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Igapó is a word used in Brazil for blackwater-flooded Amazonian forests. These forests and similar freshwater swamp forests are seasonally inundated with freshwater. They normally occur along the lower reaches of rivers and around freshwater lakes. Freshwater swamp forests are found in a range of climate zones, from boreal through temperate and subtropical to tropical. In the Amazon Basin of Brazil, a seasonally whitewater-flooded forest is known as a várzea, which is similar to igapo in many regards; the key difference between the two habitats is in the type of water that floods the forest[1].

=Characteristics=

Freshwater swamp forests are known as natural ecosystems with over 30% canopy cover. The average canopy height of these forests has been measured to range from 20-25 m, however, there have been sparse measurements of trees reaching 33-36 m. Tree composition includes a varied mixture of leaf types and seasonality. Development of tree communities within the freshwater swamp forests rely heavily on microtopography and flooding intensity. Swamp forests can have harsh environments, in which only a few adapted species are able to sustain life or dominate over other tree species. One of the tree species mostly found in freshwater swamp forests are legume trees, in which most swamp forests have high single legume species populations. Legume trees in these forests are thought to be abundant in N2-fixation, which is most common in waterlogged locations and is thought to be a favorable trait when experiencing seasonal and/or constant waterlogging. [2]

Freshwater swamp forests can be classified into two categories of forested wetlands, which are permanently and periodically flooded forests. It is possible to find these two types because of the high microtopographic variation. Due to these topographic differences, tree communities are delimited and have a distribution over small continuous areas. [2]

These forests may be able to sustain a large amount of bird, mammal, reptile, amphibian, fish and invertebrates species, however, biodiversity varies between wetland ecosystems and the species richness of the freshwater swamp forests is not entirely known. As for plant species, these forests have a lower diversity compared to other types of forests, such as the terra firme, in South America. [2]

Jaú National Park. (Source: NASA Earth Observatory News)

=Soil Chemistry=

Igapó forests are characterized by sandy acidic soils that have a low nutrient content. The color and acidity of the water is due to the acidic organic humic substances (e.g. tannins) that dissolve into the water. The acidity from the water translates into acidic soils. They are the more nutrient poor than a comparable várzea (whitewater inundated) forest. They also “carry less suspended inorganic elements and contain elevated concentrations of dissolved organic material such as humic and fulvic acids”.[3] Therefore, igapó forests support little life and the environment found within these areas tend to lack species diversity and animal biomass. Várzea forest soils have high nutrient contents because they receive a transport of high sediment loads from the whitewater rivers. In contrast, igapó forests do not receive this seasonal influx of sediments which explains the nutrient poor soils. However, they do have the highest phosphorous concentrations out of comparable várzea and terra firme forest soils. Nevertheless, the seasonal inundation of the soils leads to anoxic conditions which limits plant growth.[3]

=Flora=

Igapó and other flooded forests typically display a lower diversity of plants than that seen in terra firme forests. Similar to other forests found throughout the tropics, it is common to observe only a few dominant tree species [2]. Distribution of trees and other plants is highly dependent on inundation tolerances of species causing a non-random distribution of plants where more flood-tolerant species are found at lower elevations and less flood-tolerant species are found on higher ground [5]. Fabaceae (or Leguminosae) and Euphorbiaceae are the dominant families of plants observed in igapó forest [3,5].

=Seed dispersal=

Tree species adapted to seasonal inundation have adapted to maximize fruit production during periods of flooding in order to take advantage of newly available seed dispersal methods. Fish consume nearly all fruit that fall into the water – species that are unable to digest the seeds eventually excrete and disperse them into the water. Dispersal by other vectors such as birds and monkeys is secondary to that of fishes in igapó. An important factor in seed survival is the species of fish that consumes the seed – characins tend to destroy all seeds consumed. Fishes that lack the strong jaws found in characins, e.g. catfish, digest the fleshy material of the fruit while the seeds pass through the gut unharmed. Because many catfishes are bottom-feeders, they are critical dispersers for seeds that sink upon entering the water [6].

=Fauna=

Diversity of terrestrial wildlife and fishes in igapó is highly influenced by flooding. During periods of inundation, vast numbers of fish species migrate into the flooded forest in search of food. Fish populations are highest during maximum fruiting periods [6]. Compared to terra firme forests, igapo features lower nutrients and favors slow-growing plants with low fleshy fruit production – this has resulted in a lower diversity and abundance of animals [7]. Larger terrestrial mammals have varied strategies of dealing with flooding: red brocket deer and collared peccaries move into islands and shift diet from fruit to browsing woody plants. White-lipped peccaries and tapirs are largely unaffected by seasonal inundation; both species take advantage of the greater abundance of fruit during these times. White-lipped peccaries migrate between flooded and dry areas and tapirs are highly capable swimmers [8]. Arboreal mammals such as monkeys tend to be lower in species richness compared to terra firme forests due to the decrease in tree diversity and the resulting lack of variety of suitable food types [5]. A lower population density of birds also reflects the lack of nutrients found in igapo forests [9].

