"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)
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.
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.
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
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.
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
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
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.
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
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
low slopes and valleys between mountains (in the North), extended
depressions (“travesías”) and plains —usually
(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.
The physiognomy of the Monte is relatively simple: a shrub steppe
where small and localized open woodlands occur in areas with continuous
shrub steppe occupies almost the whole Monte area. The most extended
type of steppe is the “jarillal” or Larrea (creosotebush,
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
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.