Várzea Forest
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A várzea forest is a seasonal
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...
forest inundated by whitewater rivers that occurs in the
Amazon biome The Amazon biome () contains the Amazon rainforest, an area of tropical rainforest, and other ecoregions that cover most of the Amazon basin and some adjacent areas to the north and east. The biome contains blackwater river, blackwater and whitewa ...
. Until the late 1970s, the definition was less clear and várzea was often used for all periodically flooded Amazonian forests. Although sometimes described as consisting only of forest, várzea also contains more open, seasonally flooded habitats such as grasslands, including floating meadows.


Description

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. The result is a rise in water level, with nutrient rich waters. The
Iquitos várzea The Iquitos várzea (NT0128) is an ecoregion of flooded forest along rivers in Brazil, Peru and Bolivia in the west of the Amazon biome. The forest is seasonally flooded up to by whitewater rivers carrying nutrient-rich sediment from the Andes. T ...
ecoregion covers the margins of the upper Amazon and its tributaries. Further down are the Purus várzea in the middle Amazon, the Monte Alegre várzea and Gurupa várzea on the lower Amazon and the
Marajó várzea The Marajó várzea (NT0138) is an ecoregion of seasonally and tidally flooded várzea forest in the Amazon biome. It covers a region of sedimentary islands and floodplains at the mouth of the Amazon that is flooded twice daily as the ocean tides ...
at the mouth of the Amazon. The Marajó várzea is affected by both freshwater and tidal flows. Várzea forests can be split into two categories: low várzea and high várzea. Low várzea forests can be categorized by lower lying areas where the annual water column has an average height of more than , where the period of flooding is greater than 50 days per year. High várzea forests are categorized as the areas where the average annual water column is less than high and flooding periods are less than 50 days per year. Amazonian várzea forests are flooded by nutrient rich, high sediment whitewater rivers such as the Solimões-
Amazon Amazon most often refers to: * Amazon River, in South America * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon (company), an American multinational technology company * Amazons, a tribe of female warriors in Greek myth ...
, the Purus, and
Madeira Madeira ( ; ), officially the Autonomous Region of Madeira (), is an autonomous Regions of Portugal, autonomous region of Portugal. It is an archipelago situated in the North Atlantic Ocean, in the region of Macaronesia, just under north of ...
rivers. This makes the várzea areas distinct from igapós, floodplains from nutrient poor blackwater. The water level fluctuations that the várzea experiences result in distinct aquatic and terrestrial phases within the year. Amazonian white water river floodplains cover an area of more than , and várzea forests cover approximately of the Amazon basin. 68% of the Amazonian river basin is located in
Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...
, with the remaining areas located in
Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...
,
Colombia Colombia, officially the Republic of Colombia, is a country primarily located in South America with Insular region of Colombia, insular regions in North America. The Colombian mainland is bordered by the Caribbean Sea to the north, Venezuel ...
,
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,
French Guiana French Guiana, or Guyane in French, is an Overseas departments and regions of France, overseas department and region of France located on the northern coast of South America in the Guianas and the West Indies. Bordered by Suriname to the west ...
,
Peru Peru, officially the Republic of Peru, is a country in western South America. It is bordered in the north by Ecuador and Colombia, in the east by Brazil, in the southeast by Bolivia, in the south by Chile, and in the south and west by the Pac ...
,
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, and
Guyana Guyana, officially the Co-operative Republic of Guyana, is a country on the northern coast of South America, part of the historic British West Indies. entry "Guyana" Georgetown, Guyana, Georgetown is the capital of Guyana and is also the co ...
. The várzea extends from this basin upward into the land before reaching slopes into the terra firme forests. Várzea habitats are generally diverse, consisting of forests, grasslands, lakes, flooded fields, and swamps. Approximately 75% of the várzea areas consist of dense canopy forests, with the other 25% being represented by the remaining habitats. 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.


