Fjölrit RALA - 05.12.1999, Blaðsíða 51

OsvaldoA. Fernández et al.
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This territory is physiognomically characterized by the Caldén tree, which is the
typical and almost exclusive tree, giving its name to the area; it is endemic, xero-
phytic, deciduous and may exceed 10 m height. Presence of trees diminishes going
westward and southward, and the woody vegetation gradually becomes an open
shrubland. One of the characteristics of this region is the richness of its herbaceous
layer with grasses of high forage value with isolated shrubs and Caldén trees under
good management conditions. In some areas in the north of this region, extensive
grasslands combine with sparse large trees and shrubs to form a natural park of great
beauty. The grass herbaceous cover, locally known as “flechillas”, is dominated by
perennial species including Stipa tenuis, Piptochaetium napostaense, S. speciosa, S.
gynerioides, P. lanuginosa, Digitaria californica, Bromus brevis, Aristida subulata,
Setaria mendocina and Trichloris crinita. There are some patches of S. clarazii and
Poa ligularis. Two introduced annuals, Medicago minima and Erodium cicutarium,
are known for their high quality forage. However, their productivity is limited by a
short growing season and little rainfall (Fresnillo Fedorenko et al. 1991).
The shrub strata is very rich and the most ffequent species are Condalia micro-
phylla, Prosopis alpataco, P. flexuosa, Prosopidastrum globosum, Geoffroea decorti-
cans, Lycium chilense, L. gilliesianum, Atamisquea emarginata, Ephedra triandra,
Larrea divaricata and Bacchaqrisa ulicina. Inland saline habitats in this region con-
tain a spectrum of halophytes where shrubs like Cyclolepis genistoides, Atriplex on-
dulata and A. lampa become dominants. Other halophytes species include the grasses
Distichlis scoparia and D. spicata and the shrub Salicornia ambigua.
The cattle production industry is the most important economic activity within the
Caldenal where the grass steppe is used as the main food source for animals. Range-
lands have been overgrazed since the introduction of domestic cattle at the beginning
of the 1900s, and stocking rates currently range from 5 to 7 ha cow“' year'1 (Distel
and Boo 1996). Appropriate range management practices are seldom used. The eco-
logical system has been degraded by inappropriate use affecting both plant and animal
productivity and in many places the physical sustainability of the land itself. Another
common herbivory of the grasslands and semi-arid scrubs of Argentina are large her-
bivorous rodents named vizcachas (Lagostumus maximus); males 4.5-9 kg; females
2.5-4.5 kg. They can alter the temporal and spatial heterogeneity of shrub habitat and
the composition of plant communities in these regions (Branch et al. 1996). They live
in social groups that share a communal burrow' system and forage in a common home
range around it (Branch 1993ab, Branch and Sosa 1994, Branch et al. 1994c). Intense
vizcacha herbivory can depress forb and grass cover, and their effects are com-
pounded by increased susceptibility of open sites to erosion by heavy winds common
to the area (Branch et al. 1994b, Branch et al. 1996). Vizcachas can pull up root stock
of bunch grasses, as well as removing foliage, which may have a large effect on the
potential of grasses to regenerate afterwards (Branch et al. 1996). The effects of
vizcachas on plant communities that have not been affected by domestic herbivory are
unknown, however, because wrtuallv all grassland and semi-arid scrub in Argentina
have been subjected to domestic livestock grazing, most for many decades (e.g. Busso
1997, Guevara et al. 1997). Predation on vizcachas is very high and may restrict for-
aging distances and provide important selection pressure (Branch 1993a, Branch et al.