Arbutus occidentalis McVaugh & Rosatti
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Authority
Luteyn, James L., et al. 1995. Ericaceae, Part II. The Superior-Ovaried Genera (Monotropoideae, Pyroloideae, Rhododendroideae, and Vaccinioideae P.P.). Fl. Neotrop. Monogr. 66: 560. (Published by NYBG Press)
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Family
Ericaceae
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Scientific Name
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Type
Type. Mexico. Jalisco: Autlan, Sierra de Manantlan, along lumber rds. E of the summit between El Chante and Cuzalapa, 13°35'N, 104°8-15'W, summits of cliffs in pine forest, 2750 m, 20-21 Mar 1965 (fl), McVaugh 23129 (holotype, MICH).
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Synonyms
Arbutus occidentalis var. villosa McVaugh & Rosatti
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Description
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Species Description - Shrub, prostrate and spreading or upright, 0.3-1.5(-2) m tall, from a burl-like base or rootstock; bark on larger twigs brownish-red, smooth or slightly roughened, exfoliating in small flakes to the base of the canes. Leaves pale or dark green or glaucous-green, sometimes with a blush of red or copper color, especially along the margins, elliptic or slightly ovate-elliptic, 4-7.5(-9) × 1.5-3.5(-4.2) cm, basally tapered or rounded, rarely subtruncate, apex acute; margins flat or very slightly re volute, smooth or toothed, varying on the same shoot; upper surface glossy, rugulose, glabrous or with some pubescence along the midvein toward base of blade, lower surface glabrous or pubescent, sometimes densely so, the hairs tan or brown; petioles 0.4-1.5 cm long, l/9(-aver. 1/6-) 1/3 the length of the blade, pubescent or glabrous, usually some of the hairs gland-tipped but not conspicuously so, the gland-tipped hairs scarcely longer than the non-glandular ones. Inflorescence a terminal simple raceme or a cluster of racemes, the branches few, axes including pedicels sparsely to densely pubescent, sometimes whitened woolly, with scattered or abundant glandular hairs intermixed with non-glandular ones. Flowers at first erect then nodding slightly on curved, accrescent pedicels, subtended at the base by a clasping, reddish, accrescent bract to 5.1 mm long, often almost as long as or slightly exceeding the pedicel length (exceptional material with much-elongated pedicels), more or less enclosing 1 or 2 smaller bracteoles; calyx at first cupulate, becoming reflexed in age, lobes ca. 2.7(-3.2) mm long, obtuse, ciliolate, glabrous or sparsely pubescent, pale reddish or tan; corolla 4.8-5.4 mm long, pale pink to red, drying to reddish brown, ventrally sometimes pubescent in lines; filaments ca. 2.2 mm long including the swollen base, glabrous above, silky pubescent below, spurs 1/2-3/4 the length of the thecae, the latter ca. 1.5 mm long; ovary glabrous or pubescent, ovules several per locule. Fruit up to 8 mm diam, when ripe, bright orange-red; seeds, ca. 2.4 mm long, 1 or 2(-3) developing in each locule.
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Discussion
At the outset of the present study the two intraspecific taxa comprising McVaugh and Rosatti’s (1978) concept of Arbutus occidentalis seemed clear and distinct, but as specimens accumulated, the described discontinuities between them began to blur so that ultimately I am led to conclude that this species is a single and large polymorphic taxon. Indeed, the variation within A. occidentalis so much parallels that of the widespread and variable A. xalapensis that some specimens do not clearly fall into one or the other when foliar features alone are available. There is only a single character that may be used reliably to separate the species: the shrubby habit. Herein lies the problem of how to interpret material for which the notes lack information on habit. Fortunately, most collectors think of the genus as one of trees or treelike plants, and so note the habit when collecting from a population of prostrate or low-stature shrubs. Nevertheless, the "shrub" that is being collected may refer to a plant of A. xalapensis that is shrubby following some event as fire or woodcutting. The identity of such material is facilitated when in the fertile condition for A. xalapensis does not reach reproductive condition until it has begun to resume normal treelike growth; stump sprouts or sprouts from the base of the plant do not flower. And if the material in question is fertile, one can turn to additional characters which, if not qualitatively distinct, may collectively point to A. occidentalis. The more obvious of these are 1) flower color, which seems more often to range from pink to red rather than from white to ivory with only rare individuals showing a bit of pink at the base of the corolla; 2) fruit, which in A. occidentalis rarely develops as many as three seeds in a locule but usually bears just one or two (potentially there could be several in each chamber since the ovary at the time of anthesis contains many more ovules than routinely mature to seeds); 3) bracts at the base of the pedicels, which tend to be longer (up to 5.1 mm) in the shrubby species than in the others and often almost equal the pedicel length at the time of anthesis (and are rarely actually longer) -but this character is of less value since the pedicels increase in length as the fruits develop; 4) posture of the flower, which, while somewhat variable, quite often is pendulous; and 5) glandular hairs, which are usually present on fertile axes of A. occidentalis but are not substantially longer than the non-glandular hairs, whereas in A. xalapensis the glandular hairs are usually notably longer than the non-glandular hairs. Comparisons need not be drawn between A. occidentalis and A. tessellata, as the exceptionally long glandular hairs of the latter are unlike those of any other species.
Arbutus occidentalis inhabits higher average elevations than any of the other species within the Neotropics and is often found in somewhat bleak and windswept rocky sites, although many labels mention the habitat as pinelands or pine forests. I have collected it in the Sierra Manantlan, Jalisco, at the type locality, where the companion species included A. xalapensis. I expect that the two species are found together in many sites, but, to date, I have not seen what I would suspect to be hybrids between the two. McVaugh and Rosatti (1978) point out that there is little overlap of flowering periods of A. occidentalis and A. xalapensis in western Mexico, so phenology thus mitigates against hybridization. Elsewhere, however, the flowering periods overlap much more and hybridization may be possible. There is no information on compatibility factors, pollinators, or chromosome numbers, knowledge of all of which would help in further understanding of the biology of this interesting species.In view of the specialized features of Arbutus occidentalis versus the comparative conditions in A. xalapensis, I suggest that the former is the derived species. This is evident in its shrubby or often prostrate habit, more highly colored corolla, the higher average elevation at which it grows, and reduction of the number of seeds per chamber in its fruit. -
Distribution
Rocky cliffs and ledges and open pine woodlands or parklands in the mountains of Mexico from about 26° 50' N, 108° 5' W in NE Sinaloa then patchily southward and eastward to SE Oaxaca; 2100-3660 m, most collections from 2300-3350 m; flowering January to June, peaking in late February or early March, more or less throughout the range, rarely flowering desultorily in October or November; fruit appearing in July, ripening usually in October.
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