Familiar trees and shrubs of northern latitudes. The typically white and yellowish peeling bark of white birch and yellow birch, respectively, makes many individuals readily recognizable in the field. Yet, the taxa are extremely variable with many individuals (especially in the form of herbarium specimens) difficult to recognize. Hybridization is rather frequent, producing intermediates that are often easy to recognize in the field, but also causing difficulties in the herbarium. Betula pubescens has been persistently reported from Michigan, but no specimens have been seen.
1. Leaves with 7–12 pairs of distinct lateral veins; mature pistillate aments 11–21 (–24) mm thick, sessile or nearly so (peduncles at most ca. 5 mm); bark of twigs with flavor of wintergreen; mature plant a tree with yellowish peeling or dark yellowish, reddish, or blackish bark.
2. Leaf tips (of larger leaves, especially the paired short shoot leaves) slightly concavely tapered to a short acuminate apex; leaves usually irregularly doubly serrate; bark yellowish, peeling (occasionally dark and sometimes furrowed or checkered); abundant, often large, forest tree.
2. Leaf tips cuneately tapered to an acute tip; leaves singly serrate; bark dark red to reddish brown (especially in younger trees), not peeling, shiny; very rare small to medium tree known to date only from one site southeastern Michigan.
1. Leaves with (2–) 3–8 (–9) pairs of distinct lateral veins; mature pistillate aments (5–) 6–10 (–12) mm thick, distinctly peduncled (the peduncles sometimes as short as 2 mm in B. pumila, a peatland shrub); bark of twigs without wintergreen flavor; mature plant usually a small to large many-stemmed shrub or a tree with white peeling bark.
3. Leaf blades 1–3.5 cm long (or up to 5 cm on sprouts), mostly ± obovate and rounded at the apex; wings of fruit narrower than the body; plant a bushy shrub with dark bark.
3. Leaf blades mostly 5–8 (–10) cm long, ± ovate to broadly triangular, sometimes cordate, acute at apex; wings of fruit broader than the body; plant a tree with white bark (or often dark, especially when juvenile).
4. Leaf blades with at least some hairs in axils of lower lateral veins beneath, ± ovate, broadly cuneate to cordate at base, acute to short-acuminate; pistillate scales with middle lobe reduced to well developed and prolonged.
4. Leaf blades completely glabrous on both sides, even when young, ± broadly triangular and long-acuminate to long-caudate; pistillate scales with middle lobe reduced, scarcely prolonged.
5. Leaves with an acute to acuminate tip (longer in plants derived from cut-leaved forms), teeth mostly 18–30 per side; scales of pistillate catkins sparsely pubescent on adaxial surface.
5. Leaves with a long-caudate tip, teeth often more than 30 per side; scales of pistillate catkins densely pubescent on adaxial surface.