Muhlenbergia spatha Columbus, Aliso 28: 66. 2010, non Muhlenbergia gracilis (Kunth) Trin. (1824).
Peterson, Paul M., Arrieta, Yolanda Herrera & Romaschenko, Konstantin, 2018, Phylogeny of Muhlenbergiasubg. Pseudosporobolus, including M. spatha (Poaceae, Chloridoideae, Cynodonteae, Muhlenbergiinae) now found in Zacatecas, Mexico, PhytoKeys 103, pp. 83-94: 83
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|Muhlenbergia spatha Columbus, Aliso 28: 66. 2010, non Muhlenbergia gracilis (Kunth) Trin. (1824).|
Schaffnera gracilis Benth., Hooker’s Icon. Pl. 14: 59, t. 1378. 1882. Schaffnerella gracilis (Benth.) Nash, N. Amer. Fl. 17(2): 141. 1912, non Schaffneria Fée ex T. Moore (1857). Muhlenbergia columbi P.M. Peterson, Amer. J. Bot. 97(9): 1543. 2010, nom. illeg. superfl. Type: Mexico, San Luis Potosí, mountains of San Miguelita, Aug 1876, J.G. Schaffner 1070 (holotype: K-000309066 [image!]; isotypes PH-00022592 [image!], US-397116!, YU-063983 [image!]).
Delicate annuals, loosely caespitose. Culms 5 –20(– 30) cm tall, erect, geniculate below; nodes scaberulous, branching at lower and middle nodes; internodes 2.0 –4.5(– 8.0) cm long, strongly 4-6 ribbed. Leaves caulescent and basal; ligules 0.8-1.8 mm long, membranous, decurrent, apex obtuse, minutely erose; sheaths 0.8-1.5 cm long, much shorter than the internodes, oblong, open, chartaceous, strongly ribbed with 7-9-veins, sometimes keeled, margins hyaline; blades 0.5-4 cm long, 0.5-1.5 mm wide, flat or folded, adaxially scattered pubescent near base, the hairs antrorse leaning, apex obtuse. Inflorescences compound, fasciculate, composed of terminal and axillary condensed panicles, these branched near the base, the basal-most branch usually with a sterile floret at the base consisting of two scales, the entire panicle partially included in a spatheolate sheath; sterile florets 2-4 mm long, 0.8-1 mm wide, linear-apiculate, flat, hyaline, 1-veined; racemose branches each bearing 1-5 fertile sessile spikelets, the spikelets separated by 1-4 mm on each branch; rachis angular, 3 or 4-ribbed. Spikelets 4.4-6 mm long, lanceolate, laterally compressed, solitary, composed of one fertile floret, plumbeous-mottled; rachilla not extended; callus short, blunt, pubescent, located just below the glumes where disarticulation occurs leaving a small cuplike tip; glumes dimorphic; lower glumes absent or obscure; upper glumes 3.5 –5(– 6) mm long, about as long as the lemma, oblong, chartaceous, firmer than fertile lemma (excluding the awns), 7-9-veined, lateral veins ribbed, pubescent along the veins on lower ½, apex deeply bifid and 3(5)-awned, the awns 5-7 mm long, scabrous, recurved, arising between the bifid apex; lemmas (4 –)4.8– 6 mm long, lanceolate, membranous, 3-veined, keeled, midvein scaberulous, lateral veins ribbed, apex acute to acuminate, minutely bifid, 1-awned, the awn 3-5 mm long arising from between the teeth; paleas 3.7-5.5 mm long, shorter than the lemmas, oblong, hyaline to membranous, tightly involute, 2-veined, apex obtuse, unawned; stamens 3; anthers 2-2.5 mm long, yellowish; ovary glabrous; caryopses 1.8-2 mm long, 0.5 mm wide, narrowly fusiform, straw coloured.
The species is known only from the type locality in San Luis Potosí and Municipio Villanueva, Zacatecas.
