Fagales

▪ plant order

Introduction

  beech order of dicotyledonous woody flowering plants, comprising nearly 1,900 species in 55 genera. Members of Fagales represent some of the most important temperate deciduous or evergreen trees of both hemispheres, including oaks (oak), beeches, walnuts (walnut), hickories (hickory), and birches (birch).

      Because of the presence of catkins, or aments, Fagales, plus a number of unrelated families, were previously classified in an artificial group called Amentiferae. The chief features of the members of Amentiferae were staminate flowers, and frequently also pistillate flowers, in catkins, reduced or absent sepals and petals, and a general trend toward wind pollination (anemophily). Fagales is now placed in the Rosid I group among the core eudicots in the Angiosperm Phylogeny Group II (APG II) botanical classification system (see angiosperm).

Distribution and abundance

      Fagales contains eight families: Betulaceae, Fagaceae, Juglandaceae, Casuarinaceae, Nothofagaceae, Myricaceae, Rhoipteleaceae, and Ticodendraceae. They are described in turn in this section.

Betulaceae

 Betulaceae, or the birch family, contains 6 genera (Betula, Alnus, Corylus, Carpinus, Ostryopsis, Ostry) and 145 species. Members of the family, with the exception of Ostryopsis, are conspicuous elements in the colder parts of the Northern Hemisphere, where they are among the dominant woody plants in Arctic and Alpine regions, but the family is not restricted to those areas.

      Betula (birches (birch)), with about 60 species, is the largest genus in the family. B. pendula (silver birches) and B. nana (dwarf birches) are circumboreal (i.e., extending to the northern limit of the tree line); the two species very nearly coincide in their ranges, with the dwarf birches extending farther into the Arctic. They now occupy most areas that were glaciated until about 10,000 years ago.

 The genus Alnus (alders (alder)) is north temperate in distribution but ranges south through the Andes to about latitude 20° S. It is only in the Americas that members of the family extend along the mountains into the Southern Hemisphere.

      The greatest diversity in Betulaceae, however, is in the moist temperate forests of East Asia, particularly in China, where all six genera are well represented. It has been speculated that Alnus may have originated in southeastern, eastern, or central Asia, since the greatest morphological diversity in the genus, as well as the species considered to be the most primitive, occurs in that region.

 Corylus (filbert) contains about 15 species, including C. avellana ( filbert, also known as hazelnut), distributed throughout the Northern Hemisphere. Species of Corylus are mostly shrubs or small trees that spread by means of sucker shoots and are able to form large colonies; however, C. colurna (Turkish hazel) is a tree that may reach a height of about 24 metres (79 feet), and a close relative in China (C. chinensis) grows to about 20 metres (66 feet).

Fagaceae

 Fagaceae, or the oak and beech family, contains about 1,000 species unevenly distributed among 7 or 8 genera. The largest genus in Fagaceae is Quercus (oaks (oak)), with about 400 species, mostly limited to the warmer parts of the Northern Hemisphere. The greatest concentrations of species of oaks are in the southeastern to southwestern United States and Mexico, in eastern Asia (China and Japan), and in the area from the Mediterranean to Caucasia.

 Fagus (beeches (beech)) is a genus of about 10 species in the Northern Hemisphere, with the greatest diversity in China and Japan, where about seven species are found. A single variable species, F. grandifolia (American beech), occurs in eastern North America and Mexico, and another, F. sylvatica, is found in Europe.

      The 12 or so species of the genus Castanea (chestnut) also show a worldwide distribution in temperate areas of the Northern Hemisphere, again with the greatest diversity in eastern Asia. The two species of Chrysolepis (chinquapin) are confined to the western United States. The two remaining genera, Lithocarpus (120 species) and Castanopsis (about 110 species), are almost exclusively restricted to eastern and southeastern Asia.

Juglandaceae

 The large and economically important Juglandaceae, or the walnut and hickory family, contains 7–10 genera and 50 species, which are distributed mainly in the north temperate zone but extend through Central America along the Andes Mountains to Argentina and, in scattered stands, from temperate Asia to Java and New Guinea.

Casuarinaceae

 Casuarinaceae, or the beefwood family, contains 95 species in 4 genera (Casuarina, Allocasuarina, Ceuthostoma, and Gymnostoma). The species are restricted to the Southern Hemisphere from Southeast Asia and Malesia to the southwest Pacific, especially Australia.

