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Types of Forests
The tropical hardwood forests, including rain forests, occur throughout the lowland areas of the tropics—especially along the routes of rivers in Central and South America and in central and W Africa—and in the East Indies, the Malay Peninsula, and parts of India, Indochina, and Australia. They are characterized by an annual rainfall of 160–400 in. (406–1,000 cm) annually, with an average temperature of at least 80℉ (27℃), and support a great diversity of plant life. The foliage is a luxuriant and interlaced community from ground level to the tree canopies, and the trees support the omnipresent woody vines (see liana) and air plants (see epiphyte). Although some tropical forests are deciduous, most tropical trees are considered evergreen because their leaves are not shed simultaneously at a certain season; however, they are believed to drop and renew their leaves sporadically each year. Even though they cover only 7% of the earth's landmass, about one half of the planet's species live there.
The temperate hardwood forests of North America, Europe, and Asia are marked by seasonal rainfall distribution. The trees, typically species of beech, maple, ash, oak, elm, and basswood, are deciduous but are often mixed with conifers, especially in areas of poorer soil. The temperate hardwood forests overlap the boreal, or northern, conifer forest belts, which encircle the earth in the subarctic and cool, temperate regions south of the treeless tundra. The vegetation is typically fir and spruce in northern regions and at higher altitudes, and pine, larch, and hemlock in southern regions and at lower altitudes. In transitional areas, especially where there is a pronounced season without rain (e.g., the chaparral and tropical mountain slopes), scrub forests are frequently found in which the trees are more widely spaced and grasses intervene. Nontropical rain forests exist in New Zealand, Tasmania, Chile, and the Pacific coast of North America.
In the United States east of the prairies are the northern (boreal) forest belt, in which sugar maple, beech, and birch mix with the conifers; the hardwood forest belt, a typical temperate forest; and the warmer southern forest belt, encompassing many stands of smaller pines and cypress thickets. In the chiefly coniferous Rocky Mt. forest belt, the Ponderosa pine is most common. The Pacific forest belt has the heaviest stands of trees in America and probably in the world. The characteristic redwood and giant sequoia mingle with Douglas fir and other species.
Forested Area Today
The Importance of Forests
A highly informative account of the aboriginal North American forest is R. G. Lillard's The Great Forest (1947, repr. 1973). See also P. W. Richards, The Tropical Rain Forest (1952, repr. 1966) and The Life of the Jungle (1970); C. Caulfield, In the Rain Forest (1985); M. Williams, Americans and Their Forests (1989); K. Miller and L. Tangley, Trees of Life (1991).
one of the main types of plant cover on earth. There are many plant forms in the forest. Trees and shrubs prevail, and grasses, bushes, mosses, and lichens are secondary. It is characteristic of the forest (as it is of any other plant cover) that the plants grow together and affect one another, interact with the environment, and constitute a dynamic unity. This concept was first advanced by G. F. Morozov, who pointed out that the forest is a natural phenomenon that includes not only the forest vegetation but also its habitat. Morozov’s teaching became the basis of silvics, which is the theoretical foundation of forestry. Later, his teaching was elaborated by V. N. Sukachev, who considered the forest a biogeocenosis.
Structure. Depending on the species composition, the biological characteristics and age of the most important plants, and certain physicogeographic conditions, several strata of plants develop in a forest. In complex forests of the temperate zone the first stratum consists of first-magnitude and forest-forming trees (pine, spruce, larch, oak, and ash, for example). The second stratum is made up of second-magnitude trees, such as fir, beech, linden, and maple; and the third stratum, or undergrowth, consists of shrubs, such as the filbert, spindle tree, buckthorn, and viburnum. The lower strata (the fourth and fifth) include grasses, bushes, and moss-lichens. (Sometimes there are no moss-lichens.) In temperate forests vines and climbing plants that do not belong to any of the strata are sometimes encountered. Mosses, lichens, many fungi, and algae, which have their own ecological niches, grow on the trunks and stumps of trees.
