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Arbutus bicolor (Ericaceae, Arbuteae), a new species from Mexico.

Arbutus L. (Ericaceae, Arbuteae) includes at least 12 species, three occur in the Mediterranean Basin and along the western coast of Europe, one in the Canary Islands, and eight in North and Central America, where the genus is an important element of pine-oak forest. Arbutus menziesii Pursh is known from the west coast of North America, but the greatest species diversity of the genus is found in Mexico, where seven species with an extraordinary diversity of forms and confused taxonomy are known. Three of these Mexican species have been described in the last three decades: Arbutus occidentalis (McVaugh & Rosatti, 1978), A. tessellata (Sorensen, 1987), and A. madrensis (Gonzalez Elizondo & Gonzalez Elizondo, 1992) and a fourth species (Arbutus mollis H.B.K.) will be resurrected after almost a century of being considered as a synonym of A. xalapensis (Gonzalez-Elizondo et al., in rev.). With respect to the remaining Mexican species, A. arizonica (A. Gray) Sarg. is known from the SW United States to Central Mexico and A. xalapensis is a broadly variable taxon known from SW United States to Central America. The seventh Mexican species, twelfth for the genus, is described herein for plants that have been generally misidentified as A. glandulosa.

Arbutus bicolor S. Gonzalez, M. Gonzalez et P. D. Sorensen, sp. nov. Arbutus glandulosa of authors, non A. glandulosa M. Martens & Galeotti.

Species nova A. xalapensi H.B.K. similis a qua differt ramulis et petiolis pubescentibus trichomatibus glandulosis, foliis discoloribus subtus incanis vel ochroleucis dense trichomatibus crispatis tectis, et floribus roseis raro albis (Fig. 1, 2).

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[FIGURE 2 OMITTED]

Trees 4-25 m tall; bark exfoliating in smooth flakes over most larger limbs, the inner bark smooth, creamy, pale rose to dark cinnamon, the bark on base of trunk and eventually over the oldest parts of plant retained as pale gray, checkered, isodiametric or rectangular segments; bark of second-year branchlets soon exfoliating, leaving the inner bark smooth, reddish to dark-cinnamon sometimes with some glaucous cover; twigs of current year and petioles densely glandular pubescent, the glandular hairs 0.2-1.5(-2.5) mm long, mixed with eglandular hoary pubescence of shorter and curled hairs. Petioles (1.5-)2-6 cm long; leaf blades ovate, rarely oblong or oblong-lanceolate, 6-14 cm long, 4-9.5 cm wide, apex rounded to shortly acute, base cordate, truncate or rounded, often slightly asymmetrical, margin entire or irregularly serrate particularly on sprout leaves, blades bicolored, above dark green (turning red at senescence), opaque or slightly glossy, sparsely short glandular pubescent with or without additional eglandular whitish hairs, rarely glabrescent, the veinlets raised forming small areoles that give an irregular aspect to the minutely papillous surface, lower surface of blade creamy to ochroleucous because a dense and uniform indument of short, tightly and irregularly curled hairs hides the surface, often also with glandular hairs 0.2-0.3 mm long on the surface and longer glandular hairs (up to 1.5 mm long) along the midvein. Inflorescence a terminal cluster of racemes, showy, up to 13 cm long and to 16 cm wide, usually openly-branched, rarely with only 2-3 racemes, branches 2-9 cm long, 2-4 cm wide; axes densely hairy, with both glandular hairs and short eglandular hairs intermixed; pedicels accrescent and elongated in fruit, up to 15 mm long, with both glandular and eglandular hairs; bracts ovate to lanceolate, widened at the base, yellowish to purplish or dark reddish, densely short pubescent with glandular hairs and crispy eglandular hairs, 3-11 mm long, 2.3-4.6 mm wide including the margin that can be dark and scarious or concolorous and not scarious; two inner bracteoles 2-3 mm long, pubescent at least on the middle portion and margins. Flowers: calyx lobes 1.2-2.4(-3.5) mm long, apex acute or obtuse, often pink to pale purplish, ciliate; corolla 5.6-9 mm long, 5-7 mm wide, pink, rarely white or white with pink tinge (immature flowers sometimes recorded by collectors as white reddish or creamy with salmon tinge), pubescent or glabrous outside, lobes auriculate imbricate at the base, reflexed at anthesis; base of filaments densely villous with white hairs 0.4-0.8 mm long, anthers 1.1-1.5 mm, dehiscent by pores 1/3-1/2 the length of the thecae, spurs 0.6-1.2 mm, occasionally bearing a short spur at the base, also occasionally a diminutive (0.2-0.4 mm long) translucent appendix is present between the pores. Berries orange to deep red or purple-red, up to 9 mm long and 7-10 mm in diam.

Type: MEXICO. Durango: municipio El Mezquital: al NE de Taxicaringa, por el camino a Durango, 23[grados]14' N, 104[grados]45' W, 30 Apr 1992, 2600 m, M. Gonzalez 2636 (holotype: CIIDIR, isotypes to be distributed).

Common names: madrono, madrono rojo. Uses (rarely recorded): an infusion for drink is made with the leaves; the fruits are edible in small amounts (large quantities cause dizziness).

Arbutus bicolor is related to A. xalapensis and specimens have been usually identified as A. glandulosa M. Martens & Galeotti, a glandular morph of the former species that Sorensen (1995) appropriately reduced to synonymy. The glandular and eglandular morphs of A. xalapensis occur sometimes in the same population, a situation observed even at the type locality of the species near Xalapa, Veracruz. Arbutus bicolor can be distinguished from A. xalapensis (including its glandular morphs) by the following combination of characters: a) twigs of current year and petioles densely glandular pubescent, the glandular hairs mixed with eglandular hoary pubescence of shorter and curled hairs (vs. eglandular or glandular, villous, lacking in hoary pubescence); b) markedly bicolored leaves that are whitish to pale ochroleucous below due to a dense and uniform indument of tightly and irregularly contorted hairs (vs. sparse indument of flexuous, often twisted and/or floccose hairs); and c) pink, rarely white flowers (vs. white, creamy, yellowish or greenish, rarely rose flowers). Branch limbs in A. bicolor are somewhat ascending, forming a relatively narrow crown and the bark is usually rose creamy to cinnamon; A. xalapensis has ascendant or spreading limbs, a broad or narrow crown, and orange, rose, or creamy bark. Arbutus bicolor is prone to fungus infections, with the glands of twigs and petioles usually blackened because fungus growth and the indument of the lower surface of leaves very often spotted with dark, circular colonies of fungi. Arbutus bicolor does not fit into any of the other synonymous taxa noted in Sorensen's Flora Neotropica treatment.

