Forests
Trees are the largest and oldest of the known living plant species in the world today.
Oldest individual pine trees can be over 3000 years old. Forests are areas that have
enough rainfall to support stands of trees. Forests also have smaller forms of
vegetation growing under the threes. The three main types of forests are:
Tropical - tropical and subtropical areas; tropical rainforest, tropical dry forest. Tropical
rain forests tend to be dominated by broad-leaved evergreens (trees that do not shed
their leaves) with completely interlocking canopies (branches and leaves). In many rain
forests, there are actually multiple canopies, with some giant trees rising above the first
canopy (emergent trees). In many areas, these trees support the growth of vines that
grow up the trees to try to reach the sunlight above the canopy. There is a constant
struggle in the tropical rain forest for access to sunlight. Many of the understorey plants
are smaller versions of the dominant trees that will experience accelerated growth when
the death of giant trees allows light to penetrate to the forest floor.
The fact that little sunlight penetrates the rain forest floor generates an environment
where most of the animal species are arboreal (live in the trees). The number of species
that live in these forests is immense.
Tropical rain forests have a very different nutrient cycle in comparison with temperate
forests. In temperate forests, the nutrients are stored in the soil. In contrast, the soil of
the rain forest is relatively poor of organic matter because it decays so rapidly in the hot,
moist environment. Consequently, the tropical soils are mostly mineral soils with little
organic matter. The lack of organic matter within the soil does not mean that nutrients
are lost; however, as the first few meters of soil contain a dense layer of roots that
readily absorb the nutrients as they move through the soil and become available to the
roots. Thus, rather than being stored in the soil, the nutrients are stored in the biomass
of the plants, primarily in the trees.
Temperate - northern and southern latitudes with moderate climate; temperate
deciduous forest and temperate rain forest. Most of the trees in these forests survive
winter by dropping their leaves, which decay and produce nutrient-rich soil. The species
diversity is much lower than in tropical rain forest.
Boreal - in the northern latitudes, between tundra and temperate forest; coniferous
forest. These forests consist mostly of cone-bearing evergreen trees that keep their
needles year-round to help the trees to survive long and cold winters.
Forests provide many ecological services and resources. Ecological services provided
by forests: support energy flow and chemical cycling; reduce soil erosion; absorb, purify
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and release water; purify air, influence climate; provide habitats for wildlife and
opportunities for recreation. Natural resources provided by forests: timber, bushmeat,
berries etc.
Forests occupy about 30% of the earth’s surface (Excluding Greenland and Antarctica).
Types of forests
Forest managers and ecologists classify forests into three major types based on their
age and structure. Old-growth forest: an uncut forest that has not been seriously
disturbed by human activities or natural disasters for at least several hundred years.
22% of the world’s forests are old-growth forests. These forests are very rich in
biodiversity that they support. There is a big diversity in the age of trees in such forests.
When the old trees die they can stand for long time before falling, providing habitat for
many species. After they fall, they provide also huge ecosystem service for other
species. They create patches of light that promotes the growth of plants and the
decomposing wood material provides substrate and food for fungi and insects.
Five countries – Russia, Canada, Brazil, Indonesia, and Papua New Guinea – together
have more than 75% of the world’s remaining old-growth forests.
The second type is a second-growth forest. These forests develop after the trees in
an area have been removed by human activities (such as clear-cutting for timber or
conversion to cropland) or by natural forces (such as fire, hurricanes, or volcanic
eruption). Currently, about 63% of the world’s forests are secondary-growth forests.
A tree plantation, also called a tree farm, is a third type. It is a managed tract with
uniformly aged trees of one or two species that are harvested by clear-cutting as soon
as they become commercially valuable. The land is then replanted and clear-cut again
in a regular cycle. 5% of forests globally are tree plantations. Tree plantations produce
about 25% of the world’s commercial wood.
84% of the world’s natural forests are owned by governments.
Deforestation
It has been estimated that since the beginning of 20th century, 22% of the world’s
forests have been lost. Currently the rate of forest loss of degradation globally is 0.30.8% per year. These losses are concentrated in developing countries, especially those
in the tropics.
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Cutting down large areas of forests, especially old-growth forests, has important shortterm economic benefits (e.g. timber harvested), but it also has a number of harmful
environmental effects.