Nothofagaceae

      Nothofagaceae, or the southern or silver beech family, consists of 35 species of Nothofagus that are scattered throughout southern South America, Australia, New Zealand, New Caledonia, and the mountains of New Guinea. The history of the genus has frequently been cited as evidence of continental drift after the breakup of the single large continent of Gondwana during the Cretaceous Period (145.5–65.5 million years ago). Because the fruits of Nothofagus are highly susceptible to damage by seawater, the plants could occur where they do only by the rafting of the continents or by the unlikely event that their seeds were transported by birds across vast distances of open ocean. Previously, the southern beeches were placed in Fagaceae.

Myricaceae

      Myricaceae, or the bayberry family, consists of three genera—Myrica (bayberry), Canacomyrica, and Comptonia (sweet fern)—that are nearly cosmopolitan; though they are found in New Caledonia, they do not occur in Australia.

Rhoipteleaceae

      Rhoipteleaceae contains one genus with one species, Rhoiptelea chiliantha (horsetail tree), which is restricted to Vietnam and southern China. It has superior ovaries and inflorescences (inflorescence) with flowers in triads (the central one apparently bisexual).

Ticodendraceae

      Ticodendraceae is composed of a single species from montane areas of Central America, Ticodendron incognitum. These are wind-pollinated trees with double-serrate leaves having stipules that encircle the stem, female flowers with two long stigmas, and fruits with a thin outer pulp and a large ribbed, stony inner layer.

Economic and ecological importance

 Fagales are members of the nitrogen-fixing lineage of Rosids that includes the better-known legumes (legume). Atmospheric nitrogen is fixed into usable nitrates or ammonium by root-dwelling associates, usually the actinomycete Frankia. The nitrogen-fixing members, such as Alnus (alders), Casuarina (she-oaks), and Myrica (bayberry), often tolerate extreme nutrient-deficient habitats.

      Members of Fagaceae and Nothofagaceae form a significant part of the broad-leaved forests characteristic of mid-latitudinal areas of the Northern Hemisphere and parts of the Southern Hemisphere. Much of the mixed deciduous forest region of North America and Europe is dominated by species of Quercus, Fagus, Castanea, Juglans, and Carya. Similarly, the Southern Hemisphere temperate forest is dominated by Nothofagus. In the tropical forests of Southeast Asia, evergreen members of the family thrive in the mixed mountain forests. Betulaceae generally occupy much of the same latitudes as the oak family in the Northern Hemisphere, but they extend into more northern latitudes, where they are often part of forests at the northernmost limit of tree growth. The family extends into South America.

 The oaks (oak), walnuts, and birches (birch) are sources of some of the world's most valuable timbers. In both temperate and tropical America, many species of oak provide highly durable timber for use in construction, flooring, veneer, and millwork, as well as in the building of ships and boats. Members of the white oak group, including Q. alba, Q. macrocarpa, and Q. prinus, among others, are especially valued in temperate America. In tropical America as well as in Southeast Asia and the Pacific Islands, the southern beech, Nothofagus, is used for flooring, cabinetwork, and millwork; it is an all-purpose timber in Chile. The wood of the genus Castanopsis, while susceptible to decay, is used in India, Malaysia, and the Philippines for general indoor construction. Several species of Lithocarpus provide timber in the same region. Fagus grandifolia (American beech) and Betula alleghaniensis (yellow birch) are two of the three most important northern hardwoods of the United States and are widely employed in flooring, veneer, woodenware, and millwork; in the Pacific Northwest the principal wood is A. rubra (red alder). In East Asia F. crenata is an important source of timber. While Castanea dentata (America chestnut) was a highly valued timber in the past, its importance has diminished considerably since the advent of the chestnut blight (the fungus Cryphonectria parasitica). The Asian Castanea crenata and C. henryi, however, are important sources of timber. Juglans nigra (black walnut) and Carya illinoinensis ( pecan) are among Juglandaceae that are in great demand for their strength and beauty; however, many natural stands have been depleted.

 Since the time of Confucius, cork has been used for a variety of purposes, from insulation to decoration. Commercial cork is obtained almost exclusively from the bark of Q. suber (cork oak (cork tree)), native to and cultivated in the Mediterranean region; however, other species also produce cork. The bark is first stripped from the trees when they are about 20 years old, although the first quality cork is not produced for another 8 to 10 years. Trees may produce cork for an average of 150 years. Oaks are also valuable sources of tannins (tannin) (chemical compounds valued for, among other things, their ability to condition animal skins into leather). In the United States the bark of Lithocarpus densiflorus (tanbark oak), as well as several species of Quercus, is a source of tannin. The nutgalls produced on many Quercus species are excellent sources of tannin. Other chemical products were obtained at one time; for example, methanol (methyl alcohol), acetic acid, creosote, guaiacol, and tar were extracted from Fagus, and acetic acid was obtained from Q. robur. Several species in the order have been used in dyeing—e.g., Q. velutina in North America and C. crenata (Japanese chestnut) in East Asia.

