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

Grassland perspectives 471
Trees and shrubs
Trees and shrubs are important features of many grasslands, especially of
savannahs. Some are very useful, others are invasive weeds. Trees provide
valuable shade in hot climates and seasons and they give shelter in winter . Some
trees are browsed and may be lopped for fodder – their fruits can also provide
valuable feed. A wide range of genera is involved and their management is
still poorly understood; the tolerance of most woody species to stock-feeding
regimes and lopping still needs study, although a good deal of work has been
done on Leucaena . Woody vegetation provides branches and poles for building
and making corrals and firewood. Where firewood is scarce, excessive cutting
causes serious environmental damage, as in steppic conditions, where much
damage is due to uprooting sub-shrubs for fuel. Some trees provide fruit which
is valued by local people – such trees may be retained selectively and given
some protection. Woody vegetation is, however, often invasive, especially in
tropical and sub-tropical conditions; bush encroachment is generally taken as a
sign of poor management and overgrazing; this is dealt with below.
Pasture development methods
Clearing
After water supply, clearing is a common part of developing extensive grassland
for grazing . Where land is being developed for crops or sown pasture , clearing
may involve some removal of stones, termite hills and other obstructions, but,
for extensive grazing, clearing usually involves removing or thinning woody
vegetation to improve access and grass growth or to reduce tsetse fly habitat . In
traditional systems , fire is the commonest agent for clearing or controlling trees
and shrubs. Specialized equipment is used for large-scale commercial clearing
– tree-pushers, drag chains, bulldozers, root ploughs and root rakes, and, for
shrubs, various rollers and shredders; the debris may be burnt. The degree
of clearing or thinning will depend on the original vegetation and the use to
which it is put, but it is usually partial and selective, leaving useful trees, shade
and shelter. Selective clearing has been used to reduce tsetse habitat. Woodland
destruction for pasture development is now recognized as environmentally
undesirable, although it continues on a fairly large scale in the Amazon basin.
Strategic thinning of woody vegetation has a role in pasture development and
improvement , but it must be done within the context of the ecosystem involved.
The Australian study indicates that some leases now restrict tree clearing; it
also highlights the fact that the removal of trees and their replacement by crops
and annual pastures has made major changes to the hydrological cycle and can
lead to serious salination of soils.
Bush control
Bush control is necessary in many grassland types ; it is a maintenance activity,
while bush clearing is development . Bush encroachment usually indicates