Oaks (Quercus spp.), members of the Beech family (Fagacea), are trees and shrubs having simple, alternate leaves. Characterized by their strong, complex wood, wind-pollinated flowers, fruits called acorns, and their ability to live for centuries, oaks have played an important role in temperate landscapes. Of the 500 species in the genus Quercus, approximately 90 are found in the United States and Canada, with another 112 species in Mexico. Another member of the Beech family that is closely related to the oaks is the tanoak (L. densiflorus ), which is found in California and is the only representative of this Asian genus found in North America. It has flowers similar to the chinkapin (Castanopsis ) and bears acorns like the oaks, thus making it a possible evolutionary link between the two genera.
The Quercus genus is quite old, being one of the early angiosperms of the Miocene epoch (26-12 million years ago). Over time, oaks have divided into two main lineages, with an intermediate subgenus for less genetically distinct species. The red oaks (Erythrobalanus ) are characterized by pointed leaves with bristles or spines that can be either lobed or unlobed. The acorns have a hairy inner shell and mature in two years (except for California live oaks, Q. agrifolia which mature in one year) on the twigs of the first year’s growth. The smooth bark is dark gray, black or brown, with reddish brown wood. The white oak (Leucobalanus ) leaves are rounded and smooth but can also be lobed or unlobed. Acorns mature in one year and have a smooth inner shell. The wood is light brown or yellow and the bark is scaly or rough brown to light gray. The leaves of the intermediate oaks (Protobalanus ) are unlobed, although some may have green spines or teeth. The inner shell of the acorns can be either smooth or hairy, but does not mature until the second year. The bark can be either scaly or rough, with a wide color range. The wood is generally light brown and not as commercially valuable as that in the other oak families.
Found in a wide variety of habitats, oaks prefer loamy, well drained soils. The roots are quite extensive, reaching out at least three times the height of the tree and down as deep as 15-40 ft (4.6-12.2 m), depending on site conditions.
There are both evergreen and deciduous species. Each leaf of the evergreen oaks falls after one to two years, but there is no synchronous leaf loss. New leaves form during either the first spring growth or a smaller secondary flush of growth which can occur when conditions are favorable. Deciduous oaks follow the typical pattern of fall leaf loss in response to decreased daylight, winter dormancy, and spring flush of new leaves and flowers.
Oaks vary in size from small shrubby species to trees with majestic dimensions. The tallest oak, reaching up 123 ft (37.5 m), with a circumference of 21.6 ft (6.6 m) and a canopy spread over 83.6 ft (25.5 m) is a black oak found in Warrensville Heights, Ohio. Other oaks notable for their size are: the Wye live oak (Q. virginiana ) in Maryland standing 91.8 ft (28 m) tall; a 106.6-ft (32.5 m) tall coast live oak (Q. agrifolia ) in Chiles Va Mey, California; and a northern red oak (Q.rubra ) in Ashford, Connecticut, standing 77 ft (23.5 m) tall and spreading 105 ft (32 m).
Oaks rely primarily on wind pollination, with massive quantities of pollen produced in the male flowers (25-100 per catkins) each spring. The female flowers tucked inconspicuously in the nodes of axial twigs mature a little later, avoiding self-pollination. Either single or clusters of two to three acorns begin forming. Most trees begin acorn production after 20 years. Crop production varies yearly according to numerous factors, but an individual tree can produce over 5,000 nuts in a good year. Of these, roughly 25% are likely to be infested with weevils, making them inedible and unviable.
Because of their huge investment in acorn production, oaks play a critical role in supporting wildlife and maintaining regional biodiversity. Many migratory species of birds and bats roost or nest in them, in addition to using the food resource. In California, it is estimated that over 5,000 species of insects, more than 80 species of reptiles and amphibians, 150 species of birds, and over 60 species of mammals rely upon oaks for some part of their lifecycle. Acorn or mast production is such a significant element of most oak ecosystems that crop failure can be life threatening for many species.
