Figure 34. GEZ map for South America
Table 19. Global Ecological Zones of South America.
Surface area | |||
Km2 |
% of total land area Region |
% of GEZ worldtotal | |
Tropical rain forest |
6 631 240 |
37.3 |
45.5 |
Tropical moist deciduous forest |
4 302 306 |
24.2 |
38.9 |
Tropical dry forest |
1 681 596 |
9.5 |
22.5 |
Tropical shrubland |
103 034 |
0.6 |
1.2 |
Tropical desert |
137 638 |
0.8 |
1.2 |
Tropical mountain systems |
1 886 495 |
10.6 |
41.7 |
Subtropical humid forest |
1 199 948 |
6.8 |
25.6 |
Subtropical dry forest |
100 504 |
0.6 |
6.3 |
Subtropical steppe |
639 738 |
3.6 |
13.0 |
Subtropical desert |
0 |
0 |
0 |
Subtropical mountain systems |
238 162 |
1.3 |
4.9 |
Temperate oceanic forest |
259 147 |
1.4 |
14.4 |
Temperate continental forest |
0 |
0 |
0 |
Temperate steppe |
498 298 |
2.8 |
8.5 |
Temperate desert |
0 |
0 |
0 |
Temperate mountain systems |
76 895 |
0.4 |
1.1 |
Total land area |
17 755 001 |
99.9 |
|
The tropical rain forest zone of South America extends over the whole Amazonian basin as well as on the Pacific coast of Colombia and Ecuador, crossed by the Equator. It also extends on the Atlantic coast of Brazil and the median valleys of Iguaçu and Parana in Brazil.
Climate
The trade winds blowing from the southeast in the southern hemisphere are responsible of the wet climate of the Atlantic coast. The regime is axeric, with huge amounts of rain in the heart of the Amazonian basin and the western coast (more than 3000 mm, even up to 8000 mm). Everywhere else, rainfall is between 1000 and 3000 mm, with often a short dry period in winter, even up to 4 months in some places. Temperatures are high, especially in the Amazonian region, where mean temperature of the coldest month is always more than 20°C. On the Atlantic coast, it decreases when latitude increases (15°-20oC), as the Ecological Zone extends beyond the Tropic of Capricorn, up to 30°S.
Physiography
The Amazon Basin lies at less than 200 m above sea level and flat or gently rolling lowlands stretch from the eastern slopes of the Andes to the Atlantic coast. There, the Guiana Shield and the Brazilian Shield, to the north and south of the Amazon valley respectively, are covered with Tertiary continental sediments and Quaternary sands. The soils are mostly ferralsols. Along the proper Amazon depression, many swamps and flooded valleys occur.
The Pacific coastal region is an irregular strip, narrow near Panama, 100 km wide near the border with Ecuador, on the foothill of the Andes.
On the Atlantic coast, behind wide sand beaches, low plateaux with shallow soils lead to the crystalline hills, mostly lower than 1000 m, carved by numerous valleys filled with fertile clayey soils. These hills form a barrier receiving the rains and clouds from the Atlantic Ocean.
Vegetation
The Amazon Basin contains by far the largest area of tropical rain forest in the world. The area of Amazonian and Pacific forests is more than 4 000 000 km2. In this vast extent, at least 10 to 20 different vegetation types might be distinguished, ranging from several associations of dense humid evergreen forest to edaphic grasslands. Even if deforestation is rather active in the southern border of the Amazonian basin, it is estimated to encroach upon a relatively small portion of the original extent. On the Atlantic coast, dense evergreen forest only remains at the top of the hills. Deforestation began almost as soon as Europeans arrived in Brazil. It was first exploited for timber, then affected by mining, coffee, banana and rubber plantations. In the low fertile valleys, they were converted to agriculture, especially sugar cane. Five ecofloristic zones have been distinguished within this Ecological Zone, according to variation in climate humidity, geographical location and flora. Edaphic vegetation types are represented by riverine ombrophilous (evergreen) forest, swamp ombrophilous forest, mangrove forest and flooded grassland.
The wettest type (annual rainfall > 3000 mm, locally up to 8000 mm) is found in the upper basin of the Amazon River, Amapà and west coast of Colombia. The vegetation is luxuriant evergreen forest, multi-layered, up to 50 m tall, with emergent trees. Buttresses and stilt roots are fairly frequent. The biggest trunks reach 2 meters in diameter. Undergrowth is fairly sparse, but epiphytes are very abundant. It has a very rich flora and presents many local facies. Most important tree families are: Annonaceae, Bombacaceae, Burseraceae, Clusiaceae, Euphorbiaceae, Leguminosae, Moraceae, Sterculiaceae.
