Forest types of the Amazon region.The great extent of the Amazonian forests, together with the regional peculiarities of the soil types and river systems within the Basin, allows a number of very distinctive forest types to exist, each with its own
specialized flora. By far the largest area is taken up by tierra firme forest, which is the 'standard' forest type that one usually has in mind when referring to 'the' Amazon rainforest. Tierra firme forest occupies fairly well-drained soils that are
relatively rich in available nutrients. It is tall (40m or more) and usually very rich in tree species, with a dense upper canopy and a relatively dark, open interior.Other forest types occur under conditions on soils that are particulary poor in nutrients, or in areas subject to fire or flooding. White sand caatinga is a low, scrubby forest with small hard leaves, that occurs in places where the soil is mainly
bleached sand. It is scattered throughout the Amazon region, but particularly in the Rio Negro catchment area where the soils are formed on ancient sea beaches and other sandy deposits.Liana forest (known as cipoal in Brazil) is a relatively open forest in terms of its tree cover, but is almost smothered by huge numbers of vines growing on, over and between the trees. It often seems to occur on the intensively weathered latosols
which are associated with important ore deposits of iron (as in the Serra dos Carajas region) and aluminium, but it can also be found on the more fertile terra roxa soils. The liana forest tends to form a close patchwork with other types of forest, and
is particularly abundant in the area between Maraba and Itaituba through which the TransAmazon Highway passes.Another type of more open forest which is widespread in Amazonia is palm forest, dominated by the babacu palms (Orbignya spp). Palm forest is often partly the product of human influence, with frequent burning helping to encourage the
fire-resistant babacu. In terms of the indigenous economy of the forested regions, this forest type is important for the Brazil-nut trees (Bertholletia excelsia) which are also present.Although there are some areas of permanent swamp forest, this is a much more minor component than the seasonally inundated forests that line the riverbanks and floodplains of the Amazon and its tributaries. The highly seasonal nature of the
inundation is a result of the vast extent of the Amazon catchment. The Basin stretches north and south across the equator and westwards as far as the Andes, so when the rainy season reaches its peak in either the north or the south, the runoff is
sent rushing downstream in a great burst. Hence, the water levels of rivers even in the relatively non-seasonal areas of the Amazon Basin closest to the equator can reflect this seasonality occurring in the catchment areas far upstream.Although there are some areas of permanent swamp forest, this is a much more minor component than the seasonally inundated forests that line the riverbanks and floodplains of the Amazon and its tributaries. The highly seasonal nature of the
inundation is a result of the vast extent of the Amazon catchment. The Basin stretches north and south across the equator and westwards as far as the Andes, so when the rainy season reaches its peak in either the north or the south, the runoff is
sent rushing downstream in a great burst. Hence, the water levels of rivers even in the relatively non-seasonal areas of the Amazon Basin closest to the equator can reflect this seasonality occurring in the catchment areas far upstream.There are various types of seasonally or intermittently inundated forests along the river systems of the Amazon Basin. Where a river is fed from clay-soil areas, the suspended sediment in the water tends to be be deposited over the bank tops as
it floods. This builds up broad low ridges, known as levees, along either side of the river, channelling its flow and limiting the extent of flooding. This levee-top and floodplain forest, known as varzea, is very similar in structure to the terra firme
forest. The trees tend to be heavily buttressed (perhaps an adaptation for anchorage in the moist clayey soils), and often have seeds with special water flotation mechanisms that enable them to be dispersed when the river is in flood. The rubber
tree (Hevea brazilensis) is an example of a species which is clearly adapted to the varzea habitat, having seeds which can float for up to two months and which form an important part of the diet of certain large fish. Perhaps because of the regular
input of nutrient-rich sediment, the herbaceous understory layer is particularly lush and rich in species of such plants as gingers (Zingiberaceae) and heliconias (Scitaminae).On clearwater and blackwater rivers, in areas where the underlying sediment consists of bleached podsolized sand, a very different type of flood forest exists. This is the igapo of the Rio Negro and Rio Xingu regions, a relatively short and
species-poor forest growing under conditions of low nutrient supply. Many of the trees that are specialized to grow in the igapo are members of the family Myrtaceae (for example, Eugenia inundata), and they are zoned according to slightly
different degrees of flooding. Partly because of the lack of any constricting levees, and also because of the relatively short stature of the trees, the floods which occur in Igapo forests are often high enough to reach the crowns of the trees. Thus at the
height of the annual flood, it is possible to have the surreal experience of gliding a canoe through a rainforest canopy! The flooding often lasts for several weeks or even several months, and the trees need special biochemical adaptations to be able
to survive the lack of oxygen around their roots. During the flood season many of the trees drop their fruit into the water, where they are eaten by fish. It seems that some of these trees are strongly dependent on the fish to disperse their seeds
through the forest, requiring the seed to have passed through the gut of a fish before it will germinate.