PLEISTOCENE REFUGIA | another important factor favouring high tropical diversity

Another important factor favouring high tropical diversity is the relative stability of these areas over geological time.

Large expanses of the temperate and boreal zones were covered in ice during the latest glacial maximum, only 10 500 years ago.

Glaciation contributed directly to the extinction of many species at high latitudes, and there has been a much shorter time period over which speciation could potentially occur following glacial retreat.

Tropical forests also experienced climatic changes during the Pleistocene, mainly involving decreased precipitation and changes in sea level. In many tropical regions forests were reduced to small island-like regions called refugia.

These areas now often show particularly high levels of both species diversity and endemism. Examples of important glacial refugia include the foothills and eastern slopes of the Andes, the Choco region of Colombia, and the Mt Cameroon region in Cameroon, West Africa.

NICHE DIFFERENTIATION IN TROPICAL FOREST

Equilibrium hypotheses for the maintenance of tropical diversity generally invoke some form of niche differentiation. This hypothesis is based upon the idea that ecologically similar species are unable to coexist unless they have developed different patterns of habitat distribution and/or resource use. Within this framework, the more specialized the resource requirements of each species are, the more species can be packed into a given habitat. Niche differences among tropical animals are generally related to the type of food resources utilized, or spatial or temporal differences in habitat use.

For example, otherwise ecologically similar animals can differ in terms of height of activity in the canopy, or the time of day they are active. In contrast, all plant species utilize essentially the same set of basic resources, namely: light, water, carbon dioxide, physical space, and nutrients such as nitrogen, phosphorus and potassium.

However, plant species can differ in terms of more subtle ecological characteristics, such as the efficiency of resource use, tolerance of physiological stress, or dependence on specific pollinators, seed-dispersers or root symbionts. Many studies of tropical forest trees have emphasized differences in the "regeneration niche", or the resources and conditions required by seedlings and saplings to successfully establish in the forest. An important distinction is made between ‘pioneer’ tree species that can grow rapidly in large canopy openings or cleared areas, and "late-successional" or "primary forest" tree species that can establish under low light conditions in the understorey. Other kinds of niche differences among tropical trees include ‘structural niche’ differences related to the size reached by adult trees, and differences in ‘habitat preference’ related to soil characteristics and hydrology.

CLIMATE AND BIOLOGICAL PRODUCTIVITY OF THE TROPICS

The warm, wet, and relatively aseasonal climate of the tropics is apparently more favourable for maintaining higher diversity than anywhere else in the world. But why is this the case? One simple idea is that the high solar energy inputs and productivity of tropical regions result in greater numbers of species that can be supported energetically.


However, it is not entirely clear why a small number, or even one species, could not monopolize most or all of the incoming solar energy. Another idea is that climate stability is the main factor promoting species diversification and coexistence. In the harsher temperate and polar regions, species must be able to tolerate drastic fluctuations in seasonal temperatures. Species occurring in habitats nearer the poles are therefore adapted to a wider range of local environments in order to survive the winter months.

As a consequence, one expects a narrower range of adaptation to environmental conditions and narrower latitudinal and altitudinal distributions in the tropics, a hypothesis sometimes referred to as ‘Rapoport’s rule’. The more limited ranges of species in the tropics may allow for greater ‘species packing’ compared to temperate or boreal regions.

Although this idea has received much research attention, recent analyses give only equivocal support, at best, for Rapoport’s rule. On the other hand, it is clear that the relatively aseasonal nature of tropical rain forests allows for the evolution of highly varied and complex species interactions. This complexity itself contributes to the overall species diversity found in the tropics. For example, "dependent" ecological forms, such as specialist herbivores or predators, only persist in the community if their host species is present.

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SPECIES DIVERSITY | Tropical Rain Forests Are More Biologically Diverse

Tropical rain forests are more biologically diverse than any other biome, lying at the extreme of a latitudinal diversity gradient that extends from the poles to the tropics. High species diversity in tropical forests is perhaps most impressively illustrated by the results of surveys of insects obtained using canopy fogging with broad-spectrum insecticides. 

A landmark study by Terry Erwin sampled the canopy of a single Luehea seemannii tree in Panama, finding 163 species of beetles (Coleoptera) co-occurring in this one tree. Through a series of extrapolations (based on estimates of the total number of world tree species and the proportion of insect species comprised of beetles), Erwin estimated that there may be 30 million species of insects occurring in the tropical forests of the world. This figure implies that 495% of the earth’s species remain to be described. 

While more recent sampling efforts have tended to yield lower estimates of tropical insect diversity, there are conservatively between 5 and 10 million species. This quantity is 5-to 10-fold greater than all species described to date. Thus, while tropical forests occupy only 7% of the earth’s land surface they are thought to contain over half of all of the species on the planet. The idea that species diversity of tropical animals (which are mostly insects) is a simple function of the number of plant species also implies that any effort to explain tropical diversity in general must, first and foremost, address the problem of the origin and maintenance of plant diversity.

HISTORICAL IMPORTANCE OF TROPICAL FORESTS IN BIOLOGY

Tropical forests have played a central role in the conceptual development of biology from the time of the major biological expeditions that began at the close of the eighteenth century.

Alexander Von Humbolt initiated the study of plant ecology on his voyages through South America in the early nineteenth century. While climbing Mount Chimborazo in the Andes, Von Humbolt characterized the vegetation changes with climate as he ascended. These early observations on plant distributions provided the foundation of the field of biogeography. The most significant development in biological thinking inspired by tropical forests was the theory of evolution by natural selection. Charles Darwin and Alfred Russel Wallace independently derived this theory as a result of their scientific voyages in the tropics during the mid nineteenth century. Darwin’s inspiration was his exploration of various parts of South America as the naturalist aboard the Beagle beginning in 1831.

