Location and General Description
The southern Andean steppe ecoregion forms a continuous area along the dry southern Andes from Catamarca, La Rioja, San Juan, Mendoza and Neuquén provinces of Argentina and limiting areas of Chile between latitudes 27ºS and 39ºS. At this latitudes the Andes are formed by several parallel ranges, the most important being the Cordillera Principal, the Cordillera Frontal and the Precordillera from west to east (Taylor 1991).
The Andean steppe limits to the north with the central Andean punas and to the south with the Valdivian temperate forest and the Patagonian steppe, but it also extends as interrupted islands over mountains of these regions forming complex ecotones (Cabrera 1976, Martínez-Carretero 1995, Ferreyra et al. 1998, Chiapella & Ezcurra 1999). The eastern limit is the transition to the lower altitude and warmer Argentine Monte desert, and the western limit to the lower Chilean matorral and Chilean winter-rain forest. The lower altitudinal limits of the ecoregion range from approximately 3500 m in the north to 1800 m in the south. The highest altitudinal limits of the vegetation in this area are found at approximately 5000 m in the north and 3000 m in the south, but they also vary from west to east, rising towards the interior (Cabrera 1976).
The relief is extremely abrupt and dissected and in its northern portion the ecoregion presents some of the highest peaks of the Andes such as Mt. Ojos del Salado (6863 m), Mt. Pissis (6858 m), Mt. Aconcagua (6960 m) and Mt. Tupungato (6800 m). Many other mountains exceed 5000 m. In the south, elevations generally decrease to less than 3000 m, but high volcanoes are frequent such as Vn. Peteroa (3951), Vn. Descabezado Grande (3880 m) , Vn. Domuyo (4709 m) and Vn. Tromen (3978 m). Permanent snow, ice caps and glaciers are found in the summits of these elevations, and penitents (irregular blades of ice that point towards the sun) are characteristic of the lower parts of the glaciers. Most glaciers have receded in recent times, but during the ice ages significant expansions of the glaciers took place due to a northern shift of the wetter climate that nowadays exists more to the south (Lliboutry 1998). Soils of the ecoregion are rocky, stony or sandy, and loose screes are frequent.
The climate of this area is dry, and very cold at high elevations. Snowstorms and hailstorms can fall any time, and frosts are frequent all the year round . Precipitations in the center and the south come from winds off of the Pacific, and fall mostly in winter. In the northeast, precipitation is effected by winds coming from the Atlantic, and they tend to fall in summer. Although climatological data are scarce, mean annual precipitations vary from north to south, from approximately 200 to 600 mm, rising with elevation (Hoffmann 1975). For example, mean annual precipitation and temperature in Puente del Inca at 2720 m in Mendoza (in the central part of the ecoregion) are 342 mm and 7.4ºC, with a January (summer) mean of 12,9ºC and a July (winter) mean of 0,6ºC. In higher elevations, such as Cristo Redentor at 3892 m, temperatures can vary from an absolute maximum of 19ºC to an absolute minimum of –25ºC (Cabrera 1976).
The vegetation on the Chilean side shows clear altitudinal zonation, with a lower belt dominated by large shrubs. The intermediate belt is dominated by small shrubs and tussock grasses with cushions. The high subnival desert belt is dominated by small forbs, rosettes, and small-sized grasses. These belts have been given different names (e.g., Squeo et al. 1994, Muñoz-Schick et al. 2000). At the latitude of Santiago, shrubs such as Chuquiraga oppositifolia and Nassauvia axillaris are representative of the lower belt (aprox. 2000-2700 m), cushions such as Azorella madreporica and Laretia acaulis of the intermediate belt (aprox. 2700-3300 m), and Nassauvia lagascae, Oxalis erythrorhiza, Nassauvia pinnigera and Moschopsis leyboldii of the high Andean desert (aprox. 3300-3900 m), but there are also areas with tussock grasses of the genus Stipa termed "coironales," generally in places with less pronounced slopes (Gajardo 1994, Hoffmann et al. 1997, Muñoz-Schick et al. 2000). In humid or wet sites especially rich bog-communities called ‘vegas,’ growth is dominated by Juncaceae, Cyperaceae and grasses. These areas are frequently overgrazed by domestic stock. In the southern part of the ecoregion these humid communities may represent relicts of the peri-glacial tundras of the ice ages (Gajardo 1994).