There are usually more strata in tropical forests than in temperate ones. A stand of trees may consist of three substrata. The undergrowth may include two substrata, and the grasses and bushes, three. In addition, tropical forests include many plants that do not belong to any particular stratum—numerous lianas, as well as mosses and lichens on the soil, on tree trunks, and on shrubs. These plants may be parasites, saprophytes, or sym-bionts. They may grow on the main plants as parasites or use them for mechanical support, or they may be a member of a symbiotic relationship.
Each stratum of a forest includes species or groups of species that affect the development of the forest’s structure. In particular, they influence some ecological and physiological processes, chiefly the regeneration of the main tree species and the species of trees associated with them.
Regenerative capacity. The life-span of a forest in a particular area depends, in particular, on the extent to which it can be regenerated. A distinction is drawn between natural regeneration (by seed or vegetatively by shoots) and artificial regeneration (by planting seeds or setting out trees of different kinds and ages). The shoots occur in plant species of all strata of the forest, whereas the seedlings are only from those species which, under favorable conditions, can grow to the second or first stratum and thus maintain the mature stand. The shoots and seedlings of the main species are affected by the trees of the upper strata, which, owing to the proximity of their crowns, deprive the shoots and seedlings of much of the light, atmospheric moisture, and nutrients that they require, thereby arresting their development. As a result, the shoots and seedlings are considerably thinned out or totally destroyed. However, the partial shading provided by the trees of the upper strata also has a beneficial effect, protecting the shoots and seedlings against spring and early summer frosts and excessive moisture. It also hastens the growth and spread of the crowns in the upper strata and helps clear the trees of dead branches.
On the edges of any part of a forest that comes into contact with other kinds of plant cover, tree species may be introduced (generally by the vegetative method) into other types of vegetation. This happens quite often where a forest comes into contact with a meadow, steppe, or swamp, as well as in river valleys or water divides, where forest species may advance farther north (into the tundra) or farther south (into the steppe). The coniferous needles and leaves that fall from the trees rest in a compact layer on the soil. Because they decompose slowly, they prevent the appearance of tree shoots and weaken the grass cover. The moss and lichen cover that sometimes grows thickly on the soil, especially in coniferous forests, has the same effect. Animals also affect the regeneration of the forest.
Types of forests, growing conditions, and cutting. Because they differ in composition and development, distinctions are drawn among various types of forests, growing conditions, and cutting. Each type of forest is characterized by a definite, homogeneous composition of tree species, shrubs, grasses, bushes, mosses, and lichens. All of them develop under specific environmental conditions. Together, they determine the growing conditions characteristic of a forest, the pattern of growth and development of the trees, and their index of quality and yield, which is determined by their growth rate. Each type of forest is associated with a particular type of regeneration. If conditions are unfavorable for regeneration, the stability of a particular type of forest is impaired, and it gives way to another type. There are two types of forest: native forests, which developed without the influence of man or such phenomena as pests or diseases, windfall, and forest fires; and derivative forests, which developed as a result of the influence of the above factors and of man’s active intervention (cutting down trees, for example).
The various types of growing conditions for forests are usually determined by studying regions that were formerly treeless or regions that are temporarily treeless because of clear-cutting or forest fires or other natural disasters, such as soil erosion and washout. In such cases, scientists take into account the homogeneity of factors, including climate, soil, and water regime, as well as the condition of the forest. The natural patterns that are established for types of forests and for their growing conditions are used to classify a territory in a botanical-geographic (forest-growing) sense.
Reacting to stable and long-standing natural phenomena, in their advance to the far north and south forests enter into complex, conflicting relations with the tundra and the steppe. For example, in the north, where the forest zone gives way to the tundra, a transitional forest-tundra zone has developed. Characteristic of trees and of other plants in the north is an extremely high rate of transpiration during the short growing season, followed by an acute moisture deficiency in the long, cold winter. As a result, their life-spans are greatly shortened, they grow slowly, and the stand is thinned out considerably. The same phenomena are observed in the mountains at the timberline. Fossil remains of wood, palynological data, and the presence of the desiccated edges of forests on the boundary of tundra vegetation demonstrate that in some places the tundra encroached on the forest, which had moved southward. Later, because of changing climatic conditions, the forest gradually moved north again and invaded the tundra. The rivers that empty into the Arctic Ocean often carry tree seeds and fruits, which are deposited in the river valleys and move up to the slopes of hills and mountains, thereby changing the northernmost limit of the forest.