McVaugh & Rosatti (1978) noted that plants called A. glandulosa in western Mexico differed in several aspects from "everything in the complex of A. xalapensis", for example, they are copiously beset with gland-tipped hairs (1-)2-4(-7) mm long and rough, flaking, but persistent bark that often contrasts markedly with the smooth bark of A. xalapensis. Those features correspond to A. tessellata, another conspicuously glandular species (Sorensen, 1987). The main differences between A. bicolor and other Mexican tree species of Arbutus are presented in a key below.

The ecological preferences of A. bicolor and A. xalapensis also are different. The former species develops in sites with colder climates than A. xalapensis. In the Sierra Madre Occidental, for example, A. bicolor grows at higher elevations (2200-3300 m) on the interior-facing and eastern slopes whereas A. xalapensis is restricted to middle elevations (1500-2700 m) of the western slopes of the range, where warm-temperate climates are prevalent, although occasionally both species can be quasi-sympatric in ecotones between their preferred habitats. The habitat of the southernmost known population of A. bicolor (Puebla, Calzada et al. 04688) is described as "very cold, with strong winds from the north."

Distribution and habitat: Arbutus bicolor is widely distributed in the Sierra Madre Occidental and the Transvolcanic Belt, reaching the western slopes of the Sierra Madre Oriental in central Mexico. It is known from Chihuahua and Durango to Distrito Federal and Puebla (Fig. 3). Arbutus bicolor evidently prefers humid slopes and ravines in conifer (Abies-Pinus), pine, pine-oak, or oak forest, but also develops in sclerophylous scrub with Quercus, Arctostaphylos and Garrya and in "subalpine" grassland, as well as in rough, stony hills with pine-oak or pine-juniper dry forest. Some of the associations in which A. bicolor becomes abundant are: at the Sierra Madre Occidental: a) Abies durangensis, Pinus cooperi, P. ayacahuite, Quercus sideroxyla, Populus tremuloides; b) Quercus sideroxyla with P. durangensis, P. ayacahuite and Q. crassifolia; c) Quercus sideroxyla with P. teocote and Q. rugosa with Juniperus deppeana understory; d) Pinus teocote; e) Pinus teocote with P. arizonica, Q. crassifolia, Q. rugosa, Q. obtusata, and P. ayacahuite; f) Pinus lumholtzii-Quercus spp., all those on slopes and ravines; and g) Pinus cooperi and/or P. leiophylla on mountain valleys. In central Mexico: a) Pinus rudis-Quercus spp.-Arbutus spp. b) Pinus cembroides-Juniperus flaccida. Alt. 2200-3400 m. Flowering February-June (July), fruiting all year round, mainly May-November but fruits persisting.