The diverse plant communities found at different latitudes of the Argentinian side vary in relation to elevation, humidity, continentality, slope angle, slope aspect and soil type. Vegetation belts are not clearly delimited (Hauman 1918, Hunziker 1952, Roig 1972, Martínez Carretero 1995, 2000, Böcher, Hjerting & Rahn 1972, Chiapella & Ezcurra 1999). Nevertheless, in general, the vegetation follows a similar pattern to that on the Chilean side, with tussock grasses mostly of Stipa occurring together with shrubs of Adesmia, Mulinum, Nassauvia and Chuquiraga in the lower elevations (aprox. 1900-2700 m). This belt has sometimes been described as ‘región de la leña amarilla’ due to the yellow color of the branches of these shrubs. In the intermediate elevations (aprox. 2700-3300 m), low shrubs and cushion plants are frequent, e.g., of Oxalis, Junellia, Adesmia, Laretia and Azorella, and this belt has been described as ‘región de la llareta’, the common name given to dense woody cushions in the area (Roig 1972). On the highest parts of mountains (aprox. 3300-4500 m), specially adapted perennial forbs of generas such as Senecio, Nassauvia, Chaetanthera, Draba, Barneoudia, Leucheria, and Moschopsis, withstand the extreme wind and cold of the denuded rocky terrain, or the moving scree slopes. In areas with wet soils and near streams Cyperaceae and Juncaceae dominate, with some genera forming dense and hard wind-pollinated cushions such as Oxychloe and Patosia together with beautiful large-flowered forbs such as Mimulus, Euphrasia and Senecio species. High elevation lakes of clear waters are frequent, sometimes with species of Potamogeton and Myriophyllum in their margins (Roig 1972).
The flora of the ecoregion is diverse and has several characteristic genera that are much diversified, many with attractive colored flowers such as Adesmia, Astragalus, Cajophora, Loasa, Junellia, Jaborosa, Calceolaria, Calandrinia, Chaetanthera, Chuquiraga and Senecio (Hoffmann et al. 1997) that are mostly insect-pollinated (Kalin-Arroyo et al 1983). In terms of the phytogeoghraphical classification of Cabrera (1976, 1980), the ecoregion is part of the Altoandina phytogeographical province, which is related floristically to the closely connected Puneña and Patagónica provinces (Cabrera 1978).
Plants of this high montane region have developed considerable adaptations for survival in this particularly harsh environment. During the winter, the plants are covered by snow for many months. During the summer, they can withstand variations of extreme cold, heat, wind, and desiccation within a single day. At the higher elevations (3000-5000 m), many plants are pressed to the ground to form flat cushions or dense mats which can withstand the wind and cold air-temperature. Cushions can be herbaceous (e.g., Oreopolus glacialis), semiwoody (Oxalis erythrorhiza), or woody (Adesmia caespitosa, A. subterranea, Azorella monantha, Laretia acaulis).
Other plants form rosettes that can be wide and flat (e.g., Barneoudia major, Pachylaena atriplicifolia, Nastanthus spathulatus) or more or less cylindrical and vertical (e.g., Viola spp., Nassauvia spp., Chaetanthera spp.). Frequently they have deep rhizomes or tap-roots, and fleshy leaves. These plants are sometimes incredibly mimetic of stones, probably as an anti-herbivore defense (Böcher et al.1963). They are very attractive in flower. Rocks, as well as tussock grasses and cushion plants, protect softer and smaller forbs from the cold and the wind (Cabrera 1976), and in some cases rich associations of different species are formed by the ‘nurse-effect’ of cushions (Cavieres et al. 1998, Nuñez et al. 1999).
