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Facts on Forests and Forestry

  NEW: Read GreenFacts' new Digest on Forests:

1 Introduction – Measuring progress towards sustainable forest management
2 How much forest is there on the planet and at what rate is it disappearing?
3 How can forests affect climate change?
4 What is the biological diversity of the world’s forests?
5 How healthy are the world’s forests?
6 What products are extracted from forests?
7 What are the protective effects of forests?
8 What are the economic and social benefits of forests?
9 Are forests managed in a sustainable way?
10 Conclusions

  A faithful summary of the "Global Forest Resources Assessment"
  produced in 2005 by the FAO (Food and Agriculture Organization). More...



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5. Do forests influence the environment, the climate and humanity?

  • Forest cover is one of many factors which affect climate at the global level as well as regionally and locally.
  • Forest ecosystems are sources of wood and timber, of edible products including mushrooms and other fungi, meat (from wildlife), forage for livestock, fruit or honey. Some forest species have medicinal values.
  • Forests provide habitat for wildlife and they affect the volume and timing of water flowing out of the forested area as well as rates of soil formation or erosion.
  • Forests are one of a number of important elements in the global cycling of carbon, oxygen, and other gases of importance which influences the composition of the earth’s atmosphere.
  • Throughout the world, forests have been a source of inspiration for people who live in them or near them and have often identified forests, forest groves or even individual trees as sacred places or objects.

Many environmental and ecological ‘services’ are derived from forests. In most cases, the forest ecosystem is one element in a complex interaction in which factors such as geographical location, size of the forested area, geology, human and animal activities all play a role. Different interpretations of the significance of forests in issues such as global climate change, flows of water in rivers, or soil erosion are often related to the complexity of measuring the role of forest ecosystems in the phenomenon in question.

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5.1. Do forests have an effect on climate?

What is the relative importance of forests on climate?

The global climate varies with changes in the polar ice caps, in surface and subsurface temperatures of the oceans, by absorption, reflection and transmission of energy in the form of light and heat from the surface of the earth. Since nearly two thirds of the surface of the globe is covered by oceans and the polar ice caps, these, rather than forests and other land features, are the most significant physical features of the globe affecting climate.

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5.1.1. What is the effect of forests on rainfall?

Forest cover affects the absorption, reflection and transmission of light and heat from the surface of the earth, and of water from the forest canopy due to processes of evaporation and transpiration. On a global scale, only very large areas of forest appear to have a noticeable effect on climate and rainfall, although smaller areas of forest do have some effects on local microclimates.

Historically, it was believed that the presence of forests attracted rain or were instrumental in increasing rainfall. Over the last forty-five years, it has become possible to trace the movements of water vapor and atmospheric gases to develop a clearer idea of the role played by forests in moderating or regulating rainfall. In temperate regions and tropical regions such as Southeast Asia, the main source of water vapor in the atmosphere is from evaporation at the surface of the oceans. In the Amazon Basin, however, nearly 50% of water vapor in the atmosphere in the region of Manaus and Belém appears to be ‘recycled’ from the forest.

A good summary (not limited to tropical forests) of the effects of forests on rainfall is in: L. Oyebande "Effects of tropical forest on water yield" Chapter 3 in: Reynolds, E.R.C. and F.B. Thompson. 1988. "Forests, Climate, and Hydrology: Regional Impacts". Tokyo, The United Nations University.

"Lockwood (1976, 91) cited the work by Bergemann and Libby, who, in 1957, used isotopes of water to obtain the ratio of maritime water to land water in the Upper Mississippi Valley, and concluded that one-third of the average precipitation is formed of re-evaporated (i.e. continental) water and two-thirds of ocean water. One may be tempted to conclude from this study that it is unlikely that changes in the nature of land surface, such as the removal of the forest, will have any significant influence on local rainfall. In the subtropics, where continuously clear skies and large amounts of solar radiation are available to evaporate water, the main sources of water vapor are the oceans. The same assumption has been extended to South-East Asia, which receives much of its rainfall in the form of water that has evaporated from the subtropical Indian Ocean.

