Climate Change Impacts on South American Rangelands

Abstract

South America is highly heterogeneous in terms of climate, ecosystems, human population distribution, and cultural traditions. Rangelands are located within regions with climatic conditions from arid to subhumid, and with mean annual precipitation ranges from approximately 150 to 1500 mm (6-60 inches). The bound aries of these regions are subject to modification by local soil conditions, evaporative demands, elevation, and topography. Within South America, rangelands cover 33% of the total land area (Fig. 1), and include grasslands, shrublands, savan nas, and hot and cold deserts, but exclude hyperarid deserts.1 In terms of vegetation, rangeland ecosystems of South America have an important herbaceous component, and woody vegetation can range from scattered dwarf shrubs to an almost continuous canopy of small-stature trees. This coexistence of multiple life forms occurs because plants access soil resources differently. Trees, with deeper and more extensive root systems, are able to access deeper soil moisture and nutrients; grasses, with fine, intensive root systems, are better able to exploit resources in the upper soil layers. Fire and flooding play important roles in maintaining the balance between herbaceous and woody vegetation. Frequent flooding leads to open grasslands, whereas better drained areas support savanna species or woodland vegeta tion. Large grazing mammals, mainly livestock, have a pronounced effect upon the vertical/partitioned structure of savanna grasslands. The herbaceous layer in rangelands with greater rainfall is composed of both C3 and C4 species, which typically have different growth requirements; C3 species achieve maximum productivity in the cooler, early spring, whereas C4 species have maximum productivity in the warmer late spring or early summer. Rangeland in cold climates of the southern South America is populated by C3 species. South American rangelands include the Patagonian Steppe, the Monte, the Pampas, grass and savanna wood land known as Llanos or Cerrado, the open dry thorny woodlands called Chaco, and the dry thorn scrub named Caatinga2 (Fig. 1). Rangelands are associated with climates with pronounced dry seasons, high fire frequencies, and highly variable (and unusual) soil properties. Over much of South America, change in the intensity and location of tropical thunderstorms is the fundamental driver of climate, but weather patterns other than tropical storms also play a role through the year in southern South America. The Andes serve as a continental barrier along the Pacific coast of South America and the Amazon Basin has the distinction