Shrubs (e.g. Adesmia pinifolia, Chuquiraga oppositifolia) and hard tussock grasses (e.g., Stipa spp.) are a conspicuous feature on lower and intermediate slopes, and present anti-herbivore defenses such as spines and abundant schlerenchyma in their leaves. Tussock grasses in very cold areas grow in large patches of circular shape in which the interior of the patch dies, forming conspicuous circular or semi-circular patterns (Roig 1972). Many of the species on these slopes also have small or narrow leaves, or are covered by hairs to protect them against desiccation (Böcher 1979, Ezcurra 1997, Ancibor 1992).
This ecoregion is uninhabited at the very high, harsh climatic elevations, with the exception of herdsmen that bring animals from lower elevations in the summer, and mining activities. On Mt. Aconcagua, and on several of the other high peaks, seasonal mountain tourism in the form of climbers is important, particularly in summer. Ski resorts are active in winter in areas such as Las Leñas. In some localities, volcanic activity produces thermal water that is used therapeutically. In Villavicencio and in Domuyo, geysers can be observed. A few roads and routes cross the Andes at high elevation passes, such as Las Cuevas that connects the cities of Santiago, Chile, and Mendoza, Argentina.
The high Andean areas of this ecoregion contain important levels of endemism among plants. Although the percentage of endemism has not been measured, many species confined to the ecoregion, or restricted to small areas of the ecoregion have been described and continue being discovered (Correa, 1969-1999, Kiesling 1994, Zuloaga & Morrone 1996, 1999, Zuloaga et al. 1994). A few examples of conspicuous endemic species are shrubs such as Adesmia pinifolia, Chuquiraga ruscifolia and Chuquiraga echegarayi, cushions such as Adesmia subterranea. Azorella cryptantha, Laretia acaulis and Oxychloe bisexualis, forbs such as Senecio uspallatensis, Chaetanthera pentacaenoides and C. spathulata, and tussock grasses such as Stipa ruiz-lealii, Stipa speciosa var. breviglumis and several forms of Stipa vaginata.
Although endemism at the generic level is low, the few phylogenetically isolated, endemic, monospecific genera such as Huarpea in the north and Lithodraba in the south (Zuloaga & Morrone 1996, 1999) are specially relevant, and may indicate relicts of an ancient mid-Tertiary arid flora. On another hand, genera such as Senecio that present so many closely related species in the area (Zuloaga & Morrone 1996, 1999), probably had a relatively recent pulse of diversification associated to the climatic fluctuations of Quaternary times. Some of the genera that have Arctic (e.g., Ribes) or Subantarctic (e.g., Ourisia) affinities, may have arrived to the area through long distance dispersal when these high elevation environments appeared, at the end of the Tertiary (Simpson 1983, Simpson & Todzia 1990). They may have later diversified on different parts of the southern Andes.
Endemism in the flora, however, is not mirrored in the fauna. Although the overall number of vertebrate species is high, there are few endemics. Among the avifauna, most birds of the south Andean steppe extend their ranges to the north into the Puna or to the south into the Patagonian Andes (Fjelså & Krabbe 1990). Several conspicuous Andean species are characteristic of the ecoregion but also appear in several other mountains of southern South America (Nores 1995). There are, however, some endemics, for example the creamy-rumped miner Geositta isabellina and the greater yellowfinch Sicalis auriventris (Fjelså & Krabbe 1990). Some birds of this ecoregion only rely on these montane ecosystems for some part of their lifecycle such as the black fronted ground tyrant Muscisaxicola frontalis that migrates to the Puna in winter (Fjelså & Krabbe 1990).