Wallace conducted expeditions in both South America (1848–1852) and the Malay Archipelago (1854–1862), where he characterized two sets of fauna distinct to the different parts of the archipelago, a division now known as Wallace’s Line. For both Darwin and Wallace, the high diversity of species in the tropics, and particularly patterns of diversification associated with island groups, allowed an appreciation of evolutionary relationships not apparent in the poorer faunas of the temperate zone. The high diversity of the tropics continues to be an important inspiration and testing ground for ideas in biology, particularly in the fields of ecology and evolutionary biology.

What is a Rainforest ? | DEFINITION OF RAINFOREST

Frequently Asked Questions About the Rainforest, What is a Rainforest? or Definition of Rainforest. Rainforest is a woodland of tall trees growing in a region of year-round warmth and abundant rainfall. Almost all rain forests lie at or near the equator. They form an evergreen belt of lush vegetation that encircles the planet. German botanist Andreas F. W. Schimper first coined the term rain forest-in German, Regenwald-in 1898.

Tropical rain forests occupy only 6 to 7 percent of the earth's surface. However, they support more than half of the world's plant and animal species (kinds). More kinds of frogs and other amphibians, birds, insects, mammals, and reptiles live in rain forests than in any other area. Scientists believe millions more rain forest species remain undiscovered.

The rain forest provides people with many benefits. Its plants produce timber, foods, medicines, and such industrial products as dyes, fibers, gums, oils, and resins. Rain forests help regulate the earth's climate and maintain clean air. The forests' lush, green beauty and rich wildlife offer a special source of enjoyment.

In addition, rain forests provide homes to millions of people. Such groups as the Yanomami of South America, the Dayaks of Southeast Asia, and the Pygmies of central Africa have lived in rain forests for centuries. They make their living by hunting, fishing, collecting forest products, and farming. Traditional forest peoples have acquired much knowledge about the rain forest's plants and animals.

In spite of these benefits, people cut down thousands of square miles or square kilometers of rain forest each year. This destruction eliminates thousands of species of animals. A number of governments and conservation organizations are working to preserve the rain forests.

DEFINITION OF FOREST STANDS | FOREST PATCHES

Definition of Forest Stands or Forest Patches refer to the ecosystem scale at which a relatively homogenous forest unit can be identified. The composition, structure, and ecological functions within a stand are similar enough that an ecologically responsible forest use prescription can be applied uniformly within the stand, without encountering changes in ecological parameters that may produce unexpected or undesirable results.

In conventional forestry, “stands” have largely been defined by narrow timber characteristics, which were in turn driven by short-term economic variables. However, in order to plan and carry out ecologically responsible forest uses within an ecosystem-based approach, stands must be defined in relation to whole ecosystem factors that are required to maintain fully functioning forests at the landscape and stand levels.

In other words, the boundaries of a stand are not determined by rigid human management criteria such as timber size and timber quality, but by the full spectrum of ecosystem parameters that have been shaped by natural disturbance patterns and that reflect the movement of energy, nutrients, water, and animals into and out of a particular ecosystem.

Human scales are closest to forest scales at the stand or patch level. For example, the stand or patch level is the scale where visible human modification occurs. However, an ecosystem-based approach must always consider that what occurs at the stand or visible scale will also have impacts on a variety of other scales, from the large landscape to the microscopic.

DEFINITION OF FOREST LANDSCAPE

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Definition of forest landscape is the large-scale view of a forest. When industrial timber managers use the term “forest landscapes,” they are usually concerned with scenery and visual impacts. In the context of an ecosystem-based approach, however, a forest landscape is a mosaic of interconnected, interdependent stands or patches that are repeated in a pattern across the larger landscape. This pattern has both spatial and temporal components.

An ecosystem-based approach requires that all planning and activities begin at the regional/landscape level. When planning for human use, landscape level decisions are made for watersheds of small to moderate size (less than 5,000 hectares to about 50,000 hectares, or 12,000 to 125,000 acres). In sub-regional or regional planning processes, forest landscape level considerations are expanded to large watersheds encompassing hundreds of thousands of hectares/acres.

In planning and carrying out forest uses, particularly timber management, many people tend to focus on small forest parcels. This is a result of our limited spatial view, short time frames, and cultural conditioning. In contrast, an ecosystem-based approach requires that all planning and activities start at the landscape level. The character and condition of the forest landscape dictate what is ecologically possible at the stand level.

The character of a forest ecosystem refers to how a forest works, from the landscape level to the stand or patch. For example, forests that have frequent fires have a different character than forests where wind and root decay are the primary agents of disturbance. Some forests are characterized by steep slopes, shallow soils, and well-defined drainage patterns, while other forests have gentle slopes, cold soils, and diffuse drainage patterns.

Forests of a different character will have different composition and structures, and therefore differences in how they function. Different composition, structures, and functioning lead to different kinds of ecological limits to human use. Ecological limits are natural factors or processes that are easily damaged or degraded if modified by human uses. For example, steep and/or wet slopes impose ecological limits because, if disturbed, they are likely to erode, causing problems like soil loss and siltation of streams. Cold soils are an ecological limit because nutrient cycling occurs in shallow organic layers which may be easily damaged by many types of human activities.

The condition of a forest describes how human uses have modified forest functioning from the landscape level to the stand or patch level. Conventional timber management frequently results in negative impacts, like fragmentation, loss of old growth, and soil degradation. An ecosystem-based approach protects forest composition and structure and respects the ecological limits of forests to various human uses. By respecting ecological limits ecosystem-based approaches avoid degradation of short- and long-term forest functioning.
Ecological limits to human use are determined by describing and interpreting the character and condition of, first, the forest landscape, and then the forest stand or patch.