In the extreme southern regions in the dry steppe and semisteppe zones, which are characterized by a generally dry climate (due to insufficient moisture and high atmospheric temperatures in summer), saline soils in some areas, fierce dry winds, and other factors that cause the trees to consume more water, the forest gives way to the forest-steppe zone. Conditions favorable to the development of leached soils are created in depressions where moisture from summer precipitation and from large mounds of snow accumulates. In these depressions trees find the best conditions for growth, and it becomes possible for the forest to displace the steppe. The forest tundra and the forest steppe, as well as forests planted by man, help to improve natural conditions and promote the appearance of wildlife in the zone of soils heavily saturated with water and in the zone of arid sands.
Interaction with the habitat As a type of vegetation the forest depends on soil and climatic conditions, the activity of animals, and the character of man’s intervention. Economic activity, such as clearing areas of forests, causes pine and spruce to be replaced by birch or aspen forests, larch forests primarily by birch, and oak forest by stands of spruce or by shrubs. Some areas remain treeless for a long time and are covered by grasses or by grasses and bushes, particularly if natural regeneration of the main species occurs very slowly. Reforestation—an expensive remedy— is necessary in regions where the forest economy is important. It can be avoided by good forest management practices during tree harvesting (such as selective felling), which preserve the forest environment, permit natural regeneration, and promote the prompt development of the undergrowth and its rise to the first stratum.
Forest fires do a great deal of damage. Caused chiefly by man’s careless handling of fire and sometimes by various natural factors, they kill trees and other plants, often ignite the forest litter, and drive useful animals away. After a forest fire the species composition changes. Areas become overgrown with species of little value and grassy vegetation, and natural regeneration is prevented.
The forest greatly influences soil conditions, including the water regime, the structure of the soil, the accumulation of organic matter and minerals, and fertility. Because of the higher moisture content, more humus accumulates in a forest than under grassy vegetation. Moreover, the soil under a forest is leached more intensively than the soil under grasses, the pod-zolic horizons become more distinct, the soil is more acid, the root system spreads to greater depths, causing a more intensive removal of minerals, and water evaporates more rapidly. The forest markedly affects the distribution and accumulation of summer and especially of winter precipitation. On the one hand, it maintains the groundwater level and moderates the surface runoff of water. On the other, it intensifies plant transpiration and causes water vapor to condense, thereby making summer rains more frequent and creating favorable microclimatic conditions for forest plants and animals. Thus, the role of a forest in the water and soil regime of an area is varied and depends on the diversity of species and the biological characteristics and geographic distribution of the trees. The complex patterns of influence exerted on the environment by forests must be considered if forestry and other branches of the economy, such as agriculture, water supply, and city planning, are to be managed efficiently. In addition, the interaction between the forest and the environment should be taken into account when long-range plans for the organization and rational exploitation of an area are drawn up and the legislation of an optimum woodland area is at issue.
N. E. KABANOV
The forest as a planetary phenomenon. The earth’s forest cover is one of the planet’s accumulators of living matter. It holds several chemical elements and water in the biosphere, interacts with the troposphere, and determines the balance of oxygen and carbon. The destruction of a forest accelerates throughout a wide area the biological cycle of a number of elements, including carbon, which is released into the atmosphere as CO2. An unusual exchange of matter and energy takes places between all the components of a forest biogeocenosis. During this exchange, organic matter accumulates and is transformed into energy. The biomass accumulated in forests is dozens of times larger than the biomass of grassy and other plant communities. However, the difference between the annual gains in the phytomass of a forest and that of a grassy cover is not as great. The annual gain in the phytomass depends primarily on the water and heat balance. In the spruce forests of the Russian plain, the annual gain is about 60–90 centners per hectare (ha); in the beech forests of Western Europe, about 130 centners per ha; and in humid tropical forests, more than 300 centners per ha. The surface part of the forest phytomass is three to five times the size of the underground part.
