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BASIC Várzea floodplain forests are seasonal floodplain forests inundated by an annual 10-15 meter rise in water level by nutrient rich white water rivers that occur along the Amazon River Basin (Parolin et al. 2004). Along the Amazon River and many of its tributaries, high annual rainfall that occurs mostly within a rainy season results in extensive seasonal flooding of areas from stream and river discharge (Junk et al. 2010). Várzea forests can be split into two categories: low várzea and high várzea (Wittman et al. 2006). Low várzea forests can be categorized by lower lying areas where the annual water column has an average height of more than 3 meters, where the period of flooding is greater than 50 days per year (Wittman et al. 2006). High várzea forests are categorized as the areas where the average annual water column is less than 3 meters high and flooding periods are less than 50 days per year (Wittman et al. 2006).
Amazonian várzea forests are flooded by nutrient rich, high sediment white water rivers such as the Solimões-Amazon, the Purus, and Madeira Rivers (Prance 1979). This makes the várzea areas distinct from igapós, floodplains from nutrient poor black water (Junk et al. 2010). The water level fluctuations that the várzea experiences result in distinct aquatic and terrestrial phases within the year (Junk et al. 1989). Amazonian white water river floodplains cover an area of more than 300,000km2 (Junk 1997), and várzea forests cover approximately 180,000km of the Amazon basin (Thorbjarnarson & Da Silveira 2000). 68% of the Amazonian river basin is located in Brazil, with the remaining areas located in Bolivia, Columbia, Ecuador, French Guiana, Peru, Suriname, and Guyana (Junk et al. 2010). The várzea extends from this basin upward into the land before reaching slopes into the terra firme forests (Richey & Devol 1986).
Várzea habitats are generally diverse, consisting of forests, grasslands, lakes, flooded fields, and swamps (Richey & Devol 1986). Approximately 75% of the várzea areas consist of dense canopy forests, with the other 25% being represented by the remaining habitats (Melack and Hess 2010). Within the várzea, topographic variation leads to different flooding durations and severity, resulting in vegetation being separated where plants with different tolerances to flooding inhabit different areas (Parolin et al. 2004).
SPECIES RICHNESS
Due to the renewal of soil nutrients caused by the annual white water flooding, várzea forests are some of the most productive areas of Amazonia (Morán 1990, Morison et al. 2000) and serve as important breeding grounds for fish, birds, mammals, and reptiles (Goulding 1996). In order to grow and survive in this environment, both plant and animals must have a large range of morphological, anatomical, physiological and ethological adaptations (Junk 1993). For example, during the flooding season, fish and other aquatic organisms take advantage of the lower density of predators, which have migrated or are confined to smaller, dryer areas, and use this time to reproduce (Junk 1993).
According to Arraut et al. (2009) both Amazonian manatees (Trichechus inunguis) and river dolphins (Inia geoffrensis) spend time in várzea areas during high water periods of the flood season. In addition, black caiman (Melanosuchus niger) are abundant and also play an important ecological role (Da Silveira & Thorbjarnson 2000). The charismatic jaguar is also found in a variety of várzea habitat types (Ramalho & Magnusson 2008).
In the várzea, tree species richness, distribution and vegetation structure are influenced by a variety of characters such as topography and sedimentation dynamics (Parolinet al. 2004). Leguminosae and Euphorbiaceae are two of the most important and abundant families within both high and low várzea habitats (Wittman et al. 2006). Due to the cycles of both sedimentation and erosion, habitat types of different successional stages occur simultaneously within limited areas of the várzea (Wittman et al. 2002). Most species tend to grow in areas that are subjected to the river flood pulse, but not in areas that are influenced by the sea and tides (Junk et al. 1989). This results in waterlogged or flooded conditions for up to seven months of the year, and consequently plant species numbers in the várzea are lower than the non-flooded terra firme forests (Parolin et al. 2004).
SOILS AND AGRICULTURE
Like many tropical environments, in the várzea there is evidence that nitrogen is a limiting factor for primary production; however, nitrogen reaches the floodplain via flooding as dissolved and particulate matter (Junk 1997).
In general, soils of the várzea initially are very fertile from the white water flooding, but after two or three years of cultivation the soil fertility declines (Fageria et al. 1991). Because of the várzea forests inherent nutrient rich environment and the proximity to rivers, which act as transport routes for local peoples, these areas have historically favored human occupation, resulting in the most densely populated environment of Amazonia (Móran 1990). People have been known to plant rice, corn, beans, peppers, and bananas in várzea floodplains bordering rivers (Williams & Kricher 2001). In terms of seed dispersal, local peoples have influenced both the establishment and regeneration of várzea forests by controlling populations of fish, birds, rodents, and other seed dispersers and also by removing natural vegetation to make room for agriculture and cattle (Pined-Vasquez 1995). Historically, cattle ranching on natural pasture areas was a principle economic activity in the Amazonian floodplains, an activity that has continued into the present (Zarin et al. 2001). However, in modern times land is usually cleared for cattle ranching. In some areas, such as the floodplains of Amapá, cattle herds have been traded in favor of water buffalo, which do better in the wetter várzea habitat (Zarin et al. 2001). Both cattle and water buffalo cause damage to river banks and can lead to extensive soil erosion (Zerin et al. 2001).