FOREST PROTECTION STRATEGY

Several forest protection strategies that can be used to achieve sustainable forest management are:

1. understand the interaction with the agent of forest destruction so that:
   • can identify the factors that cause problems in forest protection
   • can identify the cause of primary damage
2. can analyze and make decisions as a whole and not just limited to the most serious causes of damage
3. always seen as a forest protection measures are not separate from silvicultural
4. aware that forest protection is increasingly important and the approach is not limited to field crops but excluding forest products.

Protection strategies in addition to ensuring the sustainability of forest management can also ensure a low risk of forest management.

Types of Rainforests

Rainforests are found throughout the world, not only in tropical regions, but also in temperate regions like Canada, the United States, and the former Soviet Union. These forests, like tropical rainforests, receive abundant, year-round rainfall, and are characterized by an enclosed canopy and high species diversity, but lack the year-round warmth and sunlight associated with tropical rainforests. However this book focuses on tropical rainforests, and these are the only forest forms discussed here.

Tropical rainforests merge into other types of forest depending on the altitude, latitude, and various soil, flooding, and climate conditions. These forest types form a mosaic of vegetation types which contribute to the overwhelming diversity of the tropics.

EQUATORIAL EVERGREEN RAINFOREST VS. MOIST FOREST

There are two major types of wet tropical forests: equatorial evergreen rainforests and moist forests, which includes monsoon forests and montane/cloud forests. Equatorial rainforests, often considered the "real rainforest," are characterized by more than 80 inches (2,000 mm) of rain annually spread evenly throughout the year. These forests have the highest biological diversity and have a well-developed canopy "tier" form of vegetation. Roughly two-thirds of the world's tropical wet forests can be considered the equatorial type. These forests are near the equator where there is very little seasonal variation and the solar day is a constant length all year round. The greatest expanses of equatorial rainforest are found in lowland Amazonia, the Congo Basin, the Southeast Asian islands of Indonesia, and Papua New Guinea.

Tropical moist forests are found at a greater distance from the equator where rainfall and day length vary seasonally. These forests get "only" 50 inches (1,270 mm) of rain annually and are markedly distinguished from equatorial rainforests by a cooler dry season. During this dry season, many trees shed some or even all their leaves, creating a seasonal reduction of canopy cover and allowing more sunlight to reach the forest floor. The increased sunlight reaching the forest floor allows the growth of vigorous understory vegetation not found in lowland equatorial forest. Such moist forest is found in parts of South America, the Caribbean, West Africa, and Southeast Asia, especially Thailand, Burma, Vietnam, and Sri Lanka.

PRIMARY VS. SECONDARY FOREST

Throughout this site, other books, and discussions about tropical rainforest, the term "primary forest" is used. Primary forest refers to untouched, pristine forest that exists in its original condition. This forest has been relatively unaffected by human activities. Primary rainforest is often characterized by a full ceiling canopy and usually several layers of understory. The ground floor is generally clear of heavy vegetation because the full canopy allows very little light, necessary for plant growth, to penetrate. Occasionally, when a canopy tree falls, a temporary "light gap" is opened in the canopy, allowing growth of floor and understory species. Primary forest is the most biologically diverse type of forest.

Secondary forest is rainforest that has been disturbed in some way, naturally or unnaturally. Secondary forest can be created in a number of ways, from degraded forest recovering from selective logging, to areas cleared by slash-and-burn agriculture that have been reclaimed by forest. Generally, secondary forest is characterized (depending on its level of degradation) by a less developed canopy structure, smaller trees, and less diversity. Due to the lack of a full canopy, more light will reach the floor, supporting vigorous ground vegetation. "Jungle" is the term often applied to secondary forest with dense ground growth, but it is also applied to some tropical moist forests where seasonal variations permit thick ground growth.

Primary versus total forest cover for selected tropical countries

		Total land area		Total forest cover 2005				Primary forest cover 2005				Total deforestation 1990-2005		Loss of primary forest 1990-2005
Country		(1000 ha)		(1000 ha)		% of total land area		(1000 ha)		% of total land area		% of 1990 forest cover		% of 1990 primary forest cover
Bolivia		109,858		58,740		54.2		29,360		26.7		-6.5		-6.5
Cambodia		18,104		10,447		59.2		322		1.8		-19.3		-58.0
Congo		34,200		22,471		65.8		7,464		21.8		-1.1		-1.1
Costa Rica		5,110		2,391		46.8		180		3.5		-6.7		-29.4
Ghana		23,854		5,517		24.2		353		1.5		-25.9		0.0
Guatemala		10,889		3,938		36.3		1,957		18.0		-17.1		-17.0
Malawi		11,848		3,402		36.2		1,132		9.6		-12.7		-34.5
Malaysia		32,975		20,890		63.6		3,820		11.6		-6.6		0.0
Nigeria		92,377		11,089		12.2		326		0.4		-35.7		-79.0
Senegal		19,672		8,673		45		1,598		8.1		-7.2		-9.2
Thailand		51,312		14,520		28.4		6,451		12.6		-9.1		0.0
Brazil		851,488		477,698		57.2		415,890		48.8		-8.1		-9.7
Colombia		113,891		60,728		58.5		53,062		46.6		-1.2		-1.5
Côte d'Ivoire		32,246		10,405		32.7		625		1.9		1.8		0.0
Liberia		11,137		3,154		32.7		129		1.2		-22.3		0.0
Madagascar		58,704		12,838		22.1		10,347		17.6		-6.2		-1.5
Mexico		195,820		64,238		33.7		32,850		16.8		-6.9		-15.3
Panama		7,552		4,294		57.7		3,023		40.0		-1.9		-18.4
Papua New Guinea		46,284		29,437		65		25,211		54.5		-6.6		-13.7
Peru		128,522		68,742		53.7		61,065		47.5		-2.0		-2.9
Philippines		30,000		7,162		24		829		2.8		-32.3		0.0
Sri Lanka		6,561		1,933		29.9		167		2.5		-17.7		-35.0

The Food and Agriculture Organization of the United Nations estimates that primary forests now account for 36 percent of total forest area, but are being lost or modified at a rate of 6 million hectares a year through deforestation or selective logging. Selective logging, where only one or two valuable tree species are harvested from an area, was recently found to be degrading forests in the Amazon twice as fast as deforestation figures indicate.

