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Chapter 42. South America: ecological zones

Figure 42-1. South America: ecological zones

Figure 42-1 shows the distribution of ecological zones in South America. Table 42-1 contains area statistics for the zones by subregion, and Table 42-2 indicates the proportion of forest in each zone by subregion.

TROPICAL RAIN FOREST

The tropical rain forests of South America extend over the whole Amazonian Basin, the Pacific coast of Colombia and Ecuador and the Atlantic coast and Iguaçu and Parana River valleys of Brazil. Huge amounts of rain fall in the heart of the Amazon Basin and along the western coast (more than 3 000 mm, even up to 8 000 mm). Elsewhere, rainfall is between 1 000 and 3 000 mm, often with a short dry period in winter. Temperatures are high, especially in the Amazonian region, where the mean temperature of the coldest month is always above 20°C. On the Atlantic coast, mean temperatures decrease as latitude increases (15° to 20°C).

The Amazon Basin contains the world's largest area of tropical rain forest. In this vast extent at least 10 to 20 different vegetation types might be distinguished. The wettest type is found in the upper basin of the Amazon River, the State of Amapà in Brazil and the west coast of Colombia. The vegetation is luxuriant, multilayered evergreen forest, up to 50 m tall, with emergent trees. The most important tree families are Annonaceae, Bombacaceae, Burseraceae, Clusiaceae, Euphorbiaceae, Leguminosae, Moraceae and Sterculiaceae.

The most extensive rain forest is somewhat drier and occurs in the Amazon Basin and on the eastern foothills of the central Andes. It is a multilayered forest up to 40 m tall, with or without emergent trees, mainly evergreen but with marked leaf reduction during the short dry season. The chief families are Bignoniaceae, Bombacaceae, Euphorbiaceae, Moraceae and Sterculiaceae. In Brazil, Leguminosae (Parkia spp., Tachiglia spp., Hymenolobium spp., Swartzia spp. and others) are particularly important. In Peru, the most common species include Bombax munguba, Calycophyllum spruceanum, Castilla ulei and Cedrela odorata while in Venezuela Calophyllum brasiliense, Carapa guianensis, Cedrela fissilis and Ceiba pentandra are among the dominant species.

Table 42-1. South America: extent of ecological zones

Subregion

Total area of ecological zone (million ha)

Tropical

Subtropical

Temperate

Boreal

Polar

Rain forest

Moist

Dry

Shrub

Desert

Mountain

Humid

Dry

Steppe

Desert

Mountain

Oceanic

Continental

Steppe

Desert

Mountain

Coniferous

Tundra

Mountain

Non-tropical South America

3

36

36

1

9

32

74

10

64


24

26


50


8





Tropical South America

665

397

133

9

5

158

46














Total South America

668

433

169

10

14

190

120

10

64


24

26


50


8





TOTAL WORLD

1 468

1 117

755

839

1 192

459

471

156

491

674

490

182

726

593

552

729

865

407

632

564

Note: Data derived from an overlay of FRA 2000 global maps of forest cover and ecological zones.

Table 42-2. South America: proportion of forest by ecological zone

Subregion

Total area of ecological zone (million ha)

Tropical

Subtropical

Temperate

Boreal

Polar

Rain forest

Moist

Dry

Shrub

Desert

Mountain

Humid

Dry

Steppe

Desert

Mountain

Oceanic

Continental

Steppe

Desert

Mountain

Coniferous

Tundra

Mountain

Non-tropical South America

72

27

75



7

2

89

1


4

29


1


20





Tropical South America

82

27

89

15


26

19














Total South America

82

27

86

13


23

9

89

1


4

29


1


20





TOTAL WORLD

69

31

64

7

0

26

31

45

9

2

20

25

34

4

1

26

66

26

50

2

Note: Data derived from an overlay of FRA 2000 global maps of forest cover and ecological zones.
Evergreen swamp forest covers large areas in the Amazon region, particularly in the delta. Characteristic species are Bombax aquaticum, Calophyllum brasiliense, Macrolobium acaciaefolium, Triplaris surinamensis and many palms, including Euterpe oleracea, Manicaria saccifera, Mauritiella pacifica and Raphia taedigera.

Mangrove forests are well established in the larger estuaries along the Atlantic and, to a lesser extent, Pacific coasts. The largest mangroves are found in Brazil. From the sea inland is first a belt of Rhizophora mangle, then Avicennia tomentosa and A. nitida and, finally, on higher ground vegetation dominated by Laguncularia racemosa, often edged on its landward side by a fringe of palms. Other common trees and shrubs include Ardisia granatensis, Avicennia tomentosa, Conocarpus erectus, Conostegia polyandra, Rhizophora brevistyla and Rustia occidentalis.

