Grasslands of the World.pdf - Disasters and Conflicts - UNEP

Other grasslands 427
The sparse tree layer is composed of Curatella americana, Byrsonima crassifolia
and Bowdichia virgiloides. Aboveground production of the grass layer
peaks at 3 200-4 200 kg/ha when burnt, whereas yields are 30 percent lower
if protected. Fire is the only economically feasible management tool. Burning,
even in mid-dry season , induces a regrowth if water reserves allow.
The Alluvial Overflow Plains, which occupy a depression of 3 800 000 ha
in the central Llanos between the piedmont and the high plains, are very flat,
with only one to two metres between the highest and lowest points . Higher
land constitutes natural levees, where the soil is a sandy alluvium, whereas
clay particles settle in the lower parts (basins), which have slow drainage; rain
drains very slowly and the lower parts remain flooded during most of the
rainy season, but have a high carrying capacity in the dry season . The area is
used for extensive cattle and buffalo grazing (96 percent for cattle; 4 percent
under forests), although frequently wild capybara are raised with cattle. The
botanical composition of levees and basins differs, but this type of savannah
has more palatable species than the others, and has been modified by human
intervention, especially in an area of 250 000 ha enclosed by low dykes and
floodgates to regulate water levels in sections of 3 000 to 6 000 ha each. Land
permanently above water is colonized by Axonopus purpusii , A. affinis and
Leptocoryphium lanatum ; sections moderately flooded contain Panicum
laxum and Leersia hexandra as dominants; and the strongly flooded areas are
dominated by Hymenachne amplexicaulus , Reimarochloa acuta and Leersia
hexandra. Cyperaceae are also abundant. Aboveground yields vary between
the 5 t DM/ha of the levees to 2–3 t DM/ha in the basins. According to some
estimates, regulation of water level in the Modulos can increase carrying
capacity up to fivefold.
The Aeolian Plains, which extend north-east from the Colombian Andean
Piedmont into southern Venezuela , are characterized by dunes covered by
sparse vegetation , almost treeless , and dominated by Trachypogon ligularis and
Paspalum carinatum ; inter-dunal depressions are occupied by a Mesosetum
savannah. Both formations are low yielding and of low palatability.
The Gran Chaco
This section is based on Riveros (2002) and Garbulsky and Deregibus (2004).
The Gran Chaco , between 17° and 33°S and 65° and 60°W, is a vast plain in
the River Plate Basin that extends through northern Argentina , southeastern
Bolivia , northwestern Paraguay and a small area of southwestern Brazil . It
stretches for about 1 500 km from north to south, and 700 km from east to
west, without any physical barriers intervening. Its area is about 850 000 km 2 .
The Chaco, which extends into both tropical and temperate zones , is one
of the major wooded grasslands in South America, but suffers from intense
degradation through unrestricted forest and bush clearing , overgrazing and
continuous monoculture.