Although forests are classified primarily by trees, the concept of a forest ecosystem includes additional species (such as smaller plants, fungi, bacteria, and animals) as well as physical and chemical processes such as energy flow and nutrient cycling.
A typical forest is composed of the over story (canopy or upper tree layer) and the understory.
The understory is further subdivided into the shrub layer, herb layer, and also the moss layer and soil microbes. In some complex forests, there is also a well-defined lower tree layer. Forests are central to all human life because they provide a diverse range of resources: they store carbon, aid in regulating the planetary climate, purify water and mitigate natural hazards such as floods. Forests also contain roughly 90 percent of the world’s terrestrial biodiversity. Forests are habitats in which the trees are the dominant form of vegetation.
They occur in many regions and climates around the globe—the tropical rainforests of the Amazon basin, the temperate forests of eastern North America, and the boreal forests of northern Europe are just a few examples.
The species composition of a forest is often unique to that forest, with some forests consisting of many hundreds of species of trees while others consist of just a handful of species. Forests are constantly changing and progress through a series of succession stages during which species composition changes within the forest. Thus, making general statements about forest habitats can be difficult.
Yet despite the variability of our planet’s forests, there are some basic structural characteristics that many forests share—characteristics that can help us to better understand both forests and the animals and wildlife that inhabit them. The forest floor is often blanketed with decaying leaves, twigs, fallen trees, animal scat, moss, and other detritus. The forest floor is where recycling occurs, fungi, insects, bacteria, and earthworms are among the many organisms that break down waste materials and ready them for reuse and recycling throughout the forest system.
The herb layer of the forest is dominated by herbaceous (or soft-stemmed) plants such as grasses, ferns, wildflowers, and other ground cover. Vegetation in the herb layer often gets little light and in forests with thick canopies, shade tolerant species are predominant in the herb layer. The understory of a forest consists of immature trees and small trees that are shorter than the main canopy level of the tree. Understory trees provide shelter for a wide range of animals. When gaps form in the canopy, often times understory trees take advantage of the opening and grow o fill in the canopy.
The canopy is the layer where the crowns of most of the forest’s trees meet and form a thick layer. Emergent are trees whose crowns emerge above the rest of the canopy. Many systems are used to classify the world’s forests. Some systems classify a forest according to the characteristics of its dominant trees. A needle leaf 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. 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 emergent, 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. 1 Norman Myers, “The World’s Forests and Their Ecosystem Services,” Nature’s Services: Societal Dependence on Natural Ecosystems, Washington D. C, Island Press, 1997, p. 159. 2 Ibid, p. 159 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 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. 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. 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 needle leaf 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. 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 rainfall, poor soil, frequent fires low, or other environmental features limit tree growth. The latitudes 10° north and south of the Equator are mostly covered in tropical rainforest, and the latitudes between53°N and 67°N have boreal forest.
As a general rule, forests dominated by angiosperms (broadleaf forests) are more species-rich than those dominated by gymnosperms (conifer, montane, or needle leaf forests), although exceptions exist. 3N. Myers. “The World’s Forests and Their Ecosystem Services. ” Chapter 12 in G. C. Daily, ed. , Nature’s Services: Societal Dependence on Natural Ecosystems. Washington D. C. : Island Press, 1997 4W. H. Schlesinger. Biogeochemistry: An Analysis of Global Change. New York, NY: Academic Press, 1997. p. 79.
Forests sometimes contain many tree species only within a small area (as in tropical rain and temperate deciduous forests), or relatively few species over large areas (e. g. , taiga and arid montane coniferous forests). Forests are often home to many animal and plant species, and biomass per unit area is high compared to other vegetation communities. Much of this biomass occurs below ground in the root systems and as partially decomposed plant detritus. The woody component of a forest contains lignin, which is relatively slow to decompose compared with other organic materials such as cellulose or carbohydrate.
Forests are differentiated from woodlands by the extent of canopy coverage: in a forest, the branches and the foliage of separate trees often meet or interlock, although there can be gaps of varying sizes within an area referred to as forest. Woodland has a more continuously open canopy, with trees spaced farther apart, which allows more sunlight to penetrate to the ground between them. Succession is the natural replacement of plant or animal species, or species associations, in an area over time. When we discuss forest succession, we are usually talking about replacement of tree species or tree associations.
