Fjölrit RALA - 05.12.1999, Side 54

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Arid and semi-arid rangelands of Argentina
tum, Melica argyrea, Setaria gracilis, S. argentina, Thrichloris crinita, and others.
Red (Schinopsis lorentzii, Anacardiaceae) and white (Aspidosperma quebracho-
blanco, Apocynaceae) quebracho woods constitute the climax association for the re-
gion. They can be 15-20 m tall in the more humid parts. These species have been
overexploited for charcoal production, and S. lorentzii also for tannin extraction. Bul-
nesia sarmentoi (Zygophyllaceae) wood is much appreciated because of its sweet per-
fume and greenish color. Legume trees are locally abundant. For example, Prosopis
alba and P. nigra are considered to be among the most useful species of the Western
Chaco. Their pods constitute an important supply of food for humans and livestock
and they provide lumber and medicinal products. Cesalpinia paraguariensis is another
particularly interesting tree with several practical uses for men (Aronson and Saravia
Toledo 1992). Other legumes include Acacia aromo, A. caven, Prosopis chilensis, P.
flexuosa, P. ruscifolia, P. kuntzei, Geojfroea decorticans, and others. Common repre-
sentatives of the frequently dense shrub vegetation are Bulnesia foliosa, B. bonarien-
sis, Bougainvillea praecox, B. infesta, Castela coccinea, Ruprechtia apelata, R. tri-
flora, Schinus piliferus, S. sinuatus, Mimosa detinens, Acacia furcatispina and Larrea
divaricata.
The natural ecosystems of this territory have been altered because of forest exploi-
tation and overgrazing of the herbaceous grassland vegetation (Morello and Saravia
Toledo 1959a). One of the main rangeland uses is cattle production. Anywhere grass
forage tends to decrease or disappear, cattle increase browsing pressure on shrubs and
tree seedlings. In areas strongly degraded by overstocking cattle is replaced by goats,
which are capable of browsing on almost any type of available plant growth. This se-
quence in rangeland use determines disappearance of both the wood characterizing the
climax situation and the herbaceous cover in many locations. It can induce the growth
of a thomy dense chaparral almost useless for forage production. The shrab Prosopis
ruscifolia, commonly known as vinal, is a good example of this in the north of this ter-
ritory wheré it has become a serious weed. Colonizing mechanisms of vinal indicate
that natural ecosystems were exposed to periodic pulses of fire and floods prior to cat-
tle introduction which hindered the advance of this shrab (Morello 1970, Morello et
al. 1971). Elimination of these two natural influences on the system allowed the ex-
plosive invasion of this species. Other genera with similar dispersion pattems that
form dense shrubby communities are Acacia, Celtis and other Prosopis.
Although the area has overall suffered less damage than many other subtropical
semi-arid and arid forests of the world, it is clear that the whole territory is subjected
to a progressive degradation of the vegetative cover and soil erosion. Fortunately,
there are in this region a few encouraging examples of agroforestry systems that are
focused towards a more rational land use with an increased and sustained cattle in-
dustry, timber and wildlife preservation.
Puna
The arid Puna covers plateaus, high plains and slopes of the Andes at elevations be-
tween 3200 and 4400 m from Bolivian boarder to about 27°S. It constitutes one of the
coldest ecosystems of the world at these elevations. Features of this region are the
characteristic basins and the subsequent intemal drainage system. Interior mountains
create plateaus, valleys and gulches. The predominant soils have been reported by