The most extensive rain forest is somewhat drier (annual rainfall between 2000 and 3000 mm) and occurs in the Amazon Basin and at the eastern foothills of the Central Andes. It is a multi-layered forest of 40 m tall, with or without emergent trees, mainly evergreen but with marked leaf reduction during the short dry season. The forest floor is rich in shrubs and herbaceous plants. The chief families are Bignoniaceae, Bombacaceae, Euphorbiaceae, Moraceae, Sterculiaceae, etc. In Brazil, Leguminosae are particularly important: Parkia, Tachiglia, Hymenolobium, Swartzia and others. In Peru the most common species include Bombax munguba, Calycophyllum spruceanum, Castilloa ulei, Cedrela odorata and in Venezuela Calophyllum brasiliense, Carapa guianensis, Cedrela fissilis, Ceiba pentandra are among the dominants.
Evergreen swamp forest covers large areas in the Amazon region, particularly in the delta. In Brazil, this flood-forest is known as “igapo”. Characteristic species are Bombax aquaticum, Calophyllum brasiliense, Macrolobium acaciaefolium, Triplaris surinamensis, etc. and above all many palms, among others: Euterpe oleracea, Manicaria saccifera, Mauritiella pacifica, Raphia taedigera.
Mangrove forests are well established in the big estuaries, especially in the tidal-flow zone. They occur along the Atlantic coast and to a lesser extent along the Pacific coast. The largest mangroves are found in Brazil. Here we find the following zones, moving from the see inland: first a belt of Rhizophora mangle, then mangrove of Avicennia tomentosa and A. nitida and finally on higher ground a vegetation dominated by Laguncularia racemosa, often edged on its landward side by a fringe of palms. Stilt roots or pneumatophores are common. Other common trees and shrubs in South American mangroves include Ardisia granatensis, Avicennia tomentosa, Conocarpus erectus, Conostegia polyantha, Rhizophora brevistyla and Rustica occidentalis.
Climate
A wide area with rather high rainfall but an always pronounced dry season extends around the wet Amazonian Basin. A rather surprising feature is the occurrence of a large island of this Ecological Zone in the Amazonian area. It corresponds to the western slope of the uplift Guiana Shield, culminating at Mount Roraima (2810 m); this area does not receive in winter the winds blowing from the Atlantic Ocean.
Physiography
This Ecological Zone roughly corresponds with the two main shields forming eastern South America:
The Brazilian Shield, to the southeast, is a rather flat surface where very old rocks are exposed; its altitude is less than 500 m in its northern part and gently rises to 900-1000 m in its southeastern part; the Parnaiba River flows to the Atlantic Ocean through a deep depression filled with more or less ancient sediments.
The Guiana Shield to the north, on which large extents of sandstones are exposed.
Along the Andean Range, the Orinoco River flows in a depression relied with the Amazonian one.
To the south, a wide depression between the Brazilian Shield and the Andes forms the Pantanal. On the other side of the Andes, the deep depression of the Cauca River low valley also belongs to this Ecological Zone.
Vegetation
This large Ecological Zone is mainly covered by “cerrado” in Brazil, i.e. a mosaic of grasslands, tree savannas and woodlands, with patches of semi-deciduous forest. There, grazing is the main activity, but the growing of maize and leguminous soya is increasing in some parts. The prevailing physiognomy is a savanna with woody plants. However, in some areas, a real forest occurs, namely “cerradao”. It is a short semi-deciduous forest, 10 to 15 m tall, of medium density. The trees, more or less twisted, have big crowns, thick bark, broad leathery leaves and deep roots. There are no thorn-trees. In the sufficiently open undergrowth, there is a herbaceous layer. The flora includes forest species such as Bowdichia, Hymenaea, Piptadenia incurialis, Machaerium, with also “cerrado” species like Curatella americana, Qualea, Kielmeyera coriacea and others. In northern Argentina, around Salta, a forest very similar to this type grows on the foothills of the Andes. It is a semi-deciduous broad-leaved forest, taller than the “cerradao”, where the higher trees are Aspidosperma peroba, Astronium sp., Cedrela fissilis, Gallesia guararema. The “Campo cerrado” presents several facies: shrub savanna or tree savanna of varying density. Trees and shrubs are short, 3 to 8 m high and twisted with low branches and thick bark. The leaves are persistent or partially deciduous. Flora is rich, with Leguminosae and Myrtaceae very prevalent in the tree and shrub canopies. The most common species are Caryocar brasiliensis (Caryocaraceae), Curatella americana (Dilleniaceae), Kielmeyera coriacea (Guttiferae), Qualea (Vochysiaceae). In the herbaceous layer, grasses and composites predominate.