IMPACTS One of the main impacts people have had on the várzea is deforestation for logging and agricultural purposes. Várzea forests show promise for timber production and forest management within Amazonia because they are less species rich than terra firme forests, but still with great amounts of timber (Uhl et al. 1997). In addition, due to the fertile alluvial soils within várzea forests, trees typically grow more rapidly in the várzea than within terre firme forests and transport of logs is made easy by the use of the river (Uhl et al. 1997). Most likely a result of the above reasoning, within Amazonia logging has traditionally been centered in the várzea, and only in recent years has expanded into terra firme areas (Uhl et al. 1997). In addition to deforestation of the várzea via logging, landowners may also dig channels to widen or connect streams in várzea areas in order to more easily transport logs or move to new, unexploited areas of the forest, which leads to loss of forest and soil degredation within the várzea habitat (Raffles 1998). Zarin et al. (2001) found that from 1976 to 1991 the areal extent of mixed species várzea had a net decrease of 25%, while other types of surrounding habitats, such as palm forest or herbaceous cover, increased. The main impacts associated with the loss of várzea areas were logging, heart of palm extraction, and agricultural conversion (Zarin et al. 2001). People have also had a severe impact on animal populations in the varzea. For example, selective hunting within the várzea on the red howler monkey (Alouatta seniculus) and the tufted capuchin monkey (Cebus apella) have had large effects on the regeneration of Fig (Ficus spp.) trees (Pinedo-Vasquez 1995). Both species of monkey are known to be important seed dispersers within the várzea (Pinedo-Vasquez 1995).
REFERENCES
Arruat, E.M., M. Marmontel, J.E. Mantovani, E.M. Novo, D.W. Macdonald, R.E. Kenward. 2009. The lesser of two evils: seasonal migrations of Amazonian manatees in the Western Amazon. Journal of Zoology. 280(3):247-256.
Fageira, N.K., A.B. Santos, I.D.G. Lins, and S.L. Camargo. 1997. Characterization of fertility and particle size of várzea soils of Mato Grosso and Mato Grosso do sul states of Brazil. Communications in Soil Science and Plant Analysis. 28(1):37-47.
Goulding, M., N.J.H. Smith, D.J. Mahar. 1996. Floods of fortune: ecology and economy along the Amazon. Columbia University Press, New York, USA.
Junk, W.J. 1993. Wetlands of tropical South America. Wetlands of the World: Inventory, ecology, and management. Kluwer Academic Publishers, Netherlands. 1:679-739.
Junk, W.J. 1997. The Central Amazonian Floodplain: Ecology of a Pulsing System. Ecological studies Vol. 126, Springer, Berlin.
Junk, W.J., P.B. Bayley, and R.E. Sparks. 1989. The flood pulse concept in river-floodplain systems. Pages 110-127 in D.P. Dodge, editor. Proceedings of the International Large River Symposium (LARS). Canadian Special Publication of Fisheries and Aquatic Sciences, Ottawa, Canada.
Junk, W.J. and M.T.F. Piedade. 2004. Status of knowledge, ongoing research, and research needs in Amazonian wetlands. Wetlands Ecology and Management. 12:597-609.
W.J. Junk, M.T.F. Piedade, J. Schongart, M. Cohn-Haft, J.M. Adney, and F. Wittman. 2011. A Classification of Major Naturally-Occuring Amazonian Lowland Wetlands. Wetlands. 31:623-640
Morán, E.F. 1990. Ecologia Humana das populações de Amazônia. Editora Vozes, Rio de Janeiro.
Parolin, P., L.V. Ferreira, A.L.K.M. Albernaz, & S. Almeida. 2004. Tree Species Distribution in Varzea Forests of Brazilian Amazonia. Folia Geobotanica. 39(4):371-383.
Pinedo-Vasquez, M. 1995. Human impacts on várzea ecosystems in the Napo-Amazon, Peru. Dissertations and Thesis database.
Ramalho, E.E. and W.E. Magnusson. 2008. Uso do habitat por onça-pintada (Panthera onca) no etorno de lagos de várzea, Reserva de Desenvolvimento Sustentável Mamirauá, Am, Brasil. Ukari 4(2):33-39.
Thorbjarnarson, J. and R. Da Silveira. 2000. Secrets of the flooded forest. Natural History 109:70-79.
Uhl, C., P. Barreto, A. Verissimo, E. Vidal, P. Amaral, B.C. Barros, C. Souza, J. Johns, J. Gerwing. 1997. Natural resource management in the Brazilian Amazon. Bioscience. 47(3):160-168.
Zarin, Daniel J., Valeria Pereira, Hugh Raffles, Fernando G. Rabelo, Miguel Pinedo-Vasquez, Russel G. Gongalton. 2001. Landscape Change in tidal floodplains near hte mouth of the Amazon River. Forest Ecology and Management. 154:383-393.
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