      Many members of Fagales produce edible fruits, some of which have been cultivated since ancient times. The European (Castanea sativa) and Chinese (C. mollissima) chestnuts are economically important crops, although susceptibility to the chestnut blight fungus has somewhat diminished production of C. sativa. In North America both C. dentata (American chestnut) and C. pumila ( chinquapin) were used extensively by the native Indians for a variety of foods, from roasted nuts to breads; however, the advent of the blight and the subsequent destruction of vast stands of C. dentata virtually eliminated it as a source of food. Today, most of the commercial orchards in the United States grow cultivars of the Chinese species, and many of the roasted nuts sold in the United States are imported from Italy. Corylus is the source of the filbert, or hazelnut, grown for many centuries in parts of Europe. C. avellana is the source of most commercial nuts. By the 1990s about 70 percent of the world's crop came from a small area in Turkey along the Black Sea.

 Although not a commercial crop, the acorns (acorn) produced by most species of Quercus (oaks) are edible, but many require some preparation to remove the tannins. In North America acorns were used extensively by the native Indians; the nuts of the “sweet” members of the white oak group were often consumed directly, while the bitter nuts of other species were ground and soaked before being made into mush or bread. Early American farmers valued acorns as a highly nutritional animal feed. During the American Civil War, acorns of the white oak were roasted and used to make a coffee substitute; roasted acorns continue to be used as a coffee substitute. Throughout the world several species are used as feed for livestock and are important to local wildlife populations. The nuts of several Lithocarpus and Castanopsis species are a food source in their home countries of China and Southeast Asia.

      Some oak species of East Asia are of indirect importance for humans. The logs of some species are used for the cultivation of edible fungi, especially shiitake mushrooms (mushroom) (Lentinus edodes) in Japan. In Tierra del Fuego another edible fungus, Cyttaria darwinii, thrives on Nothofagus species. Some species of Fagales are used for silkworm culture in Asia—for example, Q. aliena and Q. fabri in China; Q. glauca, Q. acutissima, and C. crenata (Japanese chestnut) in Japan; and Q. semecarpifolia in India. Others, such as many species of Fagus, Quercus, Betula, Ostrya, and Corylus, are cultivated as ornamentals for their distinctive form and foliage colour.

      The bark of Alnus was used by North American Indians to relieve indigestion, and that of Q. alba has been used to make an extract for treating sore lips and mouth. In England the inner bark of A. glutinosa (black alder) is employed as a mouthwash and as a treatment for inflamed gums or toothache. Both F. grandifolia and F. sylvatica (American and European beeches, respectively) are sources of the antiseptic creosote. Chewing sticks for cleaning teeth are made from the twigs of B. lenta (American black birch, or cherry birch) and B. alleghaniensis (yellow birch) of eastern North America. For many centuries the practice of using ohaguro (“toothblack”) in Japan not only was popular but was believed to preserve teeth and prevent decay; the acorns of Q. cyclophora and the nutgalls of Chinese oaks were often the source of the tannin in this solution. North American Indians used the bark or nutgalls of several Quercus species to combat dysentery; the astringent properties of the tannins served as a powerful medicine.

      Casuarina equisetifolia (beefwood, also known as Australian pine) is widely planted in tropical areas around the world for windbreak, firewood, and shingles. Members of Myrica are used to gather wax for bayberry soap, yellow dye, and fish poisons. M. faya is an introduction into Hawaii from Macaronesia (several islands located in the Atlantic Ocean just west of Portugal and northwestern Africa) and has become a serious infestation.

      The pollen of almost all the temperate genera of Fagales—Alnus, Betula, Carpinus, Corylus, and Ostrya in the birch family, and Fagus and especially Quercus in the beech family—have been reported to cause allergic reactions.