Muhlenbergia spatha was found by the authors growing on flat table rock in open areas near arroyos associated with Bouteloua hirsuta Lag., B. curtipendula (Michx.) Torr., Schizachyrium sanguineum (Retz.) Alston, Quercus , Juniperus , Muhlenbergia implicata (Kunth) Trin., M. phleoides , M. rigida (Kunth) Kunth, Chloris submutica Kunth, Digitaria ternata (A. Rich.) Stapf, Microchloa kunthii Desv., Aristida adscensionis L., A. divaricata Humb. & Bonpl. ex Willd., Enneapogon desvauxii P. Beauv., Piptochaetium fimbriatum (Kunth) Hitchc., Eragrostis intermedia Hitchc., E. pectincea (Michx.) Nees and Eleusine multiflora Hochst. ex A. Rich.
At the southern edge of the Sierra Madre Occidental Range in the Sierra Fría de Aguascalientes, we recently found Muhlenbergia spatha in two localities south of the city of Zacatecas in a corridor located east of Villanueva: 1) 17.4 km east of Villanueva on flat table rock just above Arroyo "El Muerto" in open areas and 2) about 1 km northeast of the small village Palomas Viejas along Arroyo Juan Manuel on natural grasslands near cultivated fields. These two sites are approximately 204 km W (air distance) from the type locality southwest of San Luis Potosí in the Sierra de San Miguelito along the Río Potosino ( Columbus et al. 2001). According to Instituto Nacional de Estadística, Geografía e Informática [ INEGI] (2003, 2005), the Sierra San Miguelito is placed in the Mesa del Centro Province in the Sierra Madre Oriental. Near Villanueva, Zacatecas, the soils are Haplic Phaeozem (forming sodium carbonate, Na2CO3) over extrusive igneous rock (acidic) in a semi-dry climate with an annual mean temperature of 16°C and an annual mean precipitation of 60 cm ( INEGI 2008a, 2008b). In the Sierra de San Miguelito along the Rio Potosino, the soils are Calcic Regosol (forming calcium carbonate, CaCO3) of medium texture over extrusive igneous rock (acidic) in a semi-dry climate with an annual mean temperature of 16 °C and an annual mean precipitation of 40 cm ( INEGI 2008a, 2008b). Without field verification, it is uncertain whether the soil differences among these sites are significant, but we are reasonably certain that all are alkaline with an elevated pH.
The species is rare in Mexico and is known from only three recent collections. Since it is a diminutive, short-lived annual, the species is easily overlooked and the main concern seems to be loss of habitat via human impact, i.e. agriculture, dam and road construction.
Mexico. San Luis Potosí: 1876, J.G. Schaffner 134 (GOET-006918 [image!], US-825687); Sierra de San Miguelito, Río Potosino, 22°04'55"N, 101°03'51"W, 1980 m, 2 Oct 2001, J.T. Columbus 4040 (RSA [image! in Columbus et al. 2001]). Zacatecas: Mpio. Villanueva, Arroyo Juan Manuel ± 1 km NE of Palomas Viejas near Villanueva, 22°24'36"N, 102°43'01"W, 2112 m, 22 Oct 2006, O. Rosales 3490 & Y. Herrera Arrieta (CIIDIR); Mpio Villanueva, 10.8 mi [17.4 km] E of Villanueva, just above arroyo "El Muerto", 22°22'47.1"N, 102°43'31.5"W, 2083 m, 2 Oct 2015, P.M. Peterson 25544 & Y. Herrera Arrieta (CIIDIR, US).
The hand written script (verified by J. Rzedowski, per. comm., also see Rzedowski 1959) on the label of the holotype (K) says: “Müehlenbergia gracilis mihi", indicating that Jose Guillermo Schaffner thought he had collected a new species of Muhlenbergia . Bentham (1882) agreed with Schaffner that it was a new species but thought it had enough unique morphological features (spatheolate sheathed panicles, sessile spikelets and 3(5)-awned upper glumes) to warrant description of a new genus.
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