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Old-Growth Forests

Species Dependent On Old-growth 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 red-cockaded 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 intensivelymanaged 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 northern spotted owl is jeopardized by the logging of old-growth forest, 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 wildfire 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.

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 rainforests, 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.


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Science EncyclopediaScience & Philosophy: Octadecanoate to OvenbirdsOld-Growth Forests - Properties Of Old-growth Forests, Species Dependent On Old-growth Forests, Dead Wood