Characteristic morphological features

 Fagales are all trees or shrubs with simple leaves, or compound leaves in Juglandaceae and Rhoipteleaceae, usually arranged alternately (rarely opposite or whorled). Each plant generally contains tiny flowers (flower) of separate sexes (monoecious); unisexual flowers are primarily pollinated by the wind. The staminate is often arranged in hanging chains of flowers called catkins, and the pistillate sometimes is. The perianth is absent or represented by a single rudimentary whorl of tepals. The pistillate flowers are often subtended by a cupule (hull), which sometimes partially or completely encloses the fruit. There are generally two to many distinct stamens in each flower, although in some cases there is only one. The pistillate flowers (i.e., those without a functional stamen) contain 2, 3, or 6 (to 12) carpels, with the carpels united to form an inferior ovary with as many locules (cavities) as carpels, at least in the lower part of the ovary, but with the partitions often not reaching the summit of the ovary. The styles are distinct or united at the base, and each locule has one or two ovules. The fruit is a nut, samara, or drupe, often one or several together surrounded by a hull or cupule. The fruit of nut-bearing trees is composed of three layers: an outer skin, a middle pulpy zone, and an inner stony layer (endocarp). These features typically define a drupe, but it is generally named a drupaceous nut, or simply a nut. In areas where squirrels are present, many nuts are stored underground and are not reclaimed, and by spring many seedlings appear. The seeds contain a large embryo and little or no endosperm.

      The most distinguishing feature of Fagales is the cupule (hull) subtending or surrounding the fruit. The structure is believed to be of a different origin in most of the families. For example, in Fagaceae it is derived from a highly modified and reduced branch system with its associated modified leaves or bracts, at least in the genus Quercus; and in Betulaceae the husks are thought to be made up of several modified bracts. It also has been suggested that the four-valved cupule of Nothofagus is derived from the four stipules at the base of the two bracts borne beneath the inflorescence. The interpretation is that bracts themselves are highly reduced or absent, while the stipules are greatly enlarged and modified.

      The beeches are tardily deciduous trees, although some have rather thick, leathery leaves (leaf). Many have a tendency to retain their leaves, although in a dry, lifeless state, even through the winter, especially when young. In Lithocarpus and Chrysolepis, and in some species of Quercus the plants are evergreen with thick, leathery-textured, usually smooth-margined leaves. Outside the tropics and warm temperate areas, most members of the family are deciduous, and the leaves are frequently deeply lobed or at least coarsely toothed at the margins. Whereas many deciduous trees in temperate areas produce one major flush of new leaves each spring, many species of oaks and some plants in other families in temperate areas continue to produce new leaves throughout the growing season. These two features, the lack of a well-developed abscission layer in the leaf petiole and the production of leaves throughout the summer, are interpreted as indicating an origin for the family in the tropics or subtropics, where equitable conditions would allow growth to occur throughout the year. The rapid decrease in the diversity of species of Fagaceae north of about latitude 35° to 40° N lends support to this hypothesis. The special conditions in the Mediterranean area—summer dryness and winter rains—which also are found in part of western North America, give a dry-habitat character to members of the Fagales occurring there, including such features as small leathery leaves that are spiny or hairy.

      Nothofagus have small coarsely toothed leaves resembling the leaves of some of the birches. In contrast to Fagaceae, Nothofagus is able to grow in cold, inhospitable climates, even adjacent to the snow line in the mountains of South America. In this respect the genus is more similar in ecological preference to Betulaceae than to Fagaceae and perhaps can be thought of as the Southern Hemisphere counterpart of the birches. This fits in with other evidence that indicates a closer kinship for Nothofagus with Betulaceae than with Fagaceae.

      The inflorescences of Betulaceae comprise a central axis on which are borne a series of greatly reduced and compressed spirally arranged cymules. The hypothetical ancestral inflorescence consisted of three flowers, each on a separate stalk, with each of these joined to a main stalk, and seven leaflike bracts. The extant inflorescence in Betulaceae is believed to have been derived through the evolutionary loss of the flower stalks, peduncle, and one or more of the bracts, reduction in size of the flowers, and compression of the main axis of the inflorescence. In all genera except Betula, the cymes have been reduced to two flowers. Betula, with three-flowered cymes, retains the ancestral arrangement. All the original bracts are still present in the female flowers of Carpinus, Ostrya, and Ostryopsis; the uppermost bracts have been lost in Betula; and one bract is lost in Corylus. The tepals are still present in the flowers of Carpinus, Corylus, Ostrya, and Ostryopsis, but they have been completely lost in the course of evolution in Alnus (alder) and Betula (birch).

