Flora of North America species comparison

Some of the specimens previously referred to Quercus endemica by C. H. Muller belong here instead.

Putative hybrids between Quercus arizonica and Q. grisea (= Q. ×organensis Trelease) are problematic in local areas of contact from southeastern Arizona to western Texas. These intermediates tend to have narrower leaves than Q. arizonica, with moderately reticulate patterns of venation, and more densely hairy leaves. Quercus arizonica and Q. grisea are amply distinct elsewhere, including large areas in northern Mexico, and they appear to be more closely related to other species than to one another (e.g., Q. arizonica with Q. oblongifolia and Q. laeta Liebmann, and Q. grisea with Q. mohriana and Q. microphylla Née). Thus, Q. arizonica and Q. grisea are best treated as distinct species that hybridize, and not as conspecific populations.

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

Species ca. 400 (90 in the flora).

Quercus is without doubt one of the most important woody genera of the Northern Hemisphere. Historically, oaks have been an important source of fuel, fodder, and building materials throughout their range. Other products include tannins and dyes, and oak bark and leaves were often used for tanning leather. Acorns were historically an important food for indigenous people in North America, Central America, Europe, and Asia. In some areas, acorn consumption is still important, but in general, because of the intense preparation necessary to remove tannins and strong flavor of acorn products, they have fallen out of use as human food in developed areas. They do remain, however, an important mast for wildlife and domesticated animals in many rural areas.

Among the most important diagnostic characters within Quercus, and particularly the white oak group (Quercus sect. Quercus), are features of the foliar trichomes. Often these can be seen with a 10× or 15× hand lens; higher magnifications are sometimes required and are useful particularly when characters for a species or complex are first studied and mastered for later use in the field. Although these microscopic characters may seem intimidating, the alternative characters of leaf shape and dentition, so often used in the field, are unreliable in many cases. The large number of misidentified specimens in herbaria that can be easily identified properly with the use of trichome characters illustrates this point. Additionally, many specimens are encountered, both in field and herbarium, that lack fruit or have only immature fruit. Very few species require mature fruit for proper diagnosis; most can be adequately identified with a representative selection of mature sun leaves attached, if possible, to twigs with mature buds. The combination of leaf vestiture, form of the margin (entire, lobed, toothed, spinose), twig vestiture, and bud form and vestiture constitute the majority of diagnostic features minimally required at species level.

Staminate floral and inflorescence characters have not been used to any significant extent in the taxonomy of Quercus. Immature, flowering material is often difficult to identify with certainty, and floral features such as number and form of sepals, number of stamens, and pubescence of flowers or floral rachises seem to vary independently of species affinity within many groups. Because of these problems, descriptions of staminate features are excluded in this treatment as unreliable and of little diagnostic value. When collecting flowering oaks, make a point of gathering fallen fruit and mature leaves carefully from the ground, if available, and revisit such populations again when mature material is available to verify identifications.

The character of acorn maturation in the first year (annual maturation) or second year (biennial maturation) after pollination is commonly used to differentiate major groups within Quercus. All of the North American white oaks have annual maturation; all of the Protobalanus group have biennial maturation; and the vast majority of red oaks have biennial maturation, with one eastern North American and a few western species with annual maturation. In the field, this character can be observed throughout the growing season by examining a sample of twigs from the same tree. If developing fruits exhibit a single size class and are found only on the current year's growth, maturation is annual; if the developing fruits exhibit two size classes with small pistillate flowers on new growth and larger developing fruit on the previous year's twigs, maturation is biennial. In Quercus sect. Protobalanus, biennial maturation may be mistaken for annual maturation because all of the species are fully evergreen, and often the twigs bearing fruit do not produce new growth in the second year after pollination. In such cases, careful examination of a broad sample of twigs from within one tree and throughout a population will verify biennial maturation. Herbarium specimens are sometimes inadequate for this determination.

Hybridization among species of Quercus has been widely documented and even more widely suspected. An astounding number of hybrid combinations have been reported in the literature, and many of these have been given species names, either before or after their hybrid status was known (E. J. Palmer 1948). Hybrids are known to occur in the wild only between members of the same section, and attempts at artificial crosses between species from different sections or subgenera within Quercus have failed with very few exceptions (W. P. Cottam et al. 1982).

Hybridization in most cases results in solitary unusual trees or scattered clusters of intermediate individuals (J. W. Hardin 1975). In some cases, however, populations of fairly broad distribution and extreme variability, often with a majority of intermediate types, may occur. Such instances occur in both the red oak and white oak groups, and to a lesser extent in the Protobalanus group.

When dealing with a suspected hybrid, therefore, one should first consider the possibility of intraspecific variation or environmental plasticity, and then seek parentage among sympatric members of the same section. Because of the almost infinite number of possible hybrid combinations, and the myriad names applied to them, only those that appear to be prominent either locally or in widespread areas are dealt with here. The interested reader is referred to various discussions of oak hybridization in the literature (e.g., E. J. Palmer 1948; J. W. Hardin 1975).

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