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 and serve as important breeding grounds for fish, birds, mammals, and reptiles. In order to grow and survive in this environment, both plants and animals must have a large range of morphological, anatomical, physiological and ethological adaptations. 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. Both
Amazonian manatee The Amazonian manatee (''Trichechus inunguis'') is a species of manatee that lives in the Amazon Basin in Brazil, Peru, Colombia and Ecuador. It has thin, wrinkled brownish or gray colored skin, with fine hairs scattered over its body and a white ...
s (''Trichechus inunguis'') and river dolphins (''Inia geoffrensis'') spend time in várzea areas during high water periods of the flood season. In addition,
black caiman The black caiman (''Melanosuchus niger'') is a crocodilian reptile endemic to South America. With a maximum length of around and a mass of over , it is the largest living species of the family Alligatoridae, and the third-largest crocodilian in ...
(''Melanosuchus niger'') are abundant and also play an important ecological role. The charismatic
jaguar The jaguar (''Panthera onca'') is a large felidae, cat species and the only extant taxon, living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat spe ...
(''Panthera onca'') is also found in a variety of várzea habitat types. In the várzea, tree species richness, distribution and vegetation structure are influenced by a variety of characters such as topography and sedimentation dynamics.
Leguminosae Fabaceae () or Leguminosae,International Code of Nomen ...
and
Euphorbiaceae Euphorbiaceae (), the spurge family, is a large family of flowering plants. In English, they are also commonly called euphorbias, which is also the name of Euphorbia, the type genus of the family. Most spurges, such as ''Euphorbia paralias'', ar ...
are two of the most important and abundant families within both high and low várzea habitats. Due to the cycles of both sedimentation and erosion, habitat types of different successional stages occur simultaneously within limited areas of the várzea. 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. 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.


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. 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 Soil fertility refers to the ability of soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality.
declines. 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. People have been known to plant
rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...
,
corn Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...
,
bean A bean is the seed of some plants in the legume family (Fabaceae) used as a vegetable for human consumption or animal feed. The seeds are often preserved through drying (a ''pulse''), but fresh beans are also sold. Dried beans are traditi ...
s, peppers, and
banana A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large treelike herbaceous flowering plants in the genus '' Musa''. In some countries, cooking bananas are called plantains, distinguishing the ...
s in várzea floodplains bordering rivers. 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. Historically, cattle ranching on natural pasture areas was a principal economic activity in the Amazonian floodplains, an activity that has continued into the present. However, in modern times land is usually cleared for cattle ranching. In some areas, such as the floodplains of
Amapá Amapá (; ) is one of the 26 federative units of Brazil, states of Brazil. It is in the North Region, Brazil, North Region of Brazil. It is Federative units of Brazil#List, the second-least populous state and the eighteenth-largest state by area ...
, cattle herds have been traded in favor of
water buffalo The water buffalo (''Bubalus bubalis''), also called domestic water buffalo, Asian water buffalo and Asiatic water buffalo, is a large bovid originating in the Indian subcontinent and Southeast Asia. Today, it is also kept in Italy, the Balkans ...
, which do better in the wetter várzea habitat. Both cattle and water buffalo cause damage to river banks and can lead to extensive
soil erosion Soil erosion is the denudation or wearing away of the Topsoil, upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, Atmosphere of Ea ...
.


Human impacts

One of the main impacts people have had on the várzea is
deforestation Deforestation or forest clearance is the removal and destruction of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. Ab ...
for logging and agricultural purposes. Várzea forests show promise for timber production and
forest management Forest management is a branch of forestry concerned with overall administrative, legal, economic, and social aspects, as well as scientific and technical aspects, such as silviculture, forest protection, and forest regulation. This includes man ...
within Amazonia because they are less species rich than terra firme forests, but still with great amounts of timber. 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. 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. 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 degradation Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a soil health, stable soil. Retrogression is primarily due to soil erosion and corresponds to a phenomenon where succession revert ...
within the várzea habitat. It has been 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. An additional major impact from humans seen in the várzea is the extraction or mass production of the açaí palm (''Euterpe oleracea'') for the palm or for the well known açaí berry. The juice obtained from açaí is a major part of the diet for some regional populations within the Amazon and in some regions is the most important monetary crop. In açaí agroforests, other species surrounding clumps of the palm are commonly heavily pruned to remove competition. People have also had a severe impact on animal populations in the várzea. For example, selective hunting within the várzea of the red howler monkey (''Alouatta seniculus'') and the tufted capuchin monkey (''Cebus apella'') has had large effects on the regeneration of fig ('' Ficus sphenophylla'') trees. Both species of monkey are known to be important seed dispersers within the várzea.


See also


References


Related literature

* * {{DEFAULTSORT:Varzea Forests Amazon rainforest Tropical and subtropical moist broadleaf forests Wetlands Fluvial landforms Amazon basin