Old-growth forests, sometimes also named virgin forests, primary forests, or ancient forests, are natural ecosystems dominated by large, old trees, usually of a mixed species composition, and with all ages present in the community. Old-growth forests also contain many scattered, dead trees, both standing and lying on the forest floor. In the tropics, these forests are threatened by conversion to agriculture and by other disturbances, while old-growth forests in the temperate zones are mostly threatened by forestry. Losses of these old-growth ecosystems are the most important of the modern threats to biodiversity because of the extinctions that are caused. The special values of old-growth forests are best preserved through the designation of large, landscape-scale, protected areas.
Old-growth forests are an end-of-succession, climax ecosystem. They are dominated by trees of great age, but occurring within a mixed- species community with an uneven-aged population structure (that is, all tree ages are represented in the community). The physical structure of old-growth forests is very complex, and includes multiple horizontal layers, gaps of foliage within the canopy, great variations of tree sizes, many large, standing dead trees (called snags), and logs lying on the forest floor. In some ecological contexts, the term old-growth forest might also be used to refer to senescent stands of shorter-lived species of trees, such as cherry, birch, or poplar. However, the usual interpretation is that an old-growth forest is a late-succession or climax ecosystem, with the broad features described above.
For old-growth forests to develop, a long time must pass between events of disturbance that are severe enough to cause a stand-level mortality of dominant trees. Therefore, old-growth forests occur in places or regions where fire, hurricanes, and other catastrophic disturbances are rare. These circumstances are especially frequent where there is a great deal of rainfall throughout the year. Consequently, many of the best examples of old-growth forests are tropical and temperate rain forests.
Old-growth forests provide a habitat with particular ecological qualities. These features are not present or as well developed in mature forests that are younger than old-growth forests. Some wildlife species require
these specific habitat qualities, and therefore need extensive areas of old-growth forest as all or a major part of their range. Some well-known North American examples of species considered substantially dependent on old-growth forests are birds such as the northern spotted owl (Strix occidentalis caurina ), marbled murrelet (Brachyramphus marmoratus ), and redcockaded woodpecker (Picoides borealis ), and mammals such as marten (Martes americana ) and fisher (M. pennanti ). Some species of plants may also require or be much more abundant in old-growth forest than in younger, mature forest. Examples include Pacific yew (Taxus brevifolia ) and various species of lichens and bryophytes.
A critical habitat requirement for many of the species of old-growth forests is the presence of large trees with dead tops, and large snags and logs lying on the forest floor. These habitat features are absent or uncommon in younger natural forests and in intensively managed forests created through forestry. Snags and living but heart-rotted trees are especially important to woodpeckers, which excavate nesting cavities that may later be used by many secondary species that cannot excavate their own hollows.
The northern spotted owl is a non-migratory bird of the northwestern United States and southwestern Canada that requires large tracts of old-growth, moist-to-wet, conifer forest as its habitat. Each breeding pair of northern spotted owls requires more than about 1,600 acres (600 ha) of old-growth forest, and each breeding population needs at least 20 pairs to be viable. However, old-growth forest in this region is extremely valuable as a natural resource that can be exploited by humans for profit, and this ecosystem type has been greatly reduced in area and fragmented by logging. Consequently, populations of this bird have been reduced, and the northern spotted owl has been recognized as a threatened species in the United States. Under the U.S. Endangered Species Act, designation under this status requires that a management plan must be developed to protect the threatened species. Because the logging of trees within old-growth forests jeopardizes the northern spotted owl, the plans for its protection have resulted in the withdrawal from forestry usage of large areas of valuable timber that could otherwise be profitably exploited. The strategy to protect the spotted owl would preserve large ecological reserves of old-growth forest as its essential habitat (as well as for other species dependent on this type of habitat). However, at the same time that the owl is protected, important, shorter-term, economic opportunities are lost to the forest industry because there is less high-value, old-growth timber available for exploitation.
The red-cockaded woodpecker also has a requirement for old-growth forest, in this case certain types of pine forest (especially loblolly pine, Pinus taeda ) in the southeastern United States, in which this bird excavates nesting cavities in large, living trees that have fungal heart rot. The red-cockaded woodpecker breeds in small colonies, and it has a relatively complex social system that involves non-breeding adult birds that assist breeders in brood-rearing. Old-growth pine forests that satisfy the habitat needs of red-cockaded woodpeckers have been greatly diminished and fragmented by conversions to agriculture, forestry plantations, and residential lands. This has reduced the populations of red-cockaded woodpeckers, which are further threatened by natural disturbances such as wild-fire and hurricanes. Unlike the spotted owl, the endangered red-cockaded woodpecker is somewhat tolerant of a limited intensity of disturbances of its habitat. There is some evidence that trees can be harvested
from stands in which this species breeds, as long as its nesting colonies are protected by buffers (that is, by surrounding non-harvested strips wider than about 2,600 ft [800 m]), and sufficient foraging habitat remains available. However, there is not yet enough scientific evidence to fully support this sort of an integrated management strategy for forestry and red-cockaded woodpeckers. Until this controversy is resolved, the ecologically prudent strategy for preservation of the rare woodpecker requires setting aside large ecological reserves of its natural habitat of older pine forest.