On the edge of the Amazonian Basin and in Andean foothills grows an evergreen seasonal or semi-deciduous forest. In Argentina and Paraguay, this fairly dense forest includes 3 tree canopies, the tallest reaching 30 m. Characteristic trees include Apuleia leiocarpa, Aspidosperma polyneuron, Balfourodendron riedcianum, Cabralea spp., Cedrela spp. In Bolivia, Astronium urundeuva, Ateleia guareia, Ficus spp. and Hura crepitans are dominant species.
In Venezuela, the flora and physiognomy of llanos have some similarity with Brazilian “cerrados”. Llanos are tall grasslands with evergreen broad-leaved tree synusia where the main trees are Acacia caven, Celtis spinosa, Prosopis alba, P. nigra. A deciduous thorn forest occurs in some places, with Caesalpinia coronaria, Capparis coccolobifolia, Cercidium praecox, Mimosa, Piptadenia flava and other species in addition to the main llanos species.
To the south, the grasslands of Pantanal and those around the junction of Paraguay and Parana rivers in Argentina, belong to this Ecological Zone.
To the northwest, in Colombia, the low plain of Cauca River is largely cultivated, with palms, banana trees, sugar cane plantations. Patches of the original deciduous forest remain, surrounded by savannas.
Climate
In some areas, sheltered from the humid trade-winds, climate is drier. These regions may be close to the sea, like northeastern Brazil and Caribbean coast, or inland, like the Argentine “chaco”. The same ecological conditions are experienced around Guayaquil Gulf. Rainfall varies between 500 and 1000 mm and the long dry season is 5 to 8 months. Just behind the barrier of the Atlantic hills, rainfall may be less than 500 mm and overall extremely variable. Temperatures are always high (mean temperature of the coldest month > 20°C) in the areas close to the Equator; they are lower in Chaco which extends up to 34°S.
Physiography
In northeastern Brazil, this Ecological Zone lies on the eastern Brazilian Shield, formed of Precambrian rocks and raised on its Atlantic border, forming a barrier of uplands at a medium elevation of 1000 m, with some peaks between 2000 and 3000 m. In Argentina, the Chaco extends on the lowlands bordering the eastern side of the Andes. Parts of this Ecological Zone along the Caribbean coast take place on rather recent sediments forming the coastal plain, whereas on the border of Guayaquil Gulf, the lower slopes of the Andes belong to the same Zone
Vegetation
In Brazil, the typical vegetation of the Ecological Zone is the “caatinga”. This local name designate a wide variety of xerophytic vegetation types, with or without Cactaceae, sometimes dense and almost closed, sometimes very open and pseudo-steppe like. This dry forest, more or less deciduous, has a low tree canopy (5 to 10 m), fairly open, composed of thin-stemmed trees, a shrub layer and a sparse herbaceous layer. Flora is rich, with fairly numerous Leguminosae, specially Amburana, Caesalpinia, Mimosa. The palms Cocos commosa and Copernicia cerifera (Carnauba) assume considerable importance in flood plains. Due to the severe climate as well as very poor soils, often lithosols, this region seems unfavourable to agriculture. However, through a considerable irrigation effort, it has been largely reclaimed and farmed (food crops, cereales, cotton, stock farming).
In Argentina, the Chaco constitutes a wooded region of relative ecological homogeneity, in a transitional zone between the tropical and subtropical ones. The prevailing vegetation of the Chaco is deciduous xerophilous forest with many climatic and, above all, edaphic variations. All these types are characterized by “quebrachos” (Schinopsis, Aspidosperma). 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 occupied a large part of the plain. Due to increased dryness and overgrazing, these forests have been largely replaced by agriculture or thickets. Today, around population centres and agriculture, Cactaceae formations, with or without thorn trees, form a transition with the real sclerophyllous thorn forest. Similar woodlands with Cactaceae grow along the Guayaquil Gulf in Peru and Ecuador.
