Ñacuñán Reserve

"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"

Mendoza 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 flooded soils.

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 of Ñacuñán.

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 matter.

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 wind (Picture). 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.

Weather

General characteristics

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).

Ñacuñán’s climatogram (Figure) 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 relationship(Figure), 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).

Temperature

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.

Rain

Mean 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 values.

The 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 (Figure), and is highest than in the winter period (standard error of the mean: 15.51 vs SE = 7.32 for the winter period).

Vegetation

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”).

The 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 layer. The 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.

The 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.

Other 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.