Flora of North America species comparison

The difficulty in distinguishing Texas populations of Quercus fusiformis from Q. virginiana is reflected in a variety of taxonomic treatments, including reducing Q. fusiformis to varietal rank under Q. virginiana. The latter disposition is problematic, however, because Q. fusiformis in northeastern Mexico is amply distinct from Q. virginiana and appears to be more closely related to Q. brandegei Goldmann, an endemic of Baja California, Mexico. Thus, here we assume that the intergradation of Q. virginiana and Q. fusiformis is a result of secondary contact, and is not primary clinal variation. Under this interpretation, Q. virginiana in typical form extends into Texas only as far west as the Brazos River drainage along the coast from there to the escarpment of the Edwards Plateau; most populations elsewhere are either intermediate between the two species or show greater affinity with Q. fusiformis. On the Edwards Plateau, the live oak populations are small trees forming rhizomatous copses (shinneries) and having mostly acute acorns.

Populations of live oak on deep sands in south Texas differ from typical Quercus fusiformis in having broader, more rounded leaves, often with the secondary veins somewhat impressed abaxially, and relatively blunt, barrel-shaped acorns. These characteristics suggest introgresion from the Mexican-Central American species Q. oleoides Schlechtendal & Chamisso, which in its typical form reaches north only as far as southern Tamaulipas, Mexico. The name Q. oleoides var. quaterna C. H. Muller has been applied to what is apparently a shrub form of one of these Q. fusiformis × Q. oleoides hybrids.

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

Genera 9, species probably 600-800 (5 genera, 97 species, and numerous hybrids in the flora).

In the Western Hemisphere, Fagaceae are found from southern Canada to Colombia; they are absent or infrequent in most of the northern Great Plains and northern Rocky Mountain region.

Fagaceae are one of the most important families of Northern Hemisphere woody plants in terms of total biomass and economic use. They are widely used for lumber, firewood, and horticultural plantings; the nuts are often used for animal fodder and, in some species, for human food (particularly Castanea). As dominants in forests, woodlands, and chaparral, native stands of fagaceous trees and shrubs provide optimal wildlife habitat, often harboring an exceptionally diverse insect fauna. Most of the diversity of the family in the Western Hemisphere is concentrated in the genus Quercus, with the greatest number of species in Mexico (at least 125 species), and a secondary area of diversity in the southeastern United States.

Polyploidy has not been reported in any natural populations of species of Fagaceae. Natural interspecific hybridization is common in the family, particularly in Quercus, and also in Castanea and Lithocarpus.

The most important diagnostic feature of Fagaceae is the cupule, which occurs as the cup or cap of the acorn in Quercus and Lithocarpus and the spiny bur that surrounds the fruits of Castanea and Chrysolepis. The cupule is sometimes referred to as an involuc involucre, however, is made up of bracts, while the cupule has been shown to be a complex structure that is interpreted as an indurated, condensed, partial inflorescence formed by fusion of stem axes with several orders of branching, bearing bracts that are modified as scales and/or spines (see B. S. Fey and P. K. Endress 1983).

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