The reported results from the Amazon Basin, where the mean annual precipitation is 2,000-2,400 mm, are, however, different. Here, the mean recycling time for water vapour in 1979 was found to be 5.5 days (Salati et al. 1979 and Salati and Matsui 1981). Measurements of oxygen isotopes in the rain and river waters confirmed the importance of recycled water in the hydrological balance throughout the basin. The studies concluded that 52% of the precipitation in the Amazon region between Belém and Manaus was accounted for by inflowing moisture from the Atlantic Ocean; the remainder by recycled vapour within the area."
External linkhttp://www.unu.edu/unupress/unupbooks/80635e/80635E05.htm

Another, more popular account of Salati’s work in the Amazon is available at:
Adobe Acrobat Document http://idrinfo.idrc.ca/Archive/ReportsINTRA/pdfs/v12n4e/110782.pdf

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5.1.2. What is the forest’s role in regulating hydrology?

Does the forest have an impact on water storage and hydrology?
Rain falling on forested land is intercepted by the foliage of the canopy. Some of the rainwater that penetrates to the land surface flows into rivers, lakes and oceans. Some water penetrates the soil before resurfacing to join the surface flow, while some water becomes part of underground reserves of water or aquifers.

Forest cover plays a role in regulating hydrology (cycles of water flow). Since variables such as density of foliage, temperature, the humus (or decomposed vegetation) layer on the forest floor, permeability of soils, slope and geology all affect the flow of water, it is not possible to state categorically that forests increase or decrease water flow. It would be accurate to say, however, that forest cover does mitigate the effects of events such as tropical rainstorms or rapid snowmelt, reducing the likelihood of downstream flooding and extending the time during which water flows can recharge underground reserves.

The International Panel on Climate Change (IPCC) considers that water cycling is one of the most important environmental services provided by forests.

"Water cycling is another major environmental service of forests. One of the expected impacts that would result from a significant expansion of the extent of deforestation in Amazonia and other parts of Brazil would be a reduction in rainfall, especially during the dry season (Lean et al., 1996). Similar effects have been calculated for the effects of forests on rainfall in the Indian subcontinent (Harding, 1992), and tropical forest protection has been shown to generate drought mitigation and flood mitigation benefits in Indonesia (Pattanayak and Kramer, 2000) and Madagascar (Kramer et al., 1997)."

From: Intergovernmental Panel on Climate Change (IPCC) Special Report on Land Use, Land-Use Change And Forestry. Ch.2 2000. Geneva, UNEP and WMO.
External linkhttp://www.grida.no/climate/ipcc/land_use/100.htm

"The tropical forest, and indeed any forest cover, produces litter that protects the soil beneath from rainfall impact and filters out the fine particles that may clog the larger pores … Infiltration rates are therefore usually high under forest cover where the forest floor layer is well developed. Where it is disturbed by logging or removed by fire, protection may be decreased sufficiently to lead to overland flow. … It can also be argued, however, that the microclimate (high humidity, light wind, maintaining of low moisture fluxes) prevents the forest soils from drying (especially during the West African Harmattan). The soil does not even harden, so that its permeability is maintained, preserving its infiltration capacity. Finally, the presence of a certain amount of humus in the top soil assures a soil structure favorable to infiltration."

"… The hydrological processes in question include precipitation, interception, evaporation, and runoff. Interception splits precipitation into that delivered to the land and water surfaces and that caught on the forest canopy and returned to the atmosphere by evaporation. Water delivered to the land surface may run off directly, as overland flow into streams to drain by way of rivers and lakes back into the sea, or infiltrate the soil. This latter pathway has been considered the most important pathway for the sustenance of man (Pereira 1973, 1). From the soil, vegetation is supplied; the surplus draining further down to springs maintains the steady flow of rivers. Plants return much of the soil water through transpiration to the atmosphere. Some water also evaporates directly from the soil and from the surfaces of lakes and rivers. It is known that part of the water that infiltrates the soil moves laterally through the upper horizons until it reaches a stream channel and does not become part of the ground water reservoir. This portion of subsurface flow is known as interflow or through-flow and, together with overland flow, constitutes what is generally referred to as surface runoff or, more properly, as direct runoff or quickflow. In practice interflow and overland flow cannot be accurately separated, but several methods of hydrograph separation are available for isolating their sum from baseflow."