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Additional specimens examined: Mexico. Chihuahua: municipio Bocoyna: Bocoyna, on mesa west of Creel, between Creel and rio Oteros, 2360 m, R. Bye y W. A. Weber 8197 (MEXU, MICH); Bocoyna, ejido San Ignacio Arareco, R. A. Bye 8527 (MEXU); W of Creel toward Sierrita Blanca, on Creel-Sanchez road, R. Bye 9529 (MEXU); NE of San Ignacio Arareco, N of old road between San Ignacio and Panalachic, R. A. Bye 3058, 3061 (MEXU); above arroyo del Ojito between Choguita and Cusarare, R. A. Bye 3173 (MEXU); municipio Balleza: El Vergel, al S, 0.3 km de Cueva Blanca, km 128.5 carret. 24 Parral - Guadalupe y Calvo tramo El Vergel - Cienega Larga, 26[grados]22'41" N, 106[grados]22'5" W, S. Gonzalez s.n., D. Ramirez, S. Tena y A. Mastretta (CIIDIR); El Vergel, J. J. Luna 6 (CHAPA, ENCB); municipio Guadalupe y Calvo: Cerro Mohinora, 10 mi SW of Guadalupe y Calvo, R. Straw & M. Forman 1960 (MICH); ibid., entronque del camino a la cima con la carretera Guadalupe y Calvo - Bovorigame, 26[grados]4'16" N, 106[grados]58'32" W, S. Gonzalez 7354c A. Ayala y D. Stancik (CIIDIR, IEB, MEXU); ibid., canada por el camino a la cima, 25[grados]58'36" N, 107[grados]2'42" W, S. Gonzalez 7351, A. Ayalay D. Stancik (CHIH, CIIDIR, IEB, MEXU). Durango: municipio Guanacevi: Cerro Barajas, cima, 26[grados]23'30" N, 106[grados]5'0" W, 3310 m, 24.08.2004, S. Gonzalez 6996, M. Gonzalez, M. A. Marquez, O. Cazares (CIIDIR, DEK); municipio Tamazula: ejido El Tecuan, parte alta del Desmonte de Enrique, al SO de la comunidad San Juan del Tecuan, 25[grados]34'42" N, 106[grados]59'16" W, 2605 m, 02.04.2008, F. Mercado 254 D. Ramirez N. y E. Borgas (CIIDIR); municipio Tepehuanes: Las Conchitas, vivero, al W, cerca de arroyo, El Tarahumar, 25[grados]39'40" N, 106[grados]19'35" W, 2575 m, S. Acevedo 922 (CIIDIR); El Santo Nino, El Tarahumar, Cordon Mocho al N del 2[grados] vivero, a orilla del vivero, 25[grados]39'20" N, 106[grados]19'5" W, S. Acevedo 865 y 866 (CIIDIR); El Tarahumar, a un lado del vivero No. 1, 25[grados]39' N, 106[grados]19' W, S. Acevedo 846y 847 (CIIDIR); falda de la Manga Quemada, bajada del Santo Nino, 6 km al SE de La Atascosa, comunidad de El Tarahumar, E. Guizar N. 2149 (MEXU); El Huacal, 29 km por el camino a El Tarahuamar, 25[grados]36' N, 106[grados]19' W, M. Gonzalez 2619 (CIIDIR); Altos de Yesqueros, camino de El Huacal a Quebrada Honda, 25[grados]34' N, 106[grados]24' W, M. Gonzalez 2453, 2454, 2456, 2458, 2462 (CIIDIR); Bajio de Los Arcos, por el camino de El Huacal a Quebrada Honda, 25[grados]29' N, 106[grados]27' W, M. Gonzalez 2490 y 2495 (CIIDIR); 3 km de El Huacal, por el camino a El Tarahumar, 25[grados]28' N, 106[grados]11' W, M. Gonzalez 2448, 2449, 2450, 2451 (CIIDIR); 73 km de Tepehuanes, por el camino a El Huacal, 25[grados]25' N, 106[grados]7' W, M. Gonzalez 2438 (CIIDIR); ibid., 2439 (CIIDIR, DEK, MEXU); ibid., 2441, 2444, 2445 (CIIDIR); Tepehuanes, 56 km por el camino a El Huacal, 25[grados]21' N, 106[grados]0' W, M. Gonzalez 2427, 2428, 2431, 2433, 2434 (CIIDIR); ibid., a 3 km de la Cienega del Fraile, M. Gonzalez 2425, 2426 (CIIDIR); 44 km de Tepehuanes, por el camino a El Huacal, 25[grados]14' N, 105[grados]52' W, M. Gonzalez 2422, 2423, 2424 (CIIDIR); Llanito de Buenos Aires, Mesa de Navar, 25[grados]14' N, 105[grados]55' W, E. Guizar 2108 (CHAP, CIIDIR, MEXU); 3 km de Arroyo Chico, sobre el camino Tepehuanes - El Huacal, A. Benitez y O. Bravo 394 (CHAP, CIIDIR, MEXU); Cuevecillas, 0.5 km del campamento de la UAF Topia, 25[grados]9' N, 106[grados]25' W, O. Bravo 1154 (CIIDIR, CHAP); Buenos Aires, 36 km al W de Tepehuanes, por la brecha a Topia, P. Tenorio 1169 y C. Romero de T. (ENCB, MEXU); 21 km al W de Tepehuanes, brecha a Tabahueto, P. Tenorio et al. 4186 (CIIDIR, MEXU); Boleras, 10 km al SE, rumbo a la Sierra de la Candela, 25[grados]37' N, 105[grados]25' W, S. Gonzalez 5209, 5215 (CIIDIR); municipio Topia: Cuevecillas, 0.5 km antes de llegar al pueblo, 25[grados]9' N, 106[grados]25' W, A. Benitez 2469 (CHAP, CIIDIR); Cuevecillas, atras del campamento de la UAF Topia, 25[grados]9' N, 106[grados]25' W, A. Benitez 2335 (CHAP, CIIDIR, IEB); municipio Santiago Papasquiaro: 135 km de Santiago Papasquiaro, por el camino a Canelas, 25[grados] N, 106[grados] W, S. Gonzalez, S. Acevedo y J. Panero 4949 (CIIDIR); 54 km de Altares, por el camino a Canelas, 25[grados]3' N, 106[grados]15' W, S. Gonzalez, S. Acevedo y J. Panero 4996 (CIIDIR); Santiago Papasquiaro, aprox. 40 km al W por el camino a Canelas, 25[grados]06' N, 105[grados]38' W, S. Gonzalez 4930, 4932 S. Acevedo (CIIDIR); Las Ranas, camino Santiago Papasquiaro - Los Altares, E. Guizar 2393 (CHAP, CIIDIR, IEB, MEXU); Las Ranas, camino Santiago Papasquiaro - Los Altares, 25[grados]2' N, 105[grados]41' W, E. Guizar 1013 (CIIDIR, CHAP, MEXU); municipio El Oro: Sierra de Promontorio, al NE de El Encinal, por el camino a la mina de Promontorio, 25[grados]10'47" N, 105[grados]09'41" W, S. Gonzalez 6783, P. M. Peterson, L. E. Brothers (CIIDIR, IEB, MEXU, US); municipio Otaez: El Castillito, camino a Otaez, 24[grados]57' N, 105[grados]48' W, E. Guizar 2387 (CIIDIR); proximidades a la desviacion Pielagos - Otaez (infl reducida a dos racimos), E. Guizar 2365 (CHAP, CIIDIR, IEB); proximidad de Bajio de Vacas, ej. Las Hacienditas, 24[grados]54' N, 105[grados]56' W, E. Guizar 2379 (CHAP. CIIDIR); municipio Nuevo Ideal: Canada del Gato, sierra en Nuevo Ideal, 24[grados]54'3" N, 105[grados]12'34" W, A. Garcia A. 3538 (CIIDIR); municipio San Dimas: Vencedores, area 43, 24[grados]25' N, 105[grados]42' W, S. Gonzalez 5146, M. A. Marquez (CIIDIR); La Guitarra, Lote 4, 24[grados]25" N, 105[grados]36" W, S. Gonzalez 5579 M. A. Marquez y M. Gonzalez (CIIDIR); Los Aposentos, 2.5 km al NE de San Miguel de Cruces, A. Garcia, M. Gonzalez y S. Acevedo 427 (CIIDIR, IEB); Las