3. Quercus L. (oak)

Bark split into parallel ridges, loose plates, or strips. Leaves and buds in several irregular ranks, more or less crowded at the stem apex, Buds 2–12 mm long, ovoid to ellipsoid, all of the scales alternate, a terminal bud present (sometimes difficult to distinguish in large apical bud clusters). Stipules small and inconspicuous, shed early in leaf development. Leaf blades entire, toothed, scalloped, or lobed, the secondary veins reaching the margin or turning aside before reaching it. Inflorescences of staminate and pistillate flowers separate; the staminate catkins drooping or pendant, elongate, spikelike; the pistillate flowers sessile or nearly so (sometimes a stalk developing as the fruit matures), solitary, paired, or in small clusters. Fruits ripening the first or second autumn after flowering. Cupules (usually called cups in Quercus) unlobed and not splitting at maturity, usually enclosing only the lower portion of the nut (except in Q. lyrata), scaly, the scales short and appressed or long and spreading, overlapping and covering the surface of the cupule. Nut 1 per cupule, variously narrowly ellipsoid to more or less globose, often with a prominent attachment scar at the base, in many species with a minute to small, blunt to sharply pointed, extension at the tip, circular in cross-section, brown. About 450 species, throughout the Northern Hemisphere, most diverse in temperate regions and montane portions of the tropics.

Oaks are the most important group of trees in Missouri. They dominate most of the forests, woodlands, and savannas in the state. Acorns are a very important food for many wild mammals, birds, and insects, as well as for domestic hogs. In prehistoric times acorns were a staple food for humans in many parts of the Northern Hemisphere. Acorns are quite nutritious, but they have high concentrations of tannins, which must be removed by leaching before they are palatable. Tannin concentration varies greatly from tree to tree; generally, black oaks have higher tannin concentrations than white oaks, and are more nutritious. Oaks produce high-quality hardwood lumber, which has been important for general construction, cabinet making and furniture making, veneer, pallets, and fenceposts. Other, specialized uses of some white oaks are discussed under section Quercus. Oaks also are a major source of high-quality firewood and charcoal.

Oaks are important as shade trees and ornamentals in places where native trees have been left standing after building. However, they are extremely vulnerable to construction injury. Every effort should be made to avoid severing roots, compacting the soil in the root zone, or smothering roots by earth fill. Many species are difficult to transplant or to start in containers, so they are somewhat underutilized in the nursery trade. The young seedling puts most of its growth into a deep taproot. If this develops abnormally (as in a pot) or is cut off (for transplantation), the plant is sometimes unable to compensate and dies within a few years. Oaks are best planted in the place where they are to grow, but care must be taken to protect the cotyledons of the seedling from squirrels, which frequently dig up the remains of the old nut and eat it.

In many oaks, particularly in members of the black oak group (see the classification below) the leaves are not totally deciduous. Instead, the plants retain their dried, brown leaves through the winter, especially on lower branches. This phenomenon, which also occurs in scattered other species of woody plants, is known as marcescence.

Oaks are very variable morphologically, and the species can be difficult to separate. Significant variation is seen between populations, among trees in the same population, and even among different parts of the same tree (Baranski, 1975; Braham, 1977; R. J. Jensen, 1989). The juvenile leaves of many species of oak are quite different in form from the adult leaves, and vigorous sprouts from the roots or trunk often revert to juvenile leaf morphology. Leaves growing in the shady inner part of the crown may be quite different from those in the sunny outer part of the crown, the shaded inner leaves being larger, thinner, more shallowly lobed, and more sparsely pubescent. Unfortunately, seedling leaves, sprouts, and shade leaves are sometimes the only leaves that are easily accessible from ground level. Specimens that consist only of such leaves are difficult to identify accurately.