Each stage of succession creates the conditions for the next stage. Temporary plant communities are replaced by more stable communities until a sort of equilibrium is reached between the plants and the environment. Foresters and ecologists have long-known that the growth of forests decreases as they age; however, the causes for the age-related decline have remained a mystery until recently. What is emerging is an interesting story that suggests the decline in forest growth, and other age-related functional changes, are because of the changes in stand structure.
Most notable is the dramatic changes in the nutrient cycles of forests during succession because of the changes in litter quality. Except for forests growing in heavily polluted areas, forests derive the bulk of their annual requirement of nutrients from minerals released from decomposing leaves, branches, stems and roots. During the early stages of succession a high proportion of the litter is comprised of leaf tissue which, compared to branches and stems, is more easily decomposed by decomposers because of its greater nutrient concentration.
In the later stages of succession however, the annual production of tissue falling to the forest floor is comprised of more woody tissue (e. g. branches and stems resulting from the self-thinning stage). Woody tissue decomposes slower than foliage by a factor of 10 to 100, resulting in nutrients being sequestered (locked up) for decades in the branches, twigs and stems. 5W. H. Schlesinger. Biogeochemistry: An Analysis of Global Change. New York, NY: Academic Press, 1997. p. 79. 6S. Brown, J Sathaye, M. Cannell, and P. E. Kauppi. Mitigation of forests for mitigation of greenhouse gas emissions” in Working Group II, Second Assessment Report, Intergovernmental Panel on Climate Change. Cambridge University Press. Chapter 24. The term deforestation is often misused to describe any activity where all trees in an area are removed. However in temperate climates, the removal of all trees in an area—in conformance with sustainable forestry practices—is correctly described as regeneration harvest. In temperate mesic climates, natural regeneration of forest stands often will not occur in the absence of disturbance, whether natural or anthropogenic.
Furthermore, biodiversity after regeneration harvest often mimics that found after natural disturbance, including biodiversity loss after naturally occurring rainforest destruction. Deforestation occurs for many reasons: trees or derived charcoal are used as, or sold, for fuel or as timber, while cleared land is used as pasture for livestock, plantations of commodities, and settlements. The removal of trees without sufficient reforestation has resulted in damage to habitat, biodiversity loss and aridity.
It has adverse impacts on biosequestration of atmospheric carbon dioxide. Deforestation has also been used in war to deprive an enemy of cover for its forces and also vital resources. A modern example of this, for example, was the use of Agent Orange in Vietnam. Deforested regions typically incur significant adverse soil erosion and frequently degrade into wasteland. Disregard or ignorance of intrinsic value, lack of ascribed value, lax forest management and deficient environmental laws are some of the factors that allow deforestation to occur on a large scale.
In many countries, deforestation, both naturally occurring and human induced, is an ongoing issue. Deforestation causes extinction, changes to climatic conditions, desertification, and displacement of populations as observed by current conditions and in the past through the fossil record. 7″Earth Heating Up at a Record Pace” in The New York Times. June 8, 1998. 8 X. Xiao, et al. “Transient Climate Changes and Net Ecosystem Production of the Terrestrial Biosphere” in Global Biogeochemical Cycles 12(2), 1998. pp. 345-360. 9 R. Condit, et al. Changes in Tree Species Abundance in a Geotropically Forest: Impact of Climate Change” in Journal of Tropical Ecology, 12 (part 2). pp. 231-256.
Through the centuries it had accumulated tests about how the human intervention can produce innumerable Earth damages. One of the forms that that intervention assumes, the deforestation, has seriously affected the forests of the planet. About 10,000 years ago – before the beginning of agriculture – that type of biome extended on about 4,200 million hectares, the two third parts of the terrestrial surface.
Today, in extensive regions of Asia, Europe and North America the natural forests had disappear, and the deforestation threatens to one of the most extensive forest, the Amazonian. The deforestation, which consists on the destruction of forests by burning or cutting them, goes accompanied by the technological progress, that creates new and more serious problems as well. The fire and the necessity to qualify the ground for agriculture and the pasturing devastate great forest spaces, but in the long term the grounds are eroded and impoverished by the absence of protective natural vegetation.
In America there were great wooded extensions to the arrival of the Spaniards. The action of more and more destructive technologies did that at the present time only one small part of the territory is covered by forests. The rest was destroyed to obtain firewood and wood of construction and for the manufacture of furniture, or simply burned to disassemble the grounds that could be used in the agricultural production. That way, complete populations of certain species were considerably reduced.