Rainforests are temperate or tropical forests that usually occur as old-growth ecosystems. The world sustains many type so frain for ests that differ geographically in terms of their species composition and the environmental conditions in which they occur. Temperate and tropical rainforests are considered to represent biomes, or widespread kinds of natural ecosystems having broad similarities of structure and function.
Rainforests require a humid climate, with more than about 80 to 100 in/yr (200 to 250 cm/yr) of precipitation distributed nearly equally across the seasons, so there is no pronounced dry period. This kind
of precipitation regime does not allow any but the rarest occurrences of wildfire. Other catastrophic events of stand-level tree mortality are also rare in rainforests. As a result, this ecosystem usually develops into old-growth forest containing some exceptionally old and large trees. However, the population structure of trees in old-growth rainforest is unevenly aged because of the micro-successional dynamics associated with the deaths of individual large trees, which result in canopy gaps below which there are relatively young trees. Old-growth rainforests also have a complex physical structure, with multiple layers within the canopy, and with large, standing dead trees and decomposing logs lying on the forest floor. Although old-growth rainforests support a very large biomass, trees within the ecosystem are dying and decaying about as quickly as new productivity is occurring. Consequently, the net ecosystem productivity of these old-growth forests is very small or zero. Temperate rainforests are dominated by a few species of coniferous trees, while tropical rainforests are characterized by a much greater diversity of tree species, along with an enormous richness of species of other plants, animals, and microorganisms.
Tropical rainforests are distributed in equatorial regions of Central and South America (most extensively in Amazonia), west-central equatorial Africa, and South and Southeast Asia through to New Guinea and the northeastern coast of Australia. Tropical rainforests are the most complex of the world’s ecosystems in terms of the physical structure that they develop, and also in their tremendous biodiversity of species and community types. Because of these characteristics, tropical rainforests represent the acme of ecosystem development on Earth.
Tropical rainforests have a complex canopy consisting of multiple intermeshed layers of foliage. The area of this canopy can be equivalent to 12 to 13 sq yds (10-11 m2) of foliage per sq yd (m2) of ground surface. This is among the densest foliar surfaces maintained by any of Earth’s ecosystems, a characteristic that allows a relatively great efficiency of capture of solar energy and its conversion into plant biomass. The most important foliar layer of the tropical rainforest consists of the upper canopy of the largest trees, which extends to more than 330 ft (100 m) in height in some cases. However, there are also lower canopies associated with the foliage of shorter, subdominant trees, and with lianas (or vines), shrubs, and ground vegetation. These subordinate canopies are everywhere, but they are best developed where gaps in the overstory allow some sunlight to penetrate deeper into the forest.
Tropical rainforests also have a uniquely rich canopy of epiphytes, or plants that use other plants as a substrate upon which to grow. There are especially large numbers of epiphytic species in the orchid (Orchidaceae) and air-plant (Bromeliaceae) families, of ferns and their relatives (Pteridophytes), and of mosses, liverworts, and lichens. Some species of woody plants, known as strangler figs (Ficus spp.), begin their lives as epiphytes, but if they are successful they eventually turn into full-sized trees. The sticky, bird-dispersed seeds of strangler figs are adapted to finding appropriate nooks high in the canopy of a tall tree, where they germinate and live as an epiphyte, independent of the soil far below. However, as the seedling grows into an aerial shrub, it begins to send roots down towards the ground. If the ground is eventually reached, the strangler fig is no longer a true epiphyte, although it continues to rely on the host tree for mechanical support. Over time, the strangler fig sends more and more of these roots downwards, until their coalescing biomass eventually encircles the host tree and prevents it from growing radially, while the fig pre-empts the space occupied by its foliage. Eventually the host tree is killed, and its place in the forest canopy is assumed by the hollow-trunked strangler fig.
About 80% of the ecosystem biomass of tropical rainforests occurs as woody tissues of trees, while only about 15% of the organic matter occurs in soil and litter, and about 5% is foliage. (As with all forests, the biomass of animals is much less than 1% of that of the total ecosystem.) In contrast, temperate forests maintain much larger fractions of their total ecosystem biomass as organic matter of the soil and forest floor. The reason for this difference is the relatively rapid rate of decomposition of dead biomass in the warm and humid environmental conditions of tropical rainforests. Because most of the biomass and nutrient content of tropical rainforests occurs in the biomass of living trees, and because their soils are usually highly infertile and extremely weathered, the fertility of this ecosystem is rapidly degraded after the forest is cleared. This is especially true if the site is converted to an agriculture land-use.
An enormous number of species of plants, animals, and microorganisms occurs in tropical rainforests, and this type of ecosystem accounts for a much larger fraction of Earth’s biodiversity than any other category. Of the 1.7 million species that biologists have so far identified, about 35% occur in the tropics, although less than one-half of those are from tropical rainforests. However, this is likely a gross underestimate of the importance of tropical rainforests in this regard, because relatively few of the species of this ecosystem have been identified. Some biologists have estimated that as many as 30 to 50 million species could occur on Earth and that about 90% of them inhabit tropical ecosystems, the great majority of those in rainforests. Most of the undiscovered species are insects, especially beetles. However, tropical rainforests also harbor large numbers of undiscovered species of other arthropods, as well as many new plants and microorganisms. Even new species of birds and mammals are being discovered in tropical rainforests, further highlighting the frontier nature of the biological and ecological explorations of that biodiverse natural ecosystem.
