El desierto verdadero del Centro-Oeste de Argentina: bioclimatologia, geomorfologia y vegetacion.

• Autor: Le Hou

RESUMEN

En el sector NW de Argentina entre las latitudes de Mendoza (32[grados]53 'S) y Jachal (San Juan) (30[grados]15'S), se extiende un valle tectónico, con orientación N-S, localizado entre la Precordillera y la Cordillera Frontal de los Andes. Este valle se profundiza entre 2.000 y 4.000 m con respecto a las cumbres de la Precordillera y Cordillera Frontal, que lo bordean por el este y oeste respectivamente; además posee una longitud de aproximadamente 300 km y un ancho medio de 40 km. Realmente es un típico caso de desierto de sombra, comparable al bien conocido Valle de la Muerte en California, con clima hiperárido. En el punto más bajo (Calingasta -San Juan-, 1.375 m) la precipitación media anual es de 54 mm que alcanza los 100-120 mm en las mayores alturas (Tambillos -Mendoza-, 2.164 m).

Desde el punto de vista geomorfológico, climático, florístico y de la vegetación, este graben post-tectónico constituye un verdadero desierto libre de vegetación perenne en los pedimentos, mientras que las especies perennes están contraídas siguiendo la red de drenaje y alrededor de las depresiones. La vegetación de los piedemontes se presenta como un modelo difuso de estepa en la mayor parte por encima de los 1.800 m, y como modelo contraído en alturas menores, entre 1.800 y 1.300 m.

Palabras clave: Zonas áridas, Cordillera de los Andes, desierto, vegetación, geomorfología, desierto de sombra.

THE TRUE DESERT OF THE CENTRAL-WEST ARGENTINA BIOCLIMATOLOGY, GEOMORPHOLOGY AND VEGETATION

ABSTRACT

In the NW sector of Argentina between the latitudes of Mendoza (32[degrees]53'S) and Jachal (San Juan) (30[degrees] 15' S) extends a NS oriented tectonic valley located between the Pre-Cordillera and the Front Cordillera of the Andes. This valley is 2,000-5,000 m deep with respect to the Pre-Cordillera and Cordillera summits that borders it on the East and West; it is ca. 300 km long and 40 km wide on the average. This is actually a perfect case of rain shadow desert comparable to the well known Californian Death Valley, with an hyperarid climate. At the lowest point (Calingasta -San Juan-, 1,375 m) the long-term mean annual rainfall is only 54 mm and up to 100-120 mm on the highest points (Tambillos -Mendoza-, 2,164 m). From the geomorphic, climatic, floristic and vegetational viewpoints this post-tectonic graben is a true desert void of perennial vegetation on the pediments, while perennial species are contracted along the hydraulic network and around the topographic depressions. Vegetation on the pediments is set up in a steppe-like diffuse pattern in the highest parts above 1,800 m of elevation, and on a contracted pattern in the lower (1,300-1,800 m asl).

Keywords: Arid Land, Cordillera of the Andes, deserts, vegetation, geomorphology, shadow desert.

INTRODUCTION

The southern part of the Uspallata-B arreal valley is situated a few km N of Uspallata city (Mendoza) in an area called Pampa de Yalguarraz which drains to the north into the Río San Juan basin and to the south into the Río Mendoza catchment (32[degrees]36'S- 69[degrees]20'W, Alt. 1,880m. The northernmost part, Angualasto (San Juan), is on the Rio Jachal (30[degrees]03' S-69[degrees]19' W, Alt. 1,704 m. The N-S distance is thus 300 km for an average width of 40 km, i.e. an area of 12,000 [km.sup.2]. The northernmost part is practically on the confluence of the Pre-Cordillera and the Front Cordillera. This tectonic valley is filled with fluvioglacial sediments of Mio-Plio-Pleistocene age. These may reach a thickness of 300-400 m in the central part of the Calingasta and Río Jachal valleys, and the graben's subsidence is of 1,000-1,200 m (Rodriguez et al., 1998). The pediments are made of fluvio-glacial coalescent fans of post-tectonic quaternary age similar to those described in the Mendoza pediments by Polanski (1962, 1972). In the region of Iglesia, in the Río Jachal basin, there are large deposits of diatomites of several tens of meters thickness (Las Flores, Pismanta, Tudcum, Angualasto, Rodeo, and Iglesia cities). These non-tectonized deposits, are very dissected in bad-lands and bare of perennial vegetation. They date back to a Pliocene age (Wetten, 1975; Ragona et al., 1995). They are topped by shingle and boulders from the upper pediment. These diatomites seem to be the remnants of a large Pliocene lake developed in the lower reaches of Rio Jachal, east of the Pre-Cordillera before the latter managed to find its way to the east through the Pre-Cordillera. These diatomites, albeit bare of any perennial vegetation cover, exude small springs of highly saline waters and therefore play a determinant role in the distribution of the neighbouring vegetation almost entirely made of halophytes and phreatophytes. The outcrops of the Pre-Cordillera rocks, contrary to the Front Cordillera, are almost totally of a sedimentary nature (with, however, some lightly metamorphized elements of various shales and schists) and they may be of various ages, Paleozoic (Cambrian, Ordovician, Gothlandian, Permian), Mesozoic (Triasic, Liasic and Cretaceaous) and Cenozoic (Miocene and Pliocene) in both continental and marine series. The petrographic sequences in these sediments include saline deposits: diapirs of evaporites, salt bearing marls, gypsum, anhydrite, etc. Present and past runoff on these saline sediments result downhill and downstream, gypsum crusts in the lower pediments, takyrs and salt mashes in the closed depressions.

The valleys of Calingasta on Río San Juan and of Iglesia on Río Jachal, between Villanueva and Tocota (35 km), ate separated by an upland from the upper pediment located at 2,500-2,800 m: the Llanos del Leoncito. These constitute the higher and older pediment between the Cordillera Frontal (Cerro del Diablo) and the Pre-Cordillera (Sierra del Tigre). This plateau is the local water divide between the Rio San Juan and Rio Jachal basins. This high inter-cordilleras fluvio-glacial pediment is mainly made of boulders and shingle with a high petrographic diversity of extrusive rocks. Towards 10-20 cm of depth the soil develops a calcareous crust (caliche), of medium hardness, 5-20 cm thick. Contrary to the arid and hyper arid zones to the east of the Pre-Cordillera, the tectonic valley neither shows any sanddunes nor any clay-dune systems (lunettes). The remote sensing signature images of the pediments in the eremitic zone (P

MATERIAL AND METHOD

Scarce climatological information on precipitation (P), mean temperature (T), potential evapotranspiration (ETP), was compiled from diverse sources (FAO, 1985; IIH, 1988a,b; De Fina et al., 1992; INTA, 1999). The Potential evapotranspiration, and also rainfall, temperature, the percentage of semestrial summer and winter rainfall and hydrical deficit (ETP-P) for each locality were calculated. The area was bioclimatically classified. Soils were classified according to the 8a approximation (Soil Survey, 2006).

Several floristic surveys were made considering both local and regional geomorphology: pediments, runnels, depressed areas. In each site the presence (Braun Blanquet's method, 1979) and cover (Daget et Poissonet "s linear method, 1971) of all plants were recorded. At least ten releves and transects for each environment were performed. All plants colleceted were incorporated to Geobotany and Phytogeography Herbarium (IADIZA).

RESULTS

Climatology and bioclimatology

From the climatic viewpoint, the tectonic valley...