Monte Desert

"At night in this waterless air the stars come down just out of reach of your fingers. (...) The great concept of oneness and of majestic order seems always to be born in the desert." (John Steinbeck, The log from the Sea of Cortéz)

The Biogeographical Province of the Monte comprises an extensive territory —more than 46 million ha— extended as a latitudinal strip (almost 20°, more than 2000 km) that runs along the east of the Andes and broadens to the south where it finally reaches the Atlantic Ocean (Figure). In spite of its extension, the physiognomy and the floristic composition (and in general terms the climatic features as well) are very homogeneous. The Province is characterized by the presence of shrub steppes of Zygophyllaceae plants, where small localized woodlands intermingle.

Limits

The Monte limits with the Chaco (to the NE and the E), with the Espinal (to the E), with the Puna or Prepuna (to the N, NW and W) and with the Patagonia (to the W, SW and S). Although these units are markedly different and distinguishable form each other, there are areas where the limits are not so clear bringing about several controversies. In the demarcations Monte-Chaco, Monte-Espinal and Monte-Patagonia there exist ecotonal strips where typical Monte elements coexist with elements belonging to the neighboring province. These ecotones are produced by the advance (or retrocession) of the borders between provinces. This can be attributed to:(1) current physical factors (e.g., interanual climate variations) or historical factors (e.g., Pleistocenic climate cycles; see Historical Aspects, below), or (2) the impact of human activity which modifies the composition and the physiognomy of the vegetation.

In the north, the Monte starts in Toro and Capilla gorges(24°35'S, Salta) and runs to the south along the Calchaquíes Valley from La Poma to Campo Arenal. The limit with the Prepuna lies on the foothills of the adjacent hills. To the south, this limit runs along Aconquija and Ambato ranges in the east and Fiambalá, Las Planchadas, Famatina, Umango and Punilla, among others, in the west. Further south, along the west, the Monte extends through the valleys of the rivers Blanco, Iglesias and Jáchal and the Calingasta Valley. The east of the Andes determines the limit with the Puna in this area. The limit between the Monte, the Puna and the Prepuna is altitudinal.

The ecotone Monte-Chaco, to the east, starts in the south of Ambato. The limit then moves to the west along sierra de Velazco and reaches the Bermejo River. After following this river it turns southwest (Picture) up to Capilla del Rosario (in Mendoza). From this location on it joins Capilla del Rosario, Nueva California and La Dormida along a line. Then it reaches the confluence of the rivers Tunuyán and Desaguadero. This limit, suggested by J. Morello, coincides with the western border of the distribution of Schinopsis haenkeana and with the border of various Chacoan tree species' distributions in the north of Mendoza.

The ecotone Monte-Espinal starts where the Monte-Chaco ecotone ends. The limit runs along the river Desaguadero-Salado up to the border of the Provinces of Mendoza, San Luis and La Pampa. It then crosses the latter Province to the southwest until reaching the surroundings of the Colorado river (Picture, Province of Río Negro). After that, it follows straight up to Bahía Anegada in the Atlantic Ocean. This limit, suggested by A. Cabrera, concides with the eastern border of Larrea divaricata distribution and differs from the one suggested by J. Morello who placed the limit further south and west at the hypothetical border of Prosopis caldenia historical distribution.

The western limit of the Monte (south of 32°S) runs north-south across the centre of the Province of Mendoza, on the eastern spurs of the Andes. In some areas of San Juan and in the centre and southwest of Mendoza, there are some Monte-Patagonia ecotonal areas. These areas are interrupted and reappear in the south of Neuquén, the centre of Río Negro and Península Valdés. In the south of Mendoza (Picture) the limit edges the Payunia up to Buta Ranquil (Neuquén). From there on, it follows north-south, skirting the hills and plateaus of the centre of Neuquén, up to the surroundings of Piedra del Aguila on the Limay River. In Río Negro, the limit goes to the south along the high-plateaus and plateaus. Further south, the Monte extends along the Arroyo Telsen Valley, the northern bank of the Arroyo Perdido and the lower valley of Chubut and Chico rivers up to Laguna Escondida. There, it reaches its southernmost extreme (44°20'S, Chubut). Finally, from the surroundings of Laguna Grande the limit runs west-east to Bahía Janssen on the Alantic Ocean. This limit, based on R. León and collaborators' work, is mostly altitudinal although its southern portion is also associated to climatic factors —in particular to the influence of Atlantic air masses.

Biogeography

Several biogeographical aspects of the Monte have been of interest to systematists and ecologists. The most widespread one has probably been the close relationship between the flora of the Monte and the Sonoran deserts (Mexico and southwest of the USA). This has been the gist of a long —still not solved— controversy based on the presence of a variety of plant genera and species in both units (e.g., Larrea, Prosopis, Condalia, Cercidium, Celtis, Opuntia), separated by more than 5000 km and with a wide belt of tropical and subtropical flora between them. This floristic disjunction could be explained by (1) past phytogeographical connections, (2) relatively recent long distance migrations, or (3) convergent adaptation of polymorphic transtropical taxa, which are non-exclusive causes. The similarities are not restricted to plants; in fact, the same happens with granivorous desert ants of the genus Pogonomyrmex (Picture), found in extratropical South America, and the north of Mexico and southwest of the USA. The environmental and physiognomic resemblance between areas promoted detailed studies (between 1965 and 1974) in the search for evidence of convergence at the ecosystem level. This was carried out in the framework of the 'International Biological Program' (IBP), the biggest endeavor carried out jointly by biologists from Argentina and the USA.

The Monte is intimately related biogeographically with the Chaco and the Espinal Provinces, and also with other South American areas, such as the Chilean Province, and more restricted areas in Peru and Bolivia. The Monte also gained importance in biogeographical studies, as it could constitute the contact area between two big South American biotas: the Brasilic and the Patagonian biotas.