Pubescence characters are very useful in oaks, which have a variety of conspicuous branched hairs on most of their organs. In stellate hairs, found only in white oaks, the branches of the hair (known as rays) spread in one plane, more or less parallel to the leaf or stem and often appressed to it. Other hair types include fasciculate hairs, multiradiate hairs, and dendroid hairs, all of which branch in three dimensions. These types are difficult to distinguish except under high magnification and are all referreed to in the present work as “branched spreading hairs.” Many oaks also have soft, unbranched hairs appressed to the leaf surface. These are usually inconspicuous unless they are pigmented (common in Q. marilandica and Q. stellata, not in other species), so they are easily overlooked unless the leaf is examined carefully at high power. Hair structure can be difficult to observe except with high magnification and strong illumination, so these characters have been avoided in the keys as far as possible, but they are included in the descriptions because they are important for critical work, especially the accurate recognition of hybrids.

Pubescence descriptions here apply only to mature, fully expanded leaves and associated twigs. In many species whose leaves and twigs are glabrous at maturity, including Quercus alba and many red oaks, expanding leaves and twigs in the spring are densely hairy; these hairs are shed as the leaves mature. Other organs that provide important taxonomic characters develop later in the season. The terminal buds develop over the summer, usually attaining full size around the end of August. Acorns ripen in September and October.

Natural hybridization in North American oaks was reviewed by Palmer (1948). Several hybrid combinations not known to Palmer have been described since 1948, but his general comments on hybridization are still valid. Hybrids may be expected between any two species in the same section, but they are very uncommon. Palmer estimated that fewer than one tree in every 100,000, perhaps fewer than one in a million, is a first-generation hybrid. Hybrids are usually found in areas where one of the parental species is abundant and the other is uncommon and growing in an ecologically marginal habitat. Once a first-generation hybrid reaches reproductive maturity, it may backcross with one or both of its parents. Such backcross hybrids are much more variable and difficult to distinguish from the parental species than the first-generation hybrids. Over several successive generations, this process can lead to trees that are morphologically indistinguishable from one of the original parental species, but nevertheless have some of the other parent’s genes incorporated into their genomes (Hardin, 1975; Whittemore and Schaal, 2001). Because of the large number of different hybrids documented from Missouri, it is more expedient to collect the data on these in a separate table than to disperse the information within the discussion of each species. It should be noted that in some cases one or more synonymous epithets have been published in reference to certain oak hybrids, but because of space considerations and the limited application of these names to Missouri plants, no attempt has been made here to list synonyny for these binomials. Thomson (1977) documented hybrids between the morphologically similar Q. muehlenbergii and Q. prinoides from a site in Lafayette County, but no voucher specimens to support this conclusion were discovered during the present research. Also, the putative hybrid between Q. falcata and Q. imbricaria, which Steyermark (1963) reported under the illegitimate name Q. ×anceps E.J. Palmer (a later homonym of Q. anceps Korth. [a Bornean taxon now usually treated as part of Q. spicata Sm.] that subsequently was legitimized as Q. ×palmeriana A. Camus), still requires documentation from Missouri. The specimen from Butler County cited by Steyermark has been redetermined as Q. falcata.

