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.)

Genera 6, species ca. 125 (5 genera, 33 species in the flora).

Betulaceae belong to an ancient lineage, traceable in the fossil record to the upper Cretaceous. They are easily distinguished by their woody habit; simple, pinnately veined, usually ovate, sharp-toothed leaves; long staminate catkins that often develop the season before anthesis; and (except in Corylus and Ostryopsis) strobiluslike infructescences. The family is held together on the basis of many features, including habit, leaf structure and arrangement, trichome morphology, wood anatomy, inflorescence morphology, ovary and ovule structure, pollen morphology, embryo structure, and fertilization and germination patterns. Five of the six constituent genera inhabit the boreal and cool temperate zones of Eurasia, North America, and the mountains of Mexico and Central America, and two genera also grow in the Andes to northern Argentina in South America. The remaining genus, Ostryopsis Decaisne (most closely related to Corylus Linnaeus), consists of two species of shrubs restricted to northern and western China.

The group is sometimes divided into two families, Betulaceae (Alnus and Betula) and Corylaceae (Carpinus, Ostrya, Corylus, and Ostryopsis), especially in Europe (e.g., H. K. Airy Shaw in J. C. Willis 1973; R. M. T. Dahlgren 1980; J. Hutchinson 1959). In America, this treatment has been followed by A. J. Rehder (1940), J. K. Small (1903), and a few others. Some of those writers have based their recognition of two families in part on the belief that a fundamental difference exists in the staminate inflorescences of the two groups. This view is no longer widely accepted, and most modern authors maintain the family as a single group composed of two subfamilies, Betuloideae and Coryloideae.

Betulaceae may have been derived from hamamelidaceous stock (O. Tippo 1938); this idea has been advanced by others (e.g., J. A. Doyle 1969; P. K. Endress 1977; M. W. Chase et al. 1993). The ancestor of more advanced Hamamelidae (including Betulaceae) may have been a member of the Normapolles complex, a Cretaceous fossil pollen flora that includes grains similar in structure to those of the modern Betulaceae (J. A. Doyle 1969). The pollen of modern Betulaceae is much more specialized than that of modern Hamamelidales, however, and the relationships of the Normapolles complex to Juglandaceae, Fagaceae, and other amentiferous groups, as well as to Betulaceae, are not certain. Additional fossil information will be needed if botanists are to gain stronger insights into the phylogeny of these families (P. R. Crane 1989; K. C. Nixon 1989).

The most obvious evolutionary trends within the family are those related to fruit dispersal (winged fruits in the Betuloideae, nutlets with accessory winglike bracts in Carpinus and Ostrya, and dispersal by small mammals in Corylus), and with apparent adaptations for survival in cold climates (including shrubby growth forms, small leaves, and the protection of pistillate catkins during winter by bud scales in boreal species). Some radiation has also occurred with respect to the pollination system (E. J. Jäger 1980). Please see P. R. Crane (1989) and J. J. Furlow (1990) for reviews of literature related to the origin and diversification of the family.

Members of Betulaceae are economically important as timber trees, as the source of hazelnuts and filberts (Corylus), as ornamental trees and shrubs, and as aids in soil nitrification and stabilization (Alnus). Some are important as causes of pollen allergies in regions where they grow abundantly.

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