In an interesting study in 1960s, scientists experimentally showed the effects of
deforestation on water runoff and the loss of nutrients from the soil (erosion). Dams
were built at the bottoms of several forested valleys so that all the water and minerals
flowing from each valley could be collected and measured (the valleys were located on
impenetrable rock and all the water leaving the valley had to go through openings in
dams). Then in one valley, all the trees were cut and the amount of water runoff and
nutrient content in the water was compared with undisturbed, forested valley. With no
plants to absorb water from the soil, water runoff from deforested valley increased 3040%. As this excess water ran rapidly over the surface, it eroded soil and carried
nutrients out of the ecosystem. The increase of mineral loss was 6-8 times and for
nitrate even more when compared to forested valley. In fact, in deforested valley, the
water was so saturated with nitrates that it was unsafe to drink. After few years,
however, vegetation grew back and nitrate levels returned to normal.
Haiti is an example of the effects of deforestation on a country and its society. This
island in the Caribbean was once a tropical paradise covered largely with forests. Now
only 2% of its land is covered with trees. This has led to degradation of its soil and
decreased the ability to grow crops. This has also contributed to Haiti becoming the
poorest country in Western Hemisphere (Americas and Caribbean).
There are also few positive trends in the world. The total area occupied by temperate
forests in North America, Europe and China increased slightly between 1990 and 2005
because of the spread of commercial tree plantations and ecological succession on
abandoned croplands.
One way to reduce the pressure to harvest trees for paper production is to make paper
from other materials that do not come from trees. For example, China uses tree-free
pulp that comes from rice and other agricultural wastes to make almost two-thirds of its
paper. Also a plant known as kenaf can be used to make paper. It is a fast-growing
woody annual plant and it yields more paper pulp per hectare than tree farms.
Fuelwood crisis
About half of the world’s wood harvested each year is used for fuel. This demand puts
large pressure on remaining forests. One way to reduce the severity of the fuelwood
crisis, especially in developing countries, is to plant small plantations of fast-growing
fuelwood trees and shrubs around farms.
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A second way to reduce unsustainable harvesting is to burn wood more efficiently by
providing villagers cheap, more efficient, and less-polluting wood stoves. Scientists are
also developing ways to produce charcoal (that is used for cooking) from other sources,
besides trees, for example from sugarcane leftovers. Sugarcane charcoal burns also
cleaner than wood charcoal, thus helping to reduce lung diseases that are associated
with breathing smoke from indoor cooking fires.
Few countries have also had great success in reducing fuelwood crisis. For example,
South Korea was largely deforested after the war between North and South Korea
ended in 1953. Around 1960 the government launched a national reforestation program
based on village cooperatives. Today forests cover almost two-thirds of the country and
tree plantations near villages supply fuelwood on a sustainable basis. However, most
countries suffering from fuelwood shortages are cutting trees for fuelwood and forest
products 10-20 times faster than new trees are being planted.
Harvesting trees
The first step in forest management is to build roads for access and timber removal.
Even carefully designed logging roads have a number of harmful effects – increased
erosion and sediment runoff into waterways, habitat fragmentation and biodiversity loss.
Logging roads also expose forests to invasion by nonnative pests, diseases, and wildlife
species. They also open the once inaccessible forests to farmers, miners, ranchers and
hunters.
Once loggers can reach forest, they use various methods to harvest the trees. With
selective cutting, intermediate-aged or mature trees in an uneven-aged forest are cut
singly or in small groups. Selective cutting reduces crowding, removes diseased trees,
encourages growth of younger trees, maintains a stand of trees of different species and
ages, and allows a forest to be used for multiple uses.
With trees that grow best in full sunlight, clear-cutting is often used. In this case all the
trees in an area are removed. Clear-cutting has the advantage that you can get
maximum profits in shortest time, it needs less skill and planning to execute and it
promotes tree species that need high levels of sunlight. This practice however has also
several disadvantages – reduces biodiversity, destroys and fragments wildlife habitats,
increases water pollution, flooding and erosion and eliminates the recreational value of
the forest.
A clear-cutting variation that can allow a more sustainable timber yield without
widespread destruction is strip-cutting. It involves clear-cutting a strip of trees with the
corridor narrow enough to allow natural regeneration within a few years. After
regeneration, loggers cut another strip, and so on.
