Acer glabrum Torr.

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Credits

Dan Crowley (2024)

Recommended citation
Crowley, D. (2024), 'Acer glabrum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/acer/acer-glabrum/). Accessed 2024-06-16.

Genus

Common Names

  • Rocky Mountain Maple
  • Rock Maple

Synonyms

  • Acer glabrum subsp. typicum Wesmael
  • Acer glabrum f. monophyllum Schwerin
  • Acer glabrum var. typicum (Wesmael) Keller
  • Acer glabrum subsp. neo-mexicanum var. parvifolium E. Murray

Other taxa in genus

Glossary

section
(sect.) Subdivision of a genus.
herbarium
A collection of preserved plant specimens; also the building in which such specimens are housed.
included
(botanical) Contained within another part or organ.
key
(of fruit) Vernacular English term for winged samaras (as in e.g. Acer Fraxinus Ulmus)
morphology
The visible form of an organism.
morphometric
From the measurement of shape.
trifoliolate
With three leaflets.
variety
(var.) Taxonomic rank (varietas) grouping variants of a species with relatively minor differentiation in a few characters but occurring as recognisable populations. Often loosely used for rare minor variants more usefully ranked as forms.

Credits

Dan Crowley (2024)

Recommended citation
Crowley, D. (2024), 'Acer glabrum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/acer/acer-glabrum/). Accessed 2024-06-16.

A deciduous shrub or tree to 15(–25) m. Bark reddish brown to grey, smooth. Branchlets glabrous, slender, pale green, greyish to whitish or bright red to purple, turning reddish brown with age. Buds acute, with two to four pairs of scales, scales tomentose on inner surface. Leaves broadly pentagonal to nearly orbicular in outline, base cordate to cuneate, three-parted or three- to five-lobed, 3–14 × 3–14 cm, lobes or leaflets cuneate to rhomboid, apically acute, margins doubly or irregularly serrate with acute to acuminate teeth, upper surface dark green, lower surface paler, glabrous; petiole 1–12 cm long, green or reddish, glabrous, often grooved, broadest at base; autumn colours yellow to red. Inflorescence terminal and axillary, corymbose or racemose. Flowers yellowish green, 5-merous, usually dioecious, peduncles slender, pedicels short and slender, sepals spatulate to oblong, petals linear to oblong, petals as long as sepals, stamens (6–)8(–9), inserted in the middle or on outside of the nectar disc. Samaras 1.5 to 4.5 (–6) cm long, wings spreading acutely. Nutlets flattened, strongly veined. Flowering May to July, fruiting in August to September. (Sargent 1965; Elias 1980; Justice 1995).

Distribution  Canada Alberta, British Columbia Mexico Chihuahua United States Alaska, Arizona, California, Colorado, Idaho, Montana, Nebraska, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming

Habitat Generally on extremely well-drained soils and screes in moist habitats, or areas where snow melt or mountain streams contribute to soil moisture for at least part of the growing season. In xeric areas, the species tends to grow near mountain streams, or in areas receiving appreciable snow and convectional summer rainfall. In the northern parts of the range it is commonly found as an understory plant in coniferous and mixed forests, typically at the margins or in open areas, and tolerates considerable exposure to sun. In different parts of its range it occurs with Acer circinatum, A. grandidentatum, A. macrophyllum and A. negundo. It usually grows between 1200 and 1800 m asl in moist habitats, but occurs up to 3000 m asl in southern parts of its range.

USDA Hardiness Zone 4-8

RHS Hardiness Rating H5

Conservation status Least concern (LC)

Taxonomic note Taxonomic treatments of Acer glabrum have varied over time, with authors often adopting significantly different approaches in their interpretations of the species. Numerous taxa have been described at lower ranks based on morphology and geography, often with little agreement between treatments (Justice 1995). Most recently, van Gelderen et al. (1994) treated eight infraspecifc taxa within the species, using an approach combining field and herbarium studies with morphometric and foliar flavonoid analyses. Justice (1995) recognised five varieties, with intermediates found where the ranges of these overlap. Since then, in preparation for the forthcoming account for the Flora of North America, delimitation of the varieties has been re-appraised, and one of these five – var. torreyi – discarded (D. Justice, pers. comm. 2023). In accordance with this interpretation, four varieties are included here, with synonyms of taxa treated by van Gelderen et al. (1994) and Justice (1995) listed in the accounts.

Acer glabrum is the sole member of Acer Section Glabra, and has the northernmost distribution of any maple in the Americas, extending from Alberta and British Columbia in Canada south to Chihuahua, Mexico. Its specific epithet, glabrum, comes from its hairless leaves, though in shape these display significant variation, as intimated by the accepted varieties (and see taxonomic note). Variation is largely consistent with geography, with the largest leaves found in the north of its range (var. douglasii), the smallest (and bluntest) in the southwest (var. diffusum), and becoming divided in the southeast (var. neomexicanum). While var. neomexicanum typically produces three-parted (not strictly trifoliolate) leaves, all other varieties do so on lammas growth, and only pre-formed leaves (those that emerge from winter buds) should be used to determine the varietal identity of plants.

From a horticultural standpoint, the variation inherent in the species has not translated into broad adaptability in the garden (Justice, in prep.), and while improved selections of some varieties have been trialled, reliability of performance has been difficult to achieve (Dirr & Warren 2019). Acer glabrum is however used within at least part of its native range, where, according to Dirr & Warren (2019), it has potential for use in urban environments. Though difficult to transplant, the species can be tolerant of both shade and full sun once established.

Dirr & Warren (2019) state varieties diffusum, douglasii and glabrum as the most important lower taxa, though Justice (pers. comm. 2023) considers var. neomexicanum to have the greatest potential as a garden plant.