L. Oyebande "Effects of tropical forest on water yield" Chapter 3 in: Reynolds, E.R.C. and F.B. Thompson. 1988. "Forests, Climate, and Hydrology: Regional Impacts". Tokyo, The United Nations University. See:
External linkhttp://www.unu.edu/unupress/unupbooks/80635e/80635E05.htm

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5.2. How do forests affect the process of climate change?

Forests act as important buffers that cushion the impact of ongoing climate change.
External linkhttp://www.rcfa-cfan.org/english/issues.13.html.

Through their destruction, forests can be significant sources of greenhouse gases; when managed sustainably, they can be important sinks of the same gases.

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5.2.1. How does the carbon sequestration process work?

During photosynthesis, plants absorb carbon dioxide and convert it to carbon (stored as plant tissue) and oxygen. Forests therefore have an important ecological function in fixing and storing carbon from the atmosphere. Increasing concentrations of carbon dioxide in the atmosphere appears to be one of the factors leading to observed changes in the global climate, so that there is growing interest in the role of forests as a possible factor in mitigating climate change.

"Each year, as forests grow and increase their biomass, they absorb carbon from the atmosphere and store it in plant tissue. This process is known as carbon sequestration. Despite constant exchanges of carbon between forest biomass, soils, and the atmosphere, a large amount is always present in leaves and woody tissue, roots, and soil nutrients. This quantity of carbon is known as the carbon store. Carbon sequestration and storage slow the rate at which carbon dioxide accumulates in the atmosphere and mitigate global warming. Forests sequester and store more carbon than any other terrestrial ecosystem, and constitute an important natural defense against climate change."

From Adobe Acrobat Document"Carbon Storage and Sequestration" in Pilot Analysis of Global Ecosystems (PAGE), Forest Ecosystems by Emily Matthews, Mark Rohweder, Richard Payne, Siobhan Murray, World Resources Institute, 2000. p. 55.
See also IPCC:External link "Climate Change 2001:Impacts, Adaptation and Vulnerability". Chapter about carbon storage.

External linkhttp://www.greenfacts.org/studies/climate_change/index.htm

Growing at different rates, and with different structures and different species, forest types accumulate carbon at different rates.

"More than any other kind of vegetation, forests capture vast amounts of atmospheric carbon dioxide and store it in live and dead woody tissues (especially in stems and roots) and in forest floor and soil organic matter. But different forest types and developmental stages accumulate atmospheric carbon in different ways. In general, growth rates are greatest when forests are young, whereas the total store of biomass is greatest in older forests." And, boreal forests are especially rich in soil carbon, while tropical forests probably store more in their vegetation.
Smithsonian Environmental Research Center
External linkhttp://www.serc.si.edu/forest_ecology/forest_ecology_production.htm

In 1992, the Thailand Development Research Institute (TDRI) published estimates of carbon uptake in different forest types after forest conversion. The estimates are based on research carried out in 1989 when the Royal Forestry Department had reported a total of 6,970 km² of land reforested. The report does not break down the carbon uptake by hectare, by forest type, but reports the following aggregate figures:

Forest type Carbon uptake (million tons)
Tropical Evergreen Forest 0.177
Mixed-Deciduous Forest 0.062
Dry Dipterocarp Forest 0.095
Pine Forest 0.004 0.004
Mangrove Forest 0.024
Total 0.362

External linkhttp://www.info.tdri.or.th/library/quarterly/tables/m92_2_t2.htm

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5.2.2. What happens to the carbon stored in forests?

"Forests store carbon as they grow, however, when they are degraded or cleared their stored carbon is released back to the atmosphere, thus these forests become net contributors of carbon to the atmosphere. It is estimated that the rapid destruction of tropical forests has been responsible for approximately 20% of total human-caused carbon dioxide emissions each year. The Union of Concerned Scientists has developed a graph comparing different sources of carbon dioxide emissions, including deforestation:
External linkhttp://www.ucsusa.org/global_environment/global_warming/page.cfm?pageID=526

According to the World Resources Institute’s Pilot Analysis of Global Ecosystems (PAGE):

