Ericaceae-Neotropical Blueberries
James L. Luteyn and Paola Pedraza-Peñalosa
The New York Botanical Garden




Key to Species

     Arbutus Linnaeus (Ericaceae, Vaccinioideae, Arbuteae), is a genus of 10 species of trees and shrubs.  Five of the New World species occur within the neotropics, where the widespread Arbutus xalapensis is familiar to many as a tree with red peeling bark.  However, of the less well-known ones, three do not shed their bark and a fourth is a low-growing shrub.  All species in North and Central America are called by the vernacular name Madrone or Madroño.  Superficially Arbutus and some Comarostaphylis species resemble one another, but Arbutus is readily identified by its fruit structure;  the genus is the only member of the Arbuteae in which two to several seeds develop in each of the five chambers of a slightly stony endocarp.


    Arbutus has been reviewed taxonomically several times since it was established by Linnaeus in 1753.  The many names given the madrones of the neotropics derive from the great variability of the species.  Early collectors who sampled separate populations found marked vegetative discontinuities between them and established their taxa on these distinct morphs.  Moreover, in addition to variability between populations, individual plants may vary from year to year, depending upon the circumstances of the growing season, and there may be variation on different parts of the same plant, on different shoots of the same branch, or even among the companion leaves on the same shoot.  The fertile parts, while plastic to some extent, vary far less than the foliar features.  Earlier use in some treatments of such characters as ovaries pubescent vs. glabrous have largely not withstood wide sampling. Thus, while the leaves seem the most variable of organs, the treatment presented here relies heavily on foliar features in the delimitation of the taxa, with considerable emphasis placed also on growth form, the way in which the bark peels, and the particular kind of indumentum on vegetative parts.
     The most characteristic feature marking Arbutus in the field is the exfoliating outer bark that peels away from the branches and trunk exposing a smooth, sometimes glaucous, new bark.  The exfoliation of the previous season's bark occurs well into the growing season, typically when flowering has ceased and maturing of fruit has begun.  At this time the older bark fractures, most likely due to expansion of the stem girth, then loosens and falls away.  The manner of fracturing appears to be diagnostic.  In some (A. xalapensis) it results in linear, elongated flakes, varying in size from less than one to several centimeters on the younger twigs, to large, papery flakes, 15 x 20 cm or more, on the stoutest limbs and trunk.  These flakes have a brick-red or glaucous reddish-gray color on the outside and a yellowish color inside, and when curled upon themselves expose a green, unweathered phelloderm.  The resulting display of contrasting colors produces a very handsome and decorative aspect that accounts for trees of the genus having a favored place in ornamental arboriculture.  A second form of fracturing and subsequent exfoliation takes place only on the youngest twigs of the plants where the resulting flakes are loosely squarish, or at least not linear.  Exfoliation ceases on slightly older branches.  Thereafter, the old and weathered bark persists and produces a somewhat fissured cork that otherwise typifies trees of temperate regions.  Among the neotropical species of Arbutus, the early onset of bark retention is most pronounced in A. arizonica, A. tessellata, and A. madrensis, where it serves as a diagnostic feature.  On these species the bark soon becomes checkered in appearance with the individual segments measuring 1-4 cm long and two-thirds as wide and clothing the limbs from about age six and older, except for the third species on which the bark does not exfoliate on the young twigs but is retained from their earliest age.
     People in the neotropics have found Arbutus wood very suitable for the production of charcoal.  With the demand for fuel wood in rural areas still quite high in much of the range of Arbutus, it seems that these stands of Madrone represent an important economic natural resource.  Lumber-sized trees occur rarely.  None of the neotropical species of Arbutus fall into the category of endangered or threatened plants.

ARBUTUS Linnaeus, Sp. pl.  395. 1753;  Pursh, Fl. amer. sept. 282-283. 1813;  Kunth in Humboldt, Bonpland, & Kunth, Nov. gen. sp. 3: 219-220, tab. 260. 1819;  A. P. de Candolle, Prodr. 7: 581-583. 1839;  Martens & Galeotti, Bull. Acad. Roy. Sci. Bruxelles 9: 532-536.  1842;  Klotzsch, Linnaea 24: 70-73. 1851;  Walpers, Ann. Bot. Syst. 2: 1104-1105. 1852; Hemsley, Biol. cent.-amer., Bot. 2: 276-277. 1881;  Small, N. Amer. Fl. 29: 82-85. 1914;  Standley, Trees and shrubs of Mexico, Contr. U.S. Natl. Herb. 23: 1099-1100. 1924;  McVaugh & Rosatti, Contr. Univ. Michigan Herb. 11: 301-304.  1978;  Sørensen, Fl. Neotrop. Monogr. 66: 194-221.  1995.  Type species.  Arbutus unedo Linnaeus.  Both epithets "Arbutus" and "Unedo" are classical names for the European Strawberry Tree.

Unedo Hoffmansegg & Link, Fl. portug. 1: 415.  1813-1820.  Type.  Unedo edulis Hoffmansegg & Link.  Named for its edible fruit. (=Arbutus unedo Linnaeus).