Pintas, La Pista, 24[grados]19' N, 105[grados]34' W, S. Gonzalez 7321a, R. Silva, F. Maciel, S. Ortega R., M. Soto (CIIDIR); Miravalles, UCODEFO 4, 24[grados]17' N, 105[grados]32' W, S. Gonzalez 5119 y M. A. Marquez (CIIDIR); Cerro Huehuento, vertiente N, 24[grados]4'45" N, 105[grados]44'37" W, S. Gonzalez s.n., M. Gonzalez, A. Torres, L. Ruacho, G. Medrano, S. Heines, I. Estrada, E. Larreta (CIIDIR); Cerro Huehuento, al S de Huachichiles, 1 km al NNW de la cima, 24[grados]4'42" N, 105[grados]44'35" W, S. Gonzalez 6845 (CIIDIR); Cerro Huehuento, sobre vertiente N, 24[grados]4'39" N, 105[grados]44'29" W, S. Gonzalez 7720, M. Gonzalez, A. Torres, L. Ruacho, G. Medrano, S. Heines, I. Estrada, E. Larreta (CIIDIR); Cerro Huehuento, S of Huachichiles, J. H. Maysilles 7286 (MICH); San Luis del Rio, 51 road mi northwest of Coyotes, J. H. Maysilles 7234 (MICH); ibid. 7974 (MEXU, MICH); municipio Canatlan: Sierra del Epazote, cima del cerro de las antenas, 24[grados]36'12" N, 105[grados]6'37" W, M. G. Nava Miranda 148, J. Noriega, G. M. Valtierra y L. Ruacho (CIIDIR); Sierra del Epazote, [+ or -]2.5 km de La Cieneguita por el camino a la Cienega de la Casa, 24[grados]35'26" N, 105[grados]0'22" W, M. Gonzalez 3369, R. Alvarez Z. y C. Lopez G. (CIIDIR); rancho El Durangueno, ejido Nogales, punto 33, 24[grados]26'5" N, 104[grados]53'13" W, A. Garza Herrera s.n., F. Sanchez, D. Mata, A. Arvizu, S. Gonzalez (CIIDIR, IBUG); Sierra del Epazote, rancho El Durangueno, 24[grados]22'14" N, 105[grados]0'16" W, S. Gonzalez 6292, M. A. Marquez y C. Lopez (CIIDIR); municipio Panuco de Coronado: Sierra de Gamon, ladera NW, cerca de la cima, 24[grados]35'21" N, 104[grados]16'45" W, S. Gonzalez 7383, L. Lopez, L. Resendiz, D. Ramirez, F. Mercado, M. Ramirez y L. A. Ramirez (CIIDIR); municipio Durango: ejido La Esperanza, 23[grados]55'31" N, 105[grados]17'54" W, A. Garcia 4359 (CIIDIR, HUAA); Otinapa, 23[grados]59'11" N, 104[grados]57'42" W, M. Flores 7-OT (CIIDIR); ibid., M. Flores 8-OT (CIIDIR); Otinapa, al S, 23[grados]59'11" N, 104[grados]57'42" W, S. Gonzalez 7646, J. Gimenez de Azcarate, M. A. Macias (CIIDIR); parque El Tecuan, junto a Las Conchas viejas y las cabanas abandonadas, 23[grados]54' N, 105[grados]30' W, S. Acevedo 1194 (CIIDIR, IEB); parque El Tecuan, extremo E de Mesa del Mirador, 23[grados]54'9" N, 105[grados]01'50" W, S. Gonzalez 5785, A. Garcia y S. Acevedo (CIIDIR); parque El Tecuan, alrededores, carretera Durango - El Salto, 23[grados]54' N, 105[grados]1' W, A. Roman 5 y 6 S. Gonzalez (CIIDIR); parque El Tecuan, 58 km de Durango, F. Casillas et al. 89 (CIIDIR); 35 mi W of Durango, on rt. 40, W. Hess & M. Hall (MICH); Subcuenca Santiago Bayacora, La Mesa Alta, Sitio 5, 23[grados]45'4" N, 104[grados]36'31" W, J. Acevedo Herrera 198 (CIIDIR); Subcuenca Santiago Bayacora, La Mesa de los Difuntos, sitio 4, 23[grados]41'19" N, 104[grados]33'50" W, J. Acevedo Herrera 191 (CIIDIR); Subcuenca Santiago Bayacora, Pilares, sitio 4, 23[grados]35'11" N, 104[grados]44'38" W, J. Acevedo Herrera 60 (CIIDIR); Subcuenca Santiago Bayacora, Al S de los Bajios de Don Victor, 23[grados]30'19" N, 104[grados]40'2" W, J. Acevedo Herrera 202 (CIIDIR); Mesa de Tableteros, 23[grados]37'29" N, 104[grados]44'25" W, S. Gonzalez s.n., L. Ruacho, N. Gonzalez, O. Rosales, G. Medrano V. y D. Salinas (CIIDIR); Carboneras, al NE, al SW de La Flor, 23[grados]31'29" N, 104[grados]48'32" W, S. Gonzalez 7703, L. Ruacho, N. Gonzalez, O. Rosales, G. Medrano y D. Salinas (CIIDIR); La Flor, al W, 23[grados]31'27" N, 104[grados]48'28" W, S. Gonzalez s.n. et al. (CIIDIR); municipio Pueblo Nuevo: El Salto, ca. 2 km al W, al S de la carretera 40 Durango - Mazatlan, 23[grados]46'43" N, 105[grados]23'6" W, S. Gonzalez 7633, J. Gimenez de Azcarate, M. A. Macias (CIIDIR); region de El Salto, G. Guzman et al. 18.IX.1961 (ENCB); Santa Barbara, 6 km por el camino a El Salto, 23[grados]45' N, 105[grados]25' W, A. Garcia 1093 S. Gonzalez (CIIDIR, IEB); Arroyo Santa Barbara, al SW de El Salto, 23[grados]39' N, 105[grados]25' W, S. Gonzalez 5260 (CIIDIR); Penitas, aprox. 13 km al SW de El Salto, 23[grados]39' N, 105[grados]24' W, M. Gonzalez 4010 y 4002 M. Flores V., S. Quinones (CIIDIR); Santa Barbara, al S, por el camino a El Sotano, al W del camino a Pueblo Nuevo, 23[grados]38'26" N, 105[grados]25'26" W, S. Gonzalez 6823 P. M. Peterson y G. Tena (CIIDIR); Coyotes Hacienda, 63 mi of C. Durango, J. Maysilles 7721 (MICH); ejido El Brillante, la canada de Picea, carretera Durango - Mazatlan, km 99, 23[grados]46' N, 105[grados]20' W, Equipo 2 Fac. Ciencias Forestales s.n. (CIIDIR); Km 1119, road Durango-Mazatlan, 999 ft, WeedonsM665 (MEXU); La Ermita, aprox. 100 m al W, cerca del km 158.5 de carretera Durango - Mazatlan, 23[grados]40'22" N, 105[grados]43'35" W, S. Gonzalez 6938, J. A. Villarrealy L. Chavez (CIIDIR); Buenos Aires, 7 km al W, extremo W del poblado La Ermita, 23[grados]40'13" N, 105[grados]43'25" W S. Gonzalez 6009a J. Panero (CIIDIR, IEB); Chavarria (cercanias del entronque a), km 131 carretera Durango - Mazatlan, 23[grados]44' N, 105[grados]34' W, S. y M. Gonzalez 4883 (CIIDIR); ibid., km 134, 23[grados]43' N, 105[grados]35' W, S. y M. Gonzalez 4882 (CIIDIR); 26 km al W de El Salto, S. Gonzalez y A. Lux 4435 (CIIDIR); arroyo La Tecolota, predio Las Bayas de la UJED, 23[grados]25' N, 104[grados]51' W, A. Garcia A. 1036 (CIIDIR, IEB); municipio Suchil: Reserva de la Biosfera La Michilia, Cerro Blanco, Playa Grande, 23[grados]28'48" N, 104[grados]16'26" W, G. Amacio 12-1 (CIIDIR); ibid., 23[grados]28'19" N, 104[grados]16'19" W, G. Amacio 9-2 (CIIDIR); Reserva de la Biosfera La Michilia, Cerro Blanco, Cordon de las Venadas, 23[grados]28'4" N, 104[grados]15'59" W, G. Amancio 22-1 (CIIDIR); ibid., G. Amancio 22-3 (CIIDIR); ibid., G. Amancio 23-2 (CIIDIR); ibid., G. Amancio 23-3 (CIIDIR); ibid., 23[grados]27'46" N, 