Ecosystems of South America physiognomy and floristic composition and the lowering of canopy height At the same time, the degree of deciduousness of the trees of the overstory increases The phenology of these ecosystems is determined by a seasonal climate with a period of intense summer rainfalls followed by a dry winter In the southern dry deciduous forests, the winter season also brings changes in mean temperature down to 15 1C Although between 20% and 50% of the woody species found in these ecosystems are deciduous, these forests are still dominated by Amazonian genera widely distributed across South America, such as Parapiptadenia, Peltophorum, Cariniana, Lecythis, Tabebuia, Astronium, and others of lesser physiognomic importance (Murphy and Lugo, 1986) Although tropical rain forests contain the highest diversity of species measured in terrestrial ecosystems, life-form diversity is higher in dry forest probably because of habitat heterogeneity Woody plants in tropical rain forest tend to converge to a small number of lifeforms In drier seasonal environments, the proportion of deciduous trees and shrubs increases, the presence of epiphytes decreases, and vines become more frequent The presence of epiphytes is associated with high air humidity and the presence of dew (Gentry, 1995) Along seasonality, gradients succulent plants conspicuously increase, including those with Crassulacean acid metabolism (CAM) Succulent-stemmed plants have very stable water relations and are drought resistant Evergreen woody plants dominate at both extremes of the gradient, and although they all belong to the C3 photosynthesis type, there are differences in their leaf structure and drought resistance, which allow them to function in both mesic and arid environments (Murphy and Lugo, 1986) Savannas, Grasslands, and Shrub Lands Distribution and Structure Savannas and grasslands occupy nearly 25% of South America in both tropical and subtropical regions Tropical Savannas These ecosystems are characteristic of the warm lowland tropics and occur in areas with a strongly seasonal rainfall regime and a dry period lasting from to or months Although a herbaceous cover consisting mostly of bunch grasses and sedges is dominant in savannas, the term ‘‘savanna’’ embraces a variety of vegetation types found in tropical latitudes An important characteristic of tropical savannas is a clear seasonality in their phenology and a period of lower activity associated with times of water stress According to Sarmiento (1984), tropical savannas can be divided into four functionally distinct types: (1) semiseasonal savannas, with weak water stress conditions; (2) seasonal savannas, with a distinct rainfall seasonality and common fires during the dry season; (3) hyperseasonal savannas, where in addition to a marked dry season there is also a period of water flooding; and (4) ‘‘esteros,’’ which are areas without a clear dry season, but an excess of soil water for most of the year The largest extension of savanna in South America occupies the old Precambrian Brazilian shield and has been dubbed by phytogeographers "Cerrados." Savanna ecosystems are also 113 found in the Orinoco Llanos of Colombia and Venezuela, in Suriname, Guyana, in some Amazonian regions of Brazil, and in Paraguay, Bolivia, Argentina, and the northern Chaco region (Figure 3) The vegetation of the Guyanan savannas is more related floristically to the llanos than the cerrado Perennial grasses, herbs, and shrubs are the dominant life forms in savanna ecosystems They are well adapted to the seasonal climate, soils poor in nutrients, and the fire disturbance regime that dominate in these environments At least five phenological groups of grasses have been recognized in savanna ecosystems: perennial with a seasonal semidormant period, annual ephemeral with a short cycle, annual with long cycle, perennial with a seasonal dormant period, and of continuous flowering and growth (Coupland, 1992) Tree species found in savannas have adapted to these conditions by increased allocation to underground biomass Trees in woody savannas can reach 25–30 m in height, but their diameter rarely exceeds 50 cm Some savanna tree species are described as subterranean trees because their roots can penetrate to depths of 18 m Other species like Curatella americana have shallow roots that can grow more than 20 m horizontally and secondary roots that can reach depths of m Other conspicuous components of savanna landscapes are the gallery forest along rivers and smaller streams and the seasonally inundated palm communities (morichales) dominated by the palm Mauritia flexuosa In the Brazilian Cerrado, there is a series of plant communities from open grasslands to dense woodlands and more or less recognizable stages in this continuum receive vernacular names Dry grasslands without shrubs or trees are called ‘‘campo limpio’’ (i.e., clean field); grasslands with scattered shrubs are called ‘‘campo sujo’’ (i.e., dirty field); grasslands with numerous trees and shrubs are called ‘‘campo cerrado’’ (i.e., closed field); when the vegetation is dominated by a closed canopy of trees it is called ‘‘cerradao’’ (i.e., the vegetation is closed) The latter is a woodland composed of trees often 8–12 m or even taller, with ground vegetation reduced because of the shade Many factors probably determine which of these forms of Cerrado vegetation occurs in a given locality, among them topography and soil texture seem to have the main causal effect Cerrados have a markedly seasonal climate and posses a large characteristic flora of fireresistant plants, including about 800 species of trees and large shrubs, and many times that number of herbs and subshrubs The vast majority of these species are endemic to the Cerrado, an ancient vegetation formation dating back perhaps 50 million years (Coupland, 1992) Temperate Grasslands These ecosystems occupy the eastern part of southern South America in Argentina, Uruguay, Chile, and southern Brazil They expand across the pampas of Rio de la Plata in Argentina, the Chaco region, the semiarid region west of the humid pampa (Monte), and reach the edges of desert and semidesert areas of the Patagonian region (Figure 3) The dominant landforms are large plains with a few table-shaped outcroppings no more than 500 m above the plains The dominant climate across the large grassland region is characterized by a mean temperature of 10–20 1C with an annual precipitation ranging from 400 to 1600 mm The dominant soil types