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Types of Forests

When you picture a forest in your mind, what does it look like? Your answer might depend on the biome you live in. There are many biomes, or living communities on the earth's surface such as rain forests, grasslands, deserts, woodlands, savanna or temperate deciduous forests.

Trees grow all over the world. They must adapt to different types of conditions from the icy cold to the desert dryness. For example, mangrove trees grow in tropical regions along the sea. In fact, they are sometimes called the forests of the sea. The roots of the mangrove tree are adapted to cope with the soft mud underwater by growing downwards like stilts to anchor it in place. Some roots grow upwards until they are sticking out of the water in order to reach oxygen. The roots also have a special filtering system to remove the salt from the seawater they take in. How are the trees adapted to the environment where you live?

Forests Types

Many people in the northern or mountainous regions of the United States might picture coniferous or boreal forest (RED)in their minds. Other people, from the eastern United States may envision deciduous forests (PINK), found within the United States' temperate zone, recognizable by their broad leaves which change color and fall off when the days get shorter and the weather turns colder in the fall. You most likely have heard about tropical rain forests. Did you know that there are rain forests in Oregon and Washington? These forests receive as much rain as tropical wet forests (GREEN) but they are located in northern laditudes and are called temperate rain forests (BROWN).

But in the countries to the south of the United States, other kinds of forested areas exist which you may have heard a great deal about, but have never seen.

You have probably heard and read a lot about tropical rainforests, but did you know that there are many different kinds of tropical forests? When we use the term "tropical" we refer to the area between the Tropics of Cancer and Capricorn. Because this region of the earth receives the most "direct" rays of the sun throughout the year, the temperatures usually stay fairly warm.

Between the Tropics of Capricorn and Cancer, there are many types of forests such as: Moist Tropical Forests, Dry Tropical Forests, Riverine Forests, Mangrove Forests, and Cloud Forests.

There are many factors which influence the forest type. This is true even with a specific forest biome (rainforest, coniferous, etc.) Forest type vary by rainfall, temperature, type of soil, animal life, the number of dry months, altitude, and slope. For example, short scrub forests grow in tropical regions with little rainfall. Mangroves grow along shorelines where fresh river water and sea water mix together. In high-altitude mountainous areas, coniferous forests and cloud forests ( in the tropics) can thrive.

The map below shows how forests ecosystems change as Latidude and Altitute increase

Defining the boundaries between types of tropical forests can be difficult. For example, with an increase in altitude, a tropical rainforest may gradually blend with cloud forest. As a forest nears the shore, it may slowly become dominated by mangrove forest. Therefore, the term "tropical rainforest" is sometimes just an easy way to refer to a variety of different habitats that exist between the tropics of Cancer and Capricorn. Overall, tropical forests cover about 1,700 million hectares, an area about the size of South America.

All of the forests mentioned are threatened by logging, intensive resource use, and urban development. From the coniferous forests of the Himalayas to the Guatemalan mangroves, forests are in retreat. The New Forests Project works in many capacities to reduce deforestation in tropical regions. Because many local farmers and communities need forest products such as lumber, food, and fodder to survive, it is not as simple as restricting resource use. The New Forests Project has found that it is necessary to provide the local populations with alternative sources of tree products so farmers have the opportunity to let degraded areas regenerate.

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Ecology and Trees

Trees and soil

Trees are essential to the maintenance of healthy soil. Tree roots dig deep into the ground, which helps to aerate, or provide oxygen to the soil. The roots can loosen hard mud or rocks under the topsoil, which makes it easier for burrowing animals, worms and microorganisms to tunnel and aerate the soil as well (When animals, plants and microorganisms aerate soil they create tiny air pockets that can be filled by roots or water. Healthy soil is well aerated!). Water that penetrates the topsoil can then drain deeper into the ground. This increases the rate at which rainwater infiltrates and recharges the groundwater, an underground reserve of water.

When trees lose their nuts, fruits, or leaves, these materials drop to the forest floor. If not eaten, they decompose and the organic matter is returned to the soil, which is vital in terms of keeping the soil nutrient-rich. Dead trees also decompose over time, and return their nutrients to the soil.

The shade keeps the harsh sunlight from penetrating the ground, allowing it to maintain an appropriate moisture level. The tree trunk and canopy also protect the soil from harsh winds. Trees maintain soil fertility when nutrients drawn up by their roots are recycled into the top layers of the soil by leaf decomposition.