Tropical rainforests are enormously rich in species. For example, an area of 0.25 acre (0.1 ha) in a rainforest in Ecuador had 365 species of vascular plants, while a 7.5 acre (3 ha) plot in Borneo had more than 700 species of woody plants alone. Such rainforests typically have hundreds of species of full-sized trees. In comparison, temperate rainforests typically have no more than 10 to 12 species of trees, and often fewer. Tropical rainforests also typically support more than 300 to 400 bird species, compared with fewer than about 40 in temperate forests. If we had access to accurate knowledge of the insect species of tropical rainforests, an even more enormous difference in species richness could be demonstrated, in comparison with temperate forests. The extraordinary biodiversity of tropical rainforests is probably the most critical, defining attribute of this ecosystem, and is a natural heritage that must be preserved for all time.
Temperate rainforests are most commonly found on the windward side of coastal mountain ranges. In
Biome— A geographically extensive ecosystem, usually characterized by its dominant life forms.
Climax community— The more or less stable, plant and animal community that culminates succession under a given set of conditions of climate, site, and biota.
Community— In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the sametime.
Competition— An interaction between organisms of the same or different species associated with their need for a shared resource that is present in a supply that is smaller than the potential, biological demand.
Old growth— A late-successional forest, characterized by great age, an unevenly-aged population structure, domination by long-lived species, and with a complex physical structure, including multiple layers in the canopy, large trees, and many large-dimension snags and dead logs.
Selective 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.
Species richness— The number of species occurring in a community, a landscape, or some other defined area.
such places warm, moisture-laden winds blowing from over the ocean are forced upward, where they cool, form clouds, and release their moisture as large quantities of rainfall. These forests have developed in high-rainfall, temperate regions along the west coasts of North and South America, New Zealand, and elsewhere.
There are many variants of temperate rainforests. In northern California, coastal rainforest can be dominated by stands of redwood (Sequoia sempervirens) trees older than 1,000 years. More extensive old-growth rainforests elsewhere on the western coast of North America are dominated by other conifer species, especially Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla), along with sitka spruce(Picea sitchensis), red cedar (Thuja plicata), and fir (Abies concolor). Rainforests also occur in wet, frost-free, oceanic environments of the Southern Hemisphere, for example, in parts of New Zealand, where this ecosystem type is dominated by southern beech (Nothofagus spp.) and southern pines (Podocarpus spp.).
Relatively few species have an obligate need for old-growth temperate rainforest as their habitat. In other words, most species that occur in old-growth temperate rainforests also occur in younger but mature forest of a similar tree-species composition. In the temperate rainforests of the Pacific coast of North America, the spotted owl (Strix occidentalis), marbled murrelet (Brachyramphus marmoratus), and some species of vascular plants, mosses, and lichens appear to require substantial areas of this ecosystem type as a major component of their habitat. However, the numbers of species dependent on temperate old-growth rainforests are very much smaller than in tropical rainforests. With respect to biodiversity issues, the importance of temperate rainforests is substantially associated with their intrinsic value as a natural type of ecosystem, and somewhat less so with the number of dependent species.
Rainforests are a valuable natural resource, mostly because they contain large individual trees of commercially desirable species. These trees can be cut and manufactured into lumber, plywood, paper, and other valuable wood products. Tropical rainforests, for example, contain large trees of commercially important species of tropical hardwoods, such as African mahogany (Khaya and Entandrophragma spp.), American mahogany (Swietenia spp.), Asian mahogany (Shorea spp. and Parashorea spp.), balsa (Ochroma spp.), ebony (Diospyros spp.), rosewood (Dalbergia spp.), rubber (Hevea brasiliensis), and yang (Dipterocarpus spp.). Temperate rainforests are also extremely valuable, because their large trees can be cut and converted into economic products.
Because they have little or no net production of tree biomass, it is common practice in industrial forestry to clear-cut old-growth rainforests and then convert them into more productive, secondary forests. Even though another forest regenerates on the harvested site, sometimes dominated by the same tree species that occurred initially, this practice is an ecological conversion that results in a net loss of old-growth rainforest as a natural ecosystem. All ecological conversions have attendant risks for species that require the particular habitats of the original ecosystem.
In other cases, trees may be selectively harvested from old-growth rainforests so that the physical and ecological integrity of the forest is left more or less intact. This is especially true of temperate rainforests, which unlike tropical rainforests, do not have interlocking webs of lianas in their overstory, so that the felling of one large tree can bring down or badly damage other trees in its vicinity. However, even selective harvesting changes the character of old-growth rainforests, so that they are no longer in their natural condition. As such, the selectively harvested ecosystem would no longer provide habitat for many of the species that depend on the habitats available in the original, old growth rainforest. Nevertheless, selective harvesting results in a much less intensive ecological conversion than that associated with clear-cutting.
Because old-growth rainforests are natural ecosystems, they are considered to have great intrinsic value, which is degraded when they are harvested or otherwise disturbed. The intrinsic value of rainforests is further enhanced by the enormous richness of species of plants, animals, and microorganisms that are dependent on this specific ecosystem, particularly in the tropics. Mostly because of the intrinsic biodiversity-related value of rainforests, it is critically important that not all of the world’s tracts of these natural ecosystems are converted to human uses. To prevent this terrible damage from occurring, extensive landscapes of the world’s remaining rainforests, in both tropical and temperate regions, must be protected in ecological reserves and parks, where no more than traditional uses by humans are permitted.
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