Myanmar. Kachin: Myitkyina District, Chipwi Township, Langyang Village, ca. 2100 m elev., 2 Apr 1919, R. Farrer 803 (holotype: E (E00127295) [image!]).

缅甸木瓜红 meaning “Myanmar rehderodendron”.

This species is only known from Chipwi Township, Myitkyina District, east-central Kachin State, Myanmar. It is a deciduous tree growing at ca. 2100 m elevation (7000 feet on label) and is still poorly known. The species flowers from March to April; its fruiting time is not known. Based on the IUCN Red List Criteria (IUCN Standards and Petitions Subcommittee 2019), we suggest a conservation status for this species as DD (Data Deficient).

Pterostyrax burmanicus was described on the basis of a single collection, R. Farrer 803, from Langyang, eastern Upper Burma (Smith 1920; Fig. 1). From then until the present work, it had been accepted as a species of Pterostyrax Siebold & Zucc. endemic to Myanmar (Wu and Li 1965; Chen and Chen 1996; Kress et al. 2003). The species was not included in the phylogenetic analysis of Fritsch et al. (2001). Although the fruit of P. burmanicus is unknown, its vegetative and floral characters all agree with those of Rehderodendron (Table 1) and, in combination, match no other genus of the Styracaceae (Hwang and Grimes 1996; Fritsch et al. 2001). Four critical characters differ from those of Pterostyrax, but agree with those of Rehderodendron: P. burmanicus has faintly serrulate leaf blade margins (versus dentate in Pterostyrax), fertile shoots comprising only inflorescences (Figs 1, 2A; versus leaves (proximally) and inflorescences (distally); Fig. 2B), flowers that are not strictly arranged on one side of the inflorescence branches (Figs 1, 2A; versus arranged on one side of ultimate inflorescence branches; Fig. 2B) and petals that are distinctly connate beyond their bases (versus connate at their bases only (coherent)). On this basis, we conclude that P. burmanicus is best placed in Rehderodendron (Table 1).

Figure 1. 

Holotype of Rehderodendron burmanicum.

Rehderodendron burmanicum is very similar to R. microcarpum K.M.Feng ex T.L.Ming, a species of far north-western Yunnan Province, China and northern Kachin State, Myanmar (Ming 1983; Zhao et al. 2020). Both species are deciduous trees with a paniculate and densely stellate-pubescent inflorescence, filaments connate well below the middle and a pubescent style. The young leaves of R. burmanicum have sparsely-scattered stellate pubescence and the stamens are longer than the petals (Figs 1, 2A), whereas the young leaves of R. microcarpum are densely covered with stellate pubescence and the stamens are shorter than the petals (Fig. 2C). However, the only known locality of R. burmanicum is in Chipwi Township, east-central Kachin State, Myanmar, which is well outside of the known geographic range of R. microcarpum. Properly assessing the species boundaries or whether such exist between these two species must await more specimen collections and the generation of morphological and molecular data at the population level.

Figure 2. 

Inflorescences of Rehderodendron burmanicum, Pterostyrax corymbosus Siebold & Zucc. and R. microcarpum. A inflorescence of R. burmanicum, showing flowering shoots comprising only inflorescences, flowers not strictly arranged on one side of ultimate inflorescence branches and (red arrow) stamens longer than the petals B inflorescence of P. corymbosus, showing flowering shoots comprising both inflorescences and (red arrow) leaves and flowers arranged on one side of ultimate inflorescence branches C inflorescences of R. microcarpum, showing flowering shoots comprising only inflorescences, flowers not strictly arranged on one side of ultimate inflorescence branches and stamens that are shorter than the petals. (Photographs: A from the holotype of R. burmanicum B by Wan-Yi Zhao from Jinggangshan, Jiangxi, China C by Wan-Yi Zhao from Gongshan, Yunnan, China).

“Parastyrax burmanicus W.W.Sm.” is a name listed in Kress et al. (2003). We could not find any original published literature for this species or digitised specimens identified as such and conclude that it is a nomen nudum, likely resulting from a publication error meant as Pterostyrax burmanicus.

China. Yunnan: Hekou County, Binglangzhai, ca. 150 m elev., damp valley, 1 Dec 1959, Yu Ping-Hua s.n. (lectotype, designated here: KUN (acc. # KUN0026237) [image!]; isolectotypes: KUN (acc. # KUN0026236) [image!], acc. # KUN0026238 [image!]).

大叶木瓜红 meaning “large-leaved rehderodendron”

Figure 3. 

A lectotype of Rehderodendron macrophyllum (Yu Ping-Hua s.n. [KUN (acc. # KUN0026237)] B isotype of R. truongsonense (Ulf Swenson, Do Van Truong, Hoang Manh Quyen & Sylvain Razaflimandimbison 2131 [P00937133]).

