Forests are broadly classified by latitude as either tropical, temperate, or boreal. Tropical forests, or rainforests, are predominantly evergreen and occur close to the equator, in areas with plentiful rain and little temperature variation year-round. There are tropical forests in Central and South America, Africa, South and Southeast Asia, and Australia. Tropical forests are characterized by the greatest diversity of biological species. For example, as many as 100 distinct tree species may inhabit a square kilometer (about 0.38 square miles). Vegetation is often so dense in tropical forests that very little light penetrates to the ground.
Temperate forests are found in areas with distinct warm and cold seasons, including North America, northern Asia, and western and central Europe. Many temperate forests are made up of deciduous trees—species that shed their leaves during winter. Plant diversity is not as great in temperate forests as in rainforests. There are perhaps three or four tree species per square kilometer.
Boreal forests, also known as taiga, are found at high latitudes in extremely cold climates where the growing season is short. Precipitation generally falls as snow rather than rain. Boreal forest flora include evergreen trees and lichen ground cover. Boreal forests are present in Siberia, Scandinavia, Alaska, and Canada.
North American Forests
Many U.S. forests are highly imperiled. One of the greatest threats to forests is deforestation via clear cutting, a method of logging in which all the trees in an area are cut. Serious damage to the old-growth forests of the Pacific Northwest, for example, is visible from National Aeronautics and Space Administration (NASA) satellite photos. Old-growth forests harbor many unique species, including numerous species that are threatened or endangered. An alternative to clear-cutting is selective management, in which only some trees are removed from an area. Even selective management practices, however, frequently deplete forests more quickly than they are able to recover. The lumber industry continues to battle with environmentalists and the U.S. Forest Service over the right to log national forest lands, including the unique redwood forests of the West Coast.
Huge forest fires raged through the western United States in 2000 and 2002. The Forest Service reported that these were two of the worst fire seasons in over fifty years. In 2002 forest fires scorched over seven million acres and caused over $1.7 billion in damages. The fires were partly the result of long decades of fire suppression. In response, President Bush announced the "Healthy Forest Initiative" in 2002. This initiative was immediately attacked by conservationists, who claimed that its only aim was to roll back federal regulations on logging, and that it was intended to benefit logging companies rather than to protect people or wildlife. Conservationists further argued that the Bush Administration was merely using the forest fires as an excuse for forwarding its pro-business/anti-environment agenda.
In addition to logging and fire risk, there are several other major threats to forests. These threats are highlighted by the U.S. Forest Service in America's Forests: 2003 Health Update (2003, http://www.fs.fed.us/publications/documents/foresthealthupdate2003.pdf). These threats include:
- Invasive insects and pathogens. Sudden Oak Death, caused by a new, unidentified pathogen, has killed thousands of oak and other species in coastal forests, mixed evergreen forests, and urban-wildland interfaces in California and southern Oregon. White pine blister rust is a nonnative fungus from Asia that has killed white pine trees in the western United States and Canada. The gypsy moth, first introduced from native habitats in Europe and Asia in the 1800s, continues to damage eastern U.S. forests. The hemlock woolly adelgid, native to Asia and introduced in the 1920s, continues to kill hemlock trees in the eastern United States.
- Invasive plants. About 1,400 species of nonnative plants are recognized as pest species that threaten forests and grasslands. Table 3.1 lists some of the most common ones. Invasive plant species currently affect over 100 million acres of U.S. forestland. The Forest Service spends about $16 million annually in preventing the spread of invasive plants such as the "mile-a-minute" weed, which infests northeastern forests, and leafy spurge, which affects ecosystems in southern Canada and the northern United States.
- Outbreaks of native insects. Certain native insects, including bark beetles, mountain pine beetles, and southern pine beetles, can also lay waste to native forests when they occur in large outbreaks.
TONGASS NATIONAL FOREST
During the first millennium CE, an expanse of ancient forest flourished along the entire western coast of the United States and Canada. Today, a portion of this habitat, a 500-mile expanse along the southeastern coast of Alaska, has been preserved as Tongass National Forest. Tongass National Forest represents an unblemished stretch of trees and other wildlife that has existed as a completely intact ecosystem for over a thousand years. It includes nearly eighteen million acres of virtually pristine woodland. The Tongass preserve comprises about one-fourth of the world's temperate rainforest and is the largest on earth.
