Fly south from the Amazon basin and the canopy below begins to open. Trees shorten, gaps widen, and patches of grassland appear between the forest stands. This is the Mato Grosso dry forest in its native logic. It is not one continuous cover but a mosaic - dry forest, gallery forest along the rivers, savanna patches on the ridges, dense thicket in the lowlands. The ecoregion reaches mostly across northern Mato Grosso state and spills into southern Amazonas, southern Para, and parts of Tocantins and Maranhao. In the northwest it meets the Madeira-Tapajos and Tapajos-Xingu moist forests; in the south and east it hands off to the Cerrado proper.

The terrain is not flat. The Serra do Cachimbo cradles the northeastern edge, the Serra dos Caiabis and Serra Formosa break the eastern horizon. All three are well-drained savanna ridges that sit higher than the surrounding plain. This is where the Xingu River begins - the Alto-Xingu, the upper drainage that will travel 2,000 kilometers north before it meets the Amazon. The central and western reaches of the ecoregion drain between the Serra do Roncador and the Serra do Formosa, and during the wet season from December to May whole stretches flood. In the dry months the porous soils drain the water off fast, leaving wetlands shrunken to pools and the dry forests crunching underfoot.

In 1967 and 1968 the British Royal Society and Royal Geographical Society mounted an expedition to the upper Xingu. What they found helped define what this ecoregion meant ecologically. They recorded 46 genera of rodents, including the capybara, the world's largest. They cataloged 52 genera of bats, with Artibeus and Molossus most common, plus marsupials and edentates. They logged 161 species of birds - 60 only in the cerrado patches, 33 only in the gallery forests, 25 only in the dry forest. The Serra do Cachimbo turned out to harbor an isolated population of the caatinga antwren, a species more usually associated with northeastern Brazil. The white-cheeked spider monkey, white-nosed saki, black bearded saki, and giant otter all live here, all of them endangered, all of them tied to a habitat that is shrinking.

The transformation began with pavement. The BR-070 and BR-364 highways, linking Brasilia to the state of Acre, cut through the region in the 1970s and 1980s. The BR-163, running from Cuiaba north to Santarem on the Amazon, followed. Each road was both a promise and a wound. They brought ranchers, farmers, loggers, and gold prospectors. Pasture replaced forest. Soy fields replaced pasture. A gold rush in some areas left mercury in the rivers. Between 2004 and 2011 the ecoregion lost 0.76 percent of its remaining habitat each year - a rate that compounds into a quiet calamity. The World Wildlife Fund lists the status as Vulnerable, a diplomatic word for a forest being dismantled in real time.

Not all of it is gone, and the reason is human as much as ecological. The Xingu Indigenous Park, created in 1961 by the brothers Villas Boas, sits at the heart of the ecoregion and protects both forest and the several indigenous peoples who live within it. The park is not a fortress - it faces its own pressures from encroachment, illegal logging, and fire - but it remains one of the larger islands of intact habitat in central Brazil. Around it the highways still hum. Global warming will push tropical species uphill in search of suitable temperatures, but the deforested corridors now stand between them and the places they need to go. A forest without migration routes is a forest with a countdown. The Mato Grosso dry forests are counting.