Betula populifolia Marshall

The Betula pendula aggregate forms an almost complete circumboreal population, but its easternmost component, subsp. mandshurica, peters out in central Ontario, a thousand kilometers short of the Atlantic coast. At first glance, the clinal circle might seem to be completed by Betula populifolia, the Grey Birch, which occupies the remaining gap, with a distribution that extends from Ontario to New Brunswick, and south to North Carolina and Virginia. This species is strikingly similar to B. pendula in many respects, with similar habitat preferences (for poor soils and harsh conditions) and a comparable ecological function (as an early coloniser); like the Silver Birch, it is short-lived and a useful nurse for forestry purposes (Sargent 1896); and visually it shares with that species the inverted triangular black ‘eyebrows’ on the trunks beneath each branch junction, which persist after the branch has fallen off. The two species have on occasion even been confused in the nursery trade (see under the cultivar ‘Whitespire’). However, B. populifolia is morphologically distinct, and – unlike the birches included in the B. pendula aggregate, which all intergrade – it is geographically isolated from all members of the pendula complex. Chemical evidence attests to this separation, since the species lacks the compound platyphylloside, present in all other members of the B. pendula complex which have been tested (Santamour & Lundgren 1996).

Betula populifolia is easily distinguished from all other birches by its very finely tapered leaves (unique in the genus), its usually solitary male catkins that project like a thin peg at the end of the twig, and the persistent fruiting catkins, of which the scales are more densely hairy on the adaxial surface than in those of B. pendula (Catling & Spicer 1988). In early spring, trees of B. populifolia are particularly beautiful in the freshness of their translucent, delicate, elegantly pointed leaves. Although the bark is greyish, lustreless and does not peel, it is very attractive on older trunks, and is never fissured from the base upwards as it is in B. pendula subsp. pendula. Another of the species’ common names, Wire Birch, is apt, as the twigs are thin and wiry, though the branches do not hang down as do those of B. pendula. The fruiting catkins remain on the tree into winter when they enhance the twig tracery; unlike the ripe catkins of other white-barked birches of section Betula, they are remarkably hard and difficult to crumble.

No geographical varieties appear to have been recognised and scarcely any aberrant forms, which is surprising as the leaves are so acuminate that exaggerations or reductions of this character would be expected to occur in the wild and to be selected in cultivation – only the forma incisifolia is mentioned in the literature, recorded in two locations, one at Auburndale, Massachusetts and the other on Mt Penn, Pennsylvania (Fernald 1945). The contrast with Betula pendula and its numerous cut-leaved or small-leaved forms is striking.

The name Betula populifolia was published in 1785 by Humphry Marshall (1722–1801), a cousin of the pioneer collector John Bartram, and supplier seeds of American trees and shrubs to France as well as England (Raphael 1989). The species is said by Aiton to have been introduced in 1750 by Archibald ‘The Treemonger’ Campbell, Duke of Argyll (Aiton 1789). There is no doubt that Betula populifolia is well worth planting in the garden or as a landscape tree. In climates where late frosts are a hazard, as is the case in much of lowland maritime Britain, collections from coastal New England are preferable over those from the more continental Vermont, which risk die-back of entire branches. It is said to be short-lived, rarely living longer than 50 years (Hosie 1963) so may be the shortest-lived of all birches. The handful of cultivars reported appear to be either extinct (‘Purpurea’), chimerical (Loudon’s ‘Pendula’) or of dubious – possibly hybrid – identity (‘Laciniata’); matters are not helped by confusion with B. pendula (Santamour & McArdle 1989).

Betula populifolia is very frequently seen growing with B. papyrifera and B. cordifolia – stabilised hybrid populations with the latter are so frequent that they have been given the nothospecies name B. × caerulea (Brittain & Grant 1972).