Mammals also tend to have geographic distributions that extend north and/or south to other ecoregions. The only endemic or near-endemic mammals are small ground living species, such as Euneomys noei, Akodon andinus and the nearly extinct in the wild Chinchilla lanigera (Redford & Eisenberg 1992). The puma (Puma concolor), a fox (Dusicyon culpaeus) and two camelids, vicugna (Vicugna vicugna) and guanaco (Lama guanicoe), are the largest native animals in the area and are relatively numerous in some parts of the ecoregion. These animals have reduced their range in the last 200 years (Roig 1991). Historically guanacos have become restricted from most of the wide pampas to the southern Andes and Patagonia due to hunting, habitat destruction and climatic change. During the last centuries vicugnas have also been hunted extensively for their meat and fur and their numbers have diminished alarmingly, so they currently have protected status (Franklin 1982, Redford & Eisenberg 1992, Wheeler 1995). Parque Provincial San Guillermo has been created especially to protect vicugna populations, and several of the reserves in the south Andean steppe ecoregion also protect guanaco populations.
The reptiles and amphibians in this ecoregion must be extremely well adapted to survive in the cold and unpredictable climate. Some species appear to be more or less endemic to the ecoregion, including amphibians such as Alsodes nodosus (Ubeda pers. comm.), and reptiles such as Liolaemus andinus andinus, L. ruibali, L. buergeri, L. fitzgeraldi and Homonota andicola (Cei 1986, 1993; Ibargüengoytía pers. comm.). Arthropods are relatively little known but some are ecologically important in the region, for example insects, mostly Hymenoptera, Diptera and Lepidoptera, pollinate nearly 85% of the flora of all the altitudinal belts of some areas (Kalin-Arroyo et al. 1983).
This ecoregion has its terrain at high altitudes that are not suitable for farming, and has several more or less protected areas through all its length, so there has been relatively little anthropogenic habitat loss in the area. The most important protected areas in the region from north to south are Tres Cruces, Yerba Loca, Río Clarillo, El Morado, Río de los Cipreses and Laguna del Laja in Chile, and Laguna Brava, San Guillermo, El Leoncito, Aconcagua, Tupungato, Domuyo and Tromen in Argentina.
Types and Severity of Threats
The south Andean steppe is not seriously threatened by habitat conversion at the present time. However, a marked increase in ecotourism and mountain sports such as climbing or skiing can be observed on several major mountains of the region. This could produce problems with littering, erosion, sewage disposal and the cutting of woody vegetation for fires. Burning of shrubs for fuel is generalized among local herdsmen and is probably an activity that comes from ancient times, surely performed by native hunters and gatherers before the European settlement. But if this activity increases, it could effect the vegetation of the region (e.g., Chiapella & Ezcurra 1999). Although the vegetation is also probably adapted to grazing by native camelids such as vicugna and guanaco, the relatively recent introduction of exotic cattle, especially goats, can also have an important impact (Chiapella & Ezcurra 1999). Other exotic animals that have been introduced for hunting or for fur or meat such as European rabbit (Oryctolagus cuniculus), European hare (Lepus europaeus) and Californian quail (Callipepla californica) have already invaded wide areas of the southern portion of this ecoregion, and can also alter ecosystems of the area.
Justification of Ecoregion Delineation
This steppe ecoregion occurs on the eastern slopes of the southern Andes in eastern Chile and western Argentina. At the Southern Andes, it corresponds to the Cuyano district of the Altoandina phytogeographical province of Cabrera (1976), which is separated from similar ones to the north and south (Quichua and Austral districts) on the basis of differing floral composition, climate, and endemic plants (Cabrera 1993). The precipitation is significantly lower, and elevations are generally higher here than on the wet Andes more to the south, while snow is more abundant than in the Puna, and daily temperature fluctuations are not as high. Its limits roughly follows the 3500-1800 m contour (depending on latitude), and mostly include high Andean steppe vegetation. This ecoregion forms part of the Andean-Patagonian realm of biogeographic endemism of Cabrera & Willink (1980).
Linework follows UNESCO (1980), who classify this as "caespitose herbaceous community". Classification however followed reference to the Daniele and Natenzon (1994) who term this region "high Andean steppe". Reference was also made to Morello (1968) and Cabrera (1976). Portions of the western linework were later modified at an ecoregion priority setting workshop (Valdivia, Chile (April 19-21, 1999) which extended limits of Valdivian temperate forests eastwards.
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Prepared by: Cecilia Ezcurra
Reviewed by: In process