TROPICAL MOIST DECIDUOUS FOREST

This zone roughly corresponds with the Brazilian and Guiana Shields of eastern South America. A wide area with rather high rainfall but a pronounced dry season extends around the wet Amazonian Basin.

This large zone is mainly covered by cerrado in Brazil, a mosaic of grasslands, tree savannahs and woodlands with patches of semi-deciduous forest. The flora is rich, with Leguminosae and Myrtaceae very prevalent in the tree and shrub canopies. The most common species are Caryocar brasiliense, Curatella americana, Kielmeyera coriacea and Qualea spp. In some areas a real forest occurs, the cerradao - a short semi-deciduous forest, 10 to 15 m tall, of medium density. The flora includes such forest species as Bowdichia, Hymenaea, Piptadenia inaequalis and Machaerium and also cerrado species. In northern Argentina, around Salta, a similar forest grows on the foothills of the Andes. The higher trees are Aspidosperma peroba, Astronium spp., Cedrela fissilis and Gallesia gorazema (guararema).

An evergreen seasonal or semi-deciduous forest grows on the edge of the Amazonian Basin and in the Andean foothills. In Argentina and Paraguay, this fairly dense forest includes three tree canopies, the tallest reaching 30 m. Characteristic trees include Apuleia leiocarpa, Aspidosperma polyneuron, Balfourodendron riedlianum, Cabralea spp. and Cedrela spp. In Bolivia, Astronium urundeuva, Ateleia guaraya, Ficus spp. and Hura crepitans are dominant species.

In Venezuela, the flora and physiognomy of the llanos have some similarity with Brazilian cerrados. These are tall grasslands with evergreen broad-leaved trees including Acacia caven, Celtis spinosa, Prosopis alba and P. nigra. A deciduous thorn forest occurs in some places with Caesalpinia coriaria, Capparis coccolobifolia, Cercidium praecox, Mimosa spp., Piptadenia flava and other species in addition to the main llanos species.

The zone also includes the grasslands of the Pantanal, those around the junction of the Paraguay and Parana Rivers in Argentina and the residual forest on the low plain of the Cauca River in Columbia.

TROPICAL DRY FOREST

In areas sheltered from the humid trade winds, the climate is drier. These regions may be close to the sea, as in northeastern Brazil and the Caribbean coast, or inland, such as the Argentine chaco. Rainfall varies between 500 and 1 000 mm or less with a dry season of five to eight months. Temperatures are always high near the Equator (mean temperature of the coldest month greater than 20°C) but lower in the chaco, which extends to 34°S.

In Brazil, the typical vegetation is the caatinga, xerophytic vegetation types varying from dense to very open. The trees are more or less deciduous, thin-stemmed and with a low canopy (5 to 10 m). The flora is rich, with fairly numerous Leguminosae, especially Amburana, Caesalpinia and Mimosa species, and often includes Cactaceae. The palms Cocos comosa and Copernicia cerifera (carnauba) assume considerable importance in flood plains.

In Argentina, the chaco is a wooded region of relative ecological homogeneity between the tropical and subtropical zones. The prevailing vegetation is deciduous dry forest with many climatic and, above all, edaphic variations. All these types are characterized by "quebrachos" (Schinopsis spp. and Aspidosperma spp.). The most humid forests occur in the east, a drier forest in the west and xerophilous forest on the lower Andean foothills.

In the coastal region of the Caribbean, deciduous forests and woodlands rich in Leguminosae once occupied a large part of the plain. Agriculture and thickets have largely replaced these forests. Similar woodlands with Cactaceae grow along the Gulf of Guayaquil in Peru and Ecuador.

TROPICAL SHRUBLAND

In addition to the drier parts of the Caribbean coast this zone extends along the Pacific coast of South America from south of the Gulf of Guayaquil to the Tropic of Capricorn, forming a narrow belt between the lower slopes of the Andes and the coastal desert. Rainfall is less than 500 mm, with a long dry season of eight to nine months and high temperatures (always more than 20°C). To the south, in Peru, rainfall is even less than 100 mm, but a light drizzle maintains high humidity and allows some plants to live.

Xeromorphic woodlands are represented by algarrobo, found on the southern coast of the Gulf of Guayaquil, a perennial-leaved woodland dominated by Prosopis chilensis. In western Venezuela, a deciduous thorn woodland grows under the same conditions. It is a multilayered woodland 8 to 15 m high with the canopy dominated by Bulnesia arborea, Capparis spp., Pithecellobium unguis-cati, P. saman, Prosopis spp. and Pterocarpus spp.

