Fjölrit RALA - 05.12.1999, Blaðsíða 48

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Arid and semi-arid rangelands of Argentina
square league (2500 ha). Animals have been kept on rangelands throughout the year,
except for summer and winter pastures which are managed separately. Sheep herbi-
vory for over 100 years has resulted in a broad, noticeable desertification process. De-
creased grass cover accompanied by increased shrub cover and bare ground has led to
a decrease in herbivore biomass, soil water losses through evaporation and deep
drainage, and reduced transpiration at the westem edge of Patagonia which is occu-
pied by grass steppes (Aguiar et al. 1996). Little of the original natural vegetation re-
mains, and the soil is in a progressive state of erosion, intensified by the permanent
strong winds from the west (Rostagno and del Valle 1988). A steady reduction of
plant cover induced by grazing may intensify the concentration of nutrients in the
remnant undisturbed plant patches or in newly formed patches, leaving larger areas of
bare soil with limited nutrient reserves and supplies (Mazzarino et al. 1996). This re-
gression trend induced by grazing may cause a nonsteady state of the system, where
recolonization of bare patches by grasses might be limited by fertility. Fortunately, the
actual process of desertification in large areas mav still not have reached the level of
irreversibility if simple conservation rangeland management practices are followed.
Monte
The Monte desert extends from north to south in central and westem Argentina. It is
an extensive, almost continuous and rather uniform area of shrublands comprising
about 50 million hectares. The northem portion presents a typical landscape of inter-
mountain depressions, valleys and slopes belonging to the pampean hills. Rivers are
intermittent and large salty flats are frequent. The central portion is an undulating to
depressed loessic plain of fluvial. lacustrine and Quatemaric eolic origin. The third,
southemmost region occurs on a plateau landscape forming a wide ecotone with
northem Patagonia. The Monte constitutes the most arid rangeland of the country.
Overall Monte has a dry climate, being warm in the north and gradually becoming
cooler towards the south. Aridity in the northem portion is related to its position be-
tween the Andes to the west and the pampean hills to the east both of which intercept
the humid winds coming from the Pacific and the Atlantic, respectively. Rains occur
mainly in the summer ranging frorn 80 to 200 mm yr-1, and annual potential evapo-
transpiration decreases ffom 1000 mm in the west to 700 mm in the east. Annual aver-
age temperature is between 15 and 19°C. The accentuated continental climate of the
intermediate region is influenced by warm and dry winds coming from the west. The
summers are very warm: absolute maximum temperatures may reach 40 to 45°C, and
absolute minimum temperatures may be as low as -15 to -20°C. Rainfall ranges frorn
250 to 500 mm yr-1 and potential evapotranspiration is about 800 mm annually. The
Patagonic, southern portion of the Monte has a colder climate. Its average annual tem-
perature is 12 to 14°C and rainfall is scanty with 200 to 300 mm yr_l concentrated in
winter and spring; average annual evapotranspiration is similar to that in the other two
northem regions.
Classic studies of the vegetation at the Monte are those of Hauman-Merk (1913)
and Morello (1958). Other studies include works of Roig (1970), Bocher et al. (1972),
Ruiz-Leal (1972), Cabrera (1976) and Balmaceda (1979). Monte vegetation is a
steppe scrub dominated by microphyllous xerophytic shrubs from 1 to 3 m high. It
tends to be uniform in terms of its physiognomy and floristic composition despite the