Scientists do not know how long it takes for secondary forest to attain the structure and levels of biodiversity of primary forest. A recent study by conducted as part of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) determined that trees in the Central Amazon may, on average, be several hundred years old, suggesting that primary forests take a long time to develop.


LOWLAND VS. MONTANE FOREST

Lowland tropical rainforest refers to the majority of tropical rainforest, that is, forest which grows on flat lands at elevations generally less than 3,300 feet (1,000 m)—although elevation may vary. Lowland primary forest, often characterized by more than five forest tier levels, is usually taller and more diverse than montane forest. It has a greater diversity of fruiting trees; hence more animals specially adapted to feed on their fruits and more large mammals. Lowland rainforest is far more threatened than montane forest because of its accessibility, more suitable soils for agriculture, and more hardwoods valuable as timber. In many countries, virtually all lowland primary forest is gone, while montane forest still remains.

Tropical montane rainforest is forest that grows on mountains and above an altitude of 3,300 feet. High montane forest, above 6,600-10,000 feet (2,500-3,000 meters) in elevation, is often manifested as "cloud forest," forest that receives the majority of its precipitation from mist or fog that passes up from the moist, humid lowlands. The trees of cloud forests are typically shorter than those of lowland forest resulting in a less-developed canopy. Nevertheless, cloud forest trees are heavily burdened with epiphytes that thrive with the abundance of moisture from the passing fog. Trees in places like the lower elevations of the Andes in Ecuador, Peru, Colombia, and Venezuela; Central America (Monteverde in Costa Rica in particular); Borneo (Mount Kinabalu); and Africa (Ethiopia, Kenya, Rwanda, Zaire, Uganda), are frequently green with dense moss and beautiful, often rare, orchids.

Patches of cloud forests tend to have many endemic species, because they are often isolated from other sections of cloud forest by valleys and ridges. These species are prevented from migrating to other forest areas by these obstacles to the sides, by the lowland forest below, and by steep cliffs above. Cloud forests are home to an abundance of hummingbirds, frogs, and epiphytes like orchids, bromeliads, and mosses. Many of these species are endemic to a single locality, like the Golden toad of Monteverde, Costa Rica, a species which is now believed to be extinct. Cloud forests generally lack an abundance of large-bodied mammals due to the small number of fruiting trees.

Tropical montane forests are especially in the South American Andean region, where much of the forest has been cleared for agriculture. Of the continent's endangered species, a disproportionate number of those are found in yungas, the regional name for tropical montane forests in the Andes. These forests have also been little studied.

Above 10,000 feet (3,300 m), cloud forest may give way to sub-alpine and alpine forest. These habitats have less rain, fewer trees, and reduced biodiversity compared with lower elevation forests.

Chart showing percentage cover of world forests. Rainforests are a subsection of "Tropical forests".

OTHER TYPES OF FOREST

SEASONAL OR MONSOON FOREST
Monsoon forests are tropical moist or seasonal rainforests found primarily in Asia (India/Sri Lanka to China), West and East Africa, Northern Australia, and Eastern Brazil. In this type of forest there is a distinct cooler dry season and a distinct wet season. These forests tend to be less diverse and more dwarfed in terms of tree size in comparison to typical equatorial rainforests.

Monsoon forests are highly threatened worldwide by clearing for cultivation, especially in West Africa, where over 90 percent of the coastal rainforests and the monsoon forests have been cleared.

IGAPÒ FOREST

Flooded forest in Brazil | more Photo by R. Butler

Igapò forest is rainforest that is regularly inundated for extended periods during the flood season (sometimes considered permanently flooded rainforest). The best known of such forests are found in the Amazon Basin where they make up about 2 percent of the total rainforest. Igapò forest trees are shorter than those of non-flooded forest because of the instability caused by the wet, poorly drained soils (hence it is sometimes known as "swamp forest") and characterized by certain tree species like Cecropia, Ceiba, and Mauritia palms (also known as the aguaje palm). Many igapò tree species have stilt roots and flying buttresses to lend structural support. Igapò forest is flooded (4-10 months of the year) and flooding is usually predictable. Fish play an important role in seed dispersal in this forest system.

VÀRZEA FOREST
Vàrzea forests are floodplain forests which flood seasonally. Unlike swamp forests, varzeà forests have relatively rich soils from the annual replenishment of nutrients from whitewater rivers. Because these forests are more suitable for agriculture than typical rainforest, they are some of the most threatened. Even in the Amazon where vast majority of such forests are found, vàrzea are disappearing rapidly for development.

Floodplain forests, especially those located on river banks and islands, are often short-lived due to the meandering nature of tropical lowland rivers which eat away at the forests' base. According to Amazon Headwaters, a book by Michael Goulding and his colleagues, research in Peru suggests that most floodplain forests are rarely older than 200 years and may have turnover rates exceeding 1.6 percent, implying an average tree life of 63 years. For this reason, floodplain forests are nearly always in some stage of succession with pioneer species like Cecropia being replaced with Kapok (Ceiba) and fig trees further away from the river.

