Fjölrit RALA - 05.12.1999, Blaðsíða 49

OsvaldoA. Fernández et al.
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vast area occupied for this territory with the associated variability in soils and climate.
The most characteristic plant community dominating large areas of the Monte is the
“jarillal” (Zygophyllaceae), with the presence of Larrea divaricaía, L. cuneifolia and
L. nitida as the most abundant genus. These evergreen shrubs have several mecha-
nisms to cope with the common, severe droughts in the region (Ezcurra et al. 1991).
Other evergreen shrubs are Atamisquea emarginata and Zuccagnia punctata. Decidu-
ous shrub species include Plectocarpa rougesii, Prosopidastrum globosum, Prosopis
alpataco, P. flexuosa, Lycium chilense and Geoffroea decorticans. The Cactaceae
family is well represented with the Opuntia and Cereus genera whose abundance in-
crease northward in the ecotone with the Chaco forest. Aphylly is ffequently found in
the vegetation as in Bulnesia retama, Neosparton aphyllum, Cassia aphylla, and
Monttea aphylla; photosynthesis takes place mainly in the green stems and branches
in these species. The herbaceous understory is represented by Cottea pappophoroides,
Munroa argentina, Pappophorum mucronulatum, Aristida adscensionis, Bouteloua
aristidoides, B. barbata, Euphorbia serpens, Boerahavia paniculata, Pectis sessilif-
flora, Tricholoris crinita, Tribulus terrestris, Eragrostis argentina, and others. Some
of these species are summer or winter short-lived ephemerals whose abundance is
strictly dependent on seasonal rainfalls.
The Monte vegetation has been degraded by overgrazing and wood collection for
over 100 years (Guevara et al. 1997). The net result of overgrazing has been the dis-
appearance of desirable perennial forage grasses followed by invasion of unpalatable
shrubs, weedy forbs and annual grasses, and the increase in bare soil. Rangelands are
frequently dominated by unpalatable vegetation; palatable perennial grasses are rare
and only found under the protection of spiny shrubs (Prosopis spp., Condalia micro-
phylla), and even palatable shrubs (e.g., Brachyclados lycioides, L. chilense, Atriplex
lampa, Acantholippia seriphioides, and Bredemeyera microphylla) have become rare
(Guevara et al. 1997). Changes in plant species composition have also determined
changes in the type of herbivores within the area: cattle and sheep have been replaced
by goats, although recently a 5% decline per year has been detected for goats in the
northem part of the plains (Guevara et al. 1997).
Low aboveground net primary productivity estimates for the herbaceous and woody
components of the system, with subsequent low livestock productivities, have recently
been given by Guevara et al. (1997). These low values have been the result of con-
tinuous grazing, uncontrolled fires, lack of fencing and scarcity of watering points.
Research at the central Mendoza plains during the last 10-25 years allows prediction
of primary production and carrying capacity for a given year, based on rainfall prob-
abilities, and then to determine proper stocking rates (Guevara et al. 1997). Improved
range management practices in these plains have allowed substantial increases in
stocking rate (ffom 28 to 21 ha per animal unit) and animal performance in compari-
son to the traditional management system (Guevara et al. 1997). The economic analy-
sis of cattle operations in the Mendoza plains has showed that most of them are nega-
tive which emphasizes the need for increasing the economic productivity of animal
production systems through better range and herd management.
Shrub control by hand-cutting, mainly of Larrea divaricata and Larrea cuneifolia,
has resulted in the replacement of undesirable species with productive and palatable
forage (Passera et al. 1996). This control, however, is only advisable in areas with