=Conservation=

Deforestation is the most important conservation concern because Igapó forests are characterized by slow tree growth due to low soil nutrient content and high soil acidity. Igapó and other floodplain forests have high biodiversity with some partially endemic species. That being said, Igapó forests provide humans with natural resources. However, these resources grow back more slowly than other types of forests in the Amazon because of the characteristics of the soil. This means that these forests cannot be used for timber sustainably. Igapó forests should be fully protected from timbering and deforestation because of the low productivity found in them.[10]

=Notable Areas=

Igapó forests are forests that are seasonally inundated by blackwater. These are distributed in multiple climate zones throughout the world, such as boreal, temperate, tropical and subtropic zones. They are found in the Afrotropics, Australasia, Indomalaya, and the Neotropics. However, they are most notably found in Amazonian areas. In Brazil, they are found in Cantão State Park and Jaú National Park. Notable Areas with Igapó Forests Cantão State Park Igapó forests are one of the distinct natural communities found in the park. They offer a rich bird fauna that includes some habitat specialists due to the unique ecosystem the blackwater inundation provides. The trees in the igapó bloom once the inundation season begins. They do this so they can drop their fruit into the water to be dispersed by the different fish taxa. Jaú National Park The Jaú National Park was designated in 1980 and is the largest national park in the Amazon Basin and the second largest protected tropical forest. It is known for its high level of biodiversity and varying ecosystems within. The park protects the entire watershed of the Jaú River, which happens to be one of the best examples of a blackwater ecosystem where the water is colored by acidic decomposing organic matter. The igapó forest in the Jaú National Park has 194 species of birds (58 of which are restricted to the area and 38 that are aquatic), 87 families of insects and 8 species of shrimps.[4]

=References=

[1] http://www.scribd.com/doc/58789361/Rain-Water-Harvesting-by-Freshwater-Flooded-Forests

[2] Koponen, Piia, Pekka Nygren, et al. "Tree Species Diversity and Forest Structure in Relation to Microtopography in a Tropical Freshwater Swamp Forest in French Guiana." Plant Ecology. 173.1 (2004): 17-32. Web. 9 Oct. 2012. .

[3] Haugaasen, Torbjørn and  Peres, Carlos Augusto. Floristic, edaphic and structural characteristics of flooded and unflooded forests in the lower Rio Purús region of central Amazonia, Brazil. Acta Amaz. [online]. 2006, vol.36, n.1 [cited 2012-10-07], pp. 25-35 .

[4] United Nations Environment Programme-World Conservation M. "Jaú National Park, Brazil." Jaú National Park, Brazil. The Encyclopedia of Earth, 17 Nov. 2008. Web. 09 Oct. 2012. .

[5] Parolin, P., J. Adis, et al. "Floristic study of an igapó floodplain forest in Central Amazonia, Brazil (Tarumã-Mirim, Rio Negro)." Amazoniana. XVII.1/2 (2004): 29-47. Print.

[6] Kubitzki, Klaus, and Albrecht Ziburski. "Seed Dispersal in Flood Plain Forests of Amazonia." Biotropica. 26.1 (1994): 30-43. Web. 9 Oct. 2012. .

[7] Fleagle, J. G.; Janson, Charles; and Reed, Kaye. Primate Communities. Cambridge University Press, 1999. Cambridge Books Online. http://dx.doi.org/10.1017/CBO9780511542381

[8] Bodmer, Richard E. "Responses of Ungulates to Seasonal Inundations in the Amazon Floodplain." Journal of Tropical Ecology. 6.2 (1990): 191-201. Web. 9 Oct. 2012. .

[9] Haugaasen, Torbjørn, and Carlos A. Peres. "Population abundance and biomass of large-bodied birds in Amazonian flooded and unflooded forests." Bird Conservation International. 18.2 (2008): 87-101. Web. 9 Oct. 2012. .

[10] Ferreira Da Fonseca Júnior, Sinomar, Maria Teresa Fernandez Piedade, and Jochen Schöngart. "Wood Growth of Tabebuia Barbata (E. Mey.) Sandwith (Bignoniaceae) and Vatairea Guianensis Aubl. (Fabaceae) in Central Amazonian Black-water (igapó) and White-water (várzea) Floodplain Forests." Trees 23.1 (2009): 127-34. SpringerLink. Springer Business and Science Media, 22 Aug. 2008. Web. 4 Nov. 2012.