HEATH FOREST
Heath forests are found on well-drained, sandy soils that are extremely nutrient-poor. These forests are characterized by certain tree species tolerant of the poor, acidic soil conditions and are considerably "stunted" in comparison with typical rainforests. More light reaches the forest floor making for dense tree growth. Heath forests, also known as blackwater or caatinga forests, are drained by blackwater rivers and are found primarily in the Amazon Basin (the Rio Negro drainage), but also in parts of Asia.

PEAT FOREST  [news and information on peatlands]
Peat forest is found in small parts of Africa, northeastern South America, and large areas in southeast Asia (especially Borneo and Sumatra). These swamp forests appear in places where dead vegetation becomes waterlogged and accumulates as peat. The peat acts as a sort of sponge withholding moisture at times of little rainfall and absorbing monsoon rains. When peat swamp forests are drained for agricultural projects, they become highly susceptible to combustion. Under the dry el Niño conditions of 1997-98, thousands of fires raged in the peat swamps of Indonesia. Fires in peat swamps are extraordinarily difficult to extinguish because the conflagration continues in the deeper layers of peat.

TERRA FIRME FOREST
Terra Firme literally means "firm earth" and refers to rainforest that is not inundated by flooded rivers. This forest is noticeably taller and more diverse (>400 species/hectare in some areas) than igapò or flooded forest. It is found only on dry, well-drained soils and is characterized by such species as Brazil nut trees, Rubber trees, and many tropical hardwood trees.

MANGROVE FOREST [news and information on peatlands]
Mangrove forest is found in silt-rich, saline (brackish water) habitats worldwide, generally along large river deltas, estuaries, and coastal areas. It is characterized by low tree diversity, almost exclusively mangroves, with a low broken canopy. Mangroves are evergreen trees and shrubs that are well adapted to their salty and swampy habitat by having breathing roots (pneumatophores) that emerge from the oxygen-deficient mud to absorb oxygen.

Mangroves in Honduras. Photo by R. Butler. More mangrove photos

Mangrove swamps are home to numerous bizarre amphibious fish species like the mudskippers of eastern Africa to Australia and Anableps, the so-called four-eyed fish of the New World. Mudskippers are renowned for their preference for terrestrial haunts over aquatic realms. These fish spend more time on floating debris, tree toots, and plants than they do in the water where they only go to escape predators. Watching a group of Mudskippers reminds the observer of what our ancestors must have looked like when they first left the ocean for life on land. Mudskippers are highly intelligent fish that feed primarily on insects and crustacean.

A second amphibious species found in mangrove forests is the Anableps. a species widespread in the New World from Central America to northern South America. Most notable about its physical features is its double-lobed eyes which allow it to see both above and below the water line as it swims along the water surface. The Anableps, too, regularly leaves the water to perch on tree roots and rocks.

Mangrove forests are some of the most threatened ecosystems on the planet because of their proximity to the ocean (prime resort/development property) and the tendency for local people and governments to undervalue the services they provide. A recent study by the Food and Agriculture Organization of the United Nations found that 20 percent of the world's mangrove forests have disappeared since 1980, mostly due to farming, harvesting for timber and charcoal, freshwater diversion, real estate development, and conversion for tourism.

According to the Environmental Justice Foundation, about 38 percent of global mangrove deforestation is linked to shrimp farm development. Mangrove clearing for commercial shrimp and prawn hatcheries is particularly prevalent in Southeast Asia. Ironically this form of aquaculture has come at the expense of the natural fish and shrimp hatchery.

The destruction of mangrove forest has dire implications for the fisheries industry, since these forests provide an important spawning ground and serve as a nursery for many commercially important species. In addition, mangrove forest protects coastal regions against storm damage and erosion. Research conducted following the 2004 tsunami in Asia found that areas forested with mangroves suffered considerably less damage than areas without tree vegetation.

Mangrove forests are slow to recover from clearing and degradation. For example, seismic lines only a few meters wide in the mangrove forest of Nigeria were still visible by air a decade after they were cut.


Review questions:

• What is the difference between primary and secondary forest?
• True or false—Cloud forest is found in mountainous areas.
• True or false—flooding is common in the Amazon rainforest.
• Why are mangrove forests important?
• Why are mangrove forests being destroyed?

Forest Products

Forest products have long provided people with food, shelter, clothing, and fuel. Prehistoric people ate berries and nuts that grew in forests. They built shelters from the branches of trees and wore clothing made of plant materials. About 1 1/2 million years ago, they began using wood as a fuel to make fire.


Today, wood is one of our most important raw materials. It is used in making thousands of products, from building materials, to paper, to photographic film. Despite its usefulness as a raw material, the chief use of wood throughout the world is as a fuel.





There are thousands of forest products. Most can be classified into one of five main groups: (1) wood products, (2) wood-based composite products, (3) fiber products, (4) chemical products, and (5) fuel products. Wood products are made from solid wood. Wood-based composite products contain wood and at least one other material. Manufacturers use wood fibers to produce fiber products. Chemical products are made by breaking down wood and wood fibers and chemically treating them. Such chemical products as cellophane, lacquer, and rayon are made from wood but do not feel or look like wood. Fuel products include logs, wood pellets, and charcoal. Other forest products come from the bark, fruit, gum, leaves, and sap of trees.

Wood products

Wood has many characteristics that make it an important construction material. Carpenters and woodworkers can easily shape it with tools and fasten it with nails, screws, staples, and adhesives. It is light but strong. Wood provides insulation against electric current, heat, cold, and sound. It can hold paint and other finishes, and it does not rust. Unlike metal, cement-based, or plastic construction materials, wood is a renewable resource-that is, a new supply grows after the timber has been harvested. Some of the chief wood structural materials are round timbers, lumber, and veneer products.

