Landsat data show the effects of deforestation
nearthe town of Ariquemes, Rondonia, Brazil
between 1975 and 1992

Global
Deforestation

"Here goes lumber from the Maine woods ... pine, spruce, cedar, - first, second, third, and fourth qualities, so lately all of one quality, to wave over the bear, and moose, and caribou..."
- Henry David Thoreau, 1845.

01/04/2010

Format for printing

 

In this lecture period, we wish to learn:

  • What do we mean by "deforestation"?
  • How have the old-growth forests been affected by humans?
  • What are the consequences of loss of forest habitats and ecosystems?
  • What management strategies are in place to preserve, manage and restore forests?

 

1. Introduction

It is impossible to overstate the importance of humankind's clearing of the forests. The transformation of forested lands by human actions represents one of the great forces in global environmental change and one of the great drivers of biodiversity loss. The impact of people has been and continues to be profound. Forests are cleared, degraded and fragmented by timber harvest, conversion to agriculture, road-building, human-caused fire, and in myriad other ways. The effort to use and subdue the forest has been a constant theme in the transformation of the earth, in many societies, in many lands, and at most times.  Deforestation has important implications for life on this planet.

Just think, originally, almost half of the United States, three-quarters of Canada, almost all of Europe, the plains of the Levant, and much of the rest of the world were forested. The forests have been mostly removed for fuel, building materials and to clear land for farming. The clearing of the forests has been one of the most historic and prodigious feats of humanity.

Area of Forest Ecosystems (World total:~ 34 million km2)

About one half of the forests that covered the Earth are gone. Each year, another 16 million hectares disappear. The World Resources Institute estimates that only about 22% of the world's (old growth) original forest cover remains "intact" - most of this is in three large areas: the Canadian and Alaskan boreal forest, the boreal forest of Russia, and the tropical forest of the northwestern Amazon Basin and the Guyana Shield (Guyana, Suriname, Venezuela, Columbia, etc.)

Today, forests cover more than one quarter of the world's total land area, excluding polar regions. Slightly more than 50% of the forests are found in the tropics and the rest are temperate and boreal (coniferous northern forest) zones. 

Seven countries (Russia, Brazil, Canada, the United States, China, Indonesia, and the Democratic Republic of Congo (formerly Zaire) account for more than 60% of the total.

For millennia, humankind has influenced the forests, although much of the impact has been relatively minor. Today, the impact is enormous. Deforestation is expanding and accelerating into the remaining areas of undisturbed forest, and the quality of the remaining forests is declining.  Today we examine global patterns in deforestation, assess the human and ecological costs of forest loss, and discuss some of the steps that can help to rectify this alarming situation.

Frontier Forests of the World

Red = Frontier Forests, 8,000 years ago
Green = Frontier Forests Today
Pink = Current non-frontier forests

Historical trends:

Until quite recently, most of the deforestation occurred in Europe, North Africa, and the Middle East. By the beginning of this century, these regions had been mostly converted from the original cover. Now, deforestation in these regions has stabilized and regrowth is occurring (though second growth forests have quite different character, see below). In the last few decades, the vast majority of deforestation has occurred in the tropics - and the pace still accelerates. The removal of tropical forests in Latin America is proceeding at a pace of about 2% per year. In Africa, the pace is about 0.8% per year and in Asia it is 2% per year.

The USA has already experienced its wave of deforestation, with the exception of small areas in the west and Alaska.  Our old growth forests were mostly harvested by 1920, particularly in the East.  Pacific Northwest forests and UP Michigan forests were heavily cut after 1920 until quite recently, and harvest of old growth continues today in Southeast Alaska.  Interestingly, deforestation rates at their peak in the Midwest were ~2% annually, about the rates now seen in Amazonia.  At that rate, how much of existing forest will remain in 70 years?  Just one-fourth.  However, much forest re-growth has occurred in the eastern USA during the 20th Century, although these second-growth forests differ in structure and composition from their predecessors.

Area of primary forests in the United States (lower 48)
(around 1620, top; and 1850 middle; 1920, bottom)

Since 1600, 90% of the virgin forests that once covered much of the lower 48 states have been cleared away.  Most of the remaining old-growth forests in the lower 48 states and Alaska are on public lands.  In the Pacific Northwest about 80% of this forestland is slated for logging.   

