Behling and Lichte, 1997

a Site locations are shown in Fig. 4.

terns occur mainly in the regions of southern and southeastern Brazil. One consequence of these weather patterns is that these regions have little or no marked dry season during the austral winter (Snow, 1976; Nieu-wolt, 1977; Nimer, 1989; Weischet, 1996).

18.2.2. Savanna Regions in South America

The term savanna is used by several authors in different ways (Eiten, 1992). Classifications of neotropical savannas, for example, were published by Cole (1986) and Eiten (1972, 1982). In this chapter, savanna encompasses vegetation types with a certain floristic composition ranging from tropical, treeless, grassy savanna to savanna woodland with shrubs and trees, including cerrado vegetation. These vegetation types occur under seasonal climatic conditions. Also, edaphic savannas, which occur on the sandstone plateaus in the Amazon basin, for example, are included. Most savanna regions lie below 500 m elevation. As shown in Fig. 1, major areas of neotropical savanna vegetation are distributed between 9o and 3°N and 6° and 23°S (Sarmiento and Monasterio, 1975; Sarmiento, 1983; Hueck and Seibert, 1972; Seibert, 1996).

North of the equator, the Llanos Orientales in Colombia and the Orinoco Llanos in Venezuela represent the major neotropical savanna ecosystems. The primary floristic and ecological data for these savannas were published by Cuatrecasas (1989), Gentry (1993), Huber (1987), Hueck (1966), and Vareschi (1980). For the Llanos Orientales, several groups of different savanna communities have been recognized by Blydenstein (1967): (1) savannas with scattered forested areas, (2) savannas with a gradually increasing density of shrubs, and (3) savanna trees following a gradient of increasing moisture from dry through seasonally inundated to hu

FIGURE 1 Geographical distribution of major tropical savannas in South America: (1) Llanos Orientales, (2) Orinoco Llanos, (3) Gran Sabana, (4) Rupununi-Rio Branco and Roraima-Rupununi savanna, (5) coastal Guianan savanna belt, (6) cerrado. (Adapted from Seibert, 1996.)
FIGURE 2 Photo of savanna vegetation in the Llanos Orientales of Colombia.

mid. According to Cuatrecasas (1989), characteristic taxa for grass savannas are in the family Poaceae, including the genera Andropogon, Eragrostis, Axonopus, Paspalum, Aristida, Ctenium, and Panicum. Important shrubs belong to the family Melastomataceae (Miconia, Tibouchina), Fabaceae (Clitoria), Ceasalpiniaceae (Cassia), Lamiaceae (Hyptis), Sterculiaceae (Waltheria), Malvaceae (Sida, Pavonia), and other families. Characteristic larger shrubs or trees that can be less than 1 m tall are Curatella americana (Dilleniaceae), Byrsonima crassi-folia (Malpighiaceae), Bowdichia virgilioides (Fabaceae), and Palicourea rigida (Rubiaceae). The Colombian Llanos Orientales is characterized by the predominance of grass savanna with little presence of shrubs and small trees (Fig. 2). A forest occurs mainly as a gallery forest, but also in patches between the rivers. The distribution of gallery forest in the grassy and relatively flat landscape follows the drainage system of the Andes and is mainly in a west-to-east orientation. Dense stands of the palm Mauritia (morichal) occur along waterways and lake borders and in swampy areas. Fires are frequent in this savanna ecosystem.

Other large savanna areas are the Gran Sabana in southeastern Venezuela, located on a high plain between ca. 700 and 1000 m elevation; the Rupununi-Rio Branco savanna in Suriname; and the Roraima-Rupununi savanna in Brazil. Savanna vegetation also occurs along the northeastern coast of South America (Guyana, Suriname, French Guiana, and in some smaller areas of northern Brazil in Amapa State). The coastal Guianan savanna belt has a yearly precipitation of 2000-3000 mm without a marked dry season; and therefore, it represents an edaphic savanna.

Savanna areas within the Amazon rain forest area north and south of the equator are mostly savanna islands. These areas can be divided into edaphic and nonedaph-ic savannas. Edaphic savannas occur in seasonally flooded areas (e.g., coastal savannas, Marajo savanna, riverine flooded savannas) and on white, sandy soils (e.g., coastal savannas, Amazon terra firme savannas). Nonflooded savannas in Amazonia occur on lateritic and sandy soils—e.g., the savannas of Humaita (Gotts-berger and Morawetz, 1986)—and correspond to the savanna type of the Llanos Orientales. These isolated floras might be relicts of a former, more widespread savanna area (Huber, 1982; Pires and Prance, 1985; Prance, 1996).

In the southern neotropics, the cerrado is the major savanna region (covering ca. 1.8 million km2) (Fig. 1). Cerrado vegetation, which is also a savanna type (Fig. 3), occurs between 4° and 23°S, with the core area on the highland (Planalto) in central Brazil (Ferri, 1976). Other important savanna regions are the Pantanal in eastern Brazil and the Llanos de Mojos in Bolivia. The cerrado vegetation is sensitive to frost and has its southernmost limit in southeastern Brazil, where a few isolated islands of cerrado occur within the area of semideciduous forest (Hueck, 1956; Silberbauer-

FIGURE 3 Photo of savanna vegetation (cerrado) in southeastern Brazil.

Gottsberger et al., 1977). Cerrado can be classified into five physiognomic vegetation forms (Eiten, 1972, 1982): (1) grasslands (campo limpo), (2) grasslands with small shrubs and occasionally small trees (campo sujo), (3) open or closed low tree and/ or scrub woodlands (campo cerrado), (4) tree and scrub woodland with 2- to 5-m-tall trees and an open tree canopy (cerrado, in the strict sense), and (5) arboreal woodlands with 5- to 15-m-tall trees with a semiclosed or closed tree canopy (cerradâo). The modern composition and distribution is the result of several environmental constraints, including pedological, physical, biotic, and climatic factors. Goldsmith (1974) showed that variation in cerrado communities is strongly correlated with available soil moisture and water. Soil fertility also controls the savanna types (Goodland and Pollard, 1973; Oliveira-Filho et al., 1989). It is interesting to note that the floral relationships between different savanna types and their geographical distributions are controlled by geo-morphological development of the landscape (Cole, 1960, 1982, 1986). Other authors (Ferri, 1973; Furley and Ratter, 1988) suggest that fire frequency is the most important factor in determining the physiognomy and geographical distribution of cerrado types in Brazil. The xeromorphic cerrado vegetation is well adapted to fire, which plays an important role in the savanna ecosystem, under recent conditions of human impact as well as under natural conditions (Coutinho, 1982; Eiten, 1975). The floristic composition of the extensive cerra-

do area is not homogeneous and has contact zones with other vegetation types such as the Amazon rain forest, deciduous and semideciduous forest, and caatinga. Floristic analysis of 98 cerrado sites, including Amazon savannas, showed that cerrado vegetation is very heterogeneous (Ratter et al., 1996).

0 0

Post a comment