Amazonian rainforests have been broadly classified into terra firme (unflooded forest), seasonal várzea (forest flooded periodically by white-water or water rich in suspended sediments), seasonal igapó (forest flooded periodically by black or clear water low in suspended sediments) and tidal várzea (forest flooded twice daily by freshwater backed by the incoming tide). There are also permanent white-water swamp forest and permanent igapó but tidal igapó does not seem to occur.

Lower Amazon Varzea

Varzea at the mouth of the Amazon is characterized by extensive poorly drained areas in an intricate mosaic of sediment islands and channels know as ‘the region of islands’. The floods are tidally influenced. The incoming tide pushes huge volumes of river water into the surrounding areas, and so this varzea differs from the seasonally flooded varzea in the middle and upper Amazon. It also differs in having both forests and flooded savannas. On Marajó Island (Ilha Marajó) the varzea forest has a low canopy dominated by palms such as Astrocaryum murumura, Euterpe oleracea, Jessenia bataua, Manicaria saccifera, Maurita flexuosa, Maximiliana regia, Oenocarpus bacaba, Raphia taedigera and Socratea exorrhiza together with various species of Geonoma. Other fairly common trees include Macrolobium acaciifolium, Mora paraensis, Pachira aquatica, Symphonia globulifera, Triplaris surinamensis and various important timber trees like Cedrelinga castanaeformis, Ceiba pentandra and Virola surinamensis. Several indigenous trees including Mauritia flexuosa, Mouriri ulei and Spondias mombim produce fleshy fruits that provide an important food source for the resident fauna including fruit eating fish. A number of large lianas occur such as Guatteria scandens, Landolphia paraensis and Strychnos blackii and various shrubs may be encountered. One species, Machaerium lanatum, often forms dense thickets on riverbanks. On the large sediment island of Ilha Grand de Gurupá most of the vegetation is flooded savanna rather than forest. Here grasses such as Echinochloa polystachya, Hymenachne amplexicaulis, Leersia hexandra, Luziola spruceana, Panicum elephantipes, Paspalum fasciculatum and species of Oryza occur together with various sedges like Cyperus luzulae, Scirpus cubensis and Scleria geniculata.

Central Amazon Varzea

In the river basin of the central Amazon, which includes much of the Caquetá / Japura, Juruá, Purus and Solimões river systems, seasonal flooding can last for up to 8 months and reach a depth of 12 m. Here unlike the upper Amazon varzea, the varzea forests are very distinct with a diverse under story and are generally more species-rich that lower Amazon varzea. However, unlike the lower Amazon, palms are poorly represented. On the levees the most abundant trees are Ceiba pentandra, Eschweilera albiflora, Hura crepitans, Neoxythece elegans, Parinari excelsa and Piranhea trifoliata. In addition to Ceiba pentandra, which is an enormous tree with large buttress roots, several other important timber trees occur in this part of the Amazon including Calycophyllum spruceanum, Carapa guianensis and Iryanthera surinamensis. The few palms include Astrocaryum jauari, A. murumura, Mauritia flexuosa, andspecies of Bactris such as the local endemic Bactris tefensis (Arecaceae). Other charactersitic trees are Euterpe oleraceae and Virola surinamensis, both of which are restricted to flood forests, and Ceiba burchellii, Coumarouna micrantha, Manilkara inundata, Ochroma lagopus, Parkia inundabilis and Septotheca tessmannii. Several trees, including Astrocaryum jauari, Rollinia deliciosa and Spondias mombin produce fleshy fruits that are critical to the survival of certain fruit-eating fish. Among the characteristic under storey plants are species of the families Heliconiaceae, Maranthaceae and Zingerberaceae.

