"Al promediar el día el sol no era lo único
que los torturaba: el cielo entero era una cúpula metálica
calentada al blanco. La luz implacable los aplastaba; el sol era
todo el cielo. (...) Las plantas se volvían más altas
día a día, las espinas más fuertes y más
crueles. Algunas alcanzaban ya la estatura de árboles de
copa ancha y chata, siempre tentadores, pero una nube de humo hubiera
protegido mejor de los ataques del sol." (Paul Bowles,
El cielo protector)
The "Travesía del Tunuyán"
is a Province with great geographical contrasts, but plains predominate
in the eastern part of it (Figure).
The plain of the Travesía, better known as the Travesía
del Tunuyán, is a broad flat land which limits with the
Huayquerías ('bad-lands', a group of hills and plateaus of
about 1400 m high) to the west, with the Tunuyán and Diamante
rivers to the north and south respectively, and with the Desaguadero
river to the east. It's a deep watershed, smoothly sloped to the
west, filled with eolic and fluvial sediments which came from the
mountains through the Tertiary and Quaternary. Although it currently
lacks superficial drainage, the rivers which wandered repeatedly
on this plain in the past left very complex paleo-watercourses behind
(with a predominant west-east direction). Today, phreatic waters
coming from the high occidental areas drain there. On the surface,
wind moulding predominates and forms dunes which are now fixed.
Consequently, the soils are mainly sandy, deep and poorly developed,
quickly mineralised and with scarce organic matter.
The travesías are very extended bolsones (depressions) where
the surrounding mountains are lost in the distance. In the Travesía
del Tunuyán all the typical elements of a bolsón can
be found: the Huayquerías, in the west, are mountain ranges
which undergo erosion; they are followed by a bajada or slight slope
to the east that ends in the playa of Ñacuñán.
This flat bed or alluvial plain receives the humidity that drains
from the high lands and therefore constitutes a shallow phreatic
area. The Reserve of Ñacuñán is located on
this playa. Further west, there is a sandy plain cut through by
fixed dunes and after that, the old alluvial plain of the Desaguadero
river, a depressed area with small temporary lagoons and meanders
close to the river.
The vegetation of the travesía is relatively simple. The
bajada is covered with an arbustive steppe of Larrea divaricata
with Bulnesia retama and it has an important grass cover.
In the playa of Ñacuñán there is a Prosopis
flexuosa woodland with Larrea cuneifolia areas (jarillales)
in the lower parts and dune-associated communities. The sandy plain
presents a high jarillal of Larrea divaricata and ranges
of dunes with grass cover (mainly Aristida mendocina). Close
to the Desaguadero River there are some isolated Prosopis flexuosa
woodlands and halophytic communities on poorly drained and frequently
Location and history
The Reserve of Ñacuñán (Figure)
is located in the south of the Department of Santa Rosa, Province
of Mendoza. It encompasses 12800 ha. The town of Ñacuñán
(20 houses, 78 inhabitants) is situated within the Reserve by the
route 153, which crosses it in the north-south direction. The word
“Ñacuñán” is a mapuche term that
means “lost hawk”.
Nomad aborigines, who lived on hunting and fruit collection (probably
Araucans), inhabited the area until they were displaced after the
“conquista del desierto” (“conquest of the desert”
in the mid XIX century). The fields, fiscal lands since colonial
times, were auctioned and transferred to private owners in 1907.
Since then and until 1937 these lands experienced an intensive deforestation.
Trees suffered clearcutting which left 40–60 cm high stumps
from which sprouts would recover the woodland. Products from these
woodlands were used mainly for gas distillation and Mendoza City
illumination. Between 1908 and 1935 (mostly before 1916), about
200000 tons of woodland products (firewood, wood, coal and charcoal)
departed from Ñacuñán railway station which
belonged to the southern branch of the Ferrocarril Gran Oeste Argentino
(Argentine Great West Railway). Once the woodland disappeared, these
lands were destined to livestock —sometimes through private
hands and sometimes through the government of the province —
until the Forestry Reserve of Ñacuñán was created
in 1961 by a provincial law. However, livestock raising didn’t
cease due to the lack of fences and appropriate controls. It was
not until 1966 that the reserve began to have its infrastructure
and in 1977 the IZAS (now IADIZA) started running it. The fencing
was finished in 1971 and since then no grazing occurs except for
a small, restricted area where few horses are kept. During the following
years the herbaceous cover was recovered. Nowadays the area of the
reserve contrasts with the private neighboring lands (Figure),
where cattle is still being raised extensively. In 1986, after seven
year of negotiations, the reserve was officially incorporated into
the MAB UNESCO Program, becoming the Man and the Biosphere Reserve
Geomorphology and soils
Ñacuñán is located on the “playa”
of the Huayquerías in the “Travesía de Tunuyán”
(see above). Three geomorphological units can be found in Ñacuñán,
each with a different type of vegetation: (1) smoothly sloped plain,
(2) depressions and (3) dunes.
