Overgrazing, timber and fuelwood extraction, plant collecting, desertification, irrigation, modern agricultural expansion, salinization. Each of these three regions represents a different geological province and is subject to a different climatic regime, and each has originated very distinct xeromorphic vegetation. Both originated from very old Precambrian rocks, severely degraded during the Tertiary, and overlain by more recent marine sandstones and other sediments.
Rainfall is highly irregular, leading to catastrophic droughts and floods.
The caatinga as delimited in Map 55 does not correspond to a single type of vegetation, but is a broad mosaic of types. This robust formation occurs where there is sufficient rain and the soils are deep enough. Arborescent-shrubby open caatinga , with the total ground coverage of trees, shrubs, cacti, bromeliads, etc. Shrubby open caatinga , or xerophytic open tropical scrub, which can be composed of deciduous broadleaved species, cacti and bromeliads, or mixtures of the same. Common throughout the caatinga on very shallow soils or rocky outcrops.