Várzea Forests: Seasonal Floodplain Ecoregions (South America)

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Várzea Forests: Seasonal Floodplain Ecoregions (South America)

Thu, 06/04/2020 - 18:28
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Várzeas are extensive lowland floodplain areas bordering the Amazon River and its tributaries. As a result, Várzea forests are subject to seasonal flooding and may also contain more open, seasonally flooded habitats such as grasslands, including floating meadows.

Várzea Forests: Seasonal Floodplain Ecoregions

A várzea forest is a seasonal floodplain inundated by whitewater rivers in the Amazon Biome. Although sometimes described as consisting only of the forest, várzea also contains more open, seasonally flooded habitats such as grasslands, including floating meadows.

Along the Amazon River and many tributaries, high annual rainfall that occurs mainly within a rainy season results in extensive seasonal flooding of areas from the stream and river discharge. The result is a 10 - 15 m (33 - 49 ft) rise in water level, with nutrient-rich waters.

The Iquitos várzea ecoregion covers the margins of the upper Amazon and its tributaries. Further down is the Purus várzea in the middle Amazon, the Monte Alegre várzea and Gurupa várzea on the lower Amazon and the Marajó várzea at the mouth of the Amazon. The Marajó várzea is affected by both freshwater and tidal flows.

Várzea habitats are 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 separated vegetation where plants with varying tolerances to flooding inhabit other areas.

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.

To grow and survive in this environment, plants and animals must have extensive 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.

Iquitos Várzea

The Iquitos várzea is an ecoregion of flooded forests along rivers in Brazil, Peru, and Bolivia west of the Amazon Biome. The forest is seasonally flooded up to 7 m (23 ft) by whitewater rivers carrying nutrient-rich sediment from the Andes.

The meandering rivers often shift course, creating a complex landscape of oxbow lakes, marshes, levees and bars with grasslands, shrubs and forests in different stages of succession. During extended flood periods, fish enter the forest in search of fruit.

The forest is accessible by the navigable rivers that run through it and has suffered from extensive deforestation to extract timber and create pasture for livestock.

This ecoregion comprises the low, seasonally inundated river basins of the upper Amazon, Ucayali, Marañon, and Madre de Dios in Peru and Bolivia, several smaller tributaries to the Amazon in Peru and the upper Juruá and Purus rivers in Brazil.

A large portion of the region centers around the extensive seasonally flooded plain in northeastern Peru at the confluence of the Marañon and Ucayali Rivers that join to form the Amazon.

The Pacaya and Simiria rivers bisect this plain. The large urban center of Iquitos lies at 100 m (328 ft) elevation, and the topography, on the whole, is very flat with micro undulations resulting from river meander.

Flora and Fauna

Characteristic trees of the ecoregion include the buriti palm and Jessenia batuaua. The water’s edge is sometimes dominated by shrubs of the camu-camu fruit that flourish with the flooding. Heliconia and ginger dominate parts of the understory, along with palms and epiphytes.

The Iquitos Várzea ecoregion has over 225 species of mammals, including the aquatic boto, manatee, and giant otter. Other noteworthy mammals include Goeldi’s marmosets, Andean night monkeys, monk sakis, spiny rats, and at least 4 species of opossum.

At least 624 species of birds are found in this ecoregion, which is part of the Upper Amazon-Napo lowlands Endemic Bird Area. There are 9 endemic restricted range birds, including the Cocha antshrike, black-tailed antbird, and white-masked antbird.

Map depicting the location of the Iquitos várzea (in purple)

Map depicting the location of the Iquitos várzea (in purple)

Purus Várzea

The Purus várzea covers a region of seasonally flooded forest in the central Amazon Basin. It is home to vegetation adapted to floods of up to 12 m (39 ft) that may last eight months.

There is a great variety of fish and birds but relatively fewer mammals. Ground-dwelling mammals must migrate to higher ground during the flood season. Threats include logging, cattle farming, over-fishing and mercury pollution from gold mining.