Climate
Except the drier parts of Caribbean coast, this Ecological Zone extends along the Pacific coast of South America, from the south of Guayaquil Gulf to the Tropic of Capricorn. This dry climate is dependent on general factors: the large high-pressure area, the trade winds, Humboldt’s Current, the Andean barrier. Rainfall is less than 500 mm, with a long dry season of 8-9 months and high temperatures (always more than 20°C). To the south, in Peru, rainfall is even lower than 100 mm, but a light drizzle maintains high humidity and allows some plants to live.
Physiography
This Ecological Zone is a very narrow belt between the lower slopes of the Andes and the coastal desert. The altitude may reach 1000-1200 m on the Peruvian “lomas”. Soils are poor, often squelettic.
Vegetation
Xeromorphic woodlands are represented by “algarrobo”, found on the southern coast of the Guayaquil Gulf. It is a perennial-leaved woodland dominated by Prosopis chilensis. In western Venezuela, a deciduous thorn woodland grows in the same conditions. It is a multi-layered woodland of 8 –15 m high and the canopy is dominated by Bulnesia arborea, Capparis, Pithecellobium unguiscati, P. saman, Prosopis and Pterocarpus.
Subdesert deciduous shrubland, with or without succulents, is another main vegetation in this zone. In Guajira, it is a low, open thicket, dominated by deciduous thorn trees and succulents, floristically close to the “caatinga”. In Peru and Chile, the coastal “lomas”, when not degraded, bear a xerophytic thicket with scattered trees. Among the woody plants, the following are important: Acacia macrantha, Caesalpinia spinosa (tara), Capparis pisca, Carica candicans, Prosopis chilensis and P. limensis.
Climate
This Ecological Zone comprises the Pacific coastal desert, in Peru and Chile, where rainfall is much less than 200 or even 100 mm, without fog. Temperatures are high in the north, cooler in winter to the south (mean temperature of the coldest month < 15°C).
Physiography
This coastal fringe is mainly formed of sands, or sometimes rocks.
Vegetation
It is a true desert, with scattered vegetation of deciduous dwarf shrubs (Larrea, Caesalpinia, Capparis, Prosopis) and succulent cacti.
Climate
The tropical mountains of South America are mainly the Andean range, extending from northern Colombia and Venezuela to 28-29°S. However, some uplift parts of the ancient shields, in Venezuela and Brazil, experience specific climatic features due to elevation. The common character of these mountain regions is lower temperatures leading to specific vegetation types above 1000 to 1500 m depending on the location. Precipitations greatly vary, but the region is everywhere tropical, with low annual range of temperature. Several altitudinal belts have been distinguished, defining submontane, montane and high elevation Eco-Floristic Zones.
Northern Andes
Although the Andes form a barrier which limits east-west exchanges in the northern Andes (Colombia and Venezuela), the eastern and western faces are well watered. The former receive rains from the east and northeast Atlantic, the latter, rains from the Pacific. In this the Andean forests are comparable. Above 1000 m altitude, precipitations range from 1500 to 5000 mm. Mean temperature of the coldest month is often close to 15°C, but drops down to 10°C or less with increasing elevation. There is generally no dry season, or a very short one. Moreover, in some places, there is a heavy cloud cover and very frequent fog. Frost occurs above 2000 m.
Central Andes
South of Ecuador, there is a contrast between the eastern side of the Andes, very wet, whereas Andean valleys and the western side are drier. On the eastern face, climate is similar to that of the northern Andes. In the inter-Andean valleys, even in Colombia and Venezuela, precipitations are 1000-1500 mm (sometimes less) and the dry season is 2 to 5 months. On the western face, in Peru, precipitations are even lower (less than 500 mm) and the climate becomes very dry or semi-arid. In Venezuela, the uplift southern part of the Guiana Shield reaches 1000 to 3000 m. These uplands close to the Equator receive humidity from the Atlantic Ocean and rainfall is very high. Also rather high rainfall almost all along the year is recorded on the Atlantic slopes of the uplift East-Brazilian Shield, along the Atlantic coast.