In the mid-twentieth century, however, the federal government began to negotiate with logging companies to open small portions of the ancient forest for clear-cutting. This has generated ongoing debate in Congress. In the 1990s loggers appealed to the government to open more access roads to facilitate logging, whereas environmentalists fought to preserve the area from human tampering altogether. In May 2000 the National Forest Service drafted a proposal urging renewed protection of roadless areas. A successful lawsuit brought by the Natural Resources Defense Council in 2003 appeared to protect Tongass and other roadless national forests from logging. However, the Bush Administration exempted Tongass from these protections and has continued attempts to open more acreage of the forest to logging.
In June 2005 an amendment was voted down by the U.S. Senate that would have banned federal subsidies for new roads in Tongass. Proponents of the amendment argued that taxpayers should not have to pay for building roads that allow timber producers better access to old-growth trees in the forest. Critics charged that the roads serve many purposes, including the enhancement of tourism, recreational activities, and access by Forest Service rangers. At that time approximately 4% of the Tongass National Forest was open to logging.
Deforestation
Deforestation refers to the destruction of forests through the removal of trees, most often by clear-cutting or burning. It results in habitat loss for countless species of plants as well as animals. Deforestation is occurring globally, but is proceeding at a particularly alarming rate in the world's tropical rainforests, which comprise the most diverse ecosystems in the world. Deforestation is one of the most pressing environmental issues today.
In addition to destruction of habitat for numerous plant and animal species, the loss of forests has other effects as well. For example, forests play a crucial role in the global cycling of carbon—vegetation stores two trillion tons of carbon worldwide, roughly triple the amount stored in the atmosphere. When forest trees are cleared, the carbon they contain is oxidized and released to the air, adding to the carbon dioxide in the atmosphere. The burning of the Amazon rainforests and other forests thus has a twofold effect—the release of large amounts of carbon dioxide into the atmosphere and the loss of the trees that help absorb carbon dioxide.
TABLE 3.1 Invasive plants
| TABLE 3.1 | |||
|---|---|---|---|
| Invasive plants | |||
| Species | Common name | Species | Common name |
| SOURCE: Adapted from "Invasive Plants List," in Fire Effects Information System: Invasive Plants, U.S. Department of Agriculture, U.S. Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Undated, http://www.fs.fed.us/database/feis/plants/weed/index.html (accessed March 8, 2006) | |||
| Acer platanoides | Norway maple | Imperata cylindrica | Cogon grass |
| Acroptilon repens | Russian knapweed | Kochia scoparia | Summer-cypress |
| Agropyron cristatum | Crested wheatgrass | Lepidium latifolium | Perennial pepperweed |
| Agropyron desertorum | Desert wheatgrass | Lespedeza bicolor | Bicolor lespedeza |
| Agrostis gigantea | Redtop | Lespedeza cuneata | Sericea lespedeza |
| Ailanthus altissima | Tree-of-heaven | Lespedeza striata | Common lespedeza |
| Alliaria petiolata | Garlic mustard | Ligustrum amurense | Amur privet |
| Amaranthus retroflexus | Rough pigweed | Ligustrum japonicum | Japanese privet |
| Artemisia abrotanum | Oldman wormwood | Ligustrum sinense | Chinese privet |
| Artemisia absinthium | Absinth wormwood | Ligustrum vulgare | European privet |
| Artemisia dracunculus | Tarragon | Linaria dalmatica | Dalmatian toadflax |
| Arundo donax | Giant reed | Linaria vulgaris | Yellow toadflax |
| Bromus hordeaceus | Soft chess | Lolium perenne | Perennial ryegrass |
| Bromus inermis | Smooth brome | Lolium multiflorum | Italian ryegrass |
