Rhipidocladum McClure is one of eight genera of woody bamboo found in Mexico, and one of the four Mexican bamboo genera in subtribe Arthrostylidiinae (Ruiz-Sanchez et al. 2015). The genus has 21 described species, six of which are present in the country and three of which are endemic (Ruiz-Sanchez et al. 2019, 2021a, 2021b). Taxonomically, Rhipidocladum is classified into two sections: Rhipidocladum sect. Rhipidocladum and R. sect. Racemiflorum (Clark and Londoño 1991). Rhipidocladum sect. Rhipidocladum is characterized by robust culms with an apically arching or pendulous habit, basally fused fimbriae on the foliage sheath summit, a zig-zagged synflorescence axis, and obtuse, unawned lemmas. Meanwhile, R. sect. Racemiflorum is characterized by plants with viny culms with a scandent or climbing habit, racemiform synflorescences with a straight (non-zig-zag) axes, and acute, usually awned lemmas (Clark and Londoño 1991). Only three species: Rhipidocladum arenicola C.D.Tyrrell & L.G.Clark, R. cordatum C.D.Tyrrell & L.G.Clark, and R. harmonicum (Parodi) McClure comprise the sect. Rhipidocladum (Clark and Londoño 1991; Tyrrell and Clark 2013). The rest of the species belong to the sect. Racemiflorum.

Rhipidocladum species inhabit tropical forests from sea level to 2800 m a.s.l. with species in sect. Rhipidocladum generally found at higher elevations. The geographical distribution of Rhipidocladum spans from Mexico and Trinidad in the north to Argentina and central Brazil in the south (Tyrrell and Clark 2013; Ruiz-Sanchez et al. 2019, 2021a, 2021b). The six species in Mexico are all classified within the R. sect. Racemiflorum and include: Rhipidocladum barbinode Ruiz-Sanchez, C.D.Tyrrell & Vigosa (endemic), R. bartlettii McClure, R. martinezii Davidse & Pohl (endemic), R. pittieri (Hack.) McClure, R. racemiflorum (Steud.) McClure, and R. zoqueorum Ruiz-Sanchez, C.D.Tyrrell & Sosa (endemic) (Ruiz-Sanchez et al. 2019, 2021b).

Two species are known from the state of Jalisco: R. barbinode and R. racemiflorum. The first inhabits tropical dry forest and subdeciduous tropical forest glens, mainly in the southwestern part of the state at elevations of 500–1000 m a.s.l. The second inhabits tropical subdeciduous forest at the western edge of the state at elevations of 100–1000 m a.s.l. (Ruiz-Sanchez et al. 2021b) (Fig. 1). Until now, none of these species are found in sympatry. Rhipidocladum barbinode has spicate synflorescences, bearing 3–7 spikelets spaced 5–7 mm apart, glumes are awnless, and mucronate lemmas. Meanwhile, R. racemiflorum has racemose inflorescences with 10–13 spikelets, glumes have awns, and the lemmas are aristate.

During our fieldwork carried out in Puerto Vallarta, Jalisco along the Palo María, El Nogalito, and El Pitillal rivers, we found a population of flowering bamboos that can be clearly assigned to Rhipidocladum, but cannot be assigned to any known Mexican or American Rhipidocladum species. This new species has synflorescences with only a single, terminal spikelet (compared to all other Rhipidocladum species which have two to many spikelets). Further, these spikelets were strikingly wide for a Rhipidocladum with large, inflated lemmas. A few individual branchlets had spikelets bearing numerous (> 20) florets. In this study, we present a full description of this unusual new Rhipidocladum species and provide a distribution map, illustrations, photographs, and a new key to the species of Rhipidocladum in Mexico.

Fieldwork was carried out in November and December 2021, and from January to March 2022. We collected flowering specimens growing along the Palo María, El Nogalito, and El Pitillal rivers in Puerto Vallarta, Jalisco, Mexico (Fig. 1). Specimens from the following herbaria were examined: IBUG, MEXU, and ZEA (acronyms according to Thiers continuously updated). Collection procedures followed Soderstrom and Young (1983), thus complete specimens with branch complements, culm leaves, culm nodes, and internodes were preserved. The new specimens were deposited at IBUG. Macromorphological characters were measured using a centimetre ruler and micromorphological measurements were made using a millimetre-calibrated optical micrometre in a dissecting microscope. Measurement procedures follow Ruiz-Sanchez et al. (2019, 2021b). Morphological terminology follows McClure (1973) and Tyrrell and Clark (2013). The distribution map was made using QGIS v.2.16.3 (QGIS Development Team 2016). We followed Rzedowski (2006) to classify vegetation types. For biogeographic regionalization, we followed Santiago-Alvarado et al. (2016) and Morrone et al. (2017).

