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Research Article
A striking new species of Rhipidocladum (Poaceae: Bambusoideae: Bambuseae: Arthrostylidiinae) with single, terminal-spikelet synflorescences, endemic to Jalisco, Mexico
expand article infoEduardo Ruiz-Sanchez, Christopher D. Tyrrell§|, Pablo Carrillo-Reyes, A. T. Nuño-Rubio
‡ Universidad de Guadalajara, Zapopan, Mexico
§ Milwaukee Public Museum, Milwaukee, United States of America
| Marquette University, Milwaukee, United States of America
¶ Puerto Vallarta Botanical Gardens, Puerto Vallarta, Mexico
Open Access

Abstract

Background and aimsRhipidocladum, a woody bamboo genus distributed from Mexico to Argentina, has raceme like synflorescences of multiple spikelets. Six of the 21 known species occur in Mexico. In this study, we present a full description, distribution map, illustrations, and photographs of an unusual new Rhipidocladum species endemic to Jalisco, Mexico. Additionally, we provide an updated key to the species of Rhipidocladum in Mexico.

Material and methods – This study was based on fieldwork, literature, and herbarium specimens review. Specimens collected were analysed and photographed during fieldwork. The conservation assessment is based on spatial analyses, following the IUCN guidelines and criteria.

Results – This is the first species in the genus Rhipidocladum that has synflorescences with only a single, terminal spikelet. Rhipidocladum singuliflorum occurs only in three localities in the municipality of Puerto Vallarta, Jalisco, Mexico. This species inhabits the canyon slopes of rivers in subdeciduous and tropical dry forests, at 6–150 m a.s.l. According to our IUCN assessment, this new species should be considered Critically Endangered.

Keywords

Arthrostylidiinae, Rhipidocladum sect. Racemiflorum, spicate synflorescence, tropical subdeciduous forest, woody bamboos

Introduction

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.

Figure 1. 

A. Geographical distribution of the species of Rhipidocladum in Mexico based on georeferenced localities of herbarium specimens (Ruiz-Sanchez et al. 2019, 2020, 2021b). B. Close up to the geographical distribution of R. singuliflorum (red stars) and R. racemiflorum (black circles) in western Jalisco.

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.

Material And Methods

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).

Taxonomic Treatment

Rhipidocladum singuliflorum Ruiz-Sanchez & C.D.Tyrrell, sp. nov.

Figs 2, 3, 4

Type

MEXICO – Jalisco • Puerto Vallarta, Río Palo María; 20°33’8.76”N, 105°15’34”W; 6 m; 10 Dec. 2021; fl.; E. Ruiz-Sanchez, A.T. Nuño, A. Zabalgoitia, L.A. Monroy & L. Campos 722; holotype: IBUG; isotypes: MEXU, ZEA.

Diagnosis

Rhipidocladum singuliflorum differs from R. barbinode and R. racemiflorum by having a single terminal spikelet. Meanwhile, R. barbinode has a spicate synflorescence with 3–7 spikelets and R. racemiflorum has a raceme synflorescence with 10–13 spikelets. R. singuliflorum has efimbriate or poorly developed fimbriae on the foliage leaves, meanwhile, R. barbinode and R. racemiflorum bear fimbriae at the apex of the sheath of the foliage leaves.

Description

Culm height 3–10 m. Internodes smooth, uniform in colour, 9–33 cm long, 3–9 mm in diameter, hollow (pithy when young); walls 2–3 mm thick. Culm leaves 17–19 cm long; sheaths (6.5–)7.6–9(–9.9) cm long, abaxially glabrous, adaxially glabrous and shiny; blades erect, 9–10 cm long, abaxially sparsely pubescent at the base, adaxially glabrous, margins entire; fimbriae not seen. Branch complements at mid-culm nodes with (30–)41–87(–120) branchlets; branchlets (8–)13–30(–50) cm long, glabrous or pubescent. Foliage leaves 3–4 observed on flowering branchlets; sheaths 9–22 mm long, abaxially pubescent (occasionally glabrous); mostly efimbriate, when fimbriate 0.2–0.8 mm long, white to stramineous; pseudopetioles 1.0–1.6 mm long, abaxially and adaxially pubescent; blades lanceolate to narrowly ovate with an attenuate apex, 32–75 mm long, 5–7 mm wide, abaxially and adaxially glabrous, without sparse patches of white cilia near the base on the abaxial side. Synflorescences 2.0–3.4 cm long, 0.9–10 mm wide, composed of a single spike, sometimes develop two spikelets, one of them with only two florets; rachis pubescent. Spikelets 2.0–3.4 cm long, comprising 2–4 glumes and (2–)4–5(–20) fertile florets, rachilla joints 3.5–4 mm long, glabrous. Glumes pubescent margins ciliate, awnless, lower glume, 3.2–5.8 mm long, 6-, 9-, 10-nerved, lanceolate; upper glume, 4.8–7.0 mm long, 11-, 13-nerved, lanceolate. Lemmas 8.8–12.0 mm long, 9-, 13-nerved, ovate and inflated, pubescent, apex rounded-obtuse and muticous. Palea 10.0–13.6 mm long, keels and sulcus pubescent, apex rounded-obtuse and muticous. Lodicules 3, abaxially glabrous, hyaline, margin ciliate apex fimbriate, the anterior pair 2.6–3.2 mm long, the posterior one 2.0–2.4 mm long. Anthers 3. Ovary 1.0–1.5 mm long, stigmas 2, plumose. Caryopsis 3.3–3.5 mm long, ellipsoid, pubescent with trichomes near base and apex, indented on hilum side, yellowish-golden.

