Forest habitat diversity is a reflection of geographic location of the forest, as well as the influences of humans on the forest. The broadest definition of a forest habitat is that of a habitat in which trees are a significant component. Natural forests also vary in complexity based on species composition resulting from a combination of environmental factors. A number of classification systems for natural forests have been proposed, but perhaps the most useful is that developed by the Food and Agriculture Organization of the United Nations in the 1970s. This system divides the natural forests of the world into five broad categories:
1. The cool coniferous forests are a circumpolar belt of boreal forests across northern latitudes. They occur between tundra to the north and temperate mixed forests to the south. Conifers are the dominant species in these forests, but they also include a few species of broadleaf trees.
2. The temperate mixed forests are found south of the cool coniferous forests and also are found in parts of the southern hemisphere. Pines and deciduous and evergreen broadleaf trees tend to dominate these forests.
3. Tropical moist evergreen forests are rainforests characterized by high annual precipitation (>2000 mm) evenly distributed during the year. Amazonia, western equatorial Africa, and the Indo-Malayan region are the three main regions for this forest type.
4. Tropical moist deciduous forests are found in tropical regions with 1000-2000 mm annual rainfall and a dry season for 1 or more months. These forests include monsoon forests in Asia where a dry period of 2 to 6 months is followed by heavy rains.
5. Dry forests are found in both temperate and tropical zones where annual precipitation is less than 1000 mm and are low and simply structured wooded areas.
These forest types may be further subdivided according to dominant species types and climatic differences among other factors. Additional forest types can also be recognized, including those heavily influenced by humans, such as urban forests and plantations.
The species of host plants naturally influence the species of phytophagous insects present in a forest. Additionally, human influences affect the species and density of insects in a forest. Forest management in natural forests may involve fire suppression that may favor insects that can exploit shade-tolerant tree species. For example, the fir engraver, Scolytus ventralis, has increased in density in mixed conifer forests of the Sierra Nevada in California because fire suppression has led to a dense understory of white fir, Abies concolar. Regeneration of shade-intolerant pines is suppressed and this in turn leads to an overly dense forest dominated by white fir, Abies concolar. Periodic logging or thinning may result in a build up of insect populations in cut material or associated waste. For example, neglect of forest hygiene following thinning operations in European pine forests may result in an increase in populations of the European pine shoot beetle, Tomicuspiniperda, which subsequently may damage the shoots of standing trees during its adult feeding stage. Clear-cutting and replanting or natural regeneration of forests favors insects that feed in young trees. For example, the pine weevil, Hylobius abietis, is perhaps the most significant forest insect pest in Europe. Adults feed on the bark at the base of conifer seedlings, resulting in seedling mortality. In many locations, any efforts to establish a plantation or naturally regenerated forest must have a management protocol to reduce the impact of this weevil.
In human-made forests, the species composition may be very simple, as in monocultures, or more complex, depending on the management goals of the plantation. Age and size classes of trees are frequently very limited. These plantations may comprise native or exotic tree species and may be on previously forested land or on previously unforested land. Perhaps the simplest forest habitat is a plantation monoculture of even-aged trees (such as in Monterey pine, Pinus radiata, plantations in New Zealand or loblolly pine, P. taeda, plantations in areas of the southeastern United States). This habitat is nevertheless very diverse as a result of the array of different ecological niches present on the dominant tree species. Usually, plantation monocultures involve areas of forest that are even-aged and that therefore encounter pressure from phytophagous insects that are adapted to, or more pestiferous during, certain stages of forest growth. For example, the gouty pitch midge, Cecidomyia piniinopsis, is a pest of young ponderosa pines, Pinus ponderosa, in the Sierra Nevada but is less significant on mature trees. Plantation forestry may involve tree species that are not native to the location where a forest is planted. Any insects that occur in these forests, but which were not present prior to the existence of that forest, are therefore introduced. Often these insect populations are not under natural regulatory pressures that limit their numbers. For example, as spruce forests have been planted in areas of Europe, using spruce as exotic species, the European spruce beetle, Dendroctonus micans, has moved westward from Eurasia during the 20th century. Outbreaks of this bark beetle have been most severe at the edge of its expanding range before natural enemies spread or are introduced into the areas now exploited by D. micans. In the creation of new forest habitats, the interactions among the species in the forest are unpredictable, because an essentially novel habitat has been created.
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