Forests cover about 30 percent of the earth’s land area and are found on all of the continents except Antarctica. In America and Asia there are substantial wooded areas covering more than 30 percent of the total area of each continent. Forests are least extensive in Australia, where they occupy only about 10 percent of the land. According to data provided by the FAO in 1968, the earth’s forested area (including roads, lakes, and meadows) is 4,126,000 ha, and the area covered by forest tracts is 3,779,000 ha. Forests were once distributed over a larger area than they are today. Some of them were replaced by agricultural land, industrial complexes, and rapidly growing cities. On all the continents wooded areas are now, on the average, less than half their prehistoric size. Forests still prevail over treeless expanses in some places, such as Siberia and Canada. In the eastern USA, however, the forest area is only one-tenth of what it was in the 16th and 17th centuries.
Distribution. The particular species composition of the trees and other plants, with their ecological and biological properties, growth and developmental characteristics, and longevity, depend on the zonal physicogeographic conditions that prevail in a forest. Tropical forests exhibit the greatest species diversity. They consist of different kinds of rain forests made up of xerophi-lous plants and shrubs which, in many instances, alternate with extensive, treeless herbaceous vegetation (generally coarse and weedy) or with savannas, which usually appear after tropical forests are clear-cut, owing to irrational exploitation, forest fires, and other factors. An unusual type of tropical vegetation—the mangrove forest—is found in coastal areas that are flooded periodically.
Broad-leaved deciduous (oak, beech, linden, maple, hornbeam, and ash, for example) and broad-leaved coniferous forests (with some spruce, fir, and pine) are concentrated in the temperate latitudes. Although they once grew on the broad expanses of Europe and North America, today they survive primarily in the mountains (for example, the Alps, Carpathians, Caucasus, and Appalachians). Large forest areas are concentrated in the taiga (coniferous forests of spruce, fir, larch, and Swiss pine, for example). In a continental climate the taiga forests are bounded on the north by forest tundra and tundra.
Depending on the growing conditions, many types of trees form valuable forest formations: pine forests, spruce forests, cedar forests, fir forests, and larch forests, with a rich assortment of native and derivative forests. Oak, beech, linden and other forests are common. There are extensive thickets of juniper (in Central Asia), Japanese stone pine (Eastern Siberia and the Far East), and deciduous shrubs—willow, dwarf birch, pistachio, and filbert. Considerable forest areas are found on plakors (gentle slopes above the edges of valleys and ravines and on water divides). There are also complete forest stands of varying sizes in mountains and on floodplains in various zones.
Mountain forests cover the slopes of many ranges from the foothills to the subalpine meadow and mountain tundras, provided these areas are adequately supplied with heat and moisture. They have a characteristic species diversity and exhibit pronounced vertical zonality. They perform an especially important function in conserving and regulating water and in protecting the soil. The structure of mountain forests is determined by the change in the heat supply, moisture content, and other ecological factors between the foothills and the upper boundary of forest vegetation. In the tropics and subtropics this is most evident. Thus, for example, on the southern slopes of the Himalayas tropical forests give way to subtropical ones as the altitude increases, and at approximately 1,200 m subtropical forests yield to deciduous ones. Oak and other broad-leaved forests grow between 1,900 and 2,500 m. At higher altitudes (up to 3,000 m) and under moderately cold climatic conditions there are hemlock forests and, in the subalpine zone, fir forests. In the mountains of taiga regions (in Eastern Siberia, for example) the forest may contain the same Dahurian larch that is found on the adjacent plains. In addition to larch forests, thin birch forests (dwarf birch), willow and Altai birch groves, and Siberian juniper are common in the mountain regions of the tundra and forest tundra.
In Eastern Siberia and in the Far East, Japanese stone pines are found in the region below the bare mountain peaks. They form the upper limit of arboreal vegetation (the timberline) in this area as well as in the coastal areas of the Okhotsk and Bering seas, on the Kuril Islands, and on Sakhalin. The timberline in the northern latitudes may be made up of forests of spruce or Er-man’s birch.
In the southern USSR (the Caucasus, the Crimea, and the Carpathians), the lower zone of forest vegetation includes oak groves which, in many cases, give way to highly productive beech forests at higher altitudes. The upper zone of forest vegetation in the Caucasus and Carpathians consists of tall fir and spruce forests.
