Two new species of Phyllanthus (Phyllanthaceae) from Thailand

Abstract Two Phyllanthus species are newly described from a limestone mountain in the north of Thailand. The first species, P. huamotensis Pornp., Chantar. & J.Parn., sp. nov., is one of the most distinct Phyllanthus species easily distinguished by its reddish branchlets and stem, conspicuous reddish venation, especially on the lower leaf surface, red sepals with long fimbriate margin and red capsule with papillose-puberulous surface. The second species, P. chantaranothaii Pornp., J.Parn. & Hodk., sp. nov., is similar to P. pulcher Wall. ex Müll.Arg., but it is distinguished by its puberulous upper leaf surface and pistillate flowers which have red, narrowly lanceolate sepals with a white, long fimbriate margin, puberulous outer side as well as puberulous pedicel.


Introduction
Phyllanthus L. (Phyllanthaceae) is mainly distributed in tropical and subtropical regions (Radcliffe-Smith 2001;Webster 2014). The classification of Phyllanthus is still contentious (Kawakita and Kato 2017) because molecular phylogenetic studies have indicated that Phyllanthus, as previously circumscribed, was not monophyletic . Therefore, some authors suggested merging other closely related genera in Phyllanthus, such as Breynia J.R.Forst. & G.Forst., Glochidion J.R.Forst. & G.Forst. and Sauropus Blume , while others suggested division of Phyllanthus into several monophyletic and morphologically recognisable genera (Pruesapan et al. 2012;van Welzen et al. 2014). The most recent publication by Bouman et al. (2018) recorded 880 species of Phyllanthus and, amongst these, they were able to place 837 species in 18 subgenera, 70 sections and 14 subsections; 43 species remained unassigned. Phyllanthus is, therefore, one of the most diverse genera of flowering plants and its species often have a high degree of endemism. For example, in China, there are about 32 species reported, 13 of which are endemic (Li and Gilbert 2008). Thirty-six species of Phyllanthus s.str. (excluding Breynia, Glochidion and Sauropus) were reported for the Flora of Thailand by Chantaranothai (2007). Lately, two more new species have been included by Kantachot and Chantaranothai (2013) and Pornpongrungrueng et al. (2017). Thus, the total Thai species number has increased to 38, five of which are endemic to Thailand. Recently, two Phyllanthus taxa were discovered in Umpang district, Tak province, in the northern part of Thailand; they were investigated and are described herein as new species.

Methods
Field collections and herbarium specimens from various herbaria, as well as taxonomic literature, were examined. The herbarium abbreviations follow Index Herbariorum (Thiers 2019, continuously updated). The morphological descriptions and measurements were taken from dried specimens.
Phenology. Flowering and fruiting period is June to December.
Habitat and distribution. This species grows on open limestone hills, at 880-937 m above sea level. Currently, it is known only from the type location Doi Huamot, Tak province in northern Thailand.
Conservation status. The species is only known from the type locality. It should be categorised as Critically endangered [CR, B1ab (iii)] according to the IUCN Red List Criteria and Categories version 3.1 (IUCN 2012). The extent of occurrence is estimated to be less than 20 km 2 and this species is found in a restricted area on open limestone hills which is a threatened ecosystem.
Etymology. The name of this species is given, based on the location where the plant was first discovered. Diagnosis. Phyllanthus chantaranothaii is most similar to P. pulcher, but differs in its puberulous upper leaf surface with white, simple and dendritic hairs and pistillate flowers that have red, narrowly lanceolate sepals with a white long fimbriate margin, puberulous on the outer side and puberulous pedicel, whereas in P. pulcher, the leaf is glabrous on both surfaces and the sepals of the pistillate flower are rhombic-ovate with the upper part greenish and lower part red, glabrous on the outside and glabrous pedicel.
Phenology. Flowering and fruiting from August to November.
Habitat and distribution. This species grows in mixed deciduous forest, at ca. 500 m elevation. It is currently known from the type location near Pa La Ta waterfall and Doi Huamot, Tak province, Thailand.
Conservation status. As only the type collection, which was collected from mixed deciduous forest, has been investigated in detail, more field exploration in similar habitats in the surrounding areas should be conducted in order to provide a more accurate distribution range of this species. However, based on information that is available to us, this species is preliminarily categorised here as Endangered [EN,B1ab(i,iv)], according to the IUCN Red List Criteria and Categories version 3.1 (IUCN 2012). The extent of occurrence is estimated to be less than 50 km 2 and, previously, it was found in two locations (Doi Huamot and Pa La Ta waterfall in Umpang district, Tak province), but recently, the extent of occurrence seems to be in decline, because the species has not been found in Doi Huamot since it was first photographed in November 2008.
Etymology. The name of this species honours Prof. Dr. Pranom Chantaranothai for his major contributions to plant taxonomy, in general, but especially for his extensive work on Phyllanthus in the Flora of Thailand.

Discussion
The two species described herein should be classified in subgenus Eriococcus (Hassk.) Croizat & Metcalf and Section Eriococcus (Hassk.) Croizat & Metcalf, because they have staminate flowers composed of four sepals with a long fimbriate margin, four stamens with transversely dehiscent anthers and the stigmas in pistillate flowers are free and bifid. These are diagnostic characters of the section Eriococcus which occurs predominantly in mainland Asia, especially in Indochina (Kawakita and Kato 2017;Bouman et al. 2018). Prior to this paper, there were seven species recorded in Thailand that belonged to this section, including P. elegans Wall. ex Müll.Arg., P. gracilipes (Miq.) Müll.Arg., P. pulcher, P. pulchroides Beille, P. sikkimensis Müll.Arg., P. sootepensis and P. taxodiifolius Beille (Chantaranothai 2007;Bouman et al. 2018). The two newly described species are most similar to P. pulcher, but there are a number of different characters as presented in Table 1. Actually, P. huamotensis is one of the most distinct species of Phyllanthus in Thailand. It can be easily distinguished by its reddish branchlets and stem, conspicuous reddish venation, especially on the lower leaf surface, red sepals with long fimbriate margin and red capsule with papillose-puberulous surface.