Introduced to Britain in around 1884, the species has remained largely restricted to collections here and elsewhere in Europe. A sole cultivar of German origin, ‘Rhodocarpum’, was described in 1910 and with seemingly no distinctive characters, is no longer thought to be grown (van Gelderen, de Jong & Oterdoom 1994).

A key to the varieties within Acer glabrum is provided below, adapted, with permission, from Justice (1995). In its typical form, it makes a bushy, somewhat scruffy plant of little ornamental value. The typical form is reportedly the most common in landscape use (Dirr & Warren 2019), though var. douglasii is most recorded variety in botanic gardens (Botanic Gardens Conservation International 2023).

Key to the varieties of Acer glabrum

1aLeaves mostly > 3 cm across, 4° veins visible beneath2
1bLeaves mostly < 3 cm across, 4° veins not visible beneathvar. diffusum
2aLeaves from non-extension shoots 3-5-lobed, or if 3-parted, then < 6 cm across and not glaucous beneath3
2bLeaves from non-extension shoots 3-parted or nearly so, > 6 cm across, glaucous beneath.var. neomexicanum
3aLeaves from non-extension shoots 3-5-lobed, < 6 cm across; 3° and 4° veins usually reticulatevar. glabrum
3bLeaves from non-extension shoots 3-5-lobed, > 6 cm across; 3° and 4° veins orthogonalvar. douglasii

var. diffusum (Greene) F. Smiley

Synonyms
Acer diffusum Greene
Acer bernardinum Abrams
Acer glabrum subsp. diffusum (Greene) E. Murray
Acer glabrum var. greenei Keller
Acer glabrum subsp. siskiyouense E. Murray
Acer glabrum subsp. torreyi (Greene) E. Murray
Acer glabrum var. torreyi (Greene) Smiley
Acer torreyi Greene

Var. diffusum is a shrub or small tree to <6 m. Branchlets whitish or reddish. Leaves <3–6 cm wide, three to five lobed or occasionally three-parted, margins with sparsely toothed, teeth obtuse; 3rd and higher-order veins scarcely visible beneath (Justice 1995).

Distribution

  • United States – Arizona, California, Nevada, Utah

RHS Hardiness Rating: H5

USDA Hardiness Zone: 4

Small-leaved and tolerant of long periods without water (it survives largely on snowmelt in the wild), var. diffusum makes a bushy plant of little or no ornamental value (Justice, pers. comm. 2023). It is unsurprisingly infrequent in collections, though is represented at the University of British Columbia Botanical Garden from material sourced in 2015 from Rancho Santa Ana Botanic Garden, California (American Public Gardens Association 2017).


var. douglasii (Hook) Dippel

Common Names
Douglas Maple

Synonyms
Acer douglasii Hook.
Acer barbatum Hook.
Acer glabrum subsp. douglasii (Hook.) Wesmael
Acer glabrum var. monophyllum (Schwerin) Pax
Acer subserratum Greene

Var. douglasii is a shrub or tree to 15(–25) m, often fastigiate. Branchlets reddish or purplish, becoming greyish. Leaves chartaceous, three to five lobed, 6–12(–20) cm wide, often with barely acute lobes with shallow sinuses, margins prominently toothed, mostly doubly-serrate; 3rd and 4th-order veins clearly visible beneath, orthogonal (Justice 1995).

Distribution

  • Canada – Alberta, British Columbia
  • United States – Alaska, Idaho, Montana, Oregon, Washington

RHS Hardiness Rating: H5

USDA Hardiness Zone: 4

The tallest-growing variety of Acer glabrum (exceptionally to 25 m on northern slopes in the wild), var. douglasii is also (alongside var. glabrum) the most frequently encountered form in cultivation. With age its stems become greyish and somewhat warty, but younger branchlets can be a vibrant red or purple, providing ornamental value. Adapted to winter browsing from ungulates, plants are often multi-stemmed, with individual stems short-lived but soon replaced by new growth from the base of plants (Justice, in prep.). Two plants growing on the Downs at Westonbirt from seed of H&M 1698A are fastigiate and stand at approximately 9 and 10 m tall, respectively, as of February 2024 (pers. obvs.). The seed was collected from Cougar Mountain, British Columbia, at 740 m asl.


var. neomexicanum (Greene) Kearney & Peebles

Synonyms
Acer neo-mexicanum Greene
Acer glabrum var. tripartitum (Nutt. ex Torr. & Gray) Pax
Acer glabrum subsp. neo-mexicanum (Greene) E. Murray

Var. neomexicanum is a shrub or tree to 8 m. Branchlets reddish. Leaves 5–10 cm across, mostly three-parted or nearly so (not strictly trifoliolate), margins prominently toothed, often glaucous beneath; 4th-order veins visible beneath, orthogonal or reticulate (Justice 1995).

Distribution

  • Mexico – Chihuahua
  • United States – Arizona, Colorado, New Mexico, Utah

RHS Hardiness Rating: H5

USDA Hardiness Zone: 4

Little grown, var. neomexicanum has undoubted potential for greater use, proving more adaptable than var. douglasii and offering good, orange to red autumn colour (Justice, in prep). Developing upright-arching branching, it makes a good shape and is comfortable far from its home in the southwestern United States and neighbouring northern Mexico. Plants at the University of British Columbia Botanical Garden grow well on open and partially shaded sites on dryish to moist, free-draining soils (Justice, in prep.). Individuals there derive from material collected in Bernalillo County, New Mexico (DJ 013) and Iron County, Utah (DJ 017) (D. Justice, pers. comm. 2023).