Flora of North America species comparison

The Eurasian weeping birch (Betula pendula) is extensively cultivated throughout the temperate range of the flora, and it has been known to persist or to become locally naturalized in several areas, particularly in the Northeast. In vegetative features it resembles B. populifolia Marshall, to which it is closely allied; it can easily be distinguished from the latter by its peeling bark, as well as by its mostly pubescent leaves with somewhat shorter, acuminate apices.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Betula glandulosa is the characteristic dwarf birch of upland habitats throughout much of the mountainous west, occurring as well in dry open areas across the north. Where their ranges meet, B. glandulosa intergrades with both B. pumila Linnaeus and B. nana Linnaeus subsp. exilis (Sukaczev) Hultén, creating a confusing complex of intermediate forms. In the east, it reaches its southernmost limit on the subalpine slopes of high Adirondack peaks, including Mt. Washington, where it forms low sprawling thickets and scrubs.

Specimens of Betula glandulosa have been reported from the St. Lawrence Valley, but I have not seen them.

Wherever Betula glandulosa comes in contact with B. pumila, it forms a bewildering swarm of plants, known as B. ×sargentii Dugle, having intermediate states of most vegetative characters.

Plants intermediate between Betula glandulosa and B. nana subsp. exilis make up a continuum of forms linking the typical forms of Betula nana and B. glandulosa in parts of Alaska where the ranges of these species overlap. Wherever they occur in isolation, the species remain reasonably distinct and easy to identify. In southern Greenland, Betula glandulosa hybridizes with B. nana subsp. nana and with B. pubescens.

Betula ×eastwoodiae Sargent (= B. glandulosa × occidentalis) occurs in montane meadows and marshes in Alberta, British Columbia, Northwest Territories, Saskatchewan, Yukon, Alaska, Colorado, and Wyoming, where the range of the parents overlap.

Betula ×dugleana Lepage (= Betula glandulosa Michaux × B. neoalaskana Sargent) is common throughout Alaska and the Yukon, where the parent species frequently come into contact (E. Hultén 1941–1950, vol. 4; E. Lepage 1976).

Betula ×dutillyi Lepage [= Betula glandulosa Michaux × B. minor (Tuckerman) Fernald] is a putative hybrid that occupies the same general range as Betula minor. Like that species, however, it has not been studied experimentally. Careful examination of the entire complex to which this taxon belongs will be necessary before any of its parts can be truly understood. Betula ×dutillyi exhibits many of the same characteristics as B. minor, but it is slightly smaller in habit, and its leaves are smaller with somewhat blunter tips and more cuneate bases (E. Lepage 1976).

(Discussion copyrighted by Flora of North America; reprinted with permission.)