Round timbers include pilings, poles, and posts. Pilings driven into the ground provide foundations for buildings, wharves, and other heavy structures. Poles link overhead telephone wires and power lines. People use posts chiefly to build fences. Round timbers are simply trees that have been stripped of their branches and bark, and cut into logs. The logs are dried and treated for protection against decay and insect attack.

Lumber includes boards and larger pieces of wood that have been sawed from logs. In the United States, the construction industry uses about 50 percent of the lumber production. The rest goes to make crates, furniture, railroad ties, sporting goods, toys, and thousands of other products. 

Wood scientists classify lumber as softwood or hardwood, depending on the kind of tree. This classification does not always indicate the hardness of the wood. Various softwoods produce harder lumber than do some hardwoods. Softwood lumber comes from needleleaf trees that are also called evergreens or conifers. Builders use this type of lumber for most structural work because of its straightness and length. Softwoods include pine, larch, fir, hemlock, redwood, cypress, cedar, and Douglas-fir.

Hardwood lumber comes from trees that lose their leaves every autumn. Many hardwoods have beautiful grain patterns. For this reason, builders and furniture makers use hardwoods for cabinets, flooring, furniture, and paneling. Popular hardwoods include birch, mahogany, maple, oak, sweet gum, and walnut.

Veneer products are made of thin sheets of wood called veneers. These veneers may be cut into long strips or other shapes. Veneer products include baskets, matches, tongue depressors, and toothpicks.

Wood-based composite products

Manufacturers produce many products using wood together with at least one other material. By combining materials, they can take advantage of the best properties of each. Wood-based composite products include plywood and particleboard, which are made by combining wood with adhesive resins.

Plywood consists of a number of veneers that are glued together. The veneers are arranged so that the grain direction in each layer is at a right angle to the grain direction of the next layer. This arrangement gives plywood several advantages over lumber. For example, plywood shrinks and swells less than lumber, and it can be easily nailed near the edges without splitting. The construction and furniture industries use large amounts of plywood.

Particleboard is made from wood shavings, flakes, wafers, splinters, or sawdust. Some of these materials come from scrap left over in sawmills and paper mills. Particleboard makers mix the wood with an adhesive and press it at a high temperature and pressure to form large sheets or panels. Particleboard shrinks and swells little in length and width. It may be used as a base for flooring and furniture. One type of particleboard, called oriented strand board (OSB), has the strength of plywood and many of the same uses. To make OSB, manufacturers use waxes and resins to bond layers of wood flakes positioned with their grains running in alternating directions.

Other wood-based composite products are made by combining wood with such materials as fiberglass, metals, polyvinyl chloride, polypropylene, and portland cement. Wood-based composites commonly substitute for lumber. For example, laminated veneer lumber is made of parallel laminated sheets of veneer manufactured to standard lumber dimensions.

Fiber products

Wood is made up of many tiny fibers. Manufacturers produce paper and paperboard, hardboard, and insulation board from wood fibers. Wood fiber is also used as attic insulation, as a protective soil covering called mulch, and even as a dietary fiber in breakfast cereals.

Paper and paperboard are made from wood chips that have been reduced to a fiber pulp by chemicals, heat, or mechanical treatment. The pulp is then formed into a mat, filtered, drained, and pressed. Paper products include bags, books, cartons, packaging materials, and tissue.

Medium density fiberboard (MDF) is made from wood that has been reduced to individual fibers or fiber bundles and then been bonded with adhesive. MDF is used primarily to make tops with molded edges for tables or other furniture.

Hardboard is made by pressing wood fibers into flat sheets at a high temperature and pressure. The fibers are held together primarily by lignin, a substance that naturally occurs in and between wood fibers. Hardboard is used chiefly in furniture, siding, and paneling.

Insulation board is manufactured from wood fibers that are formed into a mat, pressed lightly, and dried. It weighs less than hardboard. Insulation board is used for acoustical tile and under siding in construction.

Chemical products

Many wood products are made from wood or bark that has been broken down into such basic chemical parts as cellulose and lignin. Cellulose is the main ingredient of wood fibers.

Cellulose products. Cellulose may be chemically treated to change its properties and to produce such compounds as cellulose acetate and cellulose nitrate. Both of these compounds are used in adhesives, lacquers, and plastics. Plastic items molded from cellulose compounds include piano keys, tool handles, and table tennis balls. Cellulose nitrate is also an ingredient in explosives. Other cellulose compounds have specialized uses in such products as paint, foods, and textiles.

Textile manufacturers process cellulose to produce rayon and acetate fibers, which are used for clothing, draperies, and upholstery. Rayon cords strengthen tires. Other materials made from cellulose include cellophane and photographic film.

Lignin products. Lignin has far fewer uses than cellulose. It is used in making printing inks, dyes, and concrete. Manufacturers use it to bind (hold together) animal food pellets and textiles. Artificial vanilla, a flavoring in many foods, is also made from lignin.

Naval stores include turpentine and rosin-materials once essential to the operation of wooden sailing ships. Almost all naval stores come from the processing of pine pulp.

Fuel products

In many developing countries, wood has long served as the primary fuel for cooking and heating. In industrialized countries, wood has been burned mainly in fireplaces and charcoal grills. After petroleum prices rose in the 1970's, wood became a popular fuel in communities near forested areas. Fuel products made from wood include split, dried logs; compressed wood pellets; charcoal; and sawmill by-products. In addition, the forest products industry burns the thick liquid that results from pulping wood.

Other forest products

Although most forest products are made from wood, some come from the bark, fruit and seeds, gum, leaves, and sap of trees. By-products from sawmills include wood chips, shavings, and sawdust. These by-products may be used in making particleboard and other products, in bedding for animals, and in floor-sweeping compounds.