The conservation status was assessed based on the IUCN Red List categories and criteria (IUCN Standards and Petitions Committee 2022). The extent of occurrence (EOO) and area of occupancy (AOO), using 2 × 2 km grid cells (4 km²), were estimated using the Geospatial Conservation Assessment Tool (GeoCAT; Bachman et al. 2011).

Rhipidocladum singuliflorum is the most singular species of the genus. It is the first known to have synflorescences bearing one terminal spikelet. All other species in the genus have at least two (and usually more than three) spikelets per synflorescence axis. We observed some synflorescences in R. singuliflorum with two spikelets, but the second spikelet appeared to arise from a gemmiparous bract at the base of the first, suggesting this species is capable of producing pseudospikelets. It is possible that the single spikelets in R. singuliflorum evolved through reduction of synflorescence paraclades/co-florescences into the single spikelets observed. If true, this species may provide some insight into the evolution and origin of pseudospikelets within the bamboos.

Pseudospikelets have been evolved twice in Arthrostylidiinae: in the genera Alvimia C.E.Calderón ex Soderstr. & Londoño and Elytrostachys McClure (Tyrrell et al. 2012). Elytrostachys is nested in the same clade as Arthrostylidium Rupr., Didymogonyx (L.G.Clark & Londoño) C.D.Tyrrell, L.G.Clark & Londoño and Rhipidocladum, these last three genera have spikelets. Then R. singuliflorum could be a key species to understand spikelet-pseudospikelet evolution.

Rhipidocladum singuliflorum is nearly indistinguishable from R. racemiflorum in vegetative morphology, but strikingly different in the reproductive phase. In floral morphology, R. singuliflorum shares the inflated lemma characteristic of R. barbinode, but the latter species has spicate synflorescences with two or more spikelets, spaced 5–7 mm apart (Ruiz-Sanchez et al. 2021b). It is possible that R. singuliflorum and R. barbinode could be sister species, however, a molecular phylogenetic study would be needed to test this hypothesis.

This new species has likely been overlooked due to the long flowering cycles characteristic of many woody bamboos (Zheng et al. 2020). It is presumed that all Rhipidocladum species have gregarious and cyclical flowering events (Tyrrell and Clark 2013). According to our field observation, R. singuliflorum also exhibits a gregarious flowering pattern (Franklin 2004). In the three localities, plants were flowering simultaneously, however, we do not know the flowering cycle length as this is the first time it has been recorded in flower. Rhipidocladum racemiflorum is also present in the same region and in some places, such as Río Palo María and Río El Pitillal, and it is sympatric with R. singuliflorum (Fig. 1B). We did not see evidence of hybridization between the two species despite having collected R. racemiflorum in flower at Río Palo María in 2019. The individuals of R. racemiflorum were no longer flowering during our field investigations that uncovered R. singuliflorum in flower.

The Jalisciense-Tuito district together with the Jalisciense-Manantlán district are classified into the subprovince of Sierra Madre del Sur Occidental (Santiago-Alvarado et al. 2016). According to Aragón-Parada et al. (2021), this subprovince has 193 endemic vascular plants. The Sierra Madre del Sur is the richest woody bamboo floristic province in Mexico. Several other species of woody bamboos are known to inhabit the Jalisciense-Tuito district of the Sierra Madre del Sur (Santiago-Alvarado et al. 2016; Morrone et al. 2017), including: Chusquea circinata Soderstr. & C.E.Calderón, C. cortesii L.G.Clark & Ruiz-Sanchez, C. contrerasii Ruiz-Sanchez & L.G.Clark, C. guzmanii Ruiz-Sanchez & L.G.Clark, C. liebmannii E.Fourn., Guadua paniculata Munro, Otatea acuminata (Munro) C.E.Calderón & Soderstr., O. reynosoana Ruiz-Sanchez & L.G.Clark, and Rhipidocladum barbinode (Ruiz-Sanchez et al. 2020, 2021b, 2021c). Like R. singuliflorum, C. contrerasii is also endemic to this district (Ruiz-Sanchez et al. 2021c).

With the description of R. singuliflorum, the total number of species of Rhipidocladum increases to 22, with seven occurring in Mexico (Ruiz-Sanchez et al. 2021b). The number of native Mexican woody bamboo species also increases to 59, 42 (71%) of them endemic (Ruiz-Sanchez et al. 2022), and the number of woody bamboo species in the Sierra Madre del Sur province increases to 23 (Ruiz-Sanchez et al. 2020, 2021b, 2021c, 2022). The limited geographic distribution of R. singuliflorum and putative single population, coupled with whit its gregarious, semelparous, multiyear phenology, and the potential for roadway expansion in the area, led us to assign a preliminary IUCN category of Critically Endangered to this species. Given the prospective threats, we recommend a formal assessment to be performed to establish whether any additional subpopulations can be found and to monitor trends in their size, extent and, potentially, gene flow.