Fjölrit RALA - 05.12.1999, Page 46

44
Arid and semi-arid rangelands of Argentina
The central rangeland plateaus of Patagonia show a typical arid cold-temperate cli-
mate with four well defined seasons. Among the most noticeable characteristics of the
Patagonian climate are the prevailing strong winds from the west which blow all year
round. These winds, which average 16 km IT1 on a yearly mean basis, loose their
moisture over the Andes and subsequently rainfall ranges from 100 to 200 mm yr_1. In
the northem plateaus and the coastal oceanic border rainfall increases slightly but it
remains below 300 mm yr_l. Soil moisture is strongly influenced by the recharge
through precipitation (Coronato and Bertiller 1996). Soil water depletion begins in
spring with decrease in precipitation, and increase in temperature and evapo-
transpiration. Soil water is almost depleted by late summer. Most precipitation occurs
during autumn and winter and the highest soil moisture values are measured at the end
of winter after snowmelt. For the Patagonia area as a whole, the annual average tem-
perature varies from 6-14°C. Absolute maximum and minimum temperatures vary
from 30 to 40°C and -15 to -20°C. Dryness of Patagonia results from a combination
of low rainfall, high temperatures during the summer months, and strong winds which
cause high evaporation rates. Mean annual evapotranspiration ranges from 550 to 750
mm yr_l. All these values tend to decrease fforn the northeast to the southwest. Mean
annual frequencies are 60 or more days for frost and 5 to 20 days for snowfall which
occurs mainly in the west and south.
There is considerable variation in the vegetation within the Patagonian territory.
Soriano (1983) has published a comprehensive review of its vegetation and quoted six
floristic districts for this region. Other authors have described the vegetation for Cen-
tral Patagonia (Bertiller et al. 1977, 1981 a, 1981 b) or the ecotone vegetation between
the north of this region and the characteristic scrub of the Monte (Soriano 1949, 1950,
Ragonese and Piccinini 1969, Ruiz Leal 1972). The Patagonian vegetation is charac-
terized by a low shrubby steppe intermingled with tussock grasses. The grass family is
prevalent in its floristic composition, and the genus Stipa is dominant including S.
humilis, S. speciosa, S. ibari, S. neaei, S. psylantha, and S. subplumosa. Other signifi-
cant components of the grass flora are Poa ligularis, P. lanuginosa, Festuca argen-
tina, F. pallescens, F. gracillima, and Bromus setifolius. Large areas are physiog-
nomically characterized by shrubs with cushion-like appearance and less than 1 m
high. The most frequent shrub species are Chuquiraga avellanedae, Colliguaya inter-
gerrima, Mulinum spinosum, Senecio filagiroides, Verbena tridens, Pseudoabutilon
bicolor, Berberis heterophylla , B. cuneata, Baccharis darwinii, Anarthrophyllum ri-
gidum, Nassauvia glomerulosa, Lycium chilense and Trevoa patagonica. Total cover
varies from 15 to 60% depending on the environmental location and the particular
range management. Low lands ffequently present halophytic vegetation characterized
by communities of Frankenia patagonica, Atriplex lampa, and A. sagittifolia. Schinus
polygamus, which can reach a height of three meters and is one of the biggest shrubs
of southem Patagonia. It has been almost extinguished from large areas because of its
excellent properties as firewood.
Factors affecting species composition and diversity in the Patagonian steppe in-
clude landscapes of contrasting topography (flat vs. mountain landscapes) through
their direct effects on abiotic environmental heterogeneity (Jobbagy et al. 1996). Envi-
ronmental controls on community composition, however, appear to depend on plant
functional type since shmb and grass, but not forb, species distributions responded to