  • Forest soils and vegetation store about 40 percent of all carbon in the terrestrial biosphere, more than any other ecosystem.
  • Globally, more carbon is stored in forest soils than in forest vegetation. Boreal forests are especially rich in soil carbon, while tropical forests probably store more in their vegetation.
  • Regrowth of forests in developed countries may account in part for the increasing terrestrial sink that absorbs some of the carbon dioxide emissions released by fossil fuel combustion. However, land use change, primarily tropical deforestation, currently releases an estimated 1.6 billion tons of carbon to the atmosphere each year, equivalent to 25 percent of emissions from fossil fuel combustion.
  • Globally, deforestation far exceeds regrowth. The world's forests are therefore currently a net source of carbon.
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5.2.3. Can climate change be mitigated through forest management?

"Carbon that could be released through deforestation or forest degradation can be kept out of the atmosphere through the following land-use-based approaches:

  • Slowing or stopping the loss of existing forests, thus preserving current carbon reservoirs;
  • Adding to the planet's vegetative cover through reforestation or other means, thus enlarging living terrestrial carbon reservoirs;
  • Increasing the carbon stored in non-living carbon reservoirs such as agricultural soils;
  • Increasing the carbon stored in artificial reservoirs, including timber products; and
  • Substituting sustainable biomass energy sources for fossil fuel consumption, thus reducing energy-related carbon emissions."

Source: External link"Mitigating Climate Change through Forest Management", WRI

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5.3. Do forests provide any other environmental services?

The physical structure of forests–a canopy that shelters the soil surface, and a network of roots that anchors soil and rocks—provides protective services to the environment, especially in mountainous areas where steep slopes and geological activity are sources of instability. The degree of protection afforded by forests depends on variables such as the steepness of the slope, the root systems of the forest and undergrowth, other land uses such as grazing, the stability of the underlying soils and rock surfaces, and the intensity of winds, rainstorms and snowfall. Forest cover is important for environmental protection, but the presence of forests is not sufficient to guarantee that that there will be no erosion or other forms of environmental degradation.

The protective function of forests is particularly important in mountainous areas. In countries where hydroelectricity is a significant form of electricity, a regular flow of water free from silt (soil runoff from erosion) is vital for power generation.

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5.3.1. Do forests protect the ground from erosion?

"Forests protect the ground from erosion by covering and stabilizing it. This prevents valuable soil being washed away, rendering whole tracts of land unfruitful. Where forests have been destroyed, the risk of natural catastrophes such as floods rises. In mountainous regions forests help to protect people from avalanches and rock-fall."

The Working Group "International Relations"
Swiss Forestry Association
External linkhttp://www.forest.ch/ag/ib/agibe.htm

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5.3.2. What is the importance of mountain forests?

"Forests that grow in mountain areas play an important role in maintaining the stability of mountain systems and supporting the people who live there. Mountain forests are vital as sources of water for irrigation and power generation. They intercept and store water from rainfall, mist and snow, and release it slowly, thereby reducing soil erosion, avalanches and downstream flooding impacts. They are important sources of timber and other wood and non-wood products, and are especially important as sources of fuel for local populations and for those in nearby foothills and plains.

Mountain forests are very important as repositories of biodiversity and as a result are increasingly important for tourism and recreation as well as hunting and fishing. Because mountain forests are usually isolated from similar ecosystems by steep terrain and intervening lowlands with contrasting climates, they are frequently sites of high species endemism: that is the species occur locally and nowhere else. Local distribution tends to make species more vulnerable to extinction, and this combined with increasing pressures on mountain ecosystems has led to the inclusion of many mountain forest species on the lists of the world's most critically endangered species."

(from External linkUNEP World Conservation Monitoring Centre [WCMC] site on mountain forests)

The United Nations declared the year 2002 to be ‘The International Year of the Mountain’ (IYM). The ‘Forests’ section of the official IYM web site carries a number of commentaries about the importance of forests in mountainous regions and links to other relevant sites.
External linkhttp://www.mountains2002.org/i-forests.html
External linkhttp://www.berge2002.ch/exp/enc/living/forests/avalanche.html

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5.4. How important are forests to humanity?

Forests have always provided a wide range of goods and services to the people living in and around them. These include:

  • Timber and wood fibre
  • Food
  • Shelter
  • Energy
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5.4.1. Do plants have medicinal value?

