Evapotranspiration, Canopy Temperature, and Plant Water Relations
The reduction in leaf transpiration reduces evaporative cooling with a consequential rise in canopy temperatures. Increases of 0.3–1.7 °C at CO2 concentrations of 550 ppm (200 ppm above current concentrations) have been observed, depending on species and conditions. Such canopy temperature changes are likely to cause shifts in the optimum geographic climate areas for growth of crops and other species.
The reduction in transpiration per unit of leaf area with e[CO2] generally leads to a reduction in ET per unit of land area. However, the magnitude of such water conservation at e[CO2] varies with the degree of stimulation of plant growth and the degree of partial stomatal closure. Observed reductions in ET have ranged from near zero for cotton, a woody C3 species with large growth stimulation, to about 16 % for sorghum, a C4 grass with little growth stimulation. In the absence of global warming, such water conservation will reduce the water requirements of irrigated regions, and with global warming, it will help to keep the requirements from rising as much as the warming alone would cause.
The reductions in ET with e[CO2] will also lead to increases in soil moisture content, with consequent effects on numerous soil physical, chemical, and biological processes that are influenced by soil moisture content, such as leaching, mineralization, and soil respiration.
The reductions in ET and consequent increases in soil moisture can lead to improvements in plant water relations, such as higher plant water potentials. Water conservation with growth in e[CO2] can enable plants to maintain growth longer into drought cycles.
KeywordsSoil Water Content Soil Respiration Stomatal Conductance Canopy Temperature Stomatal Resistance
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