 

Some Definitions

  • Deforestation: The conversion of forest to another land use or the long-term reduction of the tree canopy cover below a 10 percent threshold. Deforestation implies the long-term or permanent loss of forest cover and its transformation into another land use.
     
  • Primary forest: is a forest that has never been logged and has developed following natural disturbances and under natural processes, regardless of its age.
     
  • Secondary forests: are forests regenerating largely through natural processes after significant human or natural disturbance, and which differ from primary forests in forest composition and/or canopy structure.
     
  • Disturbed forests: Any forest type that has in its interior significant areas of disturbance by people, including clearing, felling for wood extraction, anthropogenic fires, road construction, etc.
     
  • Frontier forests:  large, ecologically intact, and relatively undisturbed forests that support the natural range of species and forest functions (WRI definition).
     
  • Forest plantation is one established by planting or/and seeding in the process of afforestation or reforestation. It consists of introduced species or, in some cases, indigenous species.

 

2. Importance of Forests

Forests provide important products for human use and consumption, and they provide valuable ecosystem services.  Let's look at each in turn.

Forest Products

  • In poor areas where wood is scarce, people, usually women, walk long distances to gather wood for cooking.
     

  • Forests provide useful wood products.  Roundwood (whole logs) can be processed into building materials, or made into plywood products, furniture, etc.  Pulp is used not only for paper and boxes, but for a wide variety of products (including the “sponge” you used to wash your dishes). 
     

  • Forests are the source of numerous non-wood products, including bark, dyes, fibers, gums, incense, latexes, oils, resins, shellac, tanning compounds & waxes.  Fruits, nuts and berries are harvested as food. Maple syrup is an example of a unique non-wood product from the sap of the maple tree.

Ecosystem Services

  • Forests influence climate.  The within-year fluctuations in atmospheric CO2 in the temperate zone include a spring-through-autumn decline due to plant photosynthesis during the growing season, and an autumn-through-spring rise in CO2 as respiration and decomposition exceed photosynthetic uptake.  At a more regional scale, forests influence local climate and weather.  Rain forests transport great quantities of water to the atmosphere via plant transpiration. (Water is taken up by plant roots, bringing dissolved minerals into plant tissues.  Plants exchange gases with the atmosphere through openings in their leaves, and lose water in the same way.  That water loss provides the plant with a means to transport materials upwards, and so is beneficial, so long as water loss is not excessive).  Much of that transpired water replenishes the clouds and rain that maintain the rain forest.  If the forest is cut, much more of that rain will become river water, flow to distant seas, and the region will become permanently drier.  No rain forest can regenerate if this occurs.  Forests maintain local climate and strongly influence global fluxes of oxygen and carbon dioxide.  Before green plants appeared, it is believed that there was very little oxygen in the atmosphere.
     

  • Forests protect the top soil and husband important nutrients.  A famous study of Hubbard Brook, New Hampshire found that, after forest harvest, summer streamflows greatly increased (because the forest was no longer transpiring water) and nutrient outflow also increased greatly.  The annual flood crest of the Amazon River has increased over recent years without any concomitant increase in rainfall, presumably due to deforestation.  Damaging floods are one frequent consequence of deforestation.
     

  • Forests harbor tremendous biological diversity, and have the potential to provide us with new crop varieties and medicines. A good example of medicinal use of tropical rain forest plants is the success of the drugs vincristine and vinblastine, developed over the past 20 years from a wild periwinkle found in the forests of Madagasgar. These drugs dramatically improved the effectiveness of treatments for leukemia and other forms of cancer, Since fewer than 1% of tropical plants have been screened for possible use to medical science, ongoing deforestation results in the permanent loss to science of other species before their value can be recognized.  The winged bean is a new food crop whose value has only recently has been recognized.

