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Tim Baxter & Hugh A. McAllister (2021)
Baxter, T. & McAllister, H.A. (2021), 'Alnus acuminata' from the website Trees and Shrubs Online (treesandshrubsonline.
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Tree to 25 m, to 1 m dbh; often multistemmed, rarely shrubby or prostrate. Branchlets dull or lustrous, somewhat glaucous, sparsely pubescent and with small yellowish brown glands. Buds resinous. Leaves deciduous, 3.5–9.5(–19) × (2–)3.5–9(–11) cm, ovate to elliptic, upper surface glabrous, lower surface with limited pubescence, 10–16 lateral veins on each side of the midvein, margins slightly revolute and serrate or serrulate, apex long-acuminate to acute, obtuse or rounded; petiole glabrous or pubescent, 0.7–1.6 cm long. Staminate inflorescences catkin-like, 3.5–15 cm long; pistillate inflorescences in racemes of two to four, pedunculate, globose to oblong, 0.3–0.8 × 0.15–0.32 cm. Cones woody, 1.5–3 × 0.8–1.2 cm, bracts 0.3–0.5 cm wide. Furlow 1979a. Distribution ARGENTINA: northern provinces; BOLIVIA; COLOMBIA; ECUADOR; PERU; VENEZUELA. Habitat Stream banks and moist slopes between 1500 and 2800 m asl. USDA Hardiness Zone 8–9. Conservation status Lower Risk (whole species).
A key to the subspecies is provided below, but the morphological distinctions between them are slight and only accessions of known geographical provenance can be identified with certainty.
|1a.||Leaves lanceolate to narrowly ovate, lower surface glabrous; Mexico (Durango to Oaxaca)||subsp. glabrata|
|1b.||Leaves ovate to obovate, lower surface pubescent||2|
|2a.||Leaves finely serrate or serrulate; South America (Venezuela to Argentina)||subsp. acuminata|
|2b.||Leaves doubly serrate; Central America (Mexico to Panama)||subsp. arguta|
This subspecies differs from subsp. acuminata in that its leaves are coarser and more deeply toothed. The leaves of the type subspecies are often serrate, while those of subsp. arguta are doubly serrated. Furlow 1979a. Distribution COSTA RICA; EL SALVADOR; GUATEMALA; MEXICO: Chiapas, Chihuahua, Durango, Guanajuato, Guerrero, Hidalgo, Jalisco, México, Michoacán, Morelos, Nayarit, Oaxaca, Puebla, Querétaro, San Luis Potosí, Sinaloa, Sonora, Veracruz; PANAMA. Habitat Stream banks and moist slopes between 1500 and 3700 m asl. USDA Hardiness Zone 9. Illustration NT137.
This subspecies overlaps with subsp. arguta, from which it can be distinguished by its entirely glabrous lower leaf surface and the lanceolate to narrowly ovate, acuminate leaves. However, hybridisation between the two subspecies does occur, creating intermediate progeny. Furlow 1979a. Distribution MEXICO: central Durango, Guanajuato, Guerrero, Hidalgo, México, north-central Oaxaca, Puebla, Tlaxcala. Habitat Stream banks and moist slopes between 1500 and 3000 m asl. USDA Hardiness Zone 9.
Taken as a whole, Alnus acuminata is rare in cultivation, but all three subspecies are represented in several British collections and members of the group are cultivated in the San Francisco Bay Area botanical gardens. Subsp. acuminata is perhaps the least commonly grown, reflecting its distribution away from the normal haunts of temperate dendrologists. At Stone Lane Gardens an 11 m specimen from the Tucuman area of northern Argentina is doing well, although it is probably a little too heavily shaded for optimal growth. It has a good straight trunk (11 cm dbh, TROBI), with dark brown bark. TROBI records other specimens at Bute Park in Cardiff, Glamorgan, and at Batsford Park, Gloucestershire. Adjacent to the tree at Stone Lane Farm is a specimen of what is probably subsp. glabrata from Mexico, again doing well, at 7 m, despite the shade. It is also cultivated at Tregrehan. Subsp. arguta is the most frequently grown of the three, probably mostly from Mexican plants although there is a specimen of Guatemalan origin at the University of California Botanical Garden at Berkeley. The tallest specimen recorded in Britain was 11 m at Borde Hill, West Sussex in 1962, but in recent years 9 m specimens have been measured at both Wakehurst Place and Westonbirt (TROBI). Despite its southern origins, A. acuminata seems to have retained many of the hardiness genes associated with more northerly alders, but a warm place would seem to be advisable, and there seems to be some variation between the taxa. At Blagdon, for example, subsp. glabrata has proved to be hardier than subsp. arguta (M. Ridley, collection notes). Its principal interest in horticulture is its biogeography, as the tree and its foliage are not particularly attractive, though the male catkins are showy.