104[grados]16'4", G. Amancio 20-2 (CIIDIR); Reserva de la Biosfera La Michilia, Cerro Blanco, Encina Gorda, 23[grados]27'49" N, 104[grados]15'48" W, G. Amacio 15-3 (CIIDIR); ibid., G. Amacio 15-4 (CIIDIR); ibid., Encina Gorda, 23[grados]27'35" N, 104[grados]16'1" W, G. Amacio 13-4 (CIIDIR); Reserva de la Biosfera La Michilia, Cerro Blanco, punto Cacahuates, al S del cerro Magueycitos, 23[grados]27' N, 104[grados]16' W, J. Alvarado 290 (CIIDIR, IEB); Reserva de la Biosfera La Michilia, Cerro Blanco, 23[grados]27' N, 104[grados]17' W, S. Gonzalez 3736 (CIIDIR); Reserva de la Biosfera La Michilia, Cienega de Los Caballos, 23[grados]26'43" N, 104[grados]15'53" W, J. M. Ibarra M. A8 (CIIDIR); Reserva de la Biosfera La Michilia, Cordon de las Culebras, S. y M. Gonzalez 4795 (CIIDIR); Reserva de la Biosfera La Michilia, vados de El Taray, S. y M. Gonzalez 4737 (CIIDIR); ibid., Mesa de San Antonio, L. Jamieson 10 (CIIDIR); ibid., L. Jamieson 44 (CIIDIR); ibid., L. Jamieson 56 (CIIDIR); ibid., L. Jamieson 181 (CIIDIR); Mesa de San Antonio, cerca del Taray, L. Jamieson 227 (CIIDIR); arroyo El Taray, al SE, al pie de La Mesa El Burro, S. Acevedo 427 D. Bayona (CIIDIR); Reserva de la Biosfera La Michilia, Mesa de San Antonio hacia la Mesa El Burro, S. Gonzalez y L. Jamieson 4270 y 4273 (CIIDIR); Reserva de la Biosfera La Michilia, Mesa El Burro, 23[grados]23' N, 104[grados]17' W, R. Fernandez 1117 (CIIDIR, ENCB); Reserva de la Biosfera La Michilia, W, L. Jamieson 682 (CIIDIR); ibid., L. Jamieson 989 (CIIDIR); ibid., L. Jamieson 468 (CIIDIR); ibid., L. Jamieson 319, 354, 382, 399 (CIIDIR); Reserva de la Biosfera La Michilia, arroyo El Ranchero, bajada, al SW de la reserva, 23[grados]22'20" N, 104[grados]18'53" W, S. Gonzalez 4748, 4749M. Gonzalez (CIIDIR); Reserva de la Biosfera La Michilia, El Pujido, S. Gonzalez y L. Jamieson 4277 (CIIDIR); Reserva La Michilia, Toribia, M. E. Maury, V. Serrano y S. Gallina 23 (IEB); Reserva de la Biosfera La Michilia, arroyo El Pajonal, 23[grados] N, 104[grados] W, L. Renteria 5 y A. Garcia (CIIDIR); arroyo El Ranchero, al S de la Reserva La Michilia, S. y M. Gonzalez 4745 (CIIDIR); Cerro Las Iglesias, 7 km de El Aleman, F. Acevedo 113 (CIIDIR); cordon de la Sierra de Urica, atras del cerro Chihuahuilla, 23[grados]25' N, 104[grados]6' W, J. M. Ibarra M. Ar1 (CIIDIR); municipio El Mezquital: La Escondida, km 64 del camino que va a Los Charcos, camino a El Mezquital, S. Acevedo 435 D. Bayona (CIIDIR, IEB); Francisco I. Madero (Pajaritos), al N, en pico S de cerro, cerca de camino maderero abandonado, 22[grados]44'36" N, 104[grados]15'48" W, S. Gonzalez 6470 P. M. Peterson (CIIDIR). Zacatecas: municipio Sombrerete: Laguna Balderrama, J. D. Webster 2 (MICH). San Luis Potosi: municipio Villa de Reyes: Sierra de San Miguelito, M. F. Robert y J. Passini 891 (ENCB). Guanajuato: municipio San Jose Iturbide: mountains ESE of San Jose Iturbide and about 5 mi W of Cerro Zamorano, R. McVaugh 10389 (MICH); municipio Tierra Blanca: Cerro Zamorano, vertiente E (NE), E. Carranza y S. Zamudio 4015 (IEB); ibid., bosque de Abies-Quercus, ladera de cerro, 2800-3050 m, E. Carranza y S. Zamudio 4017 (CIIDIR, IEB). Queretaro: municipio Penamiller: parte alta del Cerro Pinguical, 2 km al W de la antena, S. Zamudio y E. Carranza 6882 (IEB); ladera S del Cerro Pinguical, 3000 m, S. Zamudio y S. Gonzalez 11727, 11728, 11729, 11730 (IEB); municipio Pinal de Amoles: cerro La Calentura, S. Zamudio 3636 (IEB, QMEX); brecha 3 km carretera a Queretaro, Puerto de los Velazquez, Ejido Barranca, J. Huerta y L. Arellanes s.n. (ENCB); municipio Colon: parte alta del Cerro Zamorano, S. Zamudio y E. Perez 7738 (IEB); ibid., 20[grados]56.01' N, 100[grados]10.50' W, 3300 m, S. Zamudio y E. Perez 11839 (IEB); parte media del Cerro Zamorano, ladera SE, 20[grados]55' N, 100[grados]16' W, 3170 m, M. Gomez S., D. Flores y L.S. Calderon 567 (IEB); en el camino de Los Trigos a las antenas de Televisa, R. Chavez 68 (QMEX); municipio Cadereyta: 4 km al E de La Laja, Sierra de El Doctor, 2940 m, S. Zamudio y E. Carranza 6407 (IEB); 2 km al S de Sombrerete, L. Hernandez 4567 (QMEX). Michoacan: municipio Quiroga: Cerro Tzirate, C. Lopez 1159 (ENCB, IEB); parte alta del Cerro Tzirate, H. Diaz Barriga y S. Zamudio 2798 (IEB); ibid., 19[grados]43'46" N, 101[grados]30'57" W, 3250 m, 29.12.2007, S. Zamudio y cols. 14057 (IEB); municipio Hidalgo: Ciudad Hidalgo, S. D. Koch 77373 y P. A. Fryxell (CHAPA); municipio Angangueo: Llano del Toro, 22.I.1987, M. Mejia s.n. (IEB); Sierra Chincua, Reserva de la Biosfera Mariposa Monarca, 19[grados]40'35" N, 100[grados]18'04" W, M. G. Cornejo Tenorio 299 y G. Ibarra Manriquez (IEB); ibid., alrededores del vivero forestal, 19[grados]40' N, 100[grados]16'53" W, 2910 m, bosque de coniferas (Abies religiosa), S. Rangel Landa 248, M. A. Garcia Guzman y G. Ibarra Manriquez (CIIDIR, IEB). Hidalgo: municipio El Chico: Las Ventanas, 5.5 km al N de Pachuca, M. Medina 2273 (ENCB, IEB); municipio Tulancingo: Cuyamaloya, norte de Tulancingo, E. Matuda 37605 (IEB); municipio Tepeapulco: cerro de Xihuingo, A. Ventura 475 (ENCB); cerro Xihuingo, A. Ventura 1047 (ENCB); municipio Zempoala: cerro de Los Pitos, A. Ventura 1641 (MEXU); municipio Epazoyucan: Sierra de Los Pitos, Tlaquilpan, G. Benitez 191 (IEB); 1 km al SW de El Guajolote, M. Medina y M. A. Barrios 2579 (IEB). Distrito Federal: Sur del Xilte Grande,M. A. Panti 22 (ENCB); Pedregal Camposanto, Temamatla, E. Ibarra 118 (MEXU); San Miguel Ajusco, Tlalpan, J. C. Soto Nunez 13011 (MEXU). Puebla: municipio Chignahuapan: ejido San Luis del Valle, 19[grados]48'08" N, 98[grados]11'28" W, E. Guizar N. y A. G. Miranda M. 5950 (MEXU); municipio Tepeyahualco: volcan de Pizarro, 6 km al 60' de Tepeyahualco, 19[grados]30' N, 97[grados]6' W, J. I. Calzada, F. Lozano, E. Martinez, y J. Gallardo 04688 (MEXU).

