6. Populus tremuloides Michx. (quaking aspen, trembling aspen)

Pl. 554 j, k; Map 2572

Plants trees 8–20 m tall, relatively open (but sometimes slender in aspect), widely branched with mostly spreading to loosely ascending branches. Bark light gray or greenish gray to white, relatively smooth, developing darker, roughened or corky spots, cross-ridges, and branch scars, occasionally becoming dark brown and longitudinally ridged and furrowed only near the base of older trees. Twigs slender to moderately stout, reddish brown, becoming grayish yellow with age, glabrous, the pith white. Winter buds 3–6(–8) mm long, reddish brown, shiny, not or only slightly resinous, glabrous. Leaves slightly heterophyllous (the blades of early-season leaves with somewhat fewer teeth than those developing later in the season), the petiole about as long as the blade, noticeably flattened on the sides, at least toward the tip, glabrous. Leaf blades 1.5–8.0(–12.0) cm long, ovate to broadly ovate or occasionally nearly circular, longer than to about as long as wide, broadly angled or abruptly tapered to a sharply pointed tip, broadly angled or broadly rounded to more commonly truncate at the base, rarely shallowly cordate, the margins with an inconspicuous, very thin, pale yellow or translucent line or band (viewed under magnification), this sometimes lacking, glabrous or sparsely pubescent with silky hairs, finely toothed, mostly with 20–80 rounded to bluntly pointed teeth 0.1–1.5 mm deep per side, the teeth incurved at their tips but not strongly thickened, sometimes also with 1 or 2 shallowly cup-shaped glands at or near the base, the upper surface dark green, glabrous, the undersurface pale green and often slightly glaucous, glabrous. Inflorescences 4–7 cm long (the pistillate ones elongating to 7–12 cm at fruiting), the bracts deeply cut into 3–7 narrow, deep lobes, the margins silky-hairy, the undersurface glabrous. Staminate flowers with 6–12 stamens. Pistillate flowers with usually 2 carpels, the style usually branched nearly to the base, the stigmas linear and often deeply 2-lobed. Fruits 2.5–5.5 mm long, narrowly ovoid, glabrous, with usually 2 valves. 2n=38, 57, 76. March–April.

Uncommon, known thus far from northeasternmost Missouri and a single historical, population in Dent County (western and northeastern U.S. (including Alaska); Canada, Mexico). Mesic upland forests (mostly in openings and along edges), edges of upland prairies, and banks of streams; also ditches and roadsides.

Quaking aspen tends to be an early-successional in montane habitats, where it colonizes areas opened by fires or other natural disturbances and gradually is replaced by other tree species. The quaking portion of the common name refers to the movement of the leaves in the wind, which (in this species and many other taxa of Populus) is enhanced by the flattened petioles. Populus tremuloides is quite uncommon in Missouri. Steyermark (1963) discussed the disjunct, historical station in Dent County, which he interpreted to represent a native occurrence.

Steyermark (1963) reported the European aspen, P. tremula L., from Missouri based on a single specimen that he collected in 1936 in Washington County. However, this specimen was redetermined as P. ×canescens Sm., the hybrid between P. alba and P. tremula, in 1990 by James Eckenwalder during his doctoral research on the genus. Eckenwader (2010) noted that P. tremula is sometimes cultivated in temperate North America, particularly a columnar cultivar that exists only as a staminate clone. The species is closely related to P. tremuloides, but differs in the following characters: terminal winter buds with cobwebby hairs toward the base (vs. glabrous or nearly so) and blades of later-season leaves with the margins relatively coarsely toothed (vs. finely toothed), the teeth mostly 8–14 (vs. 20–80) per side. Populus tremula can persist from old plantings, but has not become established outside of cultivation in Missouri thus far.