The Southwest Amazon moist forests (NT0166) is an ecoregion located in the Upper Amazon basin.
The forest is characterized by a relatively flat landscape with alluvial plains dissected by undulating hills or high terraces. The biota of the southwest Amazon moist forest is very rich because of these dramatic edaphic and topographical variations at both the local and regional levels. This ecoregion has the highest number of both mammals and birds recorded for the Amazonian biogeographic realm: 257 with 11 endemic species for mammals and 782 and 17 endemics for birds. The inaccessibility of this region, along with few roads, has kept most of the habitat intact. Also, there are a number of protected areas, which preserve this extremely biologically rich ecoregion.
Location
The southwest Amazon moist forest region covers an extensive area of the Upper Amazon Basin comprising four sub-basins: (1) both the Pastaza-Marañon and (2) Ucayali River sub-basins drain into the Upper Amazon River in Peru; (3) the Acre and (4) Madre de Dios-Beni sub-basins drain to the east into the Juruá, Purus and Madeira Rivers; which, in turn, feed into the Amazon River lower down in Brazil. The region is bisected north to south between Peru and Brazil by the small mountain range Serra do Divisor. It extends east to the edge of the Purus Arch, or ancient zone of uplift, in the southwestern area of the Brazilian State of Amazonas. It then extends southeast into northern Bolivia and in a narrow band south along the base of the Andes Mountains. Elevations range from in the west to on the eastern edge of the region.
Landforms present in this region include the upland terra firme (non-flooded) mostly on nutrient-poor lateritic soils, ancient alluvial plains (mostly non-flooded) on nutrient-rich soils, and present alluvial plains (várzea, seasonally flooded) of super-rich sediments renewed with each annual flood. Floristically, distinct lowland humid forest types occur on each of these landforms with the terra firme mature forests and late successional, seasonally flooded forest being the two major types. Permanent swamp forests are common on the alluvial plains. Pockets of nutrient-poor white sand soils are found here that host forests of lower height, a more open forest canopy, and lower alpha diversity, but with many endemics. The forests are mostly dense tropical rain forest, but some patches of open forest exist.
The ecoregion contains stretches of Iquitos várzea along the main rivers, blending into Purus várzea near the eastern border, where it adjoins the Juruá–Purus moist forests ecoregion. In the southeast, it adjoins the Purus–Madeira moist forests and Madeira–Tapajós moist forests, and in the south merges into the Beni savanna and Bolivian Yungas. In the southwest, it adjoins the Peruvian Yungas. To the west, it adjoins the Ucayali moist forests. In the north it is separated by a band of Iquitos varzea from the Solimões–Japurá moist forests.
General description of flora
Because the ecoregion covers such a vast area, there are climatic, edaphic and floristic differences within it. Generally, the wetter and less seasonal northern forests ( of rain annually) share only 44 percent of the tree species with forests in the slightly drier, more seasonal southern region. This region receives from of rain annually, in different parts. Temperatures over the year range from .
At first glance, large areas may appear to be homogeneous dense forests with a canopy high with some emergent trees to towering above the canopy. Structurally, this may be the case; however, the species composition reflects much the opposite: tree species variability reaches upwards to 300 species in a single hectare. There are a few exceptions to this high diversity, mainly where stands dominated by one or several species occur. The first are vast areas (more than ) dominated by the highly competitive arborescent bamboos Guadua sarcocarpa and Guadua weberbaueri near Acre, Brazil extending into Peru and Bolivia. Other monodominant stands include swamp forests of the economically important palms Mauritia flexuosa and Jessenia bataua.
