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Betula glandulosa Michx. Dwarf birch

Branches with leaves

Habitat

Dwarf birch occurs on a variety of sites from dry sandy hillsides and rocky ridges to stream sides and bogs. It is a northern species, so in the south it occurs only at high elevations. In New Brunswick, it is known from near the summit of Big Bald Mountain, but might be present in other high areas.

Form

Shrub in the fall.0.5 m tall

Dwarf birch is usually a short (up to 1 m tall), depressed or mat-forming, straggling shrub, but it may be a somewhat taller, more erect shrub. Its branches are frequently gnarled and twisted, and the crown is irregular.

Morphology

Male catkin (extending left), erect female catkin (center right)

The leaves are deciduous, simple, and borne alternately along the usually few long shoots, and singly, or in twos or threes close together, on the many short shoots. Each leaf is oval, almost round to obovate, 5–25 mm long and nearly as broad, coarsely toothed around the margin, and leathery. The upper surface is dark green, shiny, hairless (or mostly so), and the under surface is only slightly paler. The leaves have three or four veins per side and may be a little sticky.

The twigs are hairless or slightly and finely hairy, and bear many warty, whitish, resin glands. The twigs may have a grey, waxy surface. The long shoots bear small, ovoid, brown, lateral buds, and the short shoots, bear small, ovoid, brown terminal buds.

Fruit (left), bract (right)

The unisexual flowers are borne in male or female catkins. Male catkins are visible in unexpand-ed form at the ends of long shoots over winter. In spring they extend to lengths of 2–3 cm and hang limply to shed pollen before they drop from the twigs. Female catkins emerge beyond expanding leaves from terminal buds of short shoots. They are stalked, erect, and 10–15 mm long when their flowers are receptive to pollen. They extend another 10 mm as the fruits are developed. The bracts, which have three forward-pointing lobes and carry a resin gland on their backs, and the ripe fruits fall from the fruiting catkins beginning in the autumn.
Each fruit is a small, broadly ovoid nutlet that has two narrow lateral wings, and two tiny, hair-like, stigma remnants at its tip. Each fruit can contain a single seed.

The bark of older branches is dark brown, shiny, and speckled with scattered lenticels.

Notes

Developing fruiting catkins and fully expanding leaves

Dwarf birch may be confused with swamp birch (Betula pumila L.), which is outwardly similar. Bogs are the preferred habitat of swamp birch, but it may occur along the sides of lakes and streams. Its leaves are similar to those of dwarf birch, but are distinctly (not slightly) paler on the under surfaces and usually are dotted with tiny yellow glands. Yellowish (not whitish) warty resin glands also occur along its twigs. The bracts of its fruiting catkins have no resin glands, and the wings of its nutlets are half to almost as broad as the nutlet (not narrow).