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Effects of Acid
Rain
Forests
Over the years, scientists, foresters, and
others have watched some forests grow more slowly without knowing why. The
trees in these forests do not grow as quickly as usual. Leaves and needles
turn brown and fall off when they should be green and healthy.
Researchers suspect that acid rain may
cause the slower growth of these forests. But acid rain is not the only cause
of such conditions. Other air pollutants, insects, diseases and drought are
some other causes that harm plants. Also, some areas that receive acid rain
show a lot of damage, while other areas that receive about the same amount of
acid rain do not appear to be harmed at all. However, after many years of
collecting information on the chemistry and biology of forests, researchers
are beginning to understand how acid rain works on the forest soil, trees, and
other plants.
Acid Rain on the Forest Floor
A spring shower in the forest washes leaves
and falls through the trees to the forest floor below. Some of the water soaks
into the soil. Some trickles over the ground and runs into a stream, river or
lake. That soil may neutralize some or all of the acidity of the acid
rainwater. This ability of the soil to resist some pH change is called
buffering capacity. A buffer resists changes in pH. Without buffering
capacity, soil pH would change rapidly. Midwestern states like Nebraska and
Indiana have soils that are well buffered. Places in the mountainous
northeast, like New York's Adirondack Mountains, have soils that are less able
to buffer acids. Since there are many natural sources of acid in forest soils,
soils in these areas are more susceptible to effects from acid rain.
How Acid Rain Harms Trees
Acid rain does not usually kill trees
directly. Instead, it is more likely to weaken the trees by damaging their
leaves, limiting the nutrients available to them, or poisoning them with toxic
substances slowly released from the soil.
Scientists believe that acidic water
dissolves the nutrients and helpful minerals in the soil and then washes them
away before the trees and other plants can use them to grow. At the same time,
the acid rain causes the release of toxic substances such as aluminum into the
soil. These are very harmful to trees and plants, even if contact is limited.
Toxic substances also wash away in the runoff that carries the substances into
streams, rivers, and lakes. Less of these toxic substances are released when
the rainfall is cleaner.
Even if the soil is well buffered, there
can be damage from acid rain. Forests in high mountain regions receive
additional acid from the acidic clouds and fog that often surround them. These
clouds and fog are often more acidic than rainfall. When leaves are frequently
bathed in this acid fog, their protective waxy coating can wear away. The loss
of the coating damages the leaves and creates brown spots. Leaves turn the
energy in sunlight into food for growth. This process is called
photosynthesis. When leaves are damaged, they cannot produce enough food
energy for the tree to remain healthy.
Once trees are weak, they can be more
easily attacked by diseases or insects that ultimately kill them. Weakened
trees may also become injured more easily by cold weather.
Acid rain can harm other plants in the
same way it harms trees. Food crops are not usually seriously affected,
however, because farmers frequently add fertilizers to the soil to replace
nutrients washed away. They may also add crushed limestone to the soil.
Limestone is a basic material and increases the ability of the soil to act as
a buffer against acidity.
Water
The effects of acid rain are most clearly
seen in the aquatic, or water, environments, such as streams, lakes, and
marshes. Acid rain flows to streams, lakes, and marshes after falling on
forests, fields, buildings, and roads. Acid rain also falls directly on aquatic
habitats.
Most lakes and streams have a pH between 6
and 8. However, some lakes are naturally acidic even without the effects of acid
rain. Lakes and streams become acidic (pH value goes down) when the water itself
and its surrounding soil cannot buffer the acid rain enough to neutralize it. In
areas like the Northeastern United States where soil buffering is poor, some
lakes now have a pH value of less than 5. One of the most acidic lakes reported
is Little Echo Pond in Franklin, New York. Little Echo Pond has a pH of 4.2.
Lakes and streams in the western United States are usually not acidic. Because
of differences in emissions and wind patterns, levels of acid deposition are
generally lower in the western United States than in the eastern United States.
This chart shows that
not all fish, shellfish, or their food insects can tolerate the same amount of
acid:
Generally, the young of
most species are more sensitive than adults. Frogs may tolerate relatively
high levels of acidity, but if they eat insects like the mayfly, they may be
affected because part of their food supply may disappear. As lakes and streams
become more acidic, the numbers and types of fish and other aquatic plants and
animals that live in these waters decrease. Some types of plants and animals
are able to tolerate acidic waters. Others, however, are acid-sensitive and
will be lost as the pH declines. Some acid lakes have no fish. At pH 5, most
fish eggs cannot hatch. At lower pH levels, some adult fish die. Toxic
substances like aluminum that wash into the water from the soil may also kill
fish.
Together, biological organisms and the
environment in which they live are called an ecosystem. The plants and animals
living within an ecosystem are highly interdependent. For example, fish eat
other fish and also other plants and animals that live in the lake or stream.
If acid rain causes the loss of acid-sensitive plants and animals, then fish
that rely on these organisms for food may also be affected.
Human-Made Materials
Acid rain eats away at stone, metal, paint
-- almost any material exposed to the weather for a long period of time.
Human-made materials gradually deteriorate even when exposed to unpolluted
rain, but acid rain accelerates the process. Acid rain can cause marble
statues carved long ago to lose their features. Acid rain has the same effect
on buildings and monuments. Repairing acid rain damage to houses, buildings,
and monuments can cost billions of dollars. Ancient monuments and buildings,
such as the Parthenon in Greece, can never be replaced.
Here is a way for you to observe the
effect of acid rain on marble and limestone, two building materials commonly
used in monuments, ancient buildings, and in many modern structures.
- Place a piece of chalk in a bowl with
white vinegar.
- Place another piece in a bowl of tap
water.
- Leave the dishes overnight.
The next day, see if you can see which
piece of chalk is more worn away.
This experiment with chalk allows you to see the effect of acid rain on
marble and limestone because chalk is made of calcium carbonate, a compound
occurring in rocks, such as marble and limestone, and in animal bones,
shells, and teeth.
Effects of Acid Rain on People
Acid rain looks, feels, and tastes just
like clean rain. The harm to people from acid rain is not direct. Walking in
acid rain, or even swimming in an acid lake, is no more dangerous than walking
or swimming in clean water. The air pollution that causes acid rain is more
damaging to human health. Sulfur dioxide and nitrogen oxides, the major
sources of acid rain, can irritate or even damage our lungs.
The pollutants that cause acid rain can
also reduce visibility -- limiting how far into the distance we can see.
The primary pollutants associated with
acid rain and poor visibility are human-made sulfur dioxide emissions. These
emisisons form small sulfate particles, or aerosols, in the atmosphere. These
aerosols reduce visibility by scattering light. Sulfate aerosols are the main
cause of poor visibility in the eastern United States.
Nitrogen oxide emissions are also
associated with the acid rain problem. They, too, can form aerosols in the
atmosphere that significantly reduce visibility. Nitrate aerosols are often
the main cause for poor visibility in the western United States where sulfur
dioxide emissions and humidity are lower than in the east.
U.S. Environmental Protection Agency
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