A wide range of plants, parts of plants, and animal parts are used for medicinal purposes. Some have been incorporated into ‘conventional’ medicinal practice; others remain as ‘indigenous knowledge’ or have yet to be identified as of medicinal value. (see following sections on ‘Biodiversity’ and on ‘New Directions in Forest Management).

UNESCO has a worldwide research program studying the different uses people make of plants. An important part of this program looks at medicinal uses of plants.
External linkhttp://www.rbgkew.org.uk/peopleplants/

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5.4.2. Does the forest have spiritual value?

In November 2000, the Worldwide Fund for Nature (WWF) and the Alliance of Religions and Conservation (ARC) "joined together to develop and recognize significant new conservation actions by the world's religions. These are called External link'Sacred Gifts for a Living Planet' (Sacred Gifts)."

Religious actions which are of particular importance to the conservation of forests include Buddhist logging bans in Mongolia, the commitment of Shinto temples and shrines to sustainable management of sacred forests in Japan, Maronite protection of the Harisa forest in Lebanon, FSC certification for Swedish church forests in Sweden; Zoroastrians establishing sacred Baval tree groves in India, and also the restoration of sacred forests in Orissa, India.

The ARC web site has information about the importance of conservation to the world’s major faiths:External linkhttp://www.icorec.f9.co.uk/

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5.4.3. Can environmental economics help us to place a value on forest services?

"In addition to providing food, forests protect against wind and soil erosion and regulate climate by slowly releasing rainfall to the air and soil. They produce oxygen and reduce carbon dioxide, a source of global warming. Forests are storehouses of biodiversity. Two-thirds of all land-based species on earth live in forests, and this biodiversity is a source for many contemporary and future medicines. Most important are the many forest medicines used directly by the poor in developing countries."
External linkhttp://www.futureharvest.org/earth/forest.shtml

However, with the exception of timber, few of the services and products derived from forest ecosystems are given any monetary or market ‘value’ in conventional accounting systems, which is sometimes said to be one reason why market-oriented economies tend to promote logging for timber over and above conservation or the sustainable utilization of forests for other purposes. The field of ‘environmental economics’ has emerged over the last twenty years and focuses on deriving estimated values for non-timber products and services. Such work is still in its early stages and is criticized by some for attempting to assign a market value to values which should perhaps transcend the market and not be ‘commoditized’.

FAO has estimated and tabulated some possible ‘values’ of the following forest services: hosting diversity of species and habitats, water regulation, prevention from erosion, fisheries protection, climate stabilization and recreation purposes.
External linkThe state of food and agriculture 1994. FAO Agriculture Series, No. 27
Food and Agriculture Organization of the United Nations. Rome, 1994

A summary of the ‘state of the art’ in environmental economics can be found through the World Bank web site at:External link http://lnweb18.worldbank.org/ESSD/

There are a number of critiques of the ‘environmental economics’ analytic framework. The following abstract of a paper by E.J. Mishan summarizes the key points of criticism:

"Economic models of the cost of environmental damage are not sufficient indicators of the true cost of such damages because many factors cannot be accounted for in economic terms. Government regulations would still be needed to determine a fixed value on a case-by-case basis, and government agencies would shoulder the burden of enforcement." Mishan, E.J. Economists versus the Greens: an exposition and a critique. Political Quarterly vol. 64, no. 2 (April-June, 1993): 222 (21 pages).

Green Parties in Europe have presented more radical critiques of environmental economics.

"Here are ten interrelated principles that cover key dimensions of a green economy:

  1. The Primacy of Use-value, Intrinsic Value & Quality (…)
  2. Following Natural Flows (…)
  3. Waste Equals Food (…)
  4. Elegance and Multifunctionality (…)
  5. Appropriate Scale / Linked Scale (…)
  6. Diversity (…)
  7. Self-Reliance, Self-Organization, Self-Design (…)
  8. Participation & Direct Democracy (…)
  9. Human Creativity and Development (…)
  10. The Strategic role of the Built-environment, the Landscape & Spatial Design (…)"

External linkhttp://www.greeneconomics.net/what2f.htm

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