BACKGROUND AND AIMS: Among the genus Rhododendron growing in Asia, R. arboreum has an exceptionally wide longitudinal and elevational distribution with five subspecies and four varieties. The southern slopes of the Himalayas support subsp. arboreum at lower elevations from northern India to Bhutan, two varieties of subsp. cinnamomeum at higher elevations from central Nepal to Bhutan, and another taxonomically unresolved group of subsp. arboreum at higher elevations in western Nepal. These slopes provide continuous habitat affected by the summer monsoon and a good site to study speciation processes along an elevational gradient. METHODS: Using specimens of R. arboreum collected throughout Nepal, we studied genetic variation, indumentum types on the abaxial surfaces of leaves, and wood structure of mature stems. KEY RESULTS: Molecular identification revealed that subsp. arboreum at lower elevations and two varieties of subsp. cinnamomeum at higher elevations diverged from the unresolved group of subsp. arboreum growing at higher elevations in the least explored area of Nepal. The four groups had distinct indumentum types, one layered in subsp. arboreum and differing elevationally, and two layered in subsp. cinnamomeum and differing longitudinally. For wood structure, individuals at higher elevations had shorter tracheary elements, vessels with smaller diameters, and shorter and narrower multiseriate rays with higher frequency than those at lower elevations. In the two groups of subsp. arboreum, distinct indumentum and wood structure seem to have contributed to speciation along the elevational gradient. CONCLUSIONS: This study showed that infraspecific taxa of R. arboreum have obscure geographic origins and that two varieties of subsp. cinnamomeum diverged parapatrically at higher elevations and subsp. arboreum at lower elevations elevationally from the unresolved group of subsp. arboreum in western Nepal. The latter diversification with modification in leaf indumentum and wood structure allowed expansion of this species throughout the Himalayas.
Noshiro et al. (Thu,) studied this question.