TROPICAL MOUNTAIN SYSTEMS

The tropical mountains are mainly the Andean Range, extending from northern Colombia and Venezuela to 28° to 29°S. However, some areas in Venezuela and Brazil have similar climatic conditions. The mountain regions experience lower temperatures, leading to specific vegetation types above 1 000 to 1 500 m. Precipitation varies greatly but the region is everywhere tropical, with a low annual range of temperature. Ecofloristic zones can generally be differentiated by altitude.

In the northern Andes (Colombia and Venezuela), both the eastern and western faces of the mountains are well watered. Precipitation ranges from 1 500 to 5 000 mm. The mean temperature of the coldest month is often close to 15°C, but drops down to 10°C or lower with increasing elevation. There is generally no dry season, or a very short one. In some places there is heavy cloud cover and very frequent fog. Frost occurs above 2 000 m.

South of Ecuador there is a contrast between the very wet eastern side of the Andes and the drier Andean valleys and western side. On the eastern face, the climate is similar to that of the northern Andes. In the inter-Andean valleys, even in Colombia and Venezuela, precipitation is 1 000 to 1 500 mm (sometimes less) and the dry season is two to five months. On the western face, in Peru, precipitation is lower (less than 500 mm) and the climate is very dry or semi-arid. In Venezuela, the southern part of the Guiana Shield reaches 1 000 to 3 000 m with a fairly even annual distribution.

Between 1 000 and 1 800 to 2 400 m in the northern Andes many of the lowland taxa still persist, such as species of Licania and Eschweilera, but a number of distinctly highland elements also enter the lower montane forest. For example, in the Colombian Andes Alchornea bogotensis, Brunellia comocladifolia and Cinchona cuatrecasasii are present. The montane or upper montane forest, starting at 1 800 to 2 400 m, may extend in places up to 3 400 m. An increasing number of typical montane species enter the flora, for example, Brunellia occidentalis, Symplocos pichindensis and Weinmannia balbisiana. In the drier parts, montane forests are evergreen seasonal. Above this zone, subalpine forests may extend up to 3 800 m in some places. The characteristic highland flora includes many species of Befaria, Brunellia, Clusia, Gynoxys, Miconia, Rhamnus and Weinmannia. On the high ridges exposed to wet winds there is montane cloud forest with an "elfin woodland" of low gnarled trees with abundant mosses and lichens.

A unique submontane formation is Podocarpus spp. forest, today existing mainly in the lower montane region in northern Peru. The conifer Podocarpus oleifolius dominates this forest, where Drimys winteri, Ocotea architectorum and Weinmannia spp. are also common trees.

In Peru and Bolivia, the wet eastern face of the Andes bears submontane and montane forests similar to those of the northern Andes. In the drier inter-Andean valleys the forest often becomes deciduous, even xerophilous, but often very degraded and transformed into thicket or scrub. On the western slopes of the Andes, under a very dry climate, scrub woodland replaces forest.

In the non-Andean highlands, the submontane level is rather similar to the lowland forest but of lower stature and with a slightly different flora.

SUBTROPICAL HUMID FOREST

This zone includes plateaus and lowlands on the Atlantic side of the continent in southern Brazil, Uruguay and Argentina. The two main climatic characteristics are lower temperatures in winter (mean temperature of the coldest month less than 15°C) and rainfall evenly distributed throughout the year. However, rainfall decreases from the north (1 000 to 2 500 mm) to the south (600 to 1 000 mm).

The natural vegetation of the wetter northern parts of the zone is evergreen coniferous forest dominated by Araucaria angustifolia. The Araucaria forest, some 25 m tall, may be almost pure, but more often it dominates a dense forest with a profusion of Cedrela fissilis, Phoebe porosa, Tabebuia spp., Parapiptadenia spp. and the shrub Ilex paraguariensis. Today, only residual areas remain, as this forest has been much exploited for timber production.

Grasslands are the main vegetation in Rio Grande do Sul as well as the lowlands of Uruguay and eastern Argentina. Riparian forests fringe the main rivers.

SUBTROPICAL DRY FOREST

This zone of lowlands, less than 200 km wide, lies between the Andes foothills and the Pacific Ocean. The rainfall regime is of the Mediterranean type, with summer drought (two to seven months) and winter rains. Annual precipitation varies from 500 mm in the northern coastal region to 2 000 mm on the Andean foothills. Winter temperatures are cool (10° to 15°C).

