TheImprovement ofTropical and Subtropical Rangelands

SITB EVALUATION 101
Instantaneous measures of relative humidity are obtained from
manual observations of dry- and wet-bulb thermometers on a sling
psychrometer. Hydrographs, of which several types exist, are used
to record relative humidity on a continuous basis. These instruments
should be housed in the shelters containing thermometers.
Wind The effect of wind on evapotranspiration, the total moisture
loa from soil by evaporation and plants by transpiration, can be
critical, particularly in dry climates. When a plant is exposed to
drying winds and hot temperatures, water deficits in its leaves are
likely to occurj this situation is compounded under minimum soil
moisture conditions. The desiccating impact of wind on plants is
demonstrated by low survival rates, stunted growth, and frequently
death in many plant communities of semiarid and arid lands.
Data on wind patterns (prevailing direction, velocities, and seasonal
fluctuations, for example), characterizing a particular site, are
uncommon in many nonindustrialized countries. When this information
is available, it has generally been obtained by using an anemometer
during short-term site visits.
LigAt Another climatic factor that affects the growth of plants-an
important factor that is seldom measured extensively-is light. Solar
radiation in the visible bands of the spectrum controls photoeynthesis.
At very low light intensities, photoeynthesis may take place at
such a slow rate that all of the carbon dioxide evolved by respiration
is not usedj with these conditions, carbon dioxide is given off by the
plant, not absorbed by the plant from the atmosphere. On the other
hand, high light intensities promote rapid transpiration, which can
often have detrimental effects. In general, individual plant species
differ in their relative tolerances to either low or high intensities of
light.
The photoperiodism of plants also differs among species. Some
plants require long photoperiods (that is, length of day) to grow
and develop, while other plants do better with shorter photoperiods.
Photoperiods can be easily measured by the length of daylight at a
site.
Soil
The word soil refers, in general, to the natural surface layer
of the earth's crust in which plants grow. It is a porous medium,
comprising minerals and organic materials. Living organisms, water,