7. Betula cordifolia Regel, 13(2): 86. 1861.
Heartleaf birch, mountain white birch, bouleau à feuilles cordées, bouleau blanc
Betula alba Linnaeus var. cordifolia (Regel) Regel; B. papyrifera Marshall var. cordifolia (Regel) Fernald
Trees or shrubs , large, irregular, to 20 m; trunks often several, trees with narrow crowns. Bark of young trunks and branches dark reddish brown, close, in maturity reddish white to reddish tan or bronze, exfoliating in paper-thin sheets; lenticels dark, horizontally expanded. Twigs glabrous to sparsely pubescent, often covered with conspicuous, warty, resinous glands. Leaf blade narrowly ovate to ovate with 9--12 pairs of lateral veins, 6--10(--14) × 4--8 cm, base usually cordate, rarely rounded, margins coarsely or irregularly doubly serrate, apex short-acuminate, abaxially sparsely to moderately pubescent, sometimes velutinous or tomentose along major veins and in vein axils, covered with minute, resinous glands. Infructescences pendulous or nearly pendulous, cylindric, 2.5--5.5 × 0.6--1 cm, shattering with fruits in fall; scales glabrous to moderately pubescent, lobes diverging proximal to middle, central lobe elongate, obtuse, lateral lobes ascending, shorter and slightly broader than central lobe. Samaras with wings broader than body, broadest near summit, extended beyond body apically. 2 n = 28, 56.
Flowering late spring. Moist, rocky slopes or rich, open forest; 800--2000 m; St. Pierre and Miquelon; N.B., Nfld., N.S., Ont., P.E.I., Que.; Maine, Mass., Mich., Minn., N.H., N.Y., N.C., Pa., Vt., Va., W.Va., Wis.
Betula cordifolia has been reported from Connecticut; I have not seen specimens.
In recent years Betula cordifolia has usually been treated as a variety of B . papyrifera , and perhaps it should be considered an ecological race of that species. It differs from B . papyrifera in polyploid level (diploid and tetraploid in B . cordifolia versus tetraploid, pentaploid, and hexaploid in B . papyrifera ) and in vegetative characters, including the number of lateral veins of leaves and the color of bark (W. H. Brittain and W. F. Grant 1967; P. E. DeHond and C. S. Campbell 1989). In the Adirondacks, B . cordifolia and B . papyrifera occur in rather distinct ecological zones ( B . cordifolia mostly above 800 m and B . papyrifera generally below this elevation). The species does not appear to occur as far west (Iowa) as stated by M. L. Fernald (1950).