Key to the Mexican tree species of Arbutus

1 Twigs of current year and petioles with conspicuous glandular hairs more prominent than eglandular hairs.... 2

2 Leaves dark green above, creamy to ochroleucous below as a dense indumentum of tightly curled hairs uniformly obscures the surface, leaves at senescence red; corolla pink, rarely white.... Arbutus bicolor

2 Leaves olive green, bright or pale green above, or somewhat glaucous, pale green below, pubescent with flexuous hairs not tightly curled nor obscuring the surface, leaves at senescence yellow, orange or rarely red; corolla white, creamy, yellowish or greenish.... 3

3 Older twigs rough; bark retained as pale gray, checkered, thick isodiametric or rectangular segments on the trunk, limbs, and twigs of the previous seasons, or exfoliating on young twigs and therefore shed in small, roughened flakes; glandular hairs 1-4(-6.8) mm long, averaging ca. 2.5 mm long.... Arbutus tessellata

3 Older twigs smooth; bark exfoliating in smooth flakes at least on limbs and twigs of the previous seasons, or retained as pale gray, checkered segments only on the trunk and base of the older limbs; glandular hairs 0.2-1.5(-2) mm long, averaging ca. 1 mm long.... Arbutus xalapensis (glandular morphs)

1 Twigs of current year and petioles eglandular, or if glandular hairs present those seen only with a hand lens (10x) or at higher magnifications.