The sloped plain occupies most of the reserve. The slope is very
slight —less than 1%— and runs to the east-southeast
with an average height of 540 m above the sea level (between 490
and 600 m). The plain is a watershed filled with loess up to 150–170
m deep. The soil is sandy and slimy, reddish brown and poor in organic
The depressions are represented by dry watercourses and stripes
of clay, both oriented northwest-southeast. The formers are relatively
recent and channel the waters, which come down occasionally from
the West as alluvions. The latter are shallow stripes where clay
moved by surface runoff is accumulated. These soils are consequently
clayey, with little drainage, becoming impermeable and relatively
saline and calcareous in some places, giving place to the “peladares”.
The dunes —which reach 30 m high— are northwest-southeast
oriented and made up by coarse sand particles accumulated by the
They are almost completely fixed: the Médano Negro (the largest
in Ñacuñán, 6 km long and 1.5 km wide) hasn't
changed much since the first measurements in 1903.
There are no permanent watercourses in Ñacuñán,
only temporary superficial courses which start in the Huayquerías,
in the West. Groundwaters (which are part of the underground watershed
of the center and East of Mendoza) run deeply: from 150 m deep in
the West to 10–15 m deep to the East near the Desaguadero
river. In the reserve, ground waters are more than 70 m deep. Apparently,
in the subsoil there are slimy and sandy permeable layers alternated
with semipermeable layers placed on lenticular clayey levels. Water
which drains from the surface or comes from refilling areas in the
West would be retained for some time between the semipermeable levels,
forming aquifers which would be responsible for the subsurface humidity
which maintains the woodland.
Ñacuñán’s climate is arid-semiarid,
seasonal, with warm and relatively humid summers and cold and dry
winters. It belongs to the DB2nd category according to Thornthwaite’s
classification (semiarid, mesothermal, dry, with low percentage
of thermal efficiency in summer), Walter’s Type III (subtropical,
warm and arid, with summer rains and cold winters) and Koeppen’s
Bswk (dry, steppe climate, with summer rains and cold winters).
was drawn with temperature and rainfall data from the periods 1972–1990
and 1972–1998, respectively, recorded at the reserve’s
meteorological station. In the figure ()
the marked difference among seasons can be clearly seen. Months
grouped in the upper right hand corner of the plot belong to the
thermal summer (i.e., the period with average daily temperatures
over 20°C), which spans form November’s second fortnight
to March’s first fortnight in Ñacuñán.
On the other extreme of the plot are the months which make up the
thermal winter (i.e., the period with average daily temperatures
below 10°C), which spans from May’s second fortnight to
August’s second fortnight. Although there is a relatively
wet season between November and March according to the temperature-precipitation
the hydric balance shows a deficit all year long (i.e., evapotranspiration
always exceeds precipitation). The potential evapotranspiration
is 807.4 mm and the hydric deficit is 478 mm (1972–1992).
Mean annual temperature is 15.6°C. Mean maximum and minimum
annual temperatures are 23.8°C and 7.6°C, respectively.
These values are in the extreme of range of temperatures recorded
for the Monte. Mean monthly temperature is below 10°C during
winter months and over 20°C in summer (Figure).
Absolute maximum and minimum temperatures have been 42.5°C and
-13°C, respectively. The mean extent of the period free from
frost is between 90 and 120 days and there have been absolute minimum
values under 0°C between March and November. Daily temperature
fluctuation is large with an average of 16.2°C.
annual precipitation in Ñacuñán is 333.6 mm.
Meteorological records in the area of influence show an average
of 294.7 mm (74 years). Mean number of rainy days per year is 57.8.
Summer rains are generally convective, more intensive and shorter,
associated with localized thunderstorms (Picture),
often with hail. Winter rains, in contrast, are associated with
large scale cyclonic fronts and have a low intensity and a long
duration. The most relevant features of the rain regime in Ñacuñán
are (1) its marked seasonality and (2) its great interannual variability.
Seasonal variation in precipitation is very pronounced: 78% of
the rains are concentrated in spring-summer (October-March), although
October is markedly drier (Figure).
