Essay on Amazonian Forests

he majority of the 7 million km2 of the Amazon Forest is constituted by a dry land forest ("Floresta de Terra Firme"). This is a forest that is never flooded, spread

across a great plain of up to 130-200 metres of altitude, up to the bottom of the mountains. This great plain corresponds to the sediments left by the lake "Belterra", which occupied the greatest part of the Amazon basin during the Miocene and the Pliocene, between 25 thousand and 1.8 million years ago. The silt and clay deposited in this old lake have been submitted to a gentle mass elevation movement, while the Andes rose and modern rivers began to excavate their beds. Thus the three kinds of Amazonian forests have appeared: the highland forests of the Andes, the dry land forests and the flooded river forests, both the last ones in the Brazilian Amazon.

The climatic fluctuations of the Pleistocene Age were a repeated succession of cold / dry - hot / humid - hot / dry climates. The last cold / dry phase dates from 18 thousand to 12 thousand years ago, when the Amazon climate was semi-arid, with an average temperature reduced by up to 5Ñ"C. Following it, there was a return to the hot / humid climate, which reaches at the maximum around 7 thousand years ago. Since then, with many oscillations of a lesser scale, we live a relatively hot / dry climate.

Very important was the fact that during the semi-arid phase the great dry land forest was divided and fragmented by open vegetation formations, of the type of "cerrados", "caatingas" and "campinaranas" (open prairie-land which changes little by little into woodland), all better adapted to the dry climate. The forest survived in "refuges" situated in areas with higher soils and better water supply. In returning to the more humid climate, the forest expanded again, in detriment to the "cerrado" vegetation. Nowadays, the "cerrado" survives in its own "refuges", within the immensity of the "matas de terra firme". This floating process will undoubtedly repeat itself, unless man interferes in the situation.

The "matas de terra firme" has countless adaptations to the clay and podzol soils, poor in nutrients. The trees that form it are capable of stocking up with nitrates through nitrogen fixing bacterias, which are linked to their roots. Besides that, a great variety of fungi, also symbiont of the roots, called mycorrhizas, rapidly recycle the organic material before it is leached. The ground litter (formed by the debris vegetation that fall on the soil) is rapidly recycled by the fauna rich in insects, especially beetles, ants and termites. The insects constitute the majority of the animal biological mass in the dry land forest.

This forest, especially rich in epiphytal araceaes, is relatively poor in bromelias and orchids, in comparison to the Atlantic Forest. Among these epiphytes are the myrmecophites, plants which have a symbiotic relation with ants. In the sub-woods of the forest stand out especially palm trees and lianas; large ferns are rare.

The macrofauna of the forest grounds is relatively poor. The frogs found there have various adaptations in order to guarantee the necessary water for the development of the tadpoles. The large mammals include tapirs, "catetos" (peccaries) and "queixadas" (wild boars), and among the ground birds of distinction are "mutuns" (curassow) and inambus. Close to the ground there are also many ant-eating birds ("papa-formigas") profiting from the enormous migrations of the army ants.

The great animal diversity is found in the tree tops, between 30 and 50 metres high, an environment of difficult access to the researcher. The bird fauna is rich there, with parrots, toucans and woodpeckers. Especially eye-catching are the "pavгozinho do parб" and the "cigana" (hoatzin). The predominant mammals living in the tree tops are marsupials, bats, rodents and monkeys. The primates have very differentiated niches. The "bugio" monkey is a daytime animal, and feeds preferably on leaves. The night monkey "Aotus" is the only nocturnal monkey. There are many species and sub-species of voracious insectivorous sagoins, differentiated by the colours and shape of their faces. Alongside the classic pollinators - bees, butterflies and birds - the Amazon forest monkeys also have an important role in the pollination. The birds, bats and frugivorous monkeys of the dry land forest have an important role on disseminating the fruits and seeds from the trees.

The species and sub-species of monkeys, sloths, squirrels and others are often separated by the large tributary rivers of the Amazon. The biogeographical units formed by these rivers basins explain, in part, the great diversity of the Amazon biota. Also relevant are the areas of the forest which served as a refuge to the various differentiated populations during past periods of arid climate mentioned above, when large areas of "cerrado" used to fragment the Amazon Forest. Nowadays it is the indiscriminate deforestation that has been fragmenting the dry land forest. Without the necessary precautions, whole faunistic provinces and former centres of formation of species risk being obliterated forever.

The flooded forests are within reach of annual floods of the Amazon River and its closest tributaries. The fluctuations of the water level can get to ten metres or more. From March to September large portions of the riverside forest are flooded. The plants and animals of the flooded Amazonian Forest live because of the several special adaptations for surviving the floods.

The Amazonian waters have different characteristics, as a result of the geology of its river basins. The rivers called white or clouded water rivers, like the SolimÑ...es and the Madeira, traverse lands rich in minerals and organic suspensions. The so-called black water rivers, such as the Negro, originating from sandy soils poor in minerals, are transparent and coloured brown by the humic substances. There are also rivers of clear waters like the Tapajуs, which originate in areas of the old continental shields, also poor in minerals and nutrients.

The woodlands bathed by the white waters are normally called "florestas de vбrzea" (meadow forests) and those bathed by the clear and black waters are called "igapу" forests. The "vбrzea" vegetation is much richer than the "igapу" vegetation, due to the fertility of the white waters and the alluvial soils carried by them. The same is noticed in the fauna of the two kinds of forests,