Trees also help keep the soil in place by means of their strong root systems. This, as well as a tree's protective canopy, prevent soil erosion, which can have some very serious consequences. A half-hour thunderstorm in tropical forests can produce forty times more rain than the average northeastern U.S. thunderstorm. Therefore, these heavy downpours wash away more soil than anywhere else in the world. When it rains on slopes, hills or mountains that have been deforested, the rainwater will run off, carrying with it much of the topsoil from the hills. This will happen if you remove any type of forest (tropical, coniferous, deciduous). You may be thinking "Isn't all dirt the same?" But it takes one hundred years for nature to produce just one inch of topsoil, which holds the nutrients needed for plant regeneration.

Soil erosion has impoverished more than one-half of all agricultural land, causing farm productivity to decline sharply. When the topsoil is washed away, so too are its nutrients and its ability to sustain crops.

Where does this topsoil end up? Downstream is the easy answer. The soil continues to wash away until in ends up clogging rivers and waterways. The buildup of soil in rivers becomes so great, many dry up, and the local people and animals are deprived of a life giving resource. The riverbeds can also expand to a level where flooding occurs during heavy rains. Water reservoirs become filled with silt- the fine soil carried down the river or stream - and they are unable to store the amount of water they were designed to hold. Siltation and filling reduces their usefulness for flood protection, power generation, it also decreases fish populations and alters river flows.


Trees and the Hydrological Cycle -- or Water Cycle

Have you ever been walking on a hot day in a field or village then entered a forest or heavily shaded area? It feels much cooler and even damp!! Why do you think this is happens?

Trees are often considered natural sponges which regulate the earth's water supply by holding water. Forest waste matter, such as decomposing leaves or humus, slows down surface evaporation from the soil. Rain first falls on leaves and plants rather than directly hitting the soil, decreasing the rate of water and soil erosion.

Deforestation leads to a decrease in the amount of water absorbed by the soil. Less water seeps down into the soil to recharge the groundwater. This leads to an absence of water available during the dry season and freshwater springs can dry up. Trees also hold water with their vast root systems, by creating air pockets deep in the soil. These pockets hold water during heavy rains.

Forests help regulate the water in the air through a process called transpiration. They absorb thousands of gallons of water into their roots. The water is carried up through the trunk and into the branches and leaves for food. Excess water and nutrients are then released through the leaves back into the air.

When the air becomes saturated with enough moisture, clouds form and if enough moisture is transpired into the air, rain will fall. This begins the water cycle all over again. When a forest is destroyed, this release of moisture back into the air is lost, which results in less rainfall for that area in the future.

To see the tiny holes, or stomata, in the leaves through which leaves transpire water, you must look under a microscope. However, if you wrap a broad-leaf plant in clear plastic, and place it in the sunlight, in a few hours, you will see moisture or condensation clinging to the inside of the bag. This moisture was given off by the leaves much like the moisture excreted from your skin when you perspire.

Trees and Climate

Many human activities, such as modern farming, the burning of fossil fuels or cutting down and burning forests, release large amounts of gas into the atmosphere. These gases are referred to as greenhouse gases because they act like the glass covering of a greenhouse, absorbing solar radiation and trapping it inside. Greenhouse gases, such as Carbon dioxide (CO2), absorb some of the sun's energy which would otherwise be released back into space.

The Greenhouse Effect

Sunlight pentrates the atmosphere and warms the Earth's surface. The Earth then radiates some of this heat back into the atmosphere. Some of this heat escapes into space and some is reflected back to earth as watervapor and greenhouse gases absorb it. As greenhouse gases increase more heat can be trapped near the Earth's surface

This build-up of carbon dioxide could cause the planet to heat up. It is estimated that clearing and burning tropical forests accounts for about 20% of the increase in carbon dioxide levels in the atmosphere.

Trees absorb carbon dioxide to make food during the process of photosynthesis. Using energy from the sun, green plants break apart carbon dioxide and water molecules and rearrange the loose atoms to form sugar. Trees use sugar as food for growth and energy. Oxygen, which is what humans must inhale to survive, is left over from this process and released into the atmosphere. During the nighttime, this process is reversed. Trees absorb oxygen and emit carbon dioxide. By planting trees we can reduce the amount of CO2 in the atmosphere and possibly prevent global warming from happening.

