Since 2005, loggers, cattle ranchers, and soybean farmers have been pressing deeper into southern Amazonas state. The advance follows a pattern so consistent that scientists have given it a name: the arc of deforestation, a crescent-shaped front of forest loss that curves across the southern and eastern edges of the Amazon basin. The Aripuana National Forest was explicitly designed as part of a shield against that advance. It occupies land spanning three municipalities: Novo Aripuana holds 73.71 percent of the forest, Apui accounts for 17.03 percent, and Manicore claims the remaining 9.27 percent. To the west, the Manicore Biological Reserve and the Campos de Manicore Environmental Protection Area lock arms with the national forest. To the east, the 896,407-hectare Acari National Park extends the barrier. Together, these protected areas form a continuous wall of conservation that stretches across the southern Amazon.

The Aripuana River, a tributary of the Madeira, flows north through the heart of the forest. To its north lies the Juma Sustainable Development Reserve; to its south, the Trans-Amazonian Highway cuts its east-west line through the landscape. The forest sits in the Amazon biome's dense rainforest zone, with some pioneer formations where the canopy has been naturally disturbed. Vegetation remains largely intact here, having suffered less deforestation than other parts of the Amazon. The forest floor supports a remarkable diversity of palm species, while the upper canopy shelters endangered trees: Bertholletia excelsa, the Brazil nut tree that can live for 500 years; Swietenia macrophylla, the big-leaf mahogany prized by the timber industry; and Aniba rosaeodora, the rosewood whose essential oil has driven centuries of extraction. The Madeira River basin, which the Aripuana feeds, is estimated to harbor roughly 800 species of birds and an unusual concentration of primates, including species found nowhere else.

The Chico Mendes Institute for Biodiversity Conservation, known as ICMBio, conducted the studies that led to the forest's creation, supported by the Ministry of the Environment and financed by the Amazon Protected Areas Program, or ARPA. When the presidential decree establishing the forest was signed on May 11, 2016, it came bundled with four other conservation units: the fully protected Manicore Biological Reserve at 359,063 hectares, the Acari National Park at 896,407 hectares, the Campos de Manicore Environmental Protection Area at 151,993 hectares, and the Urupadi National Forest at 537,228 hectares. The same package expanded the existing Amana National Forest by 141,000 hectares. In total, the Dilma government created approximately 3.4 million hectares of new protected areas during her administration, a significant figure that nonetheless paled beside the roughly 26.8 million hectares designated by her predecessor, Luiz Inacio Lula da Silva.

Unlike a biological reserve, where human activity is almost entirely prohibited, a national forest in Brazil permits sustainable use. The distinction matters enormously for the people who live here. The Aripuana National Forest is designed to let the local economy grow through sustainable forest management: harvesting timber, Brazil nuts, and other forest products at rates the ecosystem can regenerate. The soils are acidic and low in fertility, poorly suited for the agriculture and cattle grazing that have consumed forest elsewhere in the Amazon. That geological fact is, paradoxically, the forest's best defense. What cannot be profitably farmed is less tempting to clear. ICMBio administers the forest, balancing the needs of traditional residents who have lived within the forest for generations against the pressures of an extractive economy that would prefer to take more than the land can give. The forest also supports scientific research, offering a vast outdoor laboratory for studying biodiversity in one of the least-explored corners of the Amazon.