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Rainforest |
Igapò forest is a rainforest that is regularly flooded by blackwater rivers for extended periods during the rainy season ( 4-10 months of the year ; sometimes considered permanently flooded rainforest). The best-known Igapò forests are found in the Amazon Basin where they make up about 2 percent of the total rainforest. The number of tree species that can grow in this ecosystem is limited by the nutrient poor blackwater that is also responsible for a lower variety of insect species that are able to pollinate trees. Igapò forest trees are shorter than those of non-flooded forests because of the instability caused by the wet, poorly drained soils (hence it is sometimes known as "swamp forest") and characterized by certain tree species like Cecropia, Ceiba, and Mauritia palms (also known as the aguaje palm). Many igapò tree species have stilt roots and flying buttresses to lend structural support. Fish play an important role in seed dispersal in this forest system. Blackwater forest areas support fewer plant or animal life forms because of the nutrient-poor water.
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Igapó Forest 
Varzeà forests are floodplain forests that flood seasonally. Unlike igapò forests, varzeà forests have relatively rich soils from the annual replenishment of nutrients from whitewater rivers. The decomposition of submerged organic matter speeded up by the high temperature and the action of bacteria and fungi, helps to fertilize the soil of these flood plains. The varzeà forests of the Amazon River can be as much as 97 km ( 60 miles) wide and can consist of groups of islands, lakes, canals, side streams, etc. depending on the water level.
Because the richer soil of these forests is more suitable for agriculture than typical rainforest, they are some of the most threatened. In the Amazon where the vast majority of such forests are found, varzeà forests are disappearing rapidly, victims of deforestation.
Terra Firme means "solid earth" and refers to a rainforest in slightly elevated terrain (60-200meters/200- 650 feet above sea level) that is not inundated by rivers. This type of forest covers most of the Amazon and its vegetation is noticeably taller and more diverse (>400 species/hectare in some areas) than that of igapò or varzeà forests. The terra firme forest has dry, well-drained soils and is characterized by such species as brazil nut trees, rubber trees, and many tropical hardwood trees.
Primary forest refers to an older forest characterized by taller trees (up to 65 m/213 ft.), often with epiphytes like bromeliads and several canopy layers. Undergrowth is less dense as less sun can reach below the forest canopies. Growth of the forest in this stage is slower and new species are added more slowly. This type of forest has the greatest biodiversity.
These are forests in the first stage of development of a forest area that has been heavily disturbed through logging, ‘slash and burn’ or fire. Forest growth is dense with shrubs, vines and tangles resembling what people refer to as “jungle”. Tree height is generally low and many plants have thorns or spines. Growth in this phase is rapid as sun can reach most plants close to the soil layer and new species enter quickly. Secondary forests typically have less biodiversity than old growth forests and usually have only one canopy layer. It takes a secondary forest typically 40 to 100 years to begin to resemble the original old growth or primary forest; however, in some cases a tropical secondary forest will not succeed due to erosion or soil nutrient loss.
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Soils |
In contrast to what the lush plant growth in the rainforest may indicate, tropical soils are nutrient poor. Soil is sandy and porous and nutrients are leached out of the soil by constant rain. In the rainforest, the climate is hot and wet all year round so plants and trees continue to grow every day of the year. Falling leaves collect on the forest floor and the hot, wet conditions help them rot quickly to release their nutrients. Fungi and bacteria also help to break down the material and the plant and tree’s network of shallow roots quickly absorb the nutrients. Tropical soils do not have fall and winter seasons like temperate climates in which fallen leaves have time to decompose and fertilize the soil. Year round growth means a quicker decomposition and absorption of nutrients by plants leaving little chance for it to penetrate deep into the soil. Poor soil highlights the devastating effects deforestation can have on this ecosystem since soils are quickly depleted of their nutrients without the rainforest to replenish it with fallen leaves and other plant matter. For perspective, an area of rainforest deforested for farming usually yields only 3 years of crops before the soil is depleted. This perpetrates the vicious cycle of further deforestation for new farmlands. A deforested area may take 40 to 100 years to return to its original state with many areas not succeeding in becoming reforested because of erosion or extreme nutrient loss. Educating the local populations to steer them away from traditional farming methods and towards “canopy farming” or farming within the forest is vital to the survival of the rainforest.
"Terra Preta do Indio" or "Indian Black Earth" is the name for dark earths found in the Amazon region. There is still considerable speculation and discussion as to its origin, distribution and properties. Many believe terra preta was most likely created by pre-Columbian Indians 500 to 2500 years ago and abandoned after the arrival of Europeans. These dark soils are not only very fertile (carbon content of 150 g of C/kg of soil vs. 20-30 g of C/kg soil for the surrounding soils) but can be as deep as 1-2 meters / 3-7 ft. or 40-50 cm/11-20 in. on average compared to 10-20 cm / 4-8 in. for surrounding soil. These fertile soils require fallow periods as short as 6 months compared to surrounding soils which need fallow periods of 8-10 years to restore fertility. The organic matter in the dark earth is very persistent (low leaching of nutrients) as levels of carbon remain high even after hundreds of years. Scientists believe the addition of black carbon or "bio-char" is responsible for black earth's desirable properties. Further research is underway to better understand these soils and develop techniques for recreating them.