4 Twigs of the current year and petioles purplish-red, rose, or pale green with rose tinge, petioles either glabrous, puberulent or shortly pubescent with white-gray hairs often only on the upper surface of mature leaves; leaves tapered to rounded at base, lower surface glabrous, puberulent, or hoary only along the midvein.... Arbutus arizonica

4 Twigs of the current year and petioles variously coloured, rarely (in A. xalapensis var. texana) rose or purplish, pubescent to villous on both upper and lower surfaces; leaves rounded, subtruncate, slightly cordate or slightly tapered at base, lower surface pubescent to villous, rarely glabrous.... 5

5 Bark exfoliating in smooth flakes at least on limbs and twigs of the previous seasons, or only retained as gray, checkered segments on the older parts of trunk and limbs; older twigs smooth, rarely slightly roughened; leaves 2.3-9 cm wide, the upper surface in various shades of green but rarely dark green, at senescence yellow, orangish, or red.... Arbutus xalapensis

5 Bark retained as gray, checkered, thick isodiametric or rectangular segments on the trunk, limbs, and twigs of the previous seasons, or rarely exfoliating on young twigs and shed in small, roughened flakes and the twigs roughened; leaves 5-9 cm wide, the upper surface dark green, at senescence bright red.... Arbutus madrensis

Hybridization is common among taxa of Arbutus. The hybrids, although fertile, are usually found as isolated occurrences. Occasionally, backcrossing occurs and several grades of introgression are found, particularly in disturbed areas. An example of a hybrid swarm between A. bicolor and A. xalapensis var. texana (Buckl.) A. Gray is found in cerro El Pinguical, Queretaro, where both taxa are found in their pure forms as well as with several grades of intermediates. Examples of sporadic hybrids between A. bicolor and other species of Arbutus are:

Arbutus madrensis x A. bicolor. Branchlets of second year rough, and lower surface of leaves with ferrugineous, somewhat floccose hairs indicate the influence of A. madrensis, whereas ovate, cordate at base leaves and densely glandular pubescent petioles are as in A. bicolor.

Durango: municipio San Dimas: Miravalles, UCODEFO 4, S. Gonzalez et al. 5118 (CIIDIR); municipio Durango: parque El Tecuan, 23[grados]54' N, 105[grados]01' W, A. Roman 5y S. Gonzalez (CIIDIR); ibid., 23[grados]54' N, 105[grados]01' W, A. Roman 6y S. Gonzalez (CIIDIR).

Arbutus bicolor x A. occidentalis. Several examples of hybrids between A. bicolor and A. occidentalis are recorded by Gonzalez Elizondo et al. (in rev.). They are shrubs 0.15-2.5 m tall with small or medium sized leaves (which reveals influence of A. occidentalis) in which the lower surface is dense and uniformly pubescent with short, irregularly curled hairs (as in A. bicolor). These plants form populations in which the morphological characters are fixed and the plants seem to represent genetically differentiated populations. Known from Durango and southern Chihuahua, from 2440 to 2700 m. An additional example is represented by shrubs to 2.5 m tall with reduced inflorescences (at most 3 cm long) but with bicolored leaves and pubescence that indicates a stronger influence of A. bicolor.

Durango: municipio Tepehuanes: Paraje Chamacueros, 30 km al W de Mesa de Navar, 2860 m, A. (Benitez) Paredes and O. Bravo 400 (CHAP, CIIDIR).

A. bicolor x A. tessellata 1. Glandular pubescence dense and short (at most 1 mm long) in branchlets and petioles as in A. bicolor, but older branchlets rough, leaves olive green, and flowers white yellowish, as in A. tessellata. The shape of leaves is intermediate between both species.

Michoacan: municipio Quiroga: ladera S del Cerro Tzirate, 2400 m S. Zamu dio 12866, (IEB).

A. bicolor x A. tessellata 2. Glandular pubescence dense in branchlets and petioles, ovate leaves to 10.5 cm long, dark green above and whitish below (A. bicolor), but lacking the dense, contorted hairs, instead having long, flexuous, twisted hairs not hiding the surface, older branchlets rough, and longer glandular hairs 1.5-3 mm long (A. tessellata).

Michoacan: municipio Contepec: 5 km del camino Santa Maria Los Angeles a Solis, 19[grados]58'35" N, 100[grados]7'3" W, 25.09.2004, J. Martinez Cruz 1086, G. Ibarra Manriquez, M. G. Cornejo Tenorio y M. A. Salinas Melgoza (IEB); ibid., 19[grados]58'26" N, 100[grados]7'41" W, (fr) J. Martinez Cruz 1055, G. Ibarra Manriquez, M. G. Cornejo Tenorio y M. A. Salinas Melgoza (IEB).

Arbutus xalapensis x A. bicolor. Peripheral populations of A. bicolor with strong introgression of A. xalapensis have lower surface of leaves with both, shaggy, ferruginous, floccose hairs as in A. xalapensis and short, appressed, curled hairs as in A. bicolor (although less dense) as well as some glandular hairs in branchlets and petioles. In the Trans Volcanic Belt in central Mexico, in Pinus or Abies forests with Baccharis conferta scrub (the last indicating disturbance). 2800-3300 m.

