Acer glabrum Torr.

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.