Into the 2000s, only about 5,000 groups of redcockaded woodpecker remain along the eastern part of the United States from Virginia to Florida and southeast Oklahoma to eastern Texas in the western part of the United States. Only about 1% of the birds remain in these regions, while the birds are completely gone from the areas around the states of Missouri, Maryland, and New Jersey.
Compared with the temperate-forest examples described above, enormously larger numbers of species are dependent on old-growth tropical forests. Because wildfire and other catastrophic disturbance are uncommon in the humid tropics, this climatic regime favors the development of old-growth rain forests. This ecosystem supports an extraordinary richness of species of plants, animals, and microorganisms that are utterly dependent on this type of forest. Because of the enormous numbers of species supported under relatively benign climatic conditions in old-growth tropical rain-forests, ecologists consider this biome to represent the acme of development of terrestrial ecosystems. Regrettably, tropical forests of all types are being rapidly lost through conversions to agriculture and other disturbances. Many of the endemic species of tropical forests have become extinct, and many others are becoming increasingly endangered.
As was noted previously, important habitat requirements of many species of wildlife relate to the numbers of dead trees in the forest, occurring as standing snags or as logs lying on the ground. These features are especially critical to some birds, which use the deadwood for nesting in excavated or natural cavities, as perches for hunting, resting, and singing, and as a substrate on which to forage for their food of insects and spiders. For example, a study in the northwestern United States found that up to 45% of the species of breeding birds are cavity nesters. These include various species of woodpeckers that actually excavate cavities, as well as other species that are secondary users of those cavities, or that use natural hollows.
Unfortunately, modern forestry does not accommodate this habitat feature very well. Because forestry plantations usually have very few snags or other types of deadwood, cavity-dependent species of wildlife are at risk in these highly managed, secondary forests. As a result, forestry-related degradation of the habitat of these animals has become an important environmental issue in many areas. This concern is especially relevant to old-growth forests, because deadwood is such a prominent characteristic of this type of ecosystem. For example, as many as six woodpecker species can co-occur along with other cavity-dependent species in old-growth forests of the Pacific Northwest of the United States. It may be possible to accommodate most of these species, while still practicing forestry, if an appropriate system of integrated management can be developed. One study done in that region suggested that about 70% of the woodpecker population could be maintained in selectively harvested old-growth forests, as long as at least four large snags remained per hectare on harvested sites.
Because of their great quantities of large-dimension timber of desired tree species, old-growth forests are an extremely valuable natural resource. However, old-growth forests are rarely managed by foresters as a renewable, natural resource. Usually, these forests are mined by harvesting, followed by a conversion of the site to a younger, second-growth forest, which is only allowed to develop into a middle-aged forest before it is harvested in turn. This management strategy is pursued because old-growth forests sustain little or no net production of new biomass, since the growth by living trees is approximately balanced by the deaths of other trees through senescence, disease, or accident. Because the primary objective of forestry is to optimize the productivity of tree biomass, it is economically preferable to harvest the secondary forests soon after their productivity starts to decrease. However, this occurs long before they become old-growth forests.
Because of this forestry practice, old-growth forests have been greatly fragmented and diminished in area. Consequently, threats of further losses of this natural ecosystem engender great controversy. To conserve some of the important qualities of old-growth temperate forests, including some of their dependent species, so-called new forestry harvesting systems are being encouraged in some areas. Compared with clear-cutting and plantation establishment, these new systems are relatively soft in terms of the intensity of the disturbance caused, and the physical integrity of the forest remains substantially intact after the harvest. For example, a system being encouraged in old-growth forests of western North America is selection-cutting with some degree of snag retention, followed by natural regeneration of trees instead of planting.
However, even the new forestry practices cause substantial changes in the character of the forest. If the societal objective in some areas is to preserve the special, natural values of old-growth forests, this can only be done by setting aside large, landscape-scale, reserves in which commercial forestry is not practiced. Only natural ecological dynamics and disturbances are allowed to occur in those ecological reserves. The landscape perspective is important to the preservation of old-growth forests because particular stands of this ecosystem cannot be preserved forever, since they are inevitably subject to the effects of unpredictable, catastrophic disturbances and/or environmental changes. However, if the ecological reserve is large enough, these stand-level dynamics can be accommodated, because a continuum of stands within the natural, old-growth successional dynamic can be sustained over the longer term.
Old-growth forests are a unique type of natural ecosystem, with great intrinsic value. If old-growth forests are to always be a component of Earth’s
Clear-cutting —A method of forest harvesting by which all trees of commercial size are removed from the site. Usually the trees are de-limbed, and the branches and foliage are left on the site as slash while the stem is removed as a commercial product.
Community —In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the same time.
Old-growth forest —A late-successional forest, characterized by great age, uneven-aged population structure, domination by long-lived species, and complex physical structure, including multiple layers in the canopy, large trees, and many large snags and dead logs.
Plantation —A tract of land on which trees have been planted and tended, often as a single-species population.
Selection cutting —A method of forest harvesting in which only trees of a desired species and size class are removed. This method leaves many trees standing, and relies on natural regeneration to replace the harvested trees.
Succession —A process of ecological change, involving the progressive replacement of earlier communities with others over time, and generally beginning with the disturbance of a previous type of ecosystem.
natural biodiversity, then human societies will have to preserve them in large ecological reserves, even if this means there will be some short-term economic losses.
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"Old–Growth Forests." The Gale Encyclopedia of Science. . Encyclopedia.com. (July 20, 2019). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/old-growth-forests
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