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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."
http://www.unu.edu/unupress/unupbooks/80635e/80635E05.htm
Another, more popular account of Salati’s work in the Amazon
is available at:
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 2.5.1.1.6. 2000. Geneva, UNEP and WMO.
http://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:
http://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.
http://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 "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:
"Climate Change 2001:Impacts, Adaptation and Vulnerability".
Chapter about carbon storage.
http://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
http://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 |
http://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:
http://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:
"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
http://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 UNEP
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.
http://www.mountains2002.org/i-forests.html
http://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.
http://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 '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:http://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."
http://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.
The
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:
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:
- The Primacy of Use-value, Intrinsic Value &
Quality (…)
- Following Natural Flows (…)
- Waste Equals Food (…)
- Elegance and Multifunctionality (…)
- Appropriate Scale / Linked Scale (…)
- Diversity (…)
- Self-Reliance, Self-Organization, Self-Design
(…)
- Participation & Direct Democracy (…)
- Human Creativity and Development (…)
- The Strategic role of the Built-environment,
the Landscape & Spatial Design (…)"
http://www.greeneconomics.net/what2f.htm
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