Várzea forest

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A várzea forest is a seasonal floodplain in the Amazon that is flooded by whitewater rivers. It includes not only dense forest but also open areas like grasslands and floating meadows. Heavy rains during the rainy season cause floods that can rise 10–15 meters and bring nutrient-rich water. The várzea stretches along the main Amazon River and its tributaries, with different named regions such as Iquitos, Purus, Monte Alegre, Gurupa, and Marajó. Marajó is influenced by both freshwater and tides.

There are two types of várzea: low várzea and high várzea. Low várzea has more than 3 meters of average annual flood depth and floods for more than 50 days a year. High várzea has less than 3 meters of flood depth and floods for fewer than 50 days a year. Floods come from very nutrient-rich whitewater rivers like the Solimões-Amazon, Purus, and Madeira, which makes várzea different from the nutrient-poor igapó (blackwater floodplain).

The water level changes create both aquatic and land phases in the year. The Amazon whitewater floodplains cover more than 300,000 square kilometers, and the várzea itself covers about 180,000 square kilometers, mostly in Brazil (about 68%), with parts in several other countries. The várzea extends into the upland terra firme forests.

Habitats in the várzea are diverse: forests, grasslands, lakes, flooded fields, and swamps. About 75% of the area has a dense canopy, while the remaining 25% includes other habitats. Topography and sedimentation shape where plants grow, so species with different flood tolerances occupy different spots.

Várzea is extremely productive because floods renew soils with nutrients. It’s a key breeding ground for fish, birds, mammals, and reptiles. Animals like river dolphins and manatees spend time there during high water, and black caimans are common. Jaguars can be found in various várzea habitats. Plant life is influenced by topography and sediment; Leguminosae and Euphorbiaceae are especially important. Because of the floods, some areas are waterlogged for up to seven months, so plant diversity is lower than in the drier terra firme forests.

Nitrogen often limits primary production, but floods bring nitrogen in dissolved and particulate form. Soils are very fertile at first but tend to lose fertility after a few years of cultivation.

People have long lived in the várzea because of its fertile soils and easy river access. They grow crops like rice, corn, beans, peppers, and bananas along the floodplains. Humans also influence seed dispersal by managing fish, birds, and other animals, and by clearing vegetation for agriculture and cattle.

Cattle ranching has been a major activity in many floodplains, and water buffalo are favored in wetter areas. Cattle and buffalo can erode riverbanks and soils. Deforestation is a major threat from logging and agricultural conversion. Historically, logging has focused on várzea, with channels cut to move logs, which damages forest and soil. From 1976 to 1991, mixed-species várzea declined by about 25% as other habitats grew.

The açaí palm (Euterpe oleracea) is widely harvested for its fruit and palm products, and in agroforests its surrounding species are often pruned to reduce competition. Hunting also affects várzea animals and their role in seed dispersal; for example, red howler monkeys and tufted capuchins are hunted, reducing seed dispersal for important fig trees.