| Bromus japonicus | Japanese brome | Lonicera × bella | Bell's honeysuckle |
| Bromus madritensis | Foxtail chess, red brome | Lonicera fragantissima | Winter honeysuckle |
| Bromus tectorum | Cheatgrass | Lonicera japonica | Japanese honeysuckle |
| Calluna vulgaris | Heather | Lonicera maackii | Amur honeysuckle |
| Cardaria chalapensis | Lens-podded hoary cress | Lonicera morrowii | Morrow's honeysuckle |
| Cardaria draba | Heart-podded hoary cress | Lonicera tatarica | Tatarian honeysuckle |
| Cardaria pubescens | Globe-podded hoary cress | Lonicera xylosteum | European fly honeysuckle |
| Carduus nutans | Musk thistle | Lygodium japonicum | Japanese climbing fern |
| Casuarina cunninghamiana | River sheoak | Lygodium microphyllum | Old World climbing fern |
| Casuarina equisetifolia | Australian-pine | Lythrum salicaria | Purple loosestrife |
| Casuarina glauca | Gray sheoak | Medicago sativa | Alfalfa |
| Celastrus orbiculatus | Oriental bittersweet | Melaleuca quinquenervia | Melaleuca |
| Centaurea diffusa | Diffuse knapweed | Melilotus alba | White sweetclover |
| Centaurea maculosa | Spotted knapweed | Melilotus officinalis | Yellow sweetclover |
| Centaurea solstitialis | Yellow starthistle | Microstegium vimineum | Nepalese browntop |
| Chondrilla juncea | Rush skeletonweed | Phalaris arundinacea | Reed canarygrass |
| Cirsium arvensis | Canada thistle | Phleum pratense | Timothy |
| Cirsium vulgare | Bull thistle | Poa pratensis | Kentucky bluegrass |
| Convolvulus arvensis | Field bindweed | Potentilla recta | Sulfur cinquefoil |
| Cynodon dactylon | Bermuda grass | Psathyrostachys juncea | Russian wildrye |
| Cynoglossum officinale | Houndstongue | Pueraria montana var. lobata | Kudzu |
| Cytisus scoparius | Scotch broom | Rosa multiflora | Multiflora rose |
| Cytisus striatus | Portuguese broom | Rubus discolor | Himalayan blackberry |
| Dactylis glomerata | Orchard grass | Rubus laciniatus | Evergreen blackberry |
| Descurainia sophia | Flixweed tansymustard | Rumex acetosella | Sheep sorrel |
| Echinochloa crus-galli | Barnyard grass | Salsola kali | Russian-thistle |
| Elaeagnus angustifolia | Russian-olive | Schinus terebinthifolius | Brazilian peppertree |
| Elaeagnus umbellata | Autumn-olive | Sisymbrium altissimum | Tumblemustard |
| Elytrigia repens | Quackgrass | Sonchus arvensis | Perennial sowthistle |
| Eragrostis curvula | Weeping lovegrass | Sorghum halepense | Johnson grass |
| Eragrostis lehmanniana | Lehmann lovegrass | Spartium junceum | Spanish broom |
| Eremochloa ophiuroides | Centipede grass | Taeniatherum caput-medusae | Medusahead |
| Erodium cicutarium | Cutleaf filaree | Tamarix aphylla | Athel tamarisk |
| Eucalyptus globulus | Bluegum eucalyptus | Tamarix chinensis | Saltcedar |
| Euphorbia esula | Leafy spurge | Tamarix gallica | French tamarisk |
| Festuca arundinaceum | Tall fescue | Tamarix parviflora | Small-flowered tamarisk |
| Genista monspessulana | French broom | Tamarix ramosissima | Saltcedar |
| Halogeton glomeratus | Halogeton | Taraxacum officinale | Dandelion |
| Hypericum perforatum | St. Johnswort | Triadica sebifera | Tallowtree |
| Imperata brasiliensis | Brazilian satintail | Xanthium strumarium | Common cocklebur |
Furthermore, deforestation also results in forest fragmentation, which is itself detrimental for several reasons. First, forest fragmentation creates more "edge" habitats and destroys habitat for deep-forest creatures. Second, fragmentation isolates plant and animal populations, making them more vulnerable to local extinction. Third, some nonnative species thrive in edge habitats, and are able to invade and displace native species in a fragmented habitat. In North America, for example, songbirds like the wood thrush and the promontory warbler are declining due to increasing numbers of blue jays and parasitic brown-headed cowbirds, both of which flourish at forest edges. Finally, most trees are more susceptible to weather at forest edges.
