The Açu National Forest sits in the municipality of Assu — also spelled Açu — in the state of Rio Grande do Norte. The terrain is flat or gently undulating, part of the low, hot landscape that rolls inland from the Atlantic toward the drier interior of the sertão. Its only water body is Lago do Piató, a shallow lake with a maximum depth of ten metres that once was fed by floodwater from the Piranhas River, the major drainage of the region. After a dam was built on the Piranhas upstream, those floods no longer reach the lake, and Lago do Piató now survives on rainfall alone. That small hydrological change tells a story about the twentieth-century transformation of northeastern Brazil — the way almost every major river in the region was dammed for hydropower or irrigation, and the way many wetlands downstream quietly adjusted or died.

The forest is covered in Caatinga, the distinctive biome that blankets about 800,000 square kilometres of the northeastern Brazilian interior. Caatinga is a xeric shrubland — drought-adapted, thorny, and unexpectedly diverse — punctuated with cactuses and hardy trees that lose their leaves in the dry season and burst into colour at the first rains. Sixty-two plant species in forty-six genera have been identified inside this small forest, mostly from four families: Fabaceae (legumes, including many species of bean-family trees), Euphorbiaceae (the spurges), Cactaceae (the cactuses), and Boraginaceae (the borages). Typical species include Croton sonderianus, the quebra-faca bush; Caesalpinia pyramidalis, the catingueira; Auxemma glazioviana; Dipteryx odorata, the cumaru; and Tabebuia impetiginosa, the ipê-roxo, whose violet blooms are among the most beautiful sights in the Brazilian dry season. Small populations of the carnauba palm — Copernicia prunifera, source of the hard wax used in polishes and food coatings — also grow here.

The Caatinga supports a surprising range of wildlife, and Açu is no exception. Armadillos shuffle through the leaf litter at night; cavies — the wild relatives of guinea pigs — forage in the undergrowth; pampas deer graze at the forest edges. Marmosets, small primates with tufted ears and long striped tails, move through the canopy in family groups. Birds and reptiles fill out the list: Caatinga is home to more than 500 bird species across the region, including many endemics, and to a rich herpetofauna of lizards, snakes, and amphibians that emerge explosively during the short rainy season. The protected area is small enough that a visitor can walk its paths in half a day, but big enough to shelter genuine wildlife in a landscape where most neighbouring land has been cleared for cattle and goats.

The forest's creation is itself worth recounting. It began life on 10 August 1950 as the Horto Florestal de Açu — the Açu Forest Park — under Law 1.175, a local initiative supported by the Defence Committee of the National Forest. That committee was a civic group, not a government agency: local residents who had watched too much of the surrounding Caatinga fall to axes and bulldozers and organised to protect one piece of it. For more than fifty years it held that status, unofficially cared for by the community. Then, on 18 July 2001, Ordinance 245 formally upgraded the area to national forest status — making it the first national forest in Rio Grande do Norte and the third in the entire northeast region. In 2008 an advisory council was appointed to help manage the area alongside the federal Chico Mendes Institute for Biodiversity Conservation (ICMBio).

The Açu National Forest is classed by the IUCN as Category VI — a protected area with sustainable use of natural resources. That category is a compromise designed for places like this: small enough that complete human exclusion would be impractical, valuable enough that some protection is essential. The category allows sustainable multiple use of forest resources — controlled extraction, scientific research, limited public visitation — within the limits of conservation. For Açu, this means that the Caatinga inside the forest is living evidence of what the surrounding landscape used to look like, and a laboratory for working out how local communities can continue to use dry-forest products without destroying the dry forest itself. It is a small experiment, on 218 hectares, in the careful balance between people and biome that the entire Caatinga needs if it is to survive the twenty-first century.