Project: Southwest Biodiversity Consortium
Trees or shrubs , evergreen or winter-deciduous, sometimes rhizomatous. Terminal buds spheric to ovoid, terete or angled, all scales imbricate. Leaves: stipules deciduous and inconspicuous (except in Quercus sadleriana ). Leaf blade lobed or unlobed, thin or leathery, margins entire, toothed, or awned-toothed, secondary veins either unbranched, ± parallel, extending to margin, or branching and anastomosing before reaching margin. Inflorescences unisexual, in axils of leaves or bud scales, usually clustered at base of new growth; staminate inflorescences lax, spicate; pistillate inflorescences usually stiff, with terminal cupule and sometimes 1-several sessile, lateral cupules. Staminate flowers: sepals connate; stamens (2-)6(-12), surrounding tuft of silky hairs (apparently a reduced pistillode). Pistillate flower 1 per cupule; sepals connate; carpels and styles 3(-6). Fruits: maturation annual or biennial; cup variously shaped (saucer- to cup- or bowl- to goblet-shaped), without indication of valves, covering base of nut (rarely whole nut), scaly, scales imbricate or reduced to tubercles, not or weakly reflexed, never hooked; nut 1 per cup, round in cross section, not winged. x = 12. 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 b
PLANT: Trees and shrubs, the wood hard, close-grained or porous; hairs of two types, both usually present, the stellate ones whitish or yellowish, non-glandular, suberect to spreading, the unbranched ones orangeish to golden, glandular, more or less appressed; young growth usually densely woolly, the older growth glabrescent or remaining woolly; buds 1-4(-5) mm long in ours. LEAVES: alternate, subcoriaceous to coriaceous, entire, toothed, or lobed. INFLORESCENCE: staminate flowers in aments; pistillate flowers solitary or in groups on spikes, these sometimes abbreviated, each pistillate flower with a separate involucre. FRUIT: an ovoid to subcylindric nut, each subtended by a cup- or bowl-shaped cap (together called an acorn), maturing after one summer in all our species except Q. hypoleucoides; caps (cupules) woody, covered with many imbricate, shortly woolly scales, variously pubescent within, the scales with thickened bases and thin tips or the entire scale thin. NOTES: ca. 450 spp. in N. Amer., n S. Amer., temperate and subtropical Eurasia, and n Afr. (Classical name for oak). Tucker, J. M. 1961. Amer. J. Bot. 48:202-208; 1963. Amer. J. Bot. 50:699-708; 1970. Amer. J. Bot. 57:71-84. Tucker, J. M. & Haskell, H. S. 1960. Brittonia 12:196- 219. Tucker, J. M. et al. 1961. Amer. J. Bot. 48:329-339. Quercus is commonly divided into three subgenera in the Western Hemisphere: the white oaks (Quercus), red oaks (Erythrobalanus) and intermediate oaks (Protobalanus). In AZ, species can be assigned to the three subgenera as follows: Quercus: Q. gambelii, Q. grisea, Q. havardii, Q. oblongifolia, Q. pungens, Q. rugosa, Q. toumeyi, and Q. turbinella; Erythrobalanus: Q. emoryi and Q. hypoleucoides; and Protobalanus: Q. chrysolepis and Q. palmeri. Hybridization between subgenera is not known, but hybridization between species of the same subgenus has been reported for all three groups in Arizona. REFERENCES: Landrum, Leslie R. Fagaceae. 1994. J. Ariz. – Nev. Acad. Sci. Volume 27, 203-214