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)

Family:

Ericaceae

Synonyms:

Arbutus xalapensis var. arizonica A.Gray

Description:

Species Description - Tree, 3-8(-10) m tall; bark light gray to reddish gray, checkered with isodiametric, squarish to rectangular segments or plates, 1-4 cm long, 1-2.5 cm wide, retained on the bole and major limbs. Twigs three years old and older with brick-red outer bark exfoliating in small flakes or sometimes in slender strips. Leaves crowded, glabrous throughout, olive-green and glossy above, slightly lighter green beneath; blades (2.5-)3-7.5(-9) × (1-)1.8-3(-4) cm, the larger on sterile shoots with long internodes; base tapered-acute, rarely slightly rounded, blades tending to be overall smaller and more lanceolate or even linear-lanceolate in the northern part of the range, broader and more nearly ovate-lanceolate southward; apex acute, less often slightly acuminate-lanceolate, sometimes ovate; margins smooth except on sprouts where sometimes coarsely serrate; petioles often red, glabrous, (1-) 1.7-3.2(-4.2) cm long. Inflorescence a cluster of racemes, often congested, axes (including pedicels) densely pubescent, the hairs sometimes glandular. Flowers obliquely erect or slightly pendulous on accrescent pedicels reaching 1.6 cm long in fruit, subtended by a scale-like reddish bract, up to 2.4 mm long, enclosing two smaller bracteoles; calyx pale green, cupulate, lobes scarious-margined, obtuse or rounded; corolla creamy white, drying to creamy tan, 5-5.6 mm long, urceolate, developing a circumferential dimple at about midway its length, the dimple becoming less prominent when approaching anthesis; lobes slightly overlapping, at first erect then becoming reflexed; stamens distinct, attached at the base of a nectariferous disc; anthers averaging 1.4 mm long, finely tuberculate spurs 1/2-2/3 the length of the sacs; ovary with ovules (2-)3 to several per locule, Fruit ripening to a rich blackish-red, 6.5-9 mm diam., varying greatly owing to seasonal conditions; seeds about 2.0 mm long.

Discussion:

Arbutus arizonica is apparently confined to the mountainous region of western and northwestern Mexico and adjacent U.S.A. So far it is not known from the Sierra Volcanica Transversal as is the closely related Arbutus tessellata. In Durango it is often the most abundant species of Arbutus. It is also abundant on the slopes of Volcán Tequila, which lies near the southern extremity of the Sierra Madre Occidental.

Two outstanding features of Arbutus arizonica distinguish it in the field: 1) the consistently narrow, glossy leaves with tapered leaf bases and 2) the nonexfoliating bark which acquires a checkered appearance within a few years (Fig. 7). These characters do vary somewhat, and the variations are correlated to a limited extent with geographic distribution. Southward, especially in Jalisco, the leaves of A. arizonica attain their greatest size; as the overall size of the leaves increases, there is a tendency for the bases to become less narrowly tapered; indeed, on rare individuals the base comes close to being rounded. Trees with the narrowest leaves overall and the most narrowly tapered bases characterize the populations in the extreme northern part of the range in Graham and Pima Counties of Arizona in the U.S.A. In Jalisco, one may encounter individual trees on which the bark exfoliates for a considerable number of years before being retained. Moreover, the exfoliating shards of bark appear linear and twistedly thread-like rather than squarish. Such specimens tend also to be more reddish than the typical gray on the trunk and major limbs.

The uses of Arbutus arizonica are not in any way unique to that species. I assume this madrone is used in charcoal making, already discussed, and that the "Carboneras" do not make a distinction between the five neotropical species.

Arbutus arizonica and A. tessellata are likely closely related. Both possess the bark retention character that results in a checkered bark on the older parts of the trees. Their inflorescences tend to be overall many fewer-flowered than A. xalapensis. The number of seeds in each chamber of their fruit is usually two or three compared to the four or five found in A. xalapensis. The distribution of the Arizona madrone farther north in North America and its present confinement to the Sierra Madre Occidental in Mexico suggest it is the more derived of either A. tessellata, A. madrensis, or A. xalapensis.

Distribution and Ecology: Southeastern Arizona and adjacent New Mexico in the U.S.A. and southward through the Sierra Madre Occidental of western Mexico into the state of Jalisco. Riverine forest and parklands along seasonally moist waterways in the northern part of its range, and southward in the seasonally dry montane zone with Pinus, Quercus, Arctostaphylos, and other species of Arbutus, 1300-2800 m. Flowering early March in the south and continuing until May and June in the north, rarely as late as July or August; through October. No specimens seen in flower nor fruit November through February.

Distribution:

Mexico North America| Chihuahua Mexico North America| Durango Mexico North America| Jalisco Mexico North America| Sinaloa Mexico North America| Sonora Mexico North America| Zacatecas Mexico North America|