Oaks are subject to infection by numerous pathogens, most of which do little more than temporary harm to the tree. The exceptions are root-related diseases caused by various fungi. Oak wilt, caused by Endoconidiphora fagacearum has become a serious problem in the eastern and central United States. Oak root rot or honey fungus (Armellaria spp.) and avocado root rot (Phytophthora spp.) are more problematic in the western region. Most oaks are resistant unless environmental stresses weaken the tree to the point that the fungus can proliferate. Little can be done to save a severely infected oak.
Because of their widespread distribution, oaks play a significant ecological role in many forest communities. Of the 90 forest-type covers described in the United States and Canada, oaks are an important element in 64 and include both evergreen and deciduous species. Some of the more important species in North America include the northern red oak (Q. rubra ), the black oak (Q. velutina ), and the white oak (Q. alba ) in the east. The coast live oak (Q. agrifolia ), the gambel oak (Q. gambelii ), and the wayleaf oak (Q. undulata ) are most widespread in the western region.
Most species are limited to either the eastern or western regions of the continent, with chinkapin oak (Q. muehlenbergii ) and shin oak (Q. havardii ) the only species to bridge the gap through the prairies. Due to the spread of development pressures and conversion of rangeland into housing and commercial uses, there are now several species of oaks facing serious decline. Of these, the Oglethorpe oak (Q. oglethorpensis ) in Georgia, the valley oak (Q. lobata ), blue oak (Q. douglasii ), and Englemann oak (Q. engelmanii ) in California and several species with limited ranges in Texas, New Mexico, and Arizona are considered either rare, threatened, or endangered.
Contributing to this problem is the lack of regeneration of many oak species. For a wide variety of reasons, including changes in understory vegetation, soil compaction, damage by grazing animals, changes in fire frequency and associated forest species, natural recruitment of new trees is limited in many areas. Mixed age stands are not as common as single age stands in many areas. More intensive management to provide light openings, reduce soil compaction, and eliminate competitive species is slowly beginning to turn this trend around. Prescribed burning to restore a more natural fire ecology is being done in many areas. The thick bark of oaks is particularly adapted to withstand forest fires and they can resprout from the root crown if the tree is burned.
The name Quercus comes from the Celtic, quer meaning “fine,” and cuez meaning “tree.” Historically, the Celtic religion as well as that of other cultures venerated old oak trees, using them as a focus for spiritual rituals. The Druids believed the oak to be a sacred tree, the symbol of their religion, and potent source of wisdom. The ancient Greeks believed the rustling leaves of a sacred oak to be oracles from Zeus. In Allonville, France, an oak 44.3 ft (13.5 m) in circumference was consecrated as a Roman Catholic church in 1696 and the chapel built into the canopy can hold 5-10 worshippers. Since many oaks live for over 300 years, their longevity and durability became the subject of literary metaphors and the trees serve as reminders of many historic events. The oldest documented oak lived for 950 years in Switzerland.
When William Penn landed in 1682, the Holly Halls white oak stood tall near Elkton, Maryland. Recently threatened by development, it has been afforded protection by the city and still remains. A bur oak (Q. macrocarpa ), known as the Council Oak, standing in Sioux City, Iowa shaded Lewis and Clark as they met with the natives. Longfellow’s famous poem “Evangeline” includes reference to a live oak (Q. virginiana ) still standing as an historic landmark in St. Martinville, Louisiana. The Jack London Oak (Q. agrifolia ) was planted near the Oakland (California) City Hall after the author died. The oak tree first visited by Spanish explorer Vizcaino in 1602 and the site of the first mass held by Father Serra in Monterey, California, in 1770 finally died and was replaced by a monument in 1896. The Oak of Peace still standing in Glendale, California, was the site of the meeting between General Andres Pico and Colonel John C. Fremont that ended the War with Mexico in 1847. Species of oak are the state trees of Connecticut, Georgia, Illinois, Iowa, Maryland, New Jersey, and the District of Columbia.