Distribution

This species is only known from three localities in the municipality of Puerto Vallarta, Jalisco, Mexico in the Jalisciense-Tuito district of the Sierra Madre del Sur (Santiago-Alvarado et al. 2016) (Fig. 1).

Habitat and ecology

This species inhabits the slopes of humid canyons of creeks and rivers at 6–150 m a.s.l. The vegetation corresponds to subdeciduous and tropical dry forests with associated species, such as Astronium graveolens Jacq., Bursera spp., Casearia arguta Kunth, Cecropia obtusifolia Bertol., Cnidoscolus tepiquensis (Cost. & Gall.) Lundell, Croton schiedeanus Schltdl., Cupania dentata Moc. & Sesse, Dendropanax arboreus (L.) Decne. & Planch., Euphorbia spp., Ficus spp., Guettarda elliptica Sw., Inga laurina (Sw.) Willd., Lonchocarpus spp., Luehea candida Mart., Lysiloma divaricatum (Jacq.) Benth., Ouratea madrensis L.Riley, Pseudobombax ellipticum (Kunth) Dugand, and Zygia conzattii (Standl.) Britton & Rose, among others (Fig. 3A).

Etymology

Latin singulus, solitary, and florum, flowered, alluding to usual occurrence of single, terminal spikelet in synflorescence (Figs 2A, 3D–F). The use of -florum instead of -spiculum is preferred here [cf. R. racemiflorum].

Figure 2. 

Rhipidocladum singuliflorum. A. Culm segment showing foliage leaves complement and synflorescences. B. Culm segment, showing culm leaf sheath and blade. C. Culm leaf sheath and blade abaxial view. D. Spikelet bearing 2 glumes and 5 florets. E. Glumes. F. Floret, lateral (right) and dorsal (left) views. G. Palea, frontal (right) and lateral (left) views. Drawing by Juvenal Aargón Parada based on E. Ruiz-Sanchez, A. Nuño, A. Zabalgoitia L.A. Monroy y L. Campos 722 (IBUG).

Figure 3. 

Rhipidocladum singuliflorum. A. Panoramic view of the subdeciduous tropical forest glen habitat at the Palo María river, culms showing scandent habit. B. Basal clump showing culms habit and culm leaves. C. Cross section and size cut of culm, showing hollow culm with thick wall. D. Synflorescence branches bearing one terminal spikelet. E. Close up of the spikelet showing glumes, lemma, palea and florets. F. Spikelet. Photos by Eduardo Ruiz-Sanchez (A, B, C, E) and Pablo Carrillo-Reyes (D, F).

Preliminary IUCN conservation assessment

Critically endangered: CR B1ab(iii). Rhipidocladum singuliflorum is known from three localities that are separated by no more than 14 km. Using GeoCAT, the Extent of Occurrence (EOO) was calculated to be 10.6 km2, and the Area of Occupancy to be 12 km2, based on 2 × 2 km cells. The three localities are canyons that are not suitable for agriculture or grazing, and are often used for recreation with one of them (El Nogalito) currently under management by an ecotourism centre. The localities, however, are adjacent to the Carretera Pacífico (Carretera Federal 200), the major west coast highway, and potentially in the path of the planned series of “vía corta” highway expansions. A preliminary category of Critically endangered: CR B1ab(iii) is proposed following the IUCN (IUCN Standards and Petitions Committee 2022) criteria.

Figure 4. 

Rhipidocladum singuliflorum. A. Caryopsis dorsal view. B. Caryopsis ventral view showing indent near the hilum.