Scientists disagree on how much these greenhouse gases will affect the global temperature. Some scientists predict that the average global temperature could increase 3.6 degrees Fahrenheit in the next seventy years due to global warming. This may not seem like a very drastic change, but some scientists believe it is a big enough change to melt the earth's polar ice caps, leading to a rise in sea levels which could flood coastal cities around the world. A rise in temperature could be enough to endanger the crops we rely on for food. It could dry up the lakes and rivers in some areas which provide water to crops, towns and cities.

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Agro forestry is a type of farming. It is a type of farming in which crops, like corn, beans and vegetables are planted alongside trees. There are a number of reasons why this is done in addition to a number of different ways it can be accomplished. It can be used on small farms or large farms.

Some farmers use a method is called live or vegetative barriers on hillsides to prevent soil erosion while they are growing their crops. The trees, mostly their trunks and roots systems, form a "barrier" which stops the soil from moving downslope. As the trees become established, a flat area of high quality top soil forms behind them. Live barrier farming systems require a great deal of maintenance, but can be highly effective for growing the crops necessary for the farmers to survive. This type of farming is also called contour farming, because farmers plant trees or vegetation along the contour lines of a hillside.

Sometimes farmers grow trees with crops for protection from the strong winds. Strong winds can carry nutrient rich topsoil away from farmers' fields and deposit it hundreds of miles away. Trees planted in shelter-belts, which shield the farmers fields, offer effective protection from the problems of wind erosion.

Shade trees such as, palm or leucaena trees can also be grown with crops that will not grow well in direct sunlight. Cocoa, coffee, pepper plants all need shade to be productive.

Other farmers grow trees with crops to provide green manure -- regular manure is usually made from horse, cow, chicken or goat dung, green manure is made of the leaves of certain kinds of tree or plant species to fertilize the soil where crops are planted. The leaves of some agroforestry tree species can also be used as feed for cattle or other livestock. When leaves are used for animal feed it is called fodder. The agroforestry tree species used by the New Forests Project are fast-growing, nitrogen-fixing tree species. Fast growth makes them an excellent choice to use in agroforestry systems because they provide farmers with large quantities of lumber, green manure and fodder in a very short time -- a year or two in most tropical areas.

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Nitrogen Fixation and Legumes

Nitrogen is the most abundant element in our atmosphere, but why is it important? Nitrogen is a vital component in the production of protein. Protein is one of the nutrients that plants and animals need to grow, survive and reproduce.

Although there is a large amount of nitrogen in the air, approximately 69,000 metric tons of nitrogen per hectare in the atmosphere, plants are unable to use it because it is chemically inert (bound to other molecules, such as oxygen). In fact in the United States and other countries, many farmers add nitrogen in the form of fertilizer to help plants grow faster and stronger. In areas that have been degraded or that lack sufficient chemical or natural fertilizer, there may be very little nitrogen in the soil for the plants to use for the production of protein. In these cases many trees have a very hard time establishing themselves and developing successfully.

The New Forests Project utilizes and distributes trees that can fix nitrogen from the air that is present in the soil surrounding their roots. But the trees cannot fix nitrogen alone they need the help of bacteria called Rhizobia to secure the nitrogen for them.

See what Rhizobia working with tree roots looks like!

These bacteria "infect" the young root cells of the trees and through a symbiotic relationship they form root nodules -- a nodule is a similar to a small compartment or pod. The bacteria live in the root nodules and produce nitrogen for the tree through nitrogen fixation. The trees supply the bacteria with nutrients and other compounds necessary for their survival. This is called a symbiotic relationship because both the tree and the Rhizobia help each other. Can you think of any other symbiotic relationships? Peas, beans, and clover are other plants called legumes which have the ability to fix nitrogen.

Only certain tress have the ability to fix nitrogen. In the United States Robinia psuedoacacia (black locust), Gleditsia triacanthos (honey locust) and Alnus species (Alder) are trees that can fix nitrogen. In the tropics Acacia, Leucaena and Prosopis species are examples of nitrogen fixing trees (there are many more than this though!).

When grown properly these species can; thrive on and regenerate degraded soils where other species may not survive, allow farmers to save money by reducing the need to purchase man-made fertilizer, stabilize soils, and provide fodder and lumber.

The New Forests Project distributes and utilizes a number of leguminous trees species in our projects to help farmers regenerate and replenish depleted soils.

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Mangrove Trees

coniferous or boreal forest


deciduous forests

green manure



"direct" rays






soil erosion

live or vegetative barriers


contour farming

Shade trees





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