Putative parentage Hybrid binomial Documented counties

Q. alba × Q. bicolor Q. ×jackiana C.K. Schneid. Lafayette

Q. alba × Q. macrocarpa Q. ×bebbiana C.K. Schneid. Cole, Jackson, Pike, Putnam, St. Louis

Q. alba × Q. michauxii Q. ×beadlei Trel. ex E.J. Palmer Dunklin

Q. alba × Q. muehlenbergii Clinton, Jackson, St. Louis

Q. alba × Q. stellata Q. ×fernowii Trel. Cole, Dade, Jackson, Lawrence, Macon, Marion, Polk, St. Clair, St. Louis City

Q. bicolor × Q. lyrata Q. ×humidicola E.J. Palmer Dunklin

Q. bicolor × Q. macrocarpa Q. ×schuettei Trel. Jackson, Johnson, St. Clair

Q. bicolor × [Q. muehlenbergii × Q. prinoides] Q. ×introgressa P.M. ThomsonLafayette

Q. bicolor ×Q. prinoides Q. ×wagneri Gaynor Clinton, Jackson

Q. coccinea × Q. velutina Carter, Oregon

Q. ellipsoidalis × Q. velutina Harrison

Q. falcata × Q. marilandica Q. ×incomita E.J. Palmer Howell

Q. falcata × Q. nigra Q. ×garlandensis E.J. Palmer Butler, Dunklin

Q. falcata × Q. phellos Q. ×subfalcata Trel. Dunklin

Q. falcata × Q. velutina Q. ×willdenowii Zabel Howell

Q. imbricaria × Q. marilandica Q. ×tridentata Engelm. ex A. DC. Jackson, St. Louis, St. Louis City, Texas, Wayne

Q. imbricaria × Q. palustris Q. ×exacta Trel.Boone, St. Louis

Q. imbricaria × Q. rubra Q. ×runcinata (A. DC.) Engelm. Franklin, Jackson

Q. imbricaria × Q. shumardii Q. ×egglestonii Trel. Dade, Jackson, Laclede

Q. imbricaria × Q. velutina Q. ×leana Nutt. Adair, Boone, Cape Girardeau, Carter, Cole, Dade, DeKalb, Greene, Iron, Jackson, Jefferson, Johnson, Knox, Madison, St. Louis, Sullivan

Q. lyrata × Q. michauxii Q. ×tottenii Melvin Ripley

Q. lyrata × Q. stellata Q. ×sterrettii Trel. Carter

Q. macrocarpa × Q. michauxii Q. ×byarsii Sudw. ex Trel. Mississippi

Q. macrocarpa × Q. muehlenbergii Q. ×deamii Trel. Butler, Clinton, Greene, Holt, Jackson, Jasper, Johnson

Q. macrocarpa × Q. prinoides Q. ×beckyae Gaynor Grundy

Q. macrocarpa × Q. stellata Q. ×guadelupensis Sarg. Jackson, Jasper

Q. marilandica × Q. velutina Q. ×bushii Sarg. Adair, Barton, Boone, Cass, Christian, Dent, Franklin, Greene, Howell, Jackson, Jasper, Johnson, Knox, Lewis, McDonald, Monroe, Montgomery, Morgan, Newton, Putnam, Ralls, Randolph, St. Francois, St. Louis, St. Louis City, Washington

Q. nigra × Q. phellos Q. ×capesii W. Wolf Dunklin

Q. pagoda × Q. velutina New Madrid, Scott

Q. palustris × Q. phellos Q. ×schochiana Dieck Stoddard, Wayne

Q. palustris × Q. rubra Benton

Q. palustris × Q. shumardii Q. ×mutabilis E.J. Palmer & Steyerm. Bates

Q. palustris × Q. texana Ripley

Q. palustris × Q. velutina Q. ×vaga E.J. Palmer & Steyerm. Nodaway

Q. phellos × Q. rubra Q. ×heterophylla F. Michx. Dunklin, Wayne

Q. phellos × Q. shumardii Q. ×moultonensis Ashe Ripley, Stoddard

Q. phellos × Q. velutina Q. ×filialis Little Butler

Q. prinoides × Q. stellata Q. ×stelloides E.J. Palmer Jackson, Lawrence, Sullivan

Q. rubra × Q. shumardii Jackson, St. Louis

Q. rubra × Q. velutina Q. ×hawkinsiae Sudw. Clark, Dunklin, Jackson

Species of Quercus are divided among several subgenera or sections, but there is still some disagreement on the best classification to use (Oh and Manos, 2008). There are only two groups native to eastern North America, the red and black oaks (section Lobatae Loudon), which are found only in the New World, and the white oaks (section Quercus), the only subgroup that is native to both hemispheres. A third group of North American oaks, the intermediate oaks (section Protobalanus (Trel.) A. Camus), is found only in the southwestern United States and northern Mexico. Groupings in the Old World oaks are less clear. A morphologically distinctive group of species in tropical and subtropical Asia have the scales of the acorn cup fused as concentric rings or dense whorls. This group is subgenus Cyclobalanopsis (Oerst.) C.K. Schneid. Some of the remaining Old World species definitely belong to sect. Quercus, but others form one or more distinct sections. A species in one of these, section Cerris Loudon, is introduced in Missouri. See the key and descriptions below for morphological distinctions among the groups found in Missouri.