The degree of seasonality is described in detail in the figure (),
where a temporal autocorrelation analysis was performed with monthly
precipitation values of the period 1972–1998. This analysis
is based on the correlation coefficients of all pairs of values
separated by different time intervals. This correlation is negative
(or positive) when a value is negatively (or positively) correlated
with values corresponding to a certain previous or posterior period
of time. An autocorrelation close to zero indicates that the values
are independent (in this case, an inability in predicting values
from the ones of the previous period considered). The
result clearly shows the seasonal rainfall pattern (Figure):
there is a significant positive autocorrelation for intervals close
to twelve months and a negative correlation for the ones close to
six months (i.e., if a month is dry, in twelve months will be dry
as well and within six months, wet). The pattern is so marked that
it is observed even without performing a data standardization to
remove the effect of great interannual differences in precipitation
interannual rainfall variability is large (Figure).
Precipitation varies between 192.6 and 532.8 mm, with a variation
coefficient of 30%. This variability is associated with erratic
trends in precipitation levels. For example, after the wettest year
(1985), one of the driest years follows. In arid seasonal environments,
total annual precipitation could be less relevant biologically than
rains falling during a profitable period for plants (i.e., the growing
season; October to March). The interannual rainfall variability
in that period is also very high in Ñacuñán
and is highest than in the winter period (standard error of the
mean: 15.51 vs SE = 7.32 for the winter period).
The three representative habitats in Ñacuñán
are: (1) the “algarrobal” or Prosopis flexuosa woodland,
which occupies the smoothly sloped plain; (2) the “jarillal”
or the Larrea cuneifolia shrubland, restricted to stripes
of clayey soil; and (3) the community of the dunes (“medanal”).
algarrobal occupies most of the reserve (Figure).
It’s an open woodland with well defined strata (Picture).
Prosopis flexuosa (“algarrobo”) and Geoffroea
decorticans (“chañar”) represent the tree
former can reach 7 m high and has a cover of 4–12%; the latter
is shorter, with less cover, and often appears in groups of numerous
individuals. Larrea divaricata dominates the shrub stratum.
It is up to 3 m high and reaches a cover of 11–32%. Other
important shrubs are Condalia microphylla, Capparis atamisquea
and Atriplex lampa. There is also a low shrub stratum
of species which are seldom more than 1 m high. Lycium chilense,
Lycium tenuispinosum, Verbena aspera and Acantholippia seriphioides
are the most abundant ones. Finally, grasses predominate in the
herbaceous stratum with a cover of 25–50%. Pappophorum
spp., Digitaria californica, Trichloris crinita, Aristida
spp., Sporobolus cryptandrus and Setaria leucopila
are abundant. All grasses —except for the less important Stipa
spp.— are C4 and most of them, perennial. There are various
dicotyledoneous species which contribute to the herbaceous stratum
(principally, Chenopodium papulosum, Phacelia artemisioides,
Sphaeralcea miniata, Parthenium hysterophorus, Glandularia mendocina
and Descurainia spp.), although their presence and cover
—usually lower than those for grasses— vary between
years in response to precipitation levels.
jarillales of Larrea cuneifolia occupy broad fringes which
alternate within the algarrobal (Figure).
The dominance of Larrea cuneifolia is marked (Picture),
with a 30–40% cover. Prosopis flexuosa has a very low
density, with isolated individuals separated from each other by
big distances. Grass cover (mainly Sporobolus cryptandrus and
Trichloris crinita) is high—similar to the one recorded
in the algarrobal— and usually much greater than for forbs.
In the medanal there is a herbaceous layer of species restricted
to dunes (Picture); among them, Panicum urvilleanum, Solanum euacanthum,
Nicotiana petunioides, Hyalis argentea and Gomphrena martiana,
which increase their cover after rains. The shrub stratum has a
low cover; Larrea divaricata, Ximenia americana and Lycium
chilense are frequent species there.
habitats in the reserve —of restricted extension (Figure)
are the “chañarales” (isolated groups of Geoffroea
decorticans, especially by the dunes and associated with dry
watercourses), the jarillales of Larrea divaricata (especially
in the northern portion of the reserve) and the “retamales”
(groups of Bulnesia retama, restricted to the southern part
of the reserve). The “zampales” of Atriplex lampa,
the peladares in low areas with poor drainage (Picture)
and the “ciénagas” (swamps) in the areas where
rainwater accumulates, are of less importance than the previous
ones but still worth mentioning.