Avifauna diversity is extraordinary, with over six hundred and thirty species. However, terrestrial mammal diversity is smaller because the habitat is often flooded.

Two narrow endemic primates inhabit this region, the white uakari monkeys (Cacajao calvus calvus) and blackish squirrel monkeys (Saimiri vanzolinii). Also, the largest snake in the world, the great anaconda (Eunectes murinus), is found here.

Much of the ecoregion is affected by human presence because of the waterways used for transportation.

Map depicting the location of the Purus várzea (in purple)

Map depicting the location of the Purus várzea (in purple)

Monte Alegre Várzea

The forests of this ecoregion extend along the low, seasonally flooded rivers of the central and lower basin of the Amazon River, including a large part of the Madeira River basin, the mouth of the Purus River, tributaries of these rivers and an isolated patch of várzea along the Mamoré River between Bolivia and Brazil.

The ecoregion adjoins the Madeira-Tapajós moist forests to the southeast and the Uatuma-Trombetas moist forests and Japurá-Solimões-Negro moist forests to the north.

The Purus-Madeira moist forests lie west of the Madeira River and south of the Amazon River. The Purus várzea is upstream along the Solimões and Purus rivers and tributaries. The Gurupa várzea is downstream along the Amazon.

Biodiversity is exceptionally high in this flooded forest along the lower Amazon. In terms of avifauna, there are 681 reported bird species, including red-shouldered macaws (Ara nobilis), sun parakeets (Aratinga solstitialis) and green-rumped parrotlets (Forpus passerinus).

Over two hundred species of mammal are found here, including jaguars (Panthera onca), ocelots (Leopardus pardalis), tapirs (Tapirus terrestris), and several primate species.

Major population centers in or near the ecoregion are Manaus, Itacoatiara, Coari and Óbidos. The few protected areas in this ecoregion are threatened by cattle ranching and large-scale agriculture.

Map depicting the location of the Monte Alegre várzea (in purple)

Map depicting the location of the Monte Alegre várzea (in purple)

Gurupa Várzea

The Gurupa várzea is an ecoregion of seasonally and tidally flooded forest along the Amazon River in the Amazon Biome. The ecoregion is named after the Ilha Grande de Gurupá, an alluvial island in the mouth of the Amazon.

The várzea extends along the lower Amazon River from the mouth of the Tapajós River down to the mouth of the Xingu River. It has an area of 984,195 ha (2,432,000 acres).

The Tapajós-Xingu moist forests lie south, and the Uatuma-Trombetas moist forests lie north. The Monte Alegre várzea is upstream, and the Marajó várzea is downstream along the Amazon.

The flooded forests of this ecoregion exemplify the incredible adaptability of species. Trees, grasses, and shrubs can be partially submerged underwater for months.

Animals and fish move to and from the area in synchronization with the floods to feast on the fruits produced by the trees. As a result, the diversity of resident species is high in these tropical savannas and includes the scaled spinetail.

Map depicting the location of the Gurupa várzea (in purple)

Map depicting the location of the Gurupa várzea (in purple)

Marajó Várzea

The Marajó várzea ecoregion covers a region of sedimentary islands and floodplains at the mouth of the Amazon River in eastern Brazil. This area is flooded twice daily as the ocean tides push the river waters onto the land. As a result, islands are numerous throughout the region.

The flooded forests provide food for fruit-eating fish, aquatic mammals, birds and other fauna. However, it has no protected areas and is threatened by cattle and water buffalo ranching, logging and fruit plantations.

This flooded area captures nutrient-rich soils carried down the river; tidal activity floods the region twice daily. As a result, vegetation is shorter than in surrounding areas, plant diversity is lower, and palms dominate. However, fauna diversity is richer; avifauna is particularly rich, with about 540 species.

There are no protected areas in this ecoregion. Cattle and water buffalo ranching and large-scale commercial logging are the main threats to this ecoregion.

Map of the Amazon River Basin

Map of the Amazon River Basin