A forest shaped by altitude and ice
The Southern Andean Yungas is a narrow band of forest clinging to the eastern slopes of the Andes in northwestern Argentina and southern Bolivia. This ecoregion exists at elevations between 800 and 3,000 meters, creating a steep environmental gradient where life must adapt to specific altitudes. The forest intercepts moisture-rich trade winds from the east, resulting in annual rainfall that can exceed 2,500 millimeters and maintaining a humid, evergreen environment. This consistent moisture supports a dense cloud forest where trees, seldom taller than 15 meters, are covered in mosses, ferns, and orchids.
These forests are remnants of larger, continuous forests that existed before the Quaternary glaciation, which began around 2.6 million years ago. During past ice ages, as glaciers advanced and climates cooled, the surrounding lowlands became progressively drier. The Yungas valleys, however, remained moist, becoming isolated refuges. This isolation allowed ancient plant and animal lineages to persist while they vanished elsewhere. The complex topography of steep ridges and deep valleys created many microclimates, further encouraging the evolution of new, localized species.