A deciduous tree up to 70 ft high and occasionally even taller, but becoming a small, dense, thicket-forming shrub at high altitudes and in exposed places; stems covered with short, erect hairs persisting two or three years. Leaves broadly ovate to broadly elliptic, 3⁄4 to 11⁄4 in. long, 1⁄2 to 7⁄8 in. wide, obtuse to rounded at the apex, the base usually obliquely cuneate, sometimes rounded or even cordate, upper surface medium green, slightly glossy, almost glabrous, underside paler, lustrous, with appressed hairs on the midrib and main veins, margins ciliate, bluntly double-toothed; lateral veins in five or six pairs, straight and parallel, raised beneath, each running out to a sinus; leaf-stalk 1⁄8 to 1⁄4 in. long. Male flowers solitary. Nutlets one only in each involucre, three-angled, clasped by two linear valves.
Native of temperate S. America, ranging from Tierra del Fuego as far north as the Chillan Andes around 36° 30′ S. It is the commonest nothofagus in Argentina, where it covers vast tracts east of the Andes at 3,000 to 6,000 ft; at the southern end of its range it descends to sea-level. It is frequently associated with N. antarctica and the two have been confused. But N. pumilio may attain a considerable size, and yields a useful timber, while N. antarctica is always a minor tree or shrub. The two are very distinct in their foliage, for whereas the latter has irregularly serrated leaves, the lenga has two large blunt teeth between each pair of veins (but usually three smaller ones between the lowermost pair). There is also a marked difference in the female inflorescence: in the lenga there is only one flower (or only one develops) and the fruit consists of a single nutlet clasped by two linear valves. In the nirre (N. antarctica) there are the normal three flowers each developing into a nutlet, and the involucre is four-valved.
Although so common in the wild, there seems to have been no recorded introduction of the lenga until the late 1950s and early 1960s, when there were three or more importations of seed. It is too early to judge whether this species will prove superior to N. antarctica as an ornamental tree, but it is certainly handsomer in foliage and grows faster. It is likely to be completely hardy.
N. gunnii (Hook, f.) Oerst. Fagus gunnii Hook. f. – This species, not yet introduced to Britain, is of interest as the only deciduous southern beech in the Australasian region. It is a shrub or small tree endemic to Tasmania, where it occurs in the mountains of the centre and west. In its foliage it is not unlike N. pumilio, but the leaves are relatively broader, often almost orbicular, and there is only one blunt tooth between each pair of veins.