The bark from the cork oak tree provides cork for such products as bottle stoppers, bulletin boards, and insulation. The bark of the hemlock and other trees furnishes tannic acid used in processing animal hides. Bark is sometimes used as fuel, ground cover, or mulch.

Fruit and seeds harvested from forest trees include many kinds of nuts. The seedpods of the kapok, or silk-cotton, tree provide kapok fibers. Kapok is widely used as a filler in jackets and sleeping bags. Latex is a milky substance produced by plants and trees of the sapodilla family. Latex is the source of natural rubber, which is used to make balloons, hoses, tires, and other items.

The leaves of some forest trees furnish ornamental greenery for Christmas wreaths and similar products. Certain evergreen and eucalyptus leaves are distilled to produce oil used in perfumes, household cleaners, soaps, and certain drugs. Sap from certain kinds of maple trees is made into maple syrup and maple sugar.

The forest products industry

The manufacture of forest products is a major industry in many industrialized countries. The United States, China, and India are the world's leading producers of forest products.

In the United States, the forest products industry employs more than 1 1/2 million people and produces more than $300 billion worth of goods annually. The industry has more than 50,000 manufacturing plants. United States forest products companies own about 70 million acres (28 million hectares) of commercially valuable forestland. They harvest timber in state and national forests under government contracts. They also buy logs from the owners of small wooded areas.

In China, economic reforms that began in 1980 have led to a greater demand for private housing. This demand has, in turn, brought a huge increase in the production of forest products for use as construction materials. In India, millions of people depend on gathering and selling forest products for cash.

Canada's forest products industry is a leading source of export income. More than 350,000 Canadians work for companies that make forest products. Each year, these firms produce goods worth more than $50 billion in U.S. dollars. Canada is the world's leading producer of newsprint, the paper on which newspapers are printed. It produces more than a fourth of the world's total supply each year. Next >>>

Contributor: Jim L. Bowyer, Ph.D., Director, Forest Products Management Development Institute, University of Minnesota.

Source : World Book 2005

The history of forests

The first forests developed in marshlands about 365 million years ago, toward the end of the Devonian Period. They consisted of tree-sized club mosses and ferns, some of which had trunks nearly 40 feet (12 meters) tall and about 3 feet (1 meter) thick. These forests became the home of early amphibians and insects.

By the beginning of the Carboniferous Period-about 360 million years ago-vast swamps covered much of North America. Forests of giant club mosses and horsetails up to 125 feet (38 meters) tall grew in these warm swamps. Ferns about 10 feet (3 meters) tall formed a thick undergrowth that sheltered huge cockroaches, dragonflies, scorpions, and spiders. In time, seed ferns and primitive conifers developed in the swamp forests.

When plants of the swamp forests died, they fell into the mud and water that covered the forest floor. The mud and water did not contain enough oxygen to support decomposers. As a result, the plants did not decay but became buried under layer after layer of mud. Over millions of years, the weight and pressure on the plants turned them into great coal deposits.

Later forests. As the Mesozoic Era began, about 248 million years ago, severe changes in climate and in the earth's surface wiped out the swamp forests. In the new, drier environment, gymnosperm trees became dominant. Gymnosperms are plants whose seeds are not enclosed in a fruit or seedcase. Such trees included seed ferns and primitive conifers like those that grew in the swamp forests. They also included cycad and ginkgo trees, which became widespread. Gymnosperm trees formed forests that covered much of the earth. Amphibians, insects, and large reptiles lived in these forests.

 

The first flowering plants appeared during the early Cretaceous Period, sometime after 145 million years ago. Flowering plants, which are called angiosperms, produce seeds enclosed in a fruit or seedcase. Many angiosperm trees became prominent in the forests. They included magnolias, maples, poplars, and willows. Flowering shrubs and herbs became common undergrowth plants.

At the start of the Cenozoic Era, about 65 million years ago, the earth's climate turned cooler. Magnificent temperate forests then spread across North America, Europe, and Asia. The forests included a wealth of flowering broadleaf trees and needleleaf conifers. Many birds and mammals lived in these forests.

Modern forests. The earth's climate continued to turn colder. By about 2.4 million years ago, the first of several great waves of glaciers had begun to advance over much of North America, Europe, and Asia. By the time the last of these glaciers had retreated-about 11,500 years ago-the ice sheets had destroyed large areas of the temperate forests in North America and Europe. Only the temperate forests of southeastern Asia remained largely untouched. 

The forests of the world took on their modern distribution after the last of the glaciers retreated. For example, the great boreal forests developed across northern Europe and North America. But the world's forest regions are not permanent. Today, for instance, temperate forests are invading the southern edge of the boreal region. Another ice age or other dramatic environmental changes could greatly alter the world's forests.

KINDS OF FORESTS

Many systems are used to classify the world's forests. Some systems classify a forest according to the characteristics of its dominant trees. A needleleaf forest, for example, consists of a forest in which the dominant trees have long, narrow, needlelike leaves. Such forests are also called coniferous (cone-bearing) because the trees bear cones. The seeds grow in these cones. A broadleaf forest is made up mainly of trees with broad, flat leaves.

Forests in which the dominant trees shed all their leaves during certain seasons of the year, and then grow new ones, are classed as deciduous forests. In an evergreen forest, the dominant trees grow new leaves before shedding the old ones. Thus, they remain green throughout the year.

In some other systems, forests are classified according to the usable qualities of the trees. A forest of broadleaf trees may be classed as a hardwood forest because most broadleaf trees have hard wood, which makes fine furniture. A forest of needleleaf trees may be classed as a softwood forest because most needleleaf trees have softer wood than broadleaf trees have.

