Tocantins–Araguaia–Maranhão moist forests

The Köppen climate classification is "Am": equatorial, monsoonal. Temperatures are fairly steady throughout the year, slightly cooler in August and slightly warmer in April. Average temperatures range from 22 °C (72 °F) to 32 °C (90 °F) with a mean temperature of 27.5 °C (81.5 °F).[4] The ecoregion experiences a dry season for five months of each year, with monthly rainfall of less than 100 millimetres (3.9 in).[4] This is most marked in the northwest of Maranhão.[1] Rainfall is lowest in September, with less than 50 millimetres (2.0 in), and greatest in March, with over 375 millimetres (14.8 in).[4] During the wet season the streams and rivers expand and flood the low lying regions. Annual rainfall is about 2,500 millimetres (98 in) in the north, and 1,500 millimetres (59 in) in the drier south where the moist forest merges into cerrado shrubland.[2]

Mahogany Swietenia macrophylla

The ecoregion may be a diversification center for many taxa of trees, and in the past the west of the region may have been a refugium. There is a wide variety of species in the dense evergreen rainforest due to the many rivers and to the transitional nature of the flora between the Amazon basin and the drier southern vegetation. The ecoregion contains flooded forests and terra firme forests.[1]

The flooded forests are either igapó, flooded daily by clear blackwater rivers, or várzea, flooded daily by whitewater rivers that take their color from suspended soil and organic matter. The igapó forests are adapted to acidic white sand soils that are poor in nutrients. The trees are lower and less diverse than in the terra firme forests. Common species in the igapó and várzea include Caraipa grandiflora, Virola surinamensis, Euterpe oleraceae, Ficus pulchella, Mauritia martiana, Symphonia globulifera, and members of the Tovomita and Clusia genera.[1]

The most common families in the terra firme forests are Lecythidaceae, Chrysobalanaceae, Burseraceae, Fabaceae, Lauraceae and Sapotaceae. Tree species include Lecythis odora, Lecythis turbinata, Cenostigma tocantina, Bombax tocantinum, and Bauhinia bombaciflora, a large liana. The legume Vouacapoua americana is the most important timber tree, growing only in the east of the Amazon region. Brazil nut (Bertholletia excelsa) is uncommon in the region and there are few orchids. The threatened mahogany (Swietenia macrophylla) may be found along the upper Capim and Guamá rivers. Other rare or threatened trees include Pilocarpus microphyllus and Dicypellium caryophyllatum.[1]

149 species of mammals have been recorded, of which over 80 are bats. Mammals include red-handed howler (Alouatta belzebul), red-handed tamarin (Saguinus midas), brown-throated sloth (Bradypus variegatus), Linnaeus's two-toed sloth (Choloepus didactylus) and nine-banded armadillo (Dasypus novemcinctus).[1] Endangered mammals include white-cheeked spider monkey (Ateles marginatus), black bearded saki (Chiropotes satanas) and giant otter (Pteronura brasiliensis).[5]

There are more than 76 species of snakes. The rivers have many fish and aquatic reptiles.[1] Endangered reptiles include green sea turtle (Chelonia mydas), hawksbill sea turtle (Eretmochelys imbricata) and Maranhão slider (Trachemys adiutrix).[5]

Yellow-rumped cacique (Cacicus cela) in the Parque Ambiental Adhemar Monteiro in Paragominas

517 species of birds have been recorded, including two species of heron that are uncommon in other parts of the Amazon region, tricolored heron (Egretta tricolor) and yellow-crowned night heron (Nyctanassa violacea). Other birds include toucans (genus Ramphastos), red-throated piping guan (Pipile cujubi), white-crested guan (Penelope pileata), parrots, parakeets and many migrant birds from the Nearctic realm.[1] Endangered birds include red-necked aracari (Pteroglossus bitorquatus) and yellow-bellied seedeater (Sporophila nigricollis). [5]