Bauhinia glauca, Clark & Mackinder & Banks, 2017

Clark, Ruth P., Mackinder, Barbara A. & Banks, Hannah, 2017, Cheniella gen. nov. (Leguminosae: Cercidoideae) from southern China, Indochina and Malesia, European Journal of Taxonomy 360, pp. 1-37: 30-32

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Bauhinia glauca

comb. nov.

Biogeography of C. glauca  comb. nov. and C. tenuiflora  comb. nov.

The decision to re-establish C. glauca subsp. tenuiflora  at the species level is supported by morphological and geographical evidence. The data obtained from the specimens studied by the authors, and those seen online via the Chinese Virtual Herbarium, shows the morphological differences between C. glauca subsp. glauca  and C. glauca subsp. tenuiflora  to be sufficiently great to merit recognition of two distinct species. The characters that most clearly separate the two species are highlighted in bold in Table 1.

Geographically, the distributions of C. glauca  comb. nov. and C. tenuiflora  comb. nov. are almost discrete, with very little overlap between the ranges ( Fig. 10View Fig). Cheniella glauca  comb. nov. has a more southerly distribution, in the Thai-Malay Peninsula, extending south as far as Sumatra and Java, whereas C. tenuiflora  comb. nov. occurs throughout Central and Northern Thailand, Cambodia, Laos, Vietnam, Myanmar, North-eastern India (the Seven Sister States), and Southern China. The transition between the two species occurs around the Isthmus of Kra, with a single record of C. glauca  comb. nov. to the north of this region, in Myanmar. This distribution coincides with that of other plant species, many of which are confined to areas north or south of the Isthmus ( Parnell 2013). The primary phytogeographical transition zone of the Thai-Malay Peninsula occurs, however, around the Kangar-Pattani Line at 6˚–7 ˚ north  , ca 450 km south of the Isthmus of Kra, where the Indochinese seasonal dry forests give way to Malesian aseasonal evergreen tropical forest ( Woodruff 2010). A secondary vegetation transition zone is found ca 500 km north of the Kangar-Pattani Line, at Wallace’s line between Mergui and Samut Songkram, where evergreen rainforest gives way to mixed moist deciduous forest ( Hughes et al. 2003; Woodruff 2010; Parnell 2013). The limits of the distributions of C. glauca  comb. nov. and C. tenuiflora  comb. nov. intersect around this transition zone. However, it must also be noted that C. glauca  comb. nov. is apparently not absolutely restricted to areas of rainforest and wetter, aseasonal climate: a single specimen is known from Myanmar, and one from Koi Samui, which are areas of higher seasonality ( Richardson et al. 2012), and the Myanmar specimen is north of Wallace's line vegetation transition zone. The western side of the Peninsula is generally wetter than the east ( Parnell 2013), which may partially explain the continuation of the range of C. glauca  comb. nov. further north on the western side. In summary, the distributions of C. glauca  comb. nov. and C. tenuiflora  comb. nov. indicate that they have distinctly divergent habitat and climate preferences, with C. glauca  comb. nov. preferring a wetter, less seasonal climate, and C. tenuiflora  comb. nov. inhabiting drier areas of greater seasonality and lower rainfall. This distinction supports their recognition as separate species.

The limited molecular based phylogenetic analysis that has been carried out including C. glauca  comb. nov. also provides some evidence to suggest that C. glauca  comb. nov. and C. tenuiflora  comb. nov. should not be considered as conspecific: the study of Hao et al. (2003) included one accession each of C. glauca  comb. nov. (as Bauhinia glauca  1) and C. tenuiflora  comb. nov. (as Bauhinia glauca  2) which were not resolved as sister lineages. Instead C. glauca  comb. nov. was placed as sister to C. didyma  comb. nov. albeit with low support (bootstrap = 63%).