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Managing forests sustainably
Biodiversity researchers have suggested ways for managing forests sustainably:
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Identify and protect forest areas high in biodiversity
Grow more timber on long rotations (in that way the forest sustains more
biodiversity)
Rely more on selective cutting and strip cutting
Don’t log old-growth forests
Leave most standing dead trees and fallen dead trees for wildlife habitat and
nutrient cycling
Certify timber grown by sustainable methods (the producer of timber can attach a
certificate to its timber, thus informing potential customers)
Include ecological services when estimating the economic value of forests
Plant tree plantations on deforested land
Shift government subsidies from harvesting trees to planting trees
In the United States forests cover about 30% of the land area. That is more than in
1920. However, old-growth forests are still under increasing pressure from loggers and
developers.
Forest fires
Depending on their intensity, fires can benefit or harm forests. Three types of fires can
affect forest ecosystems.
Surface fires usually burn only undergrowth and leaf litter on the forest floor. They may
kill seedlings (young tree emerged from seed) and small trees but spare most mature
trees and allow most wild animals to escape.
Occasional surface fires have several ecological benefits. They burn away flammable
ground material and help prevent more destructive fires. They also release valuable
mineral nutrients (tied up in slowly decomposing litter and undergrowth), release seeds
from the cones of some pines, stimulate the germination of some seeds (giant sequoia),
and help control tree pathogens and insects. The new vegetation that sprouts after
surface fires also benefits some herbivorous wildlife species – deer, elk.
Crown fires are known as extremely hot fires. These leap from treetop to treetop. They
usually occur in forests that have not experienced surface fires for several decades,
where dead wood, leaves, and other flammable ground litter have accumulated. These
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rapidly burning fires can destroy most vegetation, kill wildlife, increase soil erosion, and
burn or damage human structures.
Sometimes surface fires go underground and burn partially decayed leaves or peat.
Such ground fires are common in northern peat bogs. They may smolder for days or
weeks and are difficult to detect and extinguish.
To reduce fire damage, we can set controlled surface fires (prescribed fires) and allow
fires to burn on most public lands unless they threaten human structures and lives. The
debris that are left into forest after logging are also extremely flammable.
Tropical deforestation
Tropical forests cover about 6% of the earth’s land area. However, at least half of the
world’s terrestrial plant and animal species live in tropical rain forests. Most of the
destruction in tropical forests has happened since the 1950s. Satellite images show that
large areas of tropical rainforest and tropical dry forest are being cut every year in South
America, Africa and Southeast Asia. Brazil has about 40% of the world’s remaining
tropical rain forest but it is rapidly being deforested to make way for cattle ranching and
large plantations for growing crops. Much of this forest in Brazil is simply burned down
to clear the land. Between 1970 and 2005, about 93% of Brazil’s coastal rain forest that
was once the size of Europe was cleared. Currently less than 5% of the world’s tropical
forests are managed sustainably. Many traditional medicines come from tropical forests.
Causes of tropical deforestation
Tropical deforestation results from a number of interconnected causes, such as not
valuing ecological services, poverty and population growth.
Also government subsidies can accelerate deforestation by reducing the costs of
timber harvesting and cattle grazing. Governments of Indonesia, Mexico and Brazil
encourage poor to colonize tropical forests by giving them rights over a land that has
been cleared. This practice can help to reduce poverty but can lead to environmental
degradation unless the new settlers are taught how to use forests sustainably, which is
rarely done.
In many cases logging in tropical countries is done by foreign operations operating
under government contracts. Once a country’s forests are gone, the companies move to
another country, leaving ecological devastation behind. For example, the Philippines
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and Nigeria have lost most of their once-abundant tropical forests and now are net
importers of forest products.
After the best timber has been removed, timber companies often sell the land to
ranchers. Within a few years, they typically overgraze it. Also crop growing and rain
erosion in cleared lands results in rapid soil erosion. Then the settlers move on to newly
cleared land to repeat this environmentally destructive process. In some areas –
especially in Africa and Latin America – large sections of tropical forest are cleared for
raising crops such as sugarcane, bananas, pineapples, soybeans, palm oil for use as a
biodiesel fuel, and coffee mostly for export to developing countries.
Healthy rain forests do no burn naturally. But human-caused burning, logging and roads
result in patchy fragments of forest which dries out and is more prone to fires. It has
also been found that widespread burning of tropical forest in the Amazon is changing
weather patterns by raising temperatures and reducing rainfall. This is converting
deforested areas into tropical grassland (savanna).
Ways to reduce tropical deforestation include: planting trees, switching to small-scale
sustainable agriculture and forestry, use of strip-cut instead of clear-cut when logging,
fight against illegal logging and stop government subsidies that encourage forest
destruction.
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