Carbon Sequestration

Tropical deforestation contributes as much as 90% of the current net release of biotic carbon dioxide into the atmosphere. This change may represent as much as 20% - 30% of the total carbon flux due to humans - i.e., rivaling the carbon release due to fossil fuel burning.  Deforestation thus is an important potential source of carbon.  But what if we allow forests to regenerate?  As they grow, forests will store or sequester carbon, and so carbon sequestration has become part of the global warming debate.  What is the current balance sheet – are the world’s forests a source or a sink for atmospheric CO2?  This is uncertain for three main reasons.  We are not sure how much forest is being burnt, vs the amount of regrowth.  We don’t know enough about the fate of deforested land, ie, how much is reverting to secondary forest.  We don’t know how forest disturbance is affecting soil and forest floor carbon stores. Still, there is good evidence that the regrowth of previously-deforested areas in Europe and North America during the 20th century has sequestered considerable amounts of CO2 from the atmosphere.

Recovery time of a forest after clearing and a burn. Note that it is only after 100+ years that forest become as they were before the cut. Forest regrowth sequesters atmospheric carbon as plant biomass.


3. Tropical Deforestation

The world’s tropical forests are disappearing at an alarming rate. A recent estimate is that about 100,000 km2 are deforested each year, and another 100,000 km2 are degraded.  Estimates are constantly improving, based on satellite imagery, and deforestation rates change in response to social and economic conditions, as well as quality and accessibility of remaining forest.  NASA's Landsat (satellite) cannot see below the forest canopy, and so cannot detect below-canopy clearing, whereas radar remote sensing can detect, eg, a coffee plantation beneath overstory trees.

  • Tropical forests once occupied 16 million km2, today about 8-9 million km2 remain
  • It is estimated that Latin America and Asia have already lost 40% of their original forest; Africa a little more than half. 
  • In many countries the rate of deforestation is accelerating. For example, most of the forested areas of Bangladesh, India, the Philippines, Sri Lanka and parts of Brazil's rain forest could be gone by the end of the century.

Only in the Congo Basin and some of the more isolated areas of the Amazon Basin does the forest remain largely intact.

 

4. Brazil's Amazonian Forests

Brazil contains about 3.5 million km2 of tropical forest. This is equivalent to 30% of the world's total.  Almost all of Brazil's standing tropical forests are in the Amazon Basin, a region commonly known as Amazonia. About half of Amazonia comprises upland areas in which the original vegetation was tropical rain forest. Perhaps another 0.5 million km2 consist of transitional forests. In addition, there are large areas of savanna in the southern reaches of Amazonia.

Biological Diversity of the Amazon

  • Amazonia has been characterized as the "single richest region of the tropical biome."
  • A single hectare of rain forest near Manaus yielded 235 tree species over 5 cm in diameter and 179 species over 15 cm in diameter.
  • There are 2000 known species of fish in the waters of the Amazon Basin. This is eight times the number found in the Mississippi River system and 10 times the number found in all of Europe.

Satellite data showing the amount of land cover classified as Deforested at 4 dates between 1975 and 1988.  Note how much higher deforestation rates were in 1988 compared to earlier.

Roads

Roads usually accompany timber harvest, in order to move logs to sawmills and markets.  Even when tree harvest is highly selective, and much of the forest remains, it has been found that the roads themselves have numerous adverse side-effects.  As forests become more open through thinning, they become drier, and more susceptible to fire.  In wet areas roads become pathways for surface runoff, and carry sediments into streams, destroying aquatic life.  Culverts installed where roads cross rivers often block fish passage, and have devastated salmon populations in the western USA.  In tropical forests, roads allow hunters and poachers much greater access, resulting in the large and very serious bushmeat trade that is emptying tropical forests of their wildlife.

Forest fragmentation by roads in Central Africa. This study shows that 42% of forest area in the six countries is within 10 km of a road and more than 90% is within 50 km of a road (Source: WRI Earthtrends).


5.  Causes of Deforestation

Deforestation has many causes.  Population pressures, profits, and internal social and political forces can all push up the rate of forest loss.  Access to markets, requiring roads and capital, is an additional powerful force, recently expanded due to the suite of changes referred to as globalization.  Poor countries with expanding populations, inequitable distribution of wealth and power, and possibly corrupt governments are especially vulnerable.

