TheImprovement ofTropical and Subtropical Rangelands

196 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
drainage, the presence or absence of waterlogging, or a satisfactory
water table, and 10 forth). A simple calculation would show, for
example, that a northern slope with a 30° dip under 40° latitude N.
would have a potential evapotranspiration equal to only one-third
of a similar lOuthern slope, and therefore, if rainfall remains similar
in both cues, the water budget would be three times greater in the
former than in the latter (Le Houerou, 1972). The selection ofspecies
to be planted is therefore likely to be different in the two cues, if
optimum use of the sites is lOught.
Physiography may also strongly affect local precipitation if the
site is exposed to rain-bearing winds; is in a rain shadow; or is subject
to descending, warm, dry winds, similar to the Fohn of the lOuthern
Alps, the .am"n winds of Iran, the Santa Anu of California, the
chinooks of the eutern Rockies, the berg winds of South Africa, the
nor'westers of New Zealand, or the zontla of Argentina. Geomorphology
and slope may also play an important role in determining
rates of erosion or sedimentation and lOil depth, thus greatly afFecting
lOil fertility and water budget. Hence, species selection and plant
productivity calculations must be modified accordingly (see table 8-1
and figure 8-1).
ABRITY 01 INTRODUCED SPECIES
TO COMPETE WITH NATIVE VEGETATION
Plant competition should be considered from the viewpoint of
short-term establishment, long-term survival, and perpetuation of
stands. Competition during the establishment stage may be reduced
by the application of herbicides or mechanical treatments (mowing,
plowing) to the native vegetation until the desired species become
established. When high yields are desired, competition from weeds
may be eliminated on a continuous buis by regular or periodic
treatments.
Long-term perpetuation of stands of introduced species will depend
on their ability to reproduce either vegetatively (for example,
by suckers, runners, stolons, or rhizomes) or by teed. Some futgrowing
exotic species are not able to perpetuate themselves on the
site and may need to be replanted after a number of years. However,
some exotics have become invading pests (Opuntia spp., Pro.o,u
spp., Nicotiana glauca, Parkin.onia spp., Euphorbia spp., Jatropha
spp.).