardens, and the University of British Columbia undertook a joint expedition to Northern Vietnam in 2014. The aim of this expedition was to collect a range of species distributed in Northwest Vietnam, particularly on the Hoang Lien Son mountain range, for research and ex situ conservation. Material collected included living material, seed, herbarium specimens and DNA samples. The genus Rhododendron was a particular focus, reflecting the research and conservation interests of the group. Some of the plants raised from these seeds at Logan, Kew and the Yorkshire Arboretum first flowered in 2016. Among these, we identified two hitherto undescribed rhododendron taxa. Mackay et al. (2018) recognise 47 Rhododendron taxa from Vietnam. However, botanical exploration of the high mountains of northern Vietnam has been very limited, so our understanding of the diversity and distribution of Rhododendron species in the region remains rudimentary (Gibbs et al., 2011). Northern Vietnam is at the southeastern end of the Sino-Himalayan mountain range and is an important part of the Indo-Burma biodiversity hotspot for plants (Critical Ecosystem Partnership Fund, 2020). There are also phytogeographical affinities with the montane flora of South and Southeast China.  Ecology. Temperate montane cloud forest, in wet thin soil with high humus content and high humidity. The surrounding vegetation is characterised by Hydrangea, Illicium and Rhododendron.
Phenology. The above description has been prepared from cultivated plants grown at Logan . Two populations were discovered on Phan Xi Păng Mountain. One of these was at threat from erosion near a public path.
Cultivation. Rhododendron tephropeploides should be grown in partial shade but will tolerate exposed conditions down to a minimum temperature of approximately -8°C. It requires damp, acidic soils that are freely draining but high in organic matter. When propagated from seed it can produce flowers in its third year and should reach a maximum height of 2 m. Small shrub, to 2 m; young shoots covered with erectopatent bristles, stipitate glands and flattened strigose red-brown hairs. Leaves monomorphic, coriaceous, persistent, elliptic to oblong-ovate, 1-2 × 0.5-0.9 cm, twice as long as broad, apex acuminate, base broadly cuneate, margin entire, upper surface strigose, lower surface paler, strigose, especially on midrib; petiole 2-3 mm, strigose. Inflorescence 2-to 3-flowered; pedicels 5-7 mm, densely brown-strigose. Calyx strigose; lobes 1-2 mm, oblanceolate. Corolla funnel-shaped, lilacpurple, upper lobes flecked, 9-10 mm; tube 6 × 3 mm, with 5 rows of glands on the outer surface, glandular within but glabrous outside. Stamens 5, subequal, filaments minutely hairy   acidic soils that are freely draining but high in organic matter. When propagated from seed it can produce flowers in its third year and should reach a maximum height of 1 m.  Nguyen & Nguyen (2012) reported the existence of Rhododendron chunii in Vietnam, we assume based on the Rushforth specimens cited above. We are now aware of several collections of subsp. vietnamense that are consistent in the diagnostic morphological characters that distinguish this taxon from subsp. chunii. However, these are relatively trivial compared with the obvious similarities between the two subspecies. Furthermore, there are very few specimens available to us of subsp. chunii. We therefore cannot comment on the range of variation in the Chinese populations.
Subspecies chunii occurs at an altitude of 1100-1800 m in forested mountains in northern Guangdong Province, South China.