Historical aspects

The available information on the age of the Monte and its geological history is very general. In the North, the lands have been emerged since, at least, the end of Carboniferous. Further south, they might have been out of the marine influence since much earlier, probably since the Cambric, but suffered lacustrine sedimentation phases, particularly at the beginning of the Triassic. Some areas in the southeast, in contrast, suffered marine ingressions until the Mesozoic. The austral and central portions have been subjected recurrently to important volcanic events.

The first signs of aridity date from the Cretacic. At the beginning of the Tertiary, the environment turned arid progressively, making the mesic woodlands retreat to the southeast of Brazil, the south of Chile and the eastern slopes of the young (thus still low) Andes. Most of the ancient alluvial deposits of the plateaus come from this time. In the mid Tertiary, the Monte area might have been an open and savanna-like environment, with a warm and dry climate, similar to the present Chaco. From the Miocene on, the worldwide climate changed quickly turning the arid conditions even more marked. The lifting of the Andes (Picture) reached its end between the Pliocene and the Pleistocene, producing a barrier effect for the wet winds of the west, increasing the aridity of the area. The final establishment of the dry environment might have happened with the lifting of the Sierras Pampeanas, which blocked the Atlantic winds in the north.

The main events of the Pleistocene were associated to glacial periods. Climatic fluctuations (mainly, the alternation of dry and wet conditions) might have been much more pronounced during that period than during the subsequent periods. Temperatures changed 3–6°C. In the southeast extreme of the Monte, by the Atlantic, marine ingressions and regressions were associated with the rise and fall of the sea level. In restricted areas of the mountains there are records of the presence of glaciers. In the south and southeast of the present Monte there were glacier lakes, whereas a meltwater continental lake might have occupied part of San Luis, Mendoza, San Juan and La Rioja. The north of the Monte could have been the northwest extreme of a large lake connected with the Atlantic Ocean. However, the degree of climatic fluctuations in the Monte during the Pleistocene is subjected to controversy.

Geomorphology and climate

The Monte varies greatly physiographically: it occupies depressions (“bolsones”, Picture), low slopes and valleys between mountains (in the North), extended depressions (“travesías”) and plains —usually sandy (Picture)— (in the center and in the East) and slopes, low plateaus and extensive valleys (in the South). The topographical gradients are always pronounced to the west, where the Andes create a continuous barrier. In the North, there is also a marked gradient to the east due to the presence of the Sierras Pampeanas. These two orographic barriers ("rain shadows") are one of the main causes of the Monte aridity. The soils are mainly sandy and deep, very permeable, although other types can be found locally, from clayey to rocky, depending on the slope, the elevation or the microclimatic features.

The climate in the Monte is hot and dry. One noteworthy characteristic is the isothermia: mean temperatures vary only between 13.4° and 17.5°C, in spite of its great latitudinal extension. Mean minimum and maximum temperatures are more variable. Rains show a marked east-west gradient and vary greatly: between 80 and 300 mm a year (with some exceptional records), with a few locations exceeding the 200 mm. The dry season lasts up to nine months. Rains are restricted to the summer, except in the south (in particular, to the south of the Diamante River, Picture) where they tend to occur more regularly throughout the year.

Vegetation

The physiognomy of the Monte is relatively simple: a shrub steppe where small and localized open woodlands occur in areas with continuous water availability.

The shrub steppe occupies almost the whole Monte area. The most extended type of steppe is the “jarillal” or Larrea (creosotebush, Picture) steppe, which confers the Province a phytosociological unity. It consists of 1.5–2.5 m high (always <3 m) schrublands with evergreen and thornless branches, and always a predominant species of the genus Larrea (generally Larrea divaricata or Larrea cuneifolia). Secondarily, there are cactaceous plants (in the north), low trees or tall shrubs, such as Bulnesia spp., Monthea aphylla, Bougainvillea spinosa, Cassia aphylla, Cercidium praecox, Chuquiraga erinacea, Prosopis alpataco and Zuccagnia punctata. The herbaceous cover is spatially variable, depending on precipitation variability and the impact of livestock. In the foothills there is a thorny steppe where species of the genus Plectrocarpa predominate. Other shrublands are edaphic: “jumeales” (Suaeda divaricata) and “zampales” (Atriplex spp.), both associated with halophilous conditions; “cardonales” (giant cactuses), in the rocky slopes in the north; dune steppes; and “chilcales” (Baccharis salicifolia), in the wet soils.

In the Monte, woodlands are edaphic communities, which occupy riverbanks or areas with shallow groundwater. In both cases, water is available for roots throughout the year. The most common woodland type is the “algarrobal” or Prosopis woodland. It is an open woodland (i.e., the treetops are not in contact with each other and leave big gaps) with thorny trees of variable height (up to 15 m) and deciduous foliage. The predominant species is Prosopis flexuosa (Picture), which can occur with Prosopis chilensis, Jodina rhombifolia, Geoffroea decorticans (Picture), Prosopis alba and Prosopis nigra. The shrub stratum is generally well developed, being Capparis atamisquea, Schinus polygamus and Condalia spp. the most important species. The herbaceous stratum, in contrast, is quite poor. The Prosopis woodland occupies the riparian galleries or saline surroundings of “bolsones” (big depressions), the base of dejection cones, areas with underground water streams and areas with shallow groundwater (shallower than 20 m deep). Another type of woodland is the Salix humboldtiana woodland, with thornless trees and with an important grass cover on the banks of permanent rivers. Other woodlands are the “arcales” (Acacia visco) in the ravines of the heads of the rivers and streams, and the “maitenales” (Maytenus boaria) which follow the rivers in the central and austral portion.