TheImprovement ofTropical and Subtropical Rangelands

100 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
recently emerged plants, especially under conditions of minimal soil
moisture. Conversely, cold temperatures can delay seed germination
and subsequent early growth, placing the survival of plants in jeo~
ardy. Following establishment, temperatures of either extremes can
reduce the overall growth performance of D1Ol!Jt plant species.
For best growth, many plants require nighttime temperatures
that are considerably cooler than daytime temperatures. This difference
between nighttime and daytime temperatures, termed thermoperiod,
is important in the flowering and setting of fruit. In
general, plants will become adjusted to regular diurnal fluctuations
in temperatures and, as a result, may not exhibit "normal behavior"
when grown in foreign environments. Therefore, individual plant
species should be selected on the basis of their adaptation to temperature
regimes (including mean, maximum, and minimum temperatures)
at a site.
Reliable air temperature data are gathered from simple thermometers
(for instantaneous determinations), maximum-minimum
thermometers (to measure temperature extremes), and thermographs
(for a continuous record of temperatures). Thermometers
are generally housed in shelters with louvered sides to permit air to
circulate freely. The shelters should be located at a distance at least
two-thirds the height of any obstructions. Temperature will vary if
obtained on steep slopes or in hollow areas.
The air within plant leaves is usually saturated with moisture
under growing conditions, and vapor therefore will move from the
leaves into the surrounding atmosphere, cooling the atmosphere in
the process; this is transpiration. The rate of transpiration in plants
depends in part on the amount of atmospheric moisture present; the
drier the atmosphere, the higher the rate ofwater loss. Transpiration
is the dominant process in the water balance of plants and can cause
water deficits to occur. Under conditions of limited soil moisture,
these water deficits may be responsible for growth reductions or
death.
To characterize atmospheric moisture at a site in a given period
of time, relative humidity is often measured. With summaries of
relative humidity regimes over a growing season, it may be possible
to determine the changes of transpiration in plants resulting in water
deficits. Because certain plant species are better able to withstand
the stresses ofwater deficits, this knowledge can be useful in evaluating
the value ofplant species for revegetative purposes at a particular
site.