      In Betula and Alnus, which have the smallest fruits in Fagales, both the female and the male flowers are borne in catkins. The catkins of the female flowers of Alnus are woody and reminiscent of the cones of some gymnosperms. The scales spread apart at maturity to release the ripe fruit. In Betula the bracts of the catkins are deciduous, and the mature catkin shatters to release the winged fruit. In the remaining genera of Betulaceae, the fruits are nuts or nutlets borne in saclike husks or attached to leaflike bracts.

      The husk that surrounds the nuts of Corylus (filbert) ferox produces long spines and bears a striking resemblance to the husk of the fruits of Castanea and some species of Castanopsis in Fagaceae, but in other species of Corylus the husks are more leaflike in appearance or tubular and flask-shaped. Although they are not spiny, the husks frequently bear numerous multicellular, glandular hairs, which exude a sticky substance that may deter predation of the nuts.

      Juglandaceae are unusual in the order, with leaves pinnately compound, sometimes trifoliolate. Vegetative parts are covered with aromatic peltate (umbrella shaped) glands. Stems of black walnut trees attain heights of nearly 50 metres (about 165 feet). Massive trunks are also developed by pecans and some hickories. Tree stems consist almost entirely of secondary wood—i.e., wood produced by a cambial cylinder composed of cells that differentiate inwardly, to add to the wood already present. Cambium, also present in roots, is active mainly in the spring. Woods of this family are especially noted for their grain patterns and for the lustre of local bands of cells (wood rays) that radiate toward the perimeter of the trunk. Casuarinaceae are distinctive trees or shrubs adapted to arid habitats and have jointed branches with short internodes. The leaves are peculiar in structure, appearing as a whorl of reduced scalelike structures at each node. They are often confused with gymnosperms.

Evolution

      Nothofagaceae, or the southern beech family, are the oldest offshoot in the order. They appear to have diversified prior to the final separation of Gondwana in the late Cretaceous Period and now have various lineages in South America, Australia, and New Zealand. All the members of Nothofagus from New Guinea and New Caledonia are closely related. Fossils of Nothofagus in Australia indicate that the family was widespread in the Southern Hemisphere, except in Africa, where it apparently never occurred. Fagaceae, or the beech and oak family, represents the next diverging lineage, which diversified primarily in north temperate regions. Of the remaining families, Juglandaceae, Myricaceae, and Rhoipteleaceae appear closely related, as do Betulaceae, Casuarinaceae, and Ticodendraceae.

      Since the pollen of Fagaceae and Betulaceae shows a marked similarity to the pollen of Normapolles from the Cretaceous Period, some think that they may have originated from within that assemblage. The earliest known fossils (fossil) of Fagales are pollen specimens from the Campanian Stage (about 84 to 70 million years ago) of the Upper Cretaceous, while the Normapolles group was still in existence, but no forms transitional to modern forms are known. Various other fossils date to the end of the Cretaceous.

      Pollen characteristic of Alnus from the Santonian (Santonian Stage) (about 86 to 84 million years ago) and Campanian stages of the Upper Cretaceous in Japan and North America represents the earliest record of Betulaceae. Fossil leaves similar to those of Betuleae have been found in deposits from about 70 million years ago and from the Paleocene Epoch in the early Tertiary Period (about 60 million years ago) but are not associated with the reproductive structures that would make identification certain. Reproductive structures of Alnus have been found in Paleocene deposits in Alaska and in the former Soviet Union. The oldest fossil inflorescences and leaves to be identified with certainty as Betula are found in Eocene Epoch (about 56 to 34 million years ago) deposits in British Columbia and eastern Oregon. Fruits resembling those of Corylus and Carpinus from several localities in North America, Greenland, and western Europe have been found. Definite Carpinus fruits with attached bracts are known from the late Eocene of Japan, and Ostrya is first definitely recognized in Oligocene Epoch (about 34 to 23 million years ago) deposits in western North America.

      Despite the absence of fossils showing a clear transition, it is apparent that the catkins and the reduced and simplified flowers and inflorescences in Fagales represent the culmination of an evolutionary pathway leading to wind pollination from insect-pollinated ancestors. In members of most families, numerous staminate (pollen-producing) flowers are borne in pendulous catkins, and the pistillate flowers generally show modifications, such as elongate styles and enlarged or feathery stigmas, to receive wind-transported pollen grains. All the earliest fossils attributed to Fagales already manifest these changes, and so it is impossible to know exactly how the changes took place.

David E. Boufford Kenneth J. Sytsma

* * *