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

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Grasslands of the world
Plate 9.6
Exotic woody weeds in northern Australia . Dense stand of Cryptostegia
grandiflora , with some climbing over trees along a watercourse.
Ziziphus mauritiana ). Tothill and Gillies (1992) made a major assessment of the
condition of native pastures in northern Australia. They divided the pastures
into three classes – sustainable (main desirable species maintaining >75 percent
dominance); deteriorating (increased presence (>25 percent) of undesirable
pasture species and/or woody weeds); and degraded (predominance of undesirable
species). Overall, only 56 percent of pasturelands were rated sustainable,
with 32 percent deteriorating and 12 percent degraded.
In sown tropical pastures both legume dominance and lack of legume persistence
are problems in different regions. In the semi -arid tropics, where some
stylo (Stylosanthes spp.) pastures are now nearly 30 years old, stylo dominance
(particularly Stylosanthes scabra ) is causing some concern. The problem is
worst on infertile soils, where stylos have been over-sown into native pastures.
In stylo-grass pastures, animal preference for grass is very strong during the
early wet season (Gardener, 1980) leading to heavy selective grazing of the
grasses, which are susceptible at this stage of their growth (Hodgkinson et al.,
1989; Mott et al., 1992). A number of measures have been recommended to
reduce the stylo dominance – fire , pasture spelling, sowing grazing-tolerant
grasses, and raising soil phosphorus levels (McIvor, Noble and Orr, 1998).
In the drier subtropics, legume persistence remains a problem, and the success
of legume-based pastures has been more variable. Weeds continue to be a
problem. During the past decade, unpalatable Sporobolus species (pyramidalis,
natalensis, indicus var. major) have increased and extended inland, and Siam
weed (Chromolaena odorata ) has been recorded for the first time.
J.G. McIVOR