  • In Indonesia, powerful families allied with government rulers control large and highly valuable timber concessions.  These forests are being rapidly liquidated, at enormous profit.
     

  • In Brazil, many of the rural poor are moving to cities for work, and not finding it.  Productive farmland is controlled by a wealthy elite with a long history of land ownership, and so many of the rural poor are landless.  By opening its frontier – the Amazon forest - to its landless poor, Brazil seeks to provide a safety valve for what otherwise might be an explosive political situation.
     

  • In many areas, poor people have few options to make income, and forests have few protectors, and so land is cleared for agriculture and valuable timber is sold for profit.

6. Forest Management and Recovery

Forest management may have many goals.  Under rotation harvest, the goal typically is to maximize annual harvest while ensuring that the area harvested is consistent with forest regrowth rates and total area under management.  This might result in a second harvest of the same forest plot after some 60-100 years.  The length of time between successive harvests of a forest is called the rotation length.

Forests on federal and state lands are usually managed according to multiple use doctrine.  This means that in addition to forest harvest, the land is available for recreation and maintains a healthy forest ecosystem.  Managing to protect biodiversity and to restore pre-settlement conditions are relatively recent goals.  In many parts of the world forests may be used by indigenous people for subsistence hunting, forest harvest, and as a place to live.  These people add yet more considerations, and more stakeholders, to the challenge of forest management.

Forest restoration may seek to restore the system to a near-natural or completely natural state, or to restore many aspects of the structure and function of an undisturbed forest.  The latter is usually referred to as rehabilitation, to emphasize that the desired endpoint is not necessarily that of pre-settlement conditions.

Forest management in the USA often involves the controversial role of fire, and whether to attempt to direct the course of forest succession.  Most forests in the lower 48 states are relatively young second-growth, and may have developed under an unnatural degree of fire suppression.  In the upper Midwest, for example, aspen covers much of what was once white pine.  Roughly 80 years old, these aspen forests are at the age when they are likely to die.  It is uncertain what type of forest will replace them. Within oak-maple hardwood forests, the overstory is dominated by oaks, whereas the seedlings are dominated by maple.  To maintain oak forests, some combination of fire and thinning is needed to favor recruitment by oaks.

Fire plays a major role in many forest types, including some that are highly fire adapted.  The jackpine of Michigan, for example, releases its seeds only after fire heats its cones.  Fire suppression has been US Forest Service policy since some devastating fires of the 1950s brought calls for fire management.  Since then, fuelwood on the forest floor has accumulated and people attracted by scenic settings have increasingly built houses in wooded areas.  As a consequence, calls for fire suppression are even stronger, and the human costs of fire are even greater than before.

Trends in wildlife area burned in wildfire in U.S.

Iverson, L.R. 2002. Biological trends in the United States: an annotated on-line review.

Forests often will recover on their own, but perhaps not in the direction or as quickly as we might prefer.   The old growth spruce-hemlock forests of Southeast Alaska appear to recover toward their original state, but it is hard to say how long this will take.  After all, extensive clear-cuts first took place only some 50 years ago, so there are no forests that have fully recovered.  Best estimates from comparisons of plots of different ages, some caused by long-ago natural disturbances, suggests 200-400 years.  It often is said that tropical forests will never recover from deforestation, especially if the land is burned and the top soil is disturbed.  This may be true in some instances, but examination of lands abandoned 50, 100, or more years ago suggests that we should be more optimistic.  An ambitious plan to restore a dry forest (one that sheds its leaves in the dry season) in Guanacaste, Costa Rica, serves as a good example.  Using fire and grazing to control invasive plants, and working with a detailed knowledge of forest ecology, workers there hope to return this area to its original forested state. In other regions of the tropics, native seedlings are grown in nurseries and planted in forest openings to help speed recovery.

Summary

  • Forest ecosystems provide valuable human products and ecosystem services
  • Deforestation has already removed about half of the world’s forests, and in your lifetime threatens to eliminate most of the remaining tropical forests.  Many temperate forests are re-growing, however
  • Forests are managed for multiple uses, to restore as much of possible of natural ecosystem function, and in some cases to attempt to restore “old-growth” or pre-disturbance conditions

Suggested Readings and Links

 

 
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