Floodplain forests, which are found in many river valleys, also have great ecological and economic significance. Intrazonal floodplain (valley) forests, as well as meadow peat bogs, develop on thick alluvial soils that are rich in nutrients. Willow, poplar, Chosenia macrolepis, and other forests are common in the north. Poplar-elm and mixed broad-leaved forests are found on river floodplains in the temperate latitudes, and similar forests mixed with oak occur in certain places in the forest steppe. Ash and Manchurian walnut grow in the floodplain forests of the Far East. In the temperate latitudes hayfields occupy the sites of former floodplain forests. Tugaic tree and shrub communities are characteristic of floodplains in desert regions, where elaegnus, Turanga poplar, sea buckthorn, poplar, willow, tamarisk, and other tree species, shrubs, and grasses form dense thickets. After the trees and shrubs are destroyed, grasses grow rapidly. In the tropics floodplain forests are characterized by great species diversity. On the Amazon lowland, where large areas are inundated every year, tropical floodplain forests are a typical part of the landscape.
Within every biogeographical region the structure and productivity of floodplain forests vary with the level of the flood-plain, the duration of inundation, and the rate of deposition, chemical composition, and texture of the alluvium.
Man has made significant changes, both quantitative and qualitative, in the earth’s forest cover. Commercial forests, including those created on the sites of native ones, and cultivated stands occupy a vast area. The primeval appearance of native forests has changed greatly. For example, many of Central Europe’s spruce forests were planted on the sites of beech forests. Rapidly growing Douglas fir and cryptomeria are cultivated in North America and in Japan and a number of other countries. The area of the taiga occupied by birch and aspen forests has increased substantially.
Some idea of the spread and distribution of forests can be obtained from plans and maps of forests as well as from diagrammatic maps showing the geobotanical classification of territories according to the main features of forest vegetation. Forests are the richest source of many resources (wood, bark, branches, coniferous needles, leaves, fruits, seeds, and berries) that are used in the logging, wood products, chemicals, food-processing and pharmaceuticals industries, as well as in other branches of the economy. However, if a forest is to be exploited by man, its role as a natural phenomenon that gives aesthetic pleasure and helps regulate climate and the water supply, conserve soil and water, and improve health, should be taken into account. Forests are among this planet’s renewable biological resources. They perform a biogeochemical function and have a role in the creation of a variety of landscapes. Constant care is required to see that the forests are preserved and used rationally by applying the latest advances in biology, physicogeography, and economics.
V. B. SOCHAVA
REFERENCESDendrologiia s osnovami lesnoi geobotaniki, 2nd ed. Leningrad, 1938.
Morozov, G. F. Uchenie o lese, 2nd ed. Moscow-Leningrad, 1949.
Rastitel’nyi pokrov SSSR: Poiasnitel’nyi tekst k “Geobotanicheskoi karte SSSR,” Masshtab 1:4,000,000, part 1. Moscow-Leningrad, 1956.
Richards, P. W. Tropicheskii dozhdevoi les. Moscow, 1961. (Translated from English.)
Tsepliaev, V. P. Lesa SSSR. Moscow, 1961.
Osnovy lesnoi biogeotsenologii. Edited by V. N. Sukachev and N. V. Dylis. Moscow, 1964.
Fiziko-geograficheskii atlas mira. Moscow, 1964.
Schmidthüsen, J. Obshchaia geografiia rastitel’nosti. Moscow, 1966. (Translated from German.)
Mirovye lesnye resursy po dannym FAO OON. Moscow, 1968.
Dylis, N. V. Struktura lesnogo biogeotsenoza. Moscow, 1969. (Komarovskie chteniia, 21.)
Molchanov, A. A. Produktivnost’ organicheskoi massy v lesakh razlichnykh zon. Moscow, 1971.
What does it mean when you dream about a forest?
Forests, as a segment of nature not brought fully under the dominion of humankind, are symbols of the unconscious. Thus, traveling into the forest indicates exploration of the unconscious realm. Forests also represent a comforting refuge from the demands of everyday life. (See also Jungle).