Physiography
The ranges of the Andes result of relatively ancient foldings, eroded then raised again in the Tertiary age. Today, frequent earthquakes and numerous active volcanoes manifest recent tectonic activity. The Andes reach maximum width and height in the central portion of Bolivia and Peru, even if the highest peak (Mt Aconcagua, 6959 m) is located in subtropical Chile. In central Peru, more than 10 peaks reach 6000 m or more. Then elevation decreases northwards to about 2000 m in northern Peru. In Ecuador, the Andes split in two separate ranges (Cordillera Occidental and Cordillera Central) and a third (Cordillera Oriental) is added to the east. The eastern Cordillera continues into Venezuela following the Caribbean coast to northern Trinidad. In northern Colombia, the isolated massif of the Sierra Nevada de Santa Marta (5775 m) also belongs to the Andes. A narrow band of trans-Andean lowlands follows the Pacific coast from Chile to northwestern Colombia and here merges broadly with the Caribbean lowlands of Colombia and Venezuela. The rocks are of various origins and composition; volcanic material occurs in many places. The raised parts of the ancient shields are formed of very old rocks. Their altitude may reach 2000 m or more (2810 m at Mt Roraima, in Brazil).
Vegetation
The Andean forests depend on two main factors: precipitation, which determines, from one extreme to the other, hyperhumid forests or dry thorn forests and altitude (or temperature) which allows a distinction to be made between submontane and montane forests.
Northern Andes
In this moister part of the range, ombrophilous evergreen to seasonal forests are everywhere the climax vegetation up to 3200 to 3800 m, which is the timberline.
Between 1000 and 1800-2400 m, one finds submontane or lower montane forest. It is quite similar to lowland rain forest but a large number of species drop out and are replaced by more upland species. This forest tends to be lower than that of the surrounding lowland area, the upper canopy at 25 – 30 m, with fewer woody vines and less-buttressed trees. Epiphytes, lianas and palms are abundant. Many of the lowland taxa still persist in this belt, such as species of Licania (Chrysobalanaceae) and Eschweilera (Lecythidaceae), 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 cuatreca-sasii. The montane or upper montane forest, starting at 1800 to 2400 m, may extend in places up to 3400 m. It is usually less tall than the submontane forest, with a predominance of microphyllous trees. Vascular epiphytes are still common, but woody lianas rarer. 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 3800 m in some places. Few vascular epiphytes and climbers occur, but there are abundant mosses and lichens. The characteristic highland flora includes many species of Befaria (Ericaceae), Brunellia (Brunelliaceae), Clusia (Clusiaceae), Gynoxys (Asteraceae), Miconia (Melastomataceae), Rhamnus (Rhamnaceae) and Weinmannia (Cunoniaceae). On the high ridges exposed to the wet winds, there is montane cloud forest, with its characteristic “elfin woodland” physiognomy: low gnarled trees with extremely abundant mosses and lichens. Above the timberline, the “paramo” appears, herbaceous vegetation with a few shrubs but no trees.
A separate unique submontane formation is Podocarpus forest, which remains today mainly in the lower montane region in northern Peru. Naturally it occurred scattered along the Andes of Colombia and Peru and in the Coastal Cordillera of Venezuela. The conifer Podocarpus oleifolius dominates this forest, where Drimys winteri, Ocotea architectorum and Weinmannia are also common trees.
Central Andes
In Peru and Bolivia, the wet eastern face of the Andes bears hyperhumid to seasonal submontane and montane forests. Their structure, physiognomy and floristics are similar to these of the northern Andes. In the drier inter-Andean valleys, the forest often becomes deciduous, even xerophilous with thorn trees at the submontane level. As these Andean valleys have been populated for a long time, the forests are often very degraded and transformed into thicket or scrub. On the western slopes of the Andes, under very dry climate, scrub woodland replaces forest. Above the timberline, from central Peru southwards, “paramo” is replaced by drier “puna”.
Non-Andean mountains
In non-Andean highlands, the submontane level is mostly present, with ombrophilous rain forest rather similar to the lowland one, but of lower stature and with a slightly distinct flora.
This Ecological Zone concerns plateaux and lowlands on the Atlantic side of the continent, south of the Tropic of Capricorn up to 40°S; it lies on southern Brazil, Uruguay and Argentina
Climate
The two main climatic characters are lower temperatures in winter (mean temperature of the coldest month<15°C) and rainfall evenly distributed throughout the year. However, rainfall regularly decreases from the North (1000-2500 mm) to the South (600-1000 mm).
Physiography
The northern part, mainly the States of Parana and Santa Catarina in Brazil, is a gently rolling plateau region whose altitude varies from 300 to 1000 m. The southern part lies on lowlands generally less than 600 m.
