TheImprovement ofTropical and Subtropical Rangelands

142 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
and graues in tropical and subtropical rangelands would, of course,
be determined by the characteristics of the environmental system
considered, as well as by the specific requirements of the forms of
livestock being managed. For example, treee and shrubs increue in
importance in low rainfall regions and in regions in which liftstock
inventories contain high proportions of browsers, such as goats and
camels (Le Houerou, 1980).
The most economical method for improving deteriorated gruing
lands is through natural means, that is, methods that do not require
the planting of desirable species. This may be accomplished by
control of unwanted plants, concentrating moisture or harvesting
precipitation, or by grazing management. Gruing use of desirable
plants is discussed in more detail in chapter 6. Ifnatural revegetation
is not feasible, the planting of desirable vegetation may be needed.
The machinery used as examples in this chapter can be expensive,
difficult to obtain, and costly to operate and maintain.
However, these examples may provide an opportunity to develop
inexpensive and innovative techniques and simple equipment that
employ the concepts upon which the more expensive equipment is
based. Whether natural methods of revegetation or artificial planting
are used depends upon the residual vegetation. In any case, the
use of inexpensive and innovative techniques must be maximized.
NATURAL REVEGETATION
For natural revegetation to be effective, there must be a residue
of desirable plants to take over and occupy the site. More importantly,
reduction of unwanted plants will not result in a stand of
desirable plants if they do not already exist at the site. Undesirable
vegetation that would severely compete with the establishment of
desirable vegetation may have to be reduced or eliminated. Natural
revegetation can occur quite rapidly following control of unwanted
plants.
Plant control in range management involves reduction of unwanted
or undesirable plants that have invaded or increased in a
plant community. The migration of certain species out of their normal
habitat is a maJor problem on rangelands in arid and semiarid
regions. Each species or plant association has a habitat range to
which it is naturally restricted. The environmental factors in a given
habitat favor certain plant species and plant associations. This has
the effect of confining the favored species and associations to the