Additional specimens examined

MEXICO – Jalisco • Puerto Vallarta, 4 km al SE de Playa Grande, subiendo por el Río Pitillal (2.4 km en línea recta); 20°38’14”N, 105°10’14”W; 120 m; 18 Feb. 2022; fl.; P. Carrillo-Reyes & S. Quijas-Fonseca 10093; IBUG • Arroyo El Nogalito, 0.8 km en línea recta al SE del Ecoparque El Nogalito; 20°33’24”N, 105°14’10”W; 130 m; 20 Mar. 2022; P. Carrillo-Reyes & A.T. Nuño-Rubio 10106; IBUG • Río Palo María; 20°33’8.76”N, 105°15’34”W; 6 m; 16 Nov. 2021; fl.; A.T. Nuño & L. Campos s.n.; IBUG.

Key to the species of Rhipidocladum in Mexico

1. Plants with spikelets (reproductive stage) 2
Plants without spikelets (vegetative stage) 8
2. Synflorescence racemose or spicate with several, narrow, evenly-spaced spikelets 3
Synflorescence bearing a single, 0.9–10 mm wide spikelet, or a single spikelet-pair appearing to arise from the same terminal point R. singuliflorum
3. Spikelets with 3 glumes (the first sometimes small and acicular) 4
Spikelets with 2 glumes (more or less similarly shaped) 6
4. Mid-culm nodes with more than 100 subequal branchlets; foliage leaves linear-lanceolate (ratio of length to width 16–23) R. martinezii
Mid-culm nodes with fewer than 50 subequal branchlets; foliage leaves lanceolate (ratio of length to width 10–14) 5
5. Glumes puberulent to scabrous; foliage leaves adaxially scabrid, abaxially glabrous; sheath margins ciliolate; pseudopetioles scabrous R. bartlettii
Glumes glabrous; leaves adaxially glabrous, abaxially with a patch of hairs near the base; sheath margins entire; pseudopetioles glabrous R. pittieri
6. Glumes scabrous; lemmas 6.5–10 mm long; synflorescences with spikelets on both sides, spaced 5–20 mm apart 7
Glumes glabrous to puberulent; lemmas 5–7 mm long; synflorescences with spikelets secund, spaced 3–5 mm apart R. racemiflorum
7. Synflorescences 2.5–9 cm long, with 3–7 spikelets spaced 5–7 mm apart; glumes awnless …… R. barbinode
Synflorescences 13–22.5 cm long, with 13–21 spikelets spaced 5–20 mm apart; glumes awned … R. zoqueorum
8. Foliage leaf blades narrowly lanceolate to linear, 4–8 cm long, ratio of length to width 16–23 … R. martinezii
Foliage leaf blades lanceolate to narrowly ovate, 2.5–13 cm long, ratio of length to width 4–16 … 9
9. Foliage leaf blades abaxially retrorsely scabrous, fimbriae absent on the sheath summit ……… R. zoqueorum
Foliage leaf blades abaxially glabrous, fimbriae present or absent on the sheath summit 10
10. Foliage leaf blades sheaths with a copious band of hairs near the base R. barbinode
Foliage leaf blades sheaths glabrous or sparsely pubescent but not with a copious band of hairs near the base 11
11. Foliage leaf blades shorter than 5.5 cm and narrower than 5 mm wide; pseudopetioles less than 2 mm long; foliage leaf sheaths less than or equal to 2 cm long 12
Foliage leaf blades longer than or equal to 5.5 cm and wider than 5.5 mm; pseudopetioles more than 2 mm long; foliage leaf sheaths more than 2 cm long 13
12. Culms appearing solid or, if hollow, the wall thickness at mid-culm half or more the diameter of the lumen R. singuliflorum
Culms hollow with thin walls, the wall thickness at mid-culm (usually much) less than half the diameter of the lumen R. racemiflorum
13. Foliage leaves adaxially scabrid, abaxially glabrous; sheath margins ciliolate; pseudopetioles scabrous R. bartlettii
Foliage leaves adaxially glabrous, abaxially with a patch of hairs near the base; sheath margins entire; pseudopetioles glabrous R. pittieri

Discussion

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.

Acknowledgements

We thank Leonardo Campos, Luis Alberto Monroy, Sandra Quijas-Fonseca, and Alejandro Zabalgoitia for their assistance during the fieldwork. Juvenal Aragón Parada is thanked for the illustration. Curators and staff of the following herbaria provided access to their collections: IBUG, MEXU, WIS, and ZEA. We thank Ximena Londoño, the anonymous reviewer, and the associate editor for helpful comments.

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