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.)

Quercus macrocarpa is one of our most cold-tolerant oak species; it also endures a wide variety of other harsh conditions including poor dry soils and wet, poorly drained, and inundated locations. Putative hybrids with Q. bicolor are common in the northeastern part of its range, where the two species often occur together in wet, poorly drained habitats. The effect of this contact may be partially responsible for morphologic differences across the range of Q. macrocarpa. The large acorns are best developed in the southern part of the range, and a clinal decrease in acorn size and extent of the mossy fringe on the acorn cup seems to occur as one travels from south to north. In the northwest part of its range, Q. macrocarpa varies clinally to smaller, shrubbier forms on bluffs and hillsides, with smaller, less fringed cups, that are the basis of Q. macrocarpa var. depressa (Nuttall) Engelmann and Q. mandanensis Rydberg. These scrubby forms may merit formal recognition after more thorough study; they are treated here as clinal variants of the species. Quercus macrocarpa forms putative hybrids also with Q. alba in the savannah-type regions of the midwest. Putative hybrids with Q. gambelii occur out of the range of Q. macrocarpa.

Quercus macrocarpa is the only oak species native to Montana (in the southeast corner). Wood of Q. macrocarpa is similar to that of Q. alba and produces one of the best and most durable oak lumbers.

Native Americans used Quercus macrocarpa medicinally to treat heart troubles, cramps, diarrhea, Italian itch, and broken bones, to expel pinworms, and as an astringent (D. E. Moerman 1986).

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