Vegetation
The natural vegetation of the wetter northern parts of the zone is evergreen coniferous forest dominated by Araucaria angustifolia. These forests once covered sizeable areas, giving these regions a particular geobotanical stamp. In Brazil, this formation has several storeys. The Aracauria 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, Parapiptadenia and the shrub Ilex paraguayensis. Today, only residual areas remain, as this forest has been much exploited for timber production. However, reforestation efforts are conspicuous in this part of Brazil. The other important feature of this region is the considerable extension of coffee growing. In Argentina, the Aracauria forests are also 25 to 30 m tall and these conifers dominate a canopy of broadleaved species of 20 m tall. The humid forest is rich in tree ferns, lianas and epiphytes. Prevailing species include Apuleia leiocarpa, Aspidosperma polyneuron, Balfourodendron riedelianum, Cedrela, Nectandra, Ocotea and Peltophorum.
In Rio Grande do Sul as well as on the lowlands of Uruguay and eastern Argentina, grasslands become the main vegetation from the “campos limpos” of southern Brazil to the “pampas” of Uruguay and Argentina. They are short grasslands, 50 cm to 1m tall, more or less dense, becoming graminoid steppe to the south. Main grasses include Aristida, Bothriochloa lagurioides, Briza, Eragrostis, Poa, Piptochaetium, Stipa. In Rio Grande do Sul riparian forests are fringing the main rivers.
Climate
The Ecological Zone lies on the central “Mediterranean” part of Chile, between 32° and 38° south. Rainfall regime is of a Mediterranean type, with summer drought (2-7 months) and winter rains. Annual precipitation varies from 500 mm in the northern coastal region to 2000 mm on Andean foothills. Winter temperatures are cool (10-15°C).
Physiography
This GEZ concerns a rather narrow stretch of lowlands, less than 200 km wide, lying between the Andes foothill and the Pacific Ocean. However, the long north-south valley extending just at the foot of the Cordillera is separated from the coastal plain by the Cordillera de la Costa. The latter reaches 2000 m in front of Santiago and further south it is reduced to some tabular segments.
Vegetation
The climax is a sclerophyllous evergreen forest or woodland with xerophytic species such as Lithraea caustica (Anacardiaceae), Quillaja saponaria (Rosaceae), Peumus boldus (Monimiaceae) and the Lauraceae Cryptocarya and Beilschmiedia. The endemic palm Jubeae chilensis grows in a narrow area northeast of Valparaiso; columnar cacti (Trichocereus) and the large Puya spp. (Bromeliaceae) as well as the thorny Colletia and Prevoa are found in dry, rocky habitats. Many species of this forest are used as firewood. As a result, the forest is often degraded and replaced by secondary thorny thicket with Acacia caven. Moreover, it has been widely reclaimed for agriculture.
Toward the south or in the Andean foothill, where precipitation is higher, the sclerophyllous forest gives way to open deciduous “mesophitic” forest of “roble”, dominated by various Nothofagus species, N. obliqua, N. dombeyi, N. procera, associated with Aetoxicon punctatum, Araucaria araucana, Drymis winteri, Laurelia serrata and others.
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 Mediterranean zone and the Atacama Desert. The other is to the east of the Andes, an extensive region in central Argentina making a transition between the tropical “Chaco”, subtropical “pampa” and temperate steppes to the south.
Climate
In this large area sheltered from ocean moisture by the barrier of the Andes or the coast direction, rainfall ranges from 100 to 800 mm, the regime still being more or less tropical. The dry period is very long, up to 9 months. 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, with a Mediterranean regime. Temperatures are warmer than in Argentina, with mean temperature of the coldest month between 13° and 15°C.
Physiography
In Argentina, the Ecological Zone lies on the large depressions area stretching along the Andes, with the exception of some “sierras” around Mendoza. In Chile, the Andean foothills extend forming alluvial cones separated by flat plains. Then the coastal plain is rather wide.
Vegetation
In this GEZ, 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, it is rather a subdesert shrubland with Bougainvillea, Cercidium and various Rhamnaceae. In Chile, Acacia caven and Puya dominate the subdesert thorn scrub of the “Norte Chico”. Nevertheless, extraction of firewood over a long period, followed by agricultural reclamation, has greatly reduced the stands of Acacia caven.
Climate
The subtropical Andes roughly lie from 26° to 40°S. From 1000 m to nearly 7000 m altitude, the climate is everywhere cold. 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, precipitations are 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.