Many scientists classify forests according to various ecological systems. Under such systems, forests with similar climate, soil, and amounts of moisture are grouped into formations. Climate, soil, and moisture determine the kinds of trees found in a forest formation. One common ecological system groups the world's forests into six major formations. They are (1) tropical rain forests, (2) tropical seasonal forests, (3) temperate deciduous forests, (4) temperate evergreen forests, (5) boreal forests, and (6) savannas. 

Tropical rain forests grow near the equator, where the climate is warm and wet the year around. The largest of these forests grow in the Amazon River Basin of South America, the Congo River Basin of Africa, and throughout much of Southeast Asia.

Of the six forest formations, tropical rain forests have the greatest variety of trees. As many as 100 species-none of which is dominant-may grow in 1 square mile (2.6 square kilometers) of land. Nearly all the trees of tropical rain forests are broadleaf evergreens, though some palm trees and tree ferns can also be found. In most of the forests, the trees form three canopies. The upper canopy may reach more than 165 feet (50 meters) high. A few exceptionally tall trees, called emergents, tower above the upper canopy. The understory trees form the two lower canopies.

The shrub and herb layers are sparse because little sunlight penetrates the dense canopies. However, many climbing plants and epiphytes crowd the branches of the canopies, where the sunlight is fullest.

Most of the animals of the tropical rain forests also live in the canopies, where they can find plentiful food. These animals include such flying or climbing creatures as bats, birds, insects, lizards, mice, monkeys, opossums, sloths, and snakes.

Tropical seasonal forests grow in certain regions of the tropics and subtropics. These regions have a definite wet and dry season each year or a somewhat cooler climate than that of the tropical rain forest. Such conditions occur in Central America, central South America, southern Africa, India, eastern China, and northern Australia and on many islands in the Pacific Ocean.

Tropical seasonal forests have a great variety of tree species, though not nearly as many as the rain forests. They also have fewer climbing plants and epiphytes. Unlike the trees of the rain forest, many tropical seasonal species are deciduous. The deciduous trees are found especially in regions with distinct wet and dry seasons. The trees shed their leaves in the dry season.

Tropical seasonal forests have a canopy about 100 feet (30 meters) high. One understory grows beneath the canopy. Bamboos and palms may form a dense shrub layer, and a thick herb layer blankets the ground. The animal life resembles that of the rain forest.

Temperate deciduous forests grow in eastern North America, western Europe, and eastern Asia. These regions have a temperate climate, with warm summers and cold winters.

The canopy of temperate deciduous forests is about 100 feet (30 meters) high. Two or more kinds of trees dominate the canopy, and another 15 to 25 kinds may be present. Most of the trees in these forests are broadleaf and deciduous. They shed their leaves in fall. The understory, shrub, and herb layers may be dense. The herb layer has two growing periods each year. Plants of the first growth appear in early spring, before the trees have developed new leaves. These plants die by summer and are replaced by plants that grow in the shade of the leafy canopy.

Large animals of the temperate deciduous forests include bears, deer, and, rarely, wolves. These forests are also the home of hundreds of smaller mammals and birds. Many of the birds migrate south in fall, and some of the mammals hibernate during the winter.

Some temperate areas support mixed deciduous and evergreen forests. In the Great Lakes region of North America, for example, the cold winters promote the growth of heavily mixed forests of deciduous and evergreen trees. Forests of evergreen pine and deciduous oak and hickory grow on the dry coastal plains of the Southeastern United States.

Temperate evergreen forests. In some temperate regions, the environment favors the growth of evergreen forests. Such forests grow along coastal areas that have mild winters with heavy rainfall. These areas include the northwest coast of North America, the south coast of Chile, the west coast of New Zealand, and the southeast coast of Australia. Temperate evergreen forests also cover the lower mountain slopes in Asia, Europe, and western North America. In these regions, the cool climate favors the growth of evergreen trees.

The strata and the plant and animal life vary greatly from one temperate evergreen forest to another. For example, the mountainous evergreen forests of Asia, Europe, and North America are made up of conifers. The coastal forests of Australia and New Zealand, on the other hand, consist of broadleaf evergreen trees.

Boreal forests are found in regions that have an extremely cold winter and a short growing season. The word boreal means northern. Vast boreal forests stretch across northern Europe, Asia, and North America. Similar forests also cover the higher mountain slopes on these continents.

Boreal forests, which are also called taiga, have the simplest structure of all forest formations. They have only one uneven layer of trees, which reaches up to about 75 feet (23 meters) high. In most of the boreal forests, the dominant trees are needleleaf evergreens-either spruce and fir or spruce and pine. The shrub layer is spotty. However, mosses and lichens form a thick layer on the forest floor and also grow on the tree trunks and branches. There are few herbs.

Many small mammals, such as beavers, mice, porcupines, and snowshoe hares, live in the boreal forests. Larger mammals include bears, caribou, foxes, moose, and wolves. Birds of the boreal forests include ducks, loons, owls, warblers, and woodpeckers.

Savannas are areas of widely spaced trees. In some savannas, the trees grow in clumps. In others, individual trees grow throughout the area, forming an uneven, widely open canopy. In either case, most of the ground is covered by shrubs and herbs, especially grasses. As a result, some biologists classify savannas as grasslands. Savannas are found in regions where low rainfall, poor soil, frequent fires, or other environmental features limit tree growth. 

The largest savannas are tropical savannas. They grow throughout much of Central America, Brazil, Africa, India, Southeast Asia, and Australia. Animals of the tropical savannas include giraffes, lions, tigers, and zebras.

Temperate savannas, also called woodlands, grow in the United States, Canada, Mexico, and Cuba. They have such animals as bears, deer, elk, and pumas.

Source : World Book 2005