The climax is sclerophyllous evergreen forest or woodland with xerophytic species such as Lithraea caustica, Quillaja saponaria, Peumus boldus and species of the genera Cryptocarya and Beilschmiedia. The endemic palm Jubaea chilensis grows in a narrow area northeast of Valparaiso. Much of the forest has been degraded and replaced by secondary thorny thicket with Acacia caven or replaced by agriculture.

Towards the south or in the Andean foothills, where precipitation is higher, the sclerophyllous forest gives way to open deciduous "mesophytic" forest dominated by various Nothofagus species (N. obliqua, N. dombeyi, N. procera) associated with Aextoxicon punctatum, Araucaria araucana, Drimys winteri, Laurelia serrata and others.

SUBTROPICAL STEPPE

Two regions belong to this ecological zone. One is located to the west of the Andes, covering most of the Chilean Norte Chico and forming a transitional area between the previous zone and the Atacama Desert. The other is to the east of the Andes, an extensive region in central Argentina of transition between the tropical chaco, subtropical pampa and temperate steppes to the south. Rainfall ranges from 100 to 800 mm and the dry period is very long, up to nine months. The mean temperature of the coldest month may be less than 10°C. In Chile, rainfall is even lower, from less than 100 to 400 mm. Temperatures are warmer than in Argentina, with mean temperature of the coldest month between 13° and 15°C.

In this zone the densest vegetation type is a deciduous thicket with various species of Prosopis, turning into large areas of thorn woodland. In the drier inland plain is subdesert shrubland with Bougainvillea spp., Cercidium spp. and various Rhamnaceae. In Chile, Acacia caven and Puya spp. dominate the subdesert thorn scrub of the Norte Chico.

SUBTROPICAL MOUNTAIN SYSTEMS

The subtropical Andes lie roughly from 26° to 40°S. From 1 000 m to nearly 7 000 m altitude, the climate is cold everywhere. The area is bordered to the west by the highest peaks, forming a barrier against the winds blowing from the Pacific Ocean. As a result, precipitation is low, generally less than 300 mm. The dry season mainly occurs in spring and summer (October-December). Strong winds make the effects of aridity and cold more pronounced.

In the lower reaches of the Andes, between 1 000 m and 1 800 to 2 400 m, we find submontane beech forest on the wetter slopes. It is a deciduous low forest or woodland containing species such as Nothofagus dombeyi, N. obliqua, N. procera, Aetoxicon punctatum, Araucaria araucana, Drimys winteri, Laurelia serrata and Persea lingue. Drier slopes are covered with evergreen sclerophyllous shrubs or xerophytic deciduous woodland. Higher up, the vegetation changes gradually into a steppe.

TEMPERATE OCEANIC FOREST

South of 38°S the western side of the Andes is well watered owing to oceanic influences. The dryness decreases from north to south, together with decreasing temperatures. Rainfall ranges from 1 000 to 3 500 mm, evenly distributed throughout the year. The mean temperature of the coldest month is lower than 10°C in the north and decreases to about 0°C in the south. In eastern Patagonia, rainfall is less than 1 000 mm with mean monthly temperatures always lower than 10°C.

The northern part of the region harbours a broad-leaved, very dense evergreen forest up to 40 to 45 m tall, with equally dense undergrowth. Species of Nothofagus dominate the tree canopy, including Nothofagus obliqua, N. dombeyi and N. procera in association with Aextoxicon punctatum, Drimys winteri and Eucryphia cordifolia. A slight lowering of temperature at higher altitude or latitude gives rise to a less species-rich, mixed broad-leaved/coniferous forest with Nothofagus antarctica, N. dombeyi, N. nitida, Fitzroya cupressoides, Pilgerodendron uvifera and Podocarpus nubigena.

TEMPERATE MOUNTAIN SYSTEMS

The central part of the Patagonian Andes, up to 52°S, reaches 2 000 to 3 000 m elevation. The western upper slopes are wet, whereas the eastern side is drier. The most striking climatic features are cold, snow and winds.

Subalpine beech forest, dominated by Nothofagus betuloides, lies below the timberline on the wettest slopes. This elfin type has low multistemmed trees, greatly deformed by the weight of snow. These forests are transitional to scrub and grasslands at higher altitudes. On the drier slopes and towards the eastern drier zone a beech forest of Nothofagus betuloides and N. pumilio occurs. It is transitional between the purely evergreen lowland forests and the deciduous N. pumilio forests that lie below the timberline on the drier sites.

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