114 Ecosystems of South America are young and include Mollisols, Alfisols, Vertisols, and Entisols Vegetation in the Pampean temperate grasslands is composed of about 1000 species and varies from grass-dominated communities in the east, to xeric woodlands and semidesert communities in the west In the Argentinean Pampas, plant communities are dominated by species of Agrostis, Bouteloa, Elyonurus, Festuca, Panicum, Paspalum, and Stipa In Chile and northward in the Andes, the ecosystems of grasslands are dominated by species of Andropogon, Bromus, Calamagrostis, Poa, Sporobolus, Eragrostis, and Distichlis It is difficult to determine the proportion of Patagonia that can be classified as grasslands, because the sparse vegetation is mostly dwarf shrubs and grasses not form a continuous cover, therefore many areas are treated rather as deserts High Andean Grasslands These ecosystems are found above the tree line (3000–4500 m) of the Andean range in Colombia, Venezuela, Ecuador (Paramo), Peru, Bolivia, and Chile (Puna) The actual lower limit of paramo and puna ecosystems depends on exposure, the climatic regime, and in particular precipitation The dominant life-forms of the Andean grasslands are giantrosette plants (known locally as "frailejon"), tussocks of grasses or sedges, acaulescent rosette plants, cushion plants, cacti, and sclerophyllous shrubs The giant-rosette plants can reach heights of several meters and have developed a number of morphological and anatomical adaptations including the retention of dead leaves that cover the shoot and provide insulation At higher elevations, the vegetation cover becomes sparser and shrubs and herbs replace bunch grasses Functional Aspects Tropical Savannas Relatively homogeneous temperature and a well-defined wet season characterize the Tropical Savanna (Aw) tropical savanna climate or dry–wet tropical climate (Figure 2) The alternation of wet and dry periods coincides with the presence of tropical savanna vegetation, characterized by open spaces covered by grasses and spaced trees Soils are mainly latosols of low fertility, formed by similar processes but not as deep as those found in the Af climate Savanna ecosystems are characterized by seasonal growth cycles that follow the annual precipitation cycles However, the phenology patterns of savanna plants are very asynchronous resulting in different patterns of productivity, reproduction, and decomposition Most grasses usually start growing at the beginning of the rainy season and reach reproductive stages early in the wet season or a few months later Some vegetative growth continues throughout the year but at the end of the dry season, all aboveground biomass is dead In perennial grasses, all the aboveground biomass dies by the end of the dry season and in the following year is replaced by vegetative growth of subterranean perennial organs In evergreen species, growth of leaf and shoot biomass and reproduction generally occur during the dry season Leaf development stops at the beginning of the wet season, and before the dry season begins, leaves are shaded In most evergreen species, however, growth of stems and other woody parts only occurs during the wet season Usually there are layers of laterites under savanna soils that originated during the Cretaceous and Tertiary periods Continuous weathering has produced soils poor in nutrients and with high acidity and concentration of aluminum that immobilizes phosphorous as iron–aluminum phosphate In the soil environment, marked annual differences occur particularly in the upper soil layers where moisture levels can exceed field capacity during the wet season and diminish to wilting point during the dry period The stock of nutrients in savannas varies over the year as they move from different compartments of the ecosystem The largest fraction of nutrients is stored in the aboveground biomass Savanna trees tend to accumulate fewer nutrients than similar dry forest species and some nutrients reallocate from leaves to other organs before shedding occurs Some nutrients accumulated in grasses return to the soil after fires but large fractions of nitrogen, phosphorous, and potassium are lost to the atmosphere when biomass burns Decomposition of leaf litter by microorganisms occurs during the wet season and termites consume most of the woody material during the dry period The flora of the Brazilian cerrado and other South American savannas has typical features of pyrophytic vegetation Trees are low, of contorted form, with thick, corky, fire-resistant barks; sclerophylly is common, and many leaves have thick cuticles, silicified tissues, and are often of considerable longevity The underground organs of perennial grass species are protected from fire and their seeds as well as those of annual species are adapted to fire; in fact, the germination of many grass species is stimulated by fire events (Eiten, 1972) Because savanna vegetation is adapted to resist burning, occasional natural fires must have been an environmental factor throughout its history Frequent burning favors the herbaceous, as opposed to the woody, component of the vegetation and when this occurs, savanna woodlands are transformed into grasslands Conversely, protection from fire often has the opposite effect and a type of dense savanna woodland with considerable shade and only a very sparse grass layer is established Some authors have correlated increasing production of woody vegetation with an increasing soil fertility gradient Soil moisture is another factor that controls many of the most obvious differences in vegetation in the savanna landscape (Eiten, 1972) In general, woody vegetation occurs only on soils well drained throughout the year Swampy gallery forest is found where the water table is permanently high Temperate Grasslands There are different hypotheses that attempt to explain the physiognomic origin of the temperate grasslands An initial hypothesis proposed that pre-Columbian people burned an original scrub forest Another hypothesis points to an adaptive dominance of grasses due to a climatic effect that maintains a negative water balance during part of the year Although there are significant regional differences, edaphic factors such as the fine texture of soils are proposed as major causes for the dominance of grasses A lack of air spaces in the soil combined with summer dry periods would favor the competitive ability of grasses over trees A variety of root growth habits is observed among both grasses and shrubs and the stratification of root