Physiography
A range of volcanoes constitutes the boundary between Chile and Argentina. There are some of the highest peaks of the Andes, particularly Mount Aconcagua, which nearly reaches 7000 m. South of the Aconcagua, the Cordillera becomes narrower and its elevation is decreasing to the south, with peaks only reaching 4000 m, then 3000 m. To the east of this volcanoes range, elevation slightly decreases up to the shrubby lowlands of the previous Ecological Zone.
Vegetation
In the lower reaches of the Andes, between 1000 m and 1800 - 2400 m, we find sub-montane 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, Drymis winteri, Laurelia serrata, Persea lingue. Drier slopes are covered with evergreen sclerophyllous shrubs or xerophytic deciduous woodland. Higher up, the vegetation changes gradually into a steppe with caespitose grasses. This steppe replaces the northern dry “puna”: it is slightly moister and denser than the dry “puna”, due to less aridity. Above 4500 m, there is no more vegetation, but only glaciers and snow.
Climate
South of 38°S, the western side of the Andes is well watered due to oceanic influences, when drought decreases from north to south, with decreasing temperatures. These conditions also prevail at the very end of South America, as far as Tierra de Fuego. Rainfall ranges from 1000 to 3500 mm, evenly distributed throughout the year. 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 1000 mm, with mean monthly temperatures always lower than 10°C.
Physiography
In the northern part of the Ecological Zone, from around 38° to about 43°S, the main Chilean physical units can be recognized:
• the coastal plain, with a great extent in Chiloe Island;
• the coastal Cordillera;
• the central plain, with moraines and numerous glacial lakes in Los Lagos region, before being submerged into the sea, south of Puerto Montt;
• the volcanic Andes.
In the southern part, the Patagonian Andes are partially disaggregated, sometimes covered with glaciers, which merge into lakes or into the Ocean.
Vegetation
The northern part of the region harbours the typical Valdivian forest, which is a broadleaved very dense evergreen forest up to 40-45 m tall, with an equally dense undergrowth and many epiphytes. Species of Nothofagus dominate the tree canopy, including Nothofagus obliqua, N. dombeyi, N. procera, in association with Aextoxicon punctatum, Drymis winteri, Eucryphia cordifolia. A slight lowering of temperature accompanying higher altitude or latitude gives rise to a, less species-rich, mixed broadleaved/coniferous forest, with Nothofagus antarctica, N. dombeyi, N. nitida, Fitzroya cupressoides, Pilgerodendron uvifera, Podocarpus nubigenus. An even more marked fall in temperatures may result in increasing precipitations (particularly towards the south) giving rise to a more hygrophilous forest, the subpolar evergreen ombrophilous forest, becoming a swamp forest on gley soils. In Tierra de Fuego, this forest gradually gives way to shrub formations, then a sort of tundra.
Climate
In trans-Andean zones, precipitations are lower than in the western side: 500-600 mm, with still low temperatures (mean temperature of the coldest month = 5-7°C). It freezes almost all along the year. In the eastern part, aridity, cold and very strong winds are limiting factors for vegetation.
Physiography
This Patagonian trans-Andean zone is made of the outcrops of Patagonian and Deseado shields in the depression along the Andes.
Vegetation
On the eastern side of the Andes, decreasing precipitations induce cold deciduous forest eastward gradually becoming a thicket or shrub formation transitional to the Patagonian steppe. The eastern lower part of Patagonia is covered with a grassy steppe.
Climate
The central part of Patagonian Andes, up to 52°S, still reaches 2000 to 3000 m elevation. The western upper slopes are wet, whereas the eastern side is drier. The most striking climate features are cold, snow and winds.
Physiography
Volcanoes often form the highest peaks, surrounded by glaciers. There are numerous glacial lakes on both sides.
Vegetation
Subalpine beech forest, dominated by Nothofagus betuloides forms the timberline on the wettest slopes and borders the western sides of the Patagonian ice fields on steep slopes. It is found on thin rocky soils. This elfin type has low multi-stemmed trees of a few to 10 m, greatly deformed by the weight of the snow. The shrub layer is poor in stature and species composition. These forests are transitional to the scrub- and grasslands at higher altitudes in which N. betuloides occurs scattered among the shrubs Chiliotrichum diffusum and Empetrum rubrum. 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 which form the timberline on all but the wettest sites. Above the timberline, the prevailing vegetation is an Andean grassy steppe.
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