Betula nana (dwarf birch) is a deciduous perennial shrub with rounded leaves and dark resinous spots covering young twigs that occurs widely across Denali from lowlands into the alpine zone. One of our most common species, dwarf birch dominates many areas along the Park Road, particularly in the subalpine zone, growing up to 2m tall, but usually 1 m or less in height. Leaves are alternately arranged, dark green above, paler beneath and are orbicular in shape with crenate to toothed leaf margins. The shape of the leaf base is highly variable, ranging from acute to notched. Young twigs are covered in resin dots and turn bright red in fall. Flowers are highly reduced catkins, which appear at the same time as the leaves, early in spring. Male catkins are 10-25 mm long, female catkins are 6-10 mm long, both produced on the same plant. Fruits are winged nutlets with wings half as broad as the body. Betula nana is the only shrub with round leaves with crenate margins. This shrub can form hybrids with Alaska birch, a closely related tree. In these hybrids (known as B. occidentalis), leaf size and shape, bark characteristics and ecology are intermediate between shrub birch and tree birch.
Betula nana flowers precociously in spring; male catkins develop first from buds produced the previous year. Green-up occurs in mid to late May. Seeds develop throughout the summer and dispersed in the late fall.
Betula nana is monoecious, but male and female flowers are produced in separate catkins. Flowers are wind-pollinated. Seeds are dispersed by wind and gravity and are not long lived in the soil seed bank (Alsos et al. 2003). B. nana commonly reproduces vegetatively through layering of branches and sprouting, forming large clonal thickets. B. nana readily hybridizes with other members of the Betula genus. B. occidentalis is one such hybrid, the offspring of dwarf birch and tree birch.
The Dwarf Birch is more a shrub of variable size than a tree, and is lesser used medicinally than the related Paper Birch (B. papyrifera). However young leaves are consumed as food or used to make a tea with properties similar to aspirin by the Yup'ik (Lantis 1958, 1959 in Garibaldi 1999).
Betula nana is a circumpolar species with a widespread distribution. In North America, it ranges from Alaska, across all Canadian provinces and into the western U.S. and northern New England states. Betula nana is common and widespread across Alaska and also in Denali, where it forms dense thickets in the subalpine and lower alpine zones, occurring more frequently north of the Alaska Range crest than to the south.
Betula nana is primarily a low elevation species but can occur at high elevations in suitable sites. It is seen at elevations from 73 m to 1301 m, with an average plot elevation of 653 m. It prefers low to moderately inclined sites with an average plot slope of 7 degrees.