     Trees, large shrubs, or nearly prostrate and spreading shrubs;  bark on some brick-red or glaucous-whitened, peeling in large flakes, becoming retained only on oldest portions of trunk near the base, weathering to light or dark gray, forming an irregular checked pattern, on others, the red bark at first flaking in small, non-linear flakes, then retained on all larger parts of the trunk and main branches, from 1-6 years of age or older, becoming light or dark gray and forming a geometrically and uniformly small, rectangular checked pattern;  young branchlets glandular hairy, thinly tomentose, or both, or the branchlets glabrous;  new growth of rapidly elongating sprouts usually with some glandular hairs, indumentum of multicellular hairs, these multiseriate;  buds ovate, acute, glossy red or sometimes glabrate, usually only the terminal well-developed and conspicuous, the scales imbricate, 8-16, accrescent.  Leaves with blades flat, coriaceous, glossy green above when fresh, lighter green or even slightly glaucous beneath, ovate, widest slightly below the middle, or elliptic, larger on sterile shoots and sprouts than on fertile shoots, glabrous, pubescent, or floccose, especially beneath;  margin smooth of finely toothed, often coarsely toothed on sprouts and sterile shoots;  venation eucamptodromus, sometimes weakly so then merely reticulodromus, midvein and secondary veins usually slightly impressed above, sometimes conspicuously so, raised beneath;  petiole with slightly decurrent bases.  Inflorescence a terminal cluster of racemes.  Flowers borne erect or obliquely erect on accrescent pedicels, subtended by a bract and 2 or 3 bracteoles;  calyx continuous with the pedicel or at least not obviously articulated, at first cupulate, becoming curved back on itself and almost hidden by the ballooning of the corolla base;  lobes blunt, scarious margined, especially near summit, reflexed in fruit, becoming indurated in age;  corolla creamy-white or yellowish, soon developing a post-anthesis circumferential dimple at about midway its length, inflated basally, the lobes 5, reflexed, imbricate;  stamens 10, arising from beneath a 5-lobed nectariferous disc;  filaments slender above, abruptly expanded below into a villous swollen base, (connective sometimes prolonged into a membranous flap extending above the apparent apex of the anther);  anthers bulbous, smooth, 2-celled, inverting during floral development, the base then pointing upward, adaxially appendaged with a pair of finely tuberculate spurs from the apparent apex, dehiscing by subterminal, elliptic pores;  ovary with ovules (2-)several in each locule;  placentation axile;  style terminal, terete when fresh, shrivelling on drying to expose 5 longitudinal ribs, stigma capitate, held in the mouth of the corolla or slightly exserted.  Fruit a berry, fleshy, usually red when ripe, the surface roughened-tuberculate, glabrous or thinly pubescent;  seeds partially embedded at maturity in a maroon-colored fleshy placenta, irregularly angled, but prevailingly 3-sided in cross-section, light-colored, seed coat thin, tan-colored;  embryo straight, occupying almost the entire length of the seed, embedded in a white, slightly-granular endosperm;   chromosome number: 2n=26.

     A genus of 10 species, 3 in Europe, N. Africa, and the Middle East, 1 on the Canary Islands, and 6 in the Western Hemisphere.  Five species occur in the Neotropics, and all extend north of the Tropic of Cancer.  In the Neotropics, the plants are found primarily in montane areas associating with Pinus and Quercus spp.  Northern populations inhabit riverine woodlands (Arbutus arizonica) or progressively drier environments, associating with uniper-Piñon vegetation (A. xalapensis).

Key to Neotropical Species                                                                                               Back to Top

1.  Prostrate and spreading shrubs, forming mounds 0.3-0.6 m high
     (rarely upright and reaching 1.0-1.5 m), much branched ......... A. occidentalis.
1.  Erect shrubs or more often small or large trees, sometimes to 15 m
     and taller.
     2.  Petioles, young twigs, and inflorescence axes with glandular hairs
          up to 4 mm long (or longer) but averaging ca. 2.5 mm;  leaves glabrous
          or pubescent and/or glandular pubescent, usually with some hairs along
          the midrib near base of blade, hairs rarely dense beneath;  bark
          rough, not exfoliating on larger limbs and trunk and forming a more or
          less uniform checkered pattern .............................................  A. tessellata.
     2.  Petioles (on flowering or fruiting branches) glabrous or pubescent,
          of if glandular pubescent, then the glandular hairs averaging 1.0 mm
          long or shorter;  leaves glabrous or pubescent, sometimes densely
          woolly beneath;  bark exfoliating by smooth, reddish papery flakes or
          by small squarish flakes (on the younger twigs) and/or by long slender
          curled flakes on the intermediate-aged branches, when retained, then
          roughened only on the largest limbs and trunk or conspicuously and
          more or less evenly checkered.
          3.  Leaves usually strongly tapered at base, glabrous;  bark
               checkered, the segments or plates averaging 2-4 x 1 cm ......
               ...................................................................................... A. arizonica.
          3.  Leaves normally rounded at base, pubescent or glabrous.
              4.  Leaves glabrous or pubescent, the smooth bark peeling in large
                   flakes over most of the limbs and bole .....................  A. xalapensis.
              4.  Leaves pubescent beneath, the bark not exfoliating, all twigs
                   and limbs roughened ................................................ A. madrensis.


     This version of the taxonomic treatment of the neotropical species of Arbutus (Ericaceae), has been modified from the work by Paul D. Sørensen in "Ericaceae--Part II. The Superior-Ovaried Genera (Monotropoideae, Pyroloideae, Rhododendroideae, and Vaccinioideae p.p.)". The full treatment may be see in Flora Neotropica Monograph 66: 194-221 (1995).  This synthesis has the permission of The New York Botanical Garden and Paul D. Sørensen.


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