Because they are widespread and generally large, oaks have been used in numerous ways. The leaves, flowers, and bark were used by indigenous peoples in both Europe and North America for making medicinal drinks used to cure fevers, stop vomiting, and control diarrhea. Tannins extracted from the bark were used both for dying and tanning hides. The chestnut oak (Q. prinus ) was logged to virtual extinction due to the high quality tannin it provided for the tanning industry.
In addition to the wildlife reliant on acorns as a food source, many indigenous peoples also utilized this resource. Acorns provided a staple food supply for many Native Americans, especially in California. A valuable source of nutrients, acorns are high in fat, carbohydrates, some protein, and vitamins A, C, and E. Preparations included leaching the bitter tannins by grinding the inner nut into flour, immersing this in running water, and then making either gruel or bread. Each oak species tastes different and certain species, such as California black oak (Q. kelloggii ) were preferred, due to their lower tannin content. Acorns remain viable for several months following ripening and can be stored in granaries for years. Considered a sign of fertility by Nordic and Native American peoples, acorns were used symbolically in many ceremonies.
The structural characteristics of the wood make oak one of the most versatile hardwoods, valued by many industries. The strength of oak wood is a result of the inner structure of vessels and fibers. The ring porous nature of the woody tissue results from uneven vessel growth. During the spring and summer, large vessels and fibers grow, followed by smaller vessels as the season progresses. In deciduous species, the vessels are almost non-existent and during the fall and winter are replaced by fibers. This provides distinct growth rings and adds to the structural integrity of the wood. It also provides a distinctive grain pattern when planed into thick planks for panelling and cabinetry.
In red oaks, the vessels remain open over time, allowing fluid conduction to continue. These species are used in making railroad timbers and furniture. The vessels of the white oaks become gummed up with tyloses and are more valuable for barrel staves and flooring. Casks made of oak are in high demand for fermenting wine in France and many other countries. Many of the early sailing ships used oak timbers for hulls and ribs. A famous example is Old Ironsides, a U.S. Navy frigate whose restoration used many large timbers of live oak (Q. virginiana ). The destruction of the English oak (Q. robur ) forests in Europe to construct the navies of the 1600s was one of the many economic incentives for colonizing the New World with its untouched expanses of hardwood forests.
The cork oak (Q. suber ) is another commercially valuable species found throughout the Mediterranean region. The thick bark composed mostly of cork cells can be harvested every 10 years in early summer to provide sheets of soft, smooth cork useful in many ways. The cork cells capture air inside as they dry, making the material extremely resilient and buoyant. Cork has been used to manufacture floats, handles, stoppers, and as insulation, since it is a poor conductor of heat and sound.
Probably the most common worldwide use of oak is as fuel. Oak burns very hot, providing up to 23 million BTU per cord. Charcoal made of oak was extremely important to small local industries during the nineteenth century. Most hardwood forests are managed for fuel wood harvesting or lumber, with oaks considered the most valuable species.
In addition to ecological and aesthetic landscape value, another important role of oaks is in maintaining watershed integrity. The sometimes deep, always extensive root system of oaks stabilizes slopes, limits erosion, and allows groundwater recharge. The wide canopies dissipate the rainfall and prevent surface erosion, while allowing slow saturation into the soil. The ability of oaks and other trees to reduce air pollution and trap airborne particulates is well documented. Noise abatement and temperature modulation in urban areas is also provided by the large, dense oaks. These important contributions of oaks to the sustainability and livability of our landscapes are vital. Careful examination of the role played by oaks in maintaining watershed integrity and preservation efforts on a bioregional scale are needed to ensure that the oak woodlands endure into the future.
Lewington, Richard, and David Streeter. The Natural History of the Oak Tree. New York: Dorling Kindersley, 1993.
Logan, William Bryant. Oak: The Frame of Civilization. New York: W. W. Norton, 2006.
"Oaks." The Gale Encyclopedia of Science. . Encyclopedia.com. (February 23, 2019). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/oaks-0
"Oaks." The Gale Encyclopedia of Science. . Retrieved February 23, 2019 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/oaks-0
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