Rehderodendron macrophyllum is known only from Hekou County, Yunnan Province, where three localities of the species (Binglangzhai, Tianweizhai and Shaba) have been documented since 1953. It was recorded as growing in evergreen broad-leaved forests along damp valleys and hillsides at 120–250 m elevation. The original habitat of R. macrophyllum is severely disturbed by forestry activities. Rehderodendron macrophyllum (as Parastyrax macrophyllus) was listed as Critically Endangered by the Chinese government in 2013 (Ministry of Environmental Protection and Chinese Academy of Sciences 2013; Qin et al. 2017). In 2020, we investigated its population, finding only three trees on the hill around Binglangzhai Reservoir, now a protected area. Rehderodendron macrophyllum is still seriously threatened and research on the conservation biology of this species is urgently needed. The species flowers from November through January; the mature fruiting time is not known.

From the time of its original description until the present work, Parastyrax macrophyllus had been consistently placed in Parastyrax, including in Flora Yunnanica, Flora Reipublicae Popularis Sinicae and Flora of China (Ming 1983; Hwang 1987; Hwang and Grimes 1996). In the original description, Wu and Feng (in Wu and Li 1965) indicated that its inflorescence is very short and its fruit is drupaceous with a fleshy “exocarp” (likely the mesocarp instead), which would be consistent with the characters of Parastyrax. We re-examined the original material, including the paratype collection Cai Ke-Hua 533 (KUN [acc. # KUN0026239!]), which possesses the only fruit gathered of the material cited in the protologue (Wu and Li 1965). We found that the fruit of this collection is clearly immature, so its internal structure, which would otherwise help to confirm the placement of the species to genus, could not be determined. In March 2020, we conducted fieldwork in Binglangzhai, the type locality of P. macrophyllus. There, we discovered plants that possess the same vegetative features as those in the type material of the species, yet with fruit in which the endocarp has many irregular rays intruding into the mesocarp, as in Rehderodendron (Figs 4D, E) and unlike in P. lacei (W.W.Sm.) W.W.Sm., the type species of Parastyrax. On this basis, we conclude that P. macrophyllus is best placed in Rehderodendron. Parastyrax must be redefined on characters that exclude those of R. macrophyllum; we include diagnostic characters for this purpose in Table 1, based on several previous studies (Smith 1920; Dickison 1993; Hwang and Grimes 1996; Fritsch et al. 2001).

Figure 4. 

Morphology of Rehderodendron macrophyllum and R. macrocarpum Hu. A–D Rehderodendron macrophyllum A branchlet and leaf B fruiting branch C young fruit D transverse section of fruit showing endocarp with many rays intruding into the mesocarp E fruit transverse section of R. macrocarpum. (Photographs: A–D by Wan-Yi Zhao from Hekou, Yunnan, China E by Wan-Yi Zhao rom Wawushan, Sichuan, China).

Rehderodendron macrophyllum is most similar to R. truongsonense P.W.Fritsch, W.B.Liao & W.Y.Zhao (Fig. 3B), a species distributed in northern and central Vietnam (Zhao et al. 2019). These two species share such features as evergreen tree; inflorescences axillary, congested and densely stellate-pubescent; bracteoles foliaceous, elliptic or obovate; filaments connate at base; and style pubescent. However, R. macrophyllum can be easily distinguished from R. truongsonense by larger leaf blades 12–27 × 6–12(–15) cm (vs. 6–13 × 4–7 cm), with a greater number of lateral veins on each side (8–14 vs. 4–7).

In the protologue, Yu Ping-hua s.n., a flower gathering deposited in KUN, was indicated as the type. Of the three sheets comprising this collection, KUN0026238 and KUN0026237 were stamped as “TYPUS” and KUN0026236 as “ISOTYPUS.” A short annotation is present on KUN0026237, so we have chosen this sheet (Fig. 3A) as the lectotype.

China. Yunnan: Hekou County. Binglangzhai, ca. 260 m elev., 12 Dec 1953, Mao Pin-Yi 3299 (KUN (acc. # KUN0026241) [image!]; PE00857253 [image!]; PE00857254 [image!]; IBSC0453792 [image!]; WUK0205521 [image!]); same locality, ca. 160 m elev., 15 Dec 1953, Liu Wei-Xin 752 (KUN (acc. # KUN0025120) [image!]; PE00857255 [image!]; HITBC033059 [image!]; LBG00102925); same locality, ca. 120–140 m elev., 2 Nov 1954, Mao Pin-Yi 5240 (KUN (acc. # KUN0025114) [image!]; PE00857251 [image!]; PE00857250 [image!]; WUK0208259 [image!]); same locality, ca. 120–180 m elev., 8 Nov 1954, Mao Pin-Yi 5370 (KUN (acc. # KUN0025117) [image!]; PE00857256 [image!]; PE00857257 [image!]; WUK0206253 [image!]); Tianweizhai, ca. 150 m elev., 24 Apr 1953, Cai Ke-Hua 533 (KUN (acc. # KUN0026239) [image!]; PE00857252 [image!]); Shaba, 17 Dec 1973, Yang Zeng-Hong 7567 (HITBC033057 [image!]; HITBC033058 [image!]); 25 Nov 1958, Ding Zhi-Zun & Wang Jia-Xi 1203 (NAS00018374 [image!]); 31 Mar 2020, Zhao Wan-Yi & Ye Fan ZWY-1563 (SYS!); ditto, ZWY-1564 (SYS!); ditto, ZWY-1565 (SYS!).