Rainforests
Tropical forests are found in a band along the equator from Mexico into South America and across the Caribbean, central Africa, and parts of Asia. These forests include both lowland and upland (hill and mountainous) formations and vary in vegetative type. Most tropical forests are either dry or moist and are composed of deciduous vegetation that sheds its leaves at the end of the growing season. Lying closest to the equator are the tropical rainforests. They experience very warm temperatures year round and high amounts of precipitation. The vegetation is thick and lush and characterized by a large number of flowering and fruit-producing trees and vines. The largest concentration of tropical rainforests occurs in northern South America. However, they are also found in west central Africa and scattered across parts of Southeast Asia and northernmost Australia. Tropical rainforests are the world's most biologically rich habitats and are estimated to harbor 50% to 90% of the world's species. Biologists believe that many rainforest species have yet to be discovered and described by humans. In February 2006, the BBC reported that an international team of scientists had found a previously unknown jungle in Indonesia containing new species of butterflies, frogs, birds, and plants (http://newsvote.bbc.co.uk/mpapps/pagetools/print/news.bbc.co.uk/1/hi/sci/tech/4688000.stm).
Throughout the twentieth century tropical forests were depleted by logging and clearing for farms and ranches. The United Nations Food and Agriculture Organization (FAO) published an assessment of the world's tropical forests in Forest Resources Assessment 1990: Survey of Tropical Forest Cover and Study of Change Processes (1993). The FAO reported at that time that tropical forests covered 4.34 billion acres at the end of 1990, down from 4.72 billion acres at the end of 1980. The highest rates of deforestation were reported in Africa, Asia, Central America, and Mexico. During the 1980s an average of approximately thirty-eight million acres per year was deforested. The deforestation rate for tropical rainforests was eleven million acres per year. In an update, the FAO estimated that all tropical forests were deforested at an average of 22.7 million acres per year during the 1990s (Global Forest Resources Assessment 2000, 2005, http://www.fao.org/documents/show_cdr.asp?url_file1/4/DOCREP/004/Y1997E/ Y1997E00.HTM). The deforestation rate for tropical rainforests was 14.8 million acres per year.
The major underlying causes of tropical deforestation are economic—underdevelopment, unemployment, and poverty among the growing populations of tropical countries. Unrestricted by enforceable regulations, farmers clear forests to create meager cropland that is often useless three years after its conversion—this is because tropical forest soils are poor, because almost all available nutrients are locked up in the trees and other biomatter. Logging and the conversion of forestland to unsustainable, short-term agricultural use have resulted in the destruction of habitats, declining fisheries, erosion, and flooding. Forest loss also disrupts regional weather patterns and contributes to global climate change. Finally, it eliminates plant and animal species that may serve important medical, industrial, and agricultural purposes. However, arguments for protective measures that might not reap economic benefits for many decades are of little interest to farmers with families to feed. Developing countries frequently voice resentment over what they see as the hypocrisy of industrialized nations, which invariably engaged in similarly destructive practices to build their own economies.
Conservation of tropical forests presents a considerable challenge. The creation of protected areas alone has often proven ineffectual, mostly because the people who exploit forests are given no other options for meeting their economic needs. Many conservationists have started to focus on the promotion of sustainable development within rainforests. Agroforestry describes an agricultural strategy that involves the maintenance of diversity within developed tropical forest areas. This includes planting many different types of crops in patches that are mixed in among grazing lands and intact forest. Agroforestry often focuses on crops that produce goods for an indefinite period of time, including citrus fruits, bananas, cacao, coffee, and rubber.
Agroforestry can help to maintain soil quality as well as tropical biodiversity, allowing for a sustained productivity that makes it unnecessary to clear more and more areas of forest. In addition, rainforest conservationists have promoted the harvest of sustainable rainforest products, rather than unsustainable products such as timber. Sustainable harvests include those of medicines, food, and rubber.
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