Betula cordifolia Regel


B. papyrifera var. cordifolia (Regel) Fernald

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Betula cordifolia is difficult to differentiate from the more familiar B. papyrifera, and is often treated as an ecological variant of it. However, B. cordifolia is diploid (2n = 28) while B. papyrifera is hexaploid (2n = 84), indicating that they do not normally interbreed, and behave genetically and ecologically as quite distinct species (H. McAllister, pers. comm. 2007). Despite this, they are visually extremely similar and it is easiest to describe B. cordifolia in terms of its differences from B. papyrifera. Betula cordifolia has pinkish or brownish white to russet or bronze bark, rather than chalk-white or (rarely) dark brown as in B. papyrifera. Leaves of B. cordifolia have a cordate base and 9–12 pairs of lateral veins, while those of B. papyrifera have a cuneate, rounded or truncate base and nine or fewer pairs of lateral veins. The most reliable distinguishing characteristic is that the central lobe of the fruiting catkin scale is parallel-sided in B. cordifolia but rhombic-diamond shaped in B. papyrifera. Brittain & Grant 1965, Furlow 1997. Distribution CANADA: New Brunswick, Newfoundland, Nova Scotia, Ontario, Prince Edward Is., Quebec; ST. PIERRE & MIQUELON; USA: Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, North Carolina, Pennsylvania, Vermont, Virginia, West Virginia, Wisconsin. Habitat Open forest or moist rocky slopes between 800 and 2000 m asl. USDA Hardiness Zone 9. Conservation status Not evaluated. Illustration NT163. Cross-references B426, S117, K229.

Not acknowledged as a distinct entity by American dendrological authors, Betula cordi folia is a cryptospecies, obscured by a more familiar relative. Its generally darker bark is clearly visible in the stands of trees of Canadian origin growing at Stone Lane Gardens, where the colour varies from light reddish cream to dark red-brown. It is slower-growing in cultivation than B. papyrifera, and its young seedlings are less densely hairy than those of that species (H. McAllister, pers. comm. 2007), but there is little else to distinguish the two. Given its more northerly distribution it is probably hardier and more tolerant of a short growing season. In addition to Kenneth Ashburner’s collections, growing well in Devon and at Ness Botanic Gardens and elsewhere, it has been introduced to the United Kingdom on several occasions: for example, by Tony Schilling from a collection made on Mount Washington, New Hampshire in 1979 (Schilling 2379), where it formed a tree 10 m tall. It is also cultivated at Edinburgh from a collection made by T.E. Clarke.

A group of Betula cordifolia at Stone Lane Gardens, Devon, showing the slightly reddish bark that helps differentiate it from its whitish-barked relative B. papyrifera. Image P. Williams.



Other species in the genus