Hidalgo: municipio El Chico: Las Ventanas, 5.5 km al N de Pachuca, M. Medina 2273 (ENCB, IEB, MEXU). Distrito Federal: Canada de Contreras, por encima del Pueblo, F. Miranda s.n. 10 Dic. 1939 [other label indicates "Xochimilco", F. Miranda 39] (MEXU). Tlaxcala: municipio Tlaxco: 2 km al N de El Rosario, A. Chimal et al. 27 (MEXU); Cerro El Rosario, 11 km al NO de Tlaxco de Morelos, 24 km Apizaco - Chignahuapan, 19[grados]40'05" N, 98[grados]10'25" W, 3300 m (flores blancas y rojo-amarillentas), 20.03.1988, M. Ishiki & S. E. Perez 1989 (CHAPA, IEB); municipio Terrenates: ladera S del cerro Tlajacolo, al W del rancho Atotonilco, C. Castillejos 108 FES-ZA (MEXU). Veracruz: municipio Perote: camino a El Paisano, 19[grados]34' N, 97[grados]06' W, P. Zamora G., G. Castillo Campos 2159 (IEB).

A. bicolor x A. xalapensis var. texana. Hybrid swarm. Known from the Cerro El Pinguical, Queretaro, between 3000 and 3250 m asl in pine-oak-madrone forest as well as in disturbed Quercus scrub with Baccharis, Cercocarpus, Garrya, and Arbutus.

Both parents are present in the area in their pure forms as well as with several grades of intermediates with respect to habit and size of the plants (short shrubs to trees to 5 m tall), size of leaves, type and density of pubescence and glandularity (often with mixture of short, tightly curled hairs and long, floccose hairs).

Queretaro: municipio Penamiller: parte alta del Cerro Pinguical, S. Zamudio y S. Gonzalez 11718 (IEB); ibid., ladera SW, casi en la cima, S. Zamudio y S. Gonzalez 11722 (IEB); ibid., ladera S, S. Zamudio y S. Gonzalez 11725 (IEB); ibid., S. Zamudio y S. Gonzalez 11731 (IEB); ibid., S. Zamudio y S. Gonzalez 11732 (IEB).

Peripheral populations of Arbutus bicolor from Hidalgo (e.g. Benitez 191; Medina and Barrios 2579) have short inflorescences with only 2-3 racemes and relatively narrow leaves, the upper surface glabrescent and more or less smooth (veinlets not raised), probably due to past occurrence or continuing sporadic hybridization with A. xalapensis var. texana.

Markedly bicolor leaves are also found in two other Arbutus taxa, but they lack the dense indument of short, tightly contorted hairs characteristic of the leaves of A. bicolor: a) Arbutus xalapensis var. texana, with coriaceous to subcoriaceous leaves, very pale green to pale yellowish green or glaucous below, glabrous or with straight to wavy or twisted hairs never obscuring the surface; b) Arbutus cf. xalapensis, only known from western Chihuahua, which has ovate-lanceolate, acute, narrow (<3.8 cm wide) leaves pale green below, with sparse, fine, ferruginous hairs not hiding the lower surface, as well as long peduncles up to 1.6 cm long; the indumentum appears to indicate influence of A. xalapensis but the shape and bicolored character of the leaves and long peduncles appear to be unique.

ACKNOWLEDGMENTS

We are indebted to Drs. J. Rzedowski and Sergio Zamudio for the enriching discussions on the taxonomy of the group, Dr. A. A. Reznicek for images of specimens at MICH, Dr. Stephen D. Koch and Dr. Raquel Galvan V. for assistance with field work, Dr. Jorge A. Tena, M.C. Lorena Lopez, Biol. Flor Isela Retana Renteria, and Daniela Corral Sandoval for help in different phases of this work, Dr. Silvia Espinosa and M.C. Berenit Mendoza for the SEM images, and Lic. Martha Beatriz Campos and Damian Pina Bedolla for the distribution map. The keepers of the herbaria CIIDIR, CHAP, CHAPA, ENCB, IBUG, IEB, MEXU, MICH, NY, P, QMEX, and US are gratefully acknowledged for loans of material or for granting access to their facilities. The Comite Tecnico de Prestaciones a Becarios of the Instituto Politecnico Nacional and the Comision de Operacion y Fomento de Actividades Academicas provided support to conclude the study, which was developed as part of project 20110681, Instituto Politecnico Nacional.

Recibido en junio de 2011.

Aceptado en diciembre de 2011.

LITERATURE CITED

Gonzalez Elizondo, M. S. & M. Gonzalez Elizondo. 1992. Una nueva especie de Arbutus (Ericaceae-Arbuteae) de la Sierra Madre Occidental, Mexico. Acta Bot. Mex. 17: 7-12.

Gonzalez-Elizondo, M. S., M. Gonzalez-Elizondo & S. Zamudio. Delimitacion taxonomica de los complejos Arbutus mollis y A. occidentalis (Ericaceae). Acta Bot. Mex. (in rev.).

McVaugh, R. & T. J. Rosatti. 1978. A new species of Arbutus (Ericaceae) from western Mexico. Contr. Univ. Michigan Herb. 11: 301-304.

Sorensen, P. D. 1987. Arbutus tessellata (Ericaceae), new from Mexico. Brittonia 39: 263267.

Sorensen, P. D. 1995. Arbutus Linnaeus. In: Luteyn, J. L. (ed.). Ericaceae Part II. The superior-ovaried genera. Flora Neotropica, Monograph 66. New York Botanical Garden. New York, U.S.A. pp. 194-221.

M. SOCORRO GONZALEZ-ELIZONDO (1,3), MARTHA GONZALEZ-ELIZONDO (1) AND PAUL D. SORENSEN (2)

(1) Instituto Politecnico Nacional, Centro Interdisciplinario de Investigacion para el Desarrollo Integral Regional, Unidad Durango, Sigma 119, Fraccionamiento 20 de Noviembre II, 34220 Durango, Durango, Mexico.

(2) Northern Illinois University, Rm 140 Faraday Hall, DeKalb, IL 60115, U.S.A.

(3) Author for correspondence: herbario_ciidir@yahoo.com.mx
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Title Annotation:texto en ingles
Author:Gonzalez-Elizondo, M. Socorro; Gonzalez-Elizondo, Martha; Sorensen, Paul D.
Publication:Acta Botanica Mexicana
Date:Apr 1, 2012
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