Upper Amazon Varzea

In the river basins of the upper Amazon, including much of the Maránon, Madre de Dios and Ucayali river systems, the river floods twice a year due to seasonal input first from the Peruvian Andes and then from the Ecuadorian Andes. The floods can last for up to 10 months and reach a depth of up to 7 m. This has created a zonation of successional stages with a pioneer zones dominated by grasses such as Echinocloa polystachya, Gynerium sagittatum and Paspalum repens. In more developed areas shrubs like Adenaria floribunda, Alchornea castanaefolia and Salix martiana have become established but pioneer trees like Annona hypoglauca, Astrocaryum jauari and Cecropia latiloba eventually succeed these. The climax forest is finally dominated mainly by species of the genera Chorisia, Eschweilera, Hura, Spondias and Virola, but some of the more conspicuous trees include Calycophyllum spruceanum, Ceiba samauma, Cedrela odorata, Copaifera reticulata and Phytelephas macrocarpa. Like the central Amazon, the undergrowth here includes many species of the plant families Heliconiaceae, Maranthaceae and Zingerberaceae. However, the varzea here is less distinctive than in the central and lower Amazon, but as a consequence tends to be more species-rich. Moving slightly south to the Braga-Supay and Zobillo zones east of the town of Jenaro Herrera in Peru, the varzea appears to have a different species composition. Here the varzea (or tahuampa) forest is dominated by Eschweilera parrifolia and E. turbinata (Lecythidaceae), while other important large trees include Campsiandra angustifolia (Fabaceae), Licania micrantha (Chrysobalanaceae), Luehea cymulosa (Sparrmanniaceae) and Parinari excelsa (Chrysobalanceae), Other characteristic species of this varzea included Coccoloba densifrons (Polygonaceae), Duguetia spixiana (Annonaceae), Pouteria procera (Sapotaceae), Pseudoxandra polyphleba (Annonaceae), Sapium glandulosum (Euphorbiaceae) but palms are absent. In forest (described as high restinga) where flooding is less prolonged and may only last for about one month the vegetation is markedly different and dominated by large trees like Ceiba pentandra (Malvaceae), Guarea macrophylla (Meliaceae), Hura crepitans (Euphorbiaceae), Maquira coriacea (Moraceae), Spondias mombin (Anacardiaceae) and Terminalia oblonga (Combretaceae). Palms are also present and one species, Scheelea brachyclada (Arecaceae) was one of the dominant trees. Notable species in the lower strata included Drypetes amazonica (Putranjivaceae), Leonia glycicarpa (Violaceae), Protium nodulosum and Theobroma cacao. Surprisingly it was found that the forests with the longest flooding period contained the most species but this was not as rich as adjacent terra firma forest.

Eastern Igapó Forest

On the Rio Tapajós clear water river in eastern Amazonia flooding occurs between December and June. The most abundant trees are Campsiandra laurifolia (Fabaceae), Couepia paraensis (Chrysobalanaceae), Leopoldinia pulchra (Arecaceae) and Tabebuia barbata (Bignoniaceae), but overall the most species-rich families are Chrysobalanaceae and Fabaceae. Other typical trees, which appear to be common to all Amazonia’s Igapó forests, include Excellodendron coriaceum, Humiria balsamifera, Humiriastrum cuspidatum, Licania apetala and Panopsis rubescens. However, these forests were considered to be less species-rich than other Igapó forests studied in the Brazilian Amazon. The number of tree species ranged from 21-30 per hectare while the number of families ranged from 10-13. This relatively low diversity was mainly put down to the fact that these forests have a longer flooding period and lower nutrient soils.

Central Igapó Forest

In the igapó forest along the Rio Tarumã-Mirim an affluent of the Rio Negro about 20 km north of Manaus, floods can last for seven months (between December and June) and reach depths of 15 m submerging roots, seedlings, shrubs and small trees. The forest can be divided into four vertical layers with a canopy reaching 12 m or so. The typical canopy species include Aldinia latifolia (Chrysobalanaceae), Erisma calcaratum (Vochysiaceae), Parkia discolor (Fabaceae) and Swartzia polyphylla (Violaceae), but the two tallest species, Aldinia latifolia and Erisma calcaratum, can reach heights of up to 25 m and often emerge above the canopy. In the sub canopy, which ranges in height from about 8-10 m, the main species are Caraipa grandifolia (Clusiaceae), Eschweilera albiflora (Lecythidaceae), Parkia pectinata (Fabaceae) and Zygia cataractae (Fabaceae). The third level ranges from 4-7 m and typically includes Faramea sessilifolia, Ferdinandusa rudgeoides (Rubiaceae), Microplumeria anomala (Apocynaceae) and Virola elongata (Myristicaceae). The shrub layer at 1-3 m mainly comprises Tabernaemontana rupicola (Apocynaceae). Other characteristic igapó trees found here include Amphirrhox surinamensis, Swartzia argentea (Violaceae), Couepia paraensis (Crysobalanaceae), Eugenia inundata (Myrtaceae), Crudia amazonica, Licania heteromorpha, Macrolobium acaciifolium (Caesalpiniaceae) and Tabebuia barbata (Bignoniaceae). Many of these species have a high tolerance to flooding and are restricted to blackwater inundation areas (igapó) and form a fairly homogenous stand along much of the Rio Negro (black river), although there is a degree of zonation with species more tolerant of flooding at lower levels and less tolerant species at high levels.  To judge the species-richness of these forests a study based on four 25 x 10 m plots identified 44 tree species in 38 genera and 22 families, with the most important families being Caesalpiniaceae, Clusiaceae, Fabaceae, Lecythidaceae and Rubiaceae. This igapó forest is considered to have a high level of endemism.


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