3.2. Diurnal variation of WUE
The diurnal patterns of WUE in the growing season for the
mixed plantation were obvious (Fig. 3). In the early morning, the
augment rate of photosynthetic rate was higher than that of evapotranspiration
rate, leading to higher WUE. As the increasing solar
radiation and air temperature, VPD was rising, and hence WUE
decreased gradually. At around 10:00, WUE dropped the lowest
value. From 11:00 to 16:00, the variation of WUE was not remarkable,
varying from 5 to 8 mg CO2 g1 H2O. Before sunset, the
decreasing rate of photosynthesis was less than that of evapotranspiration
due to the reduction of solar radiation and air temperature.
Consequently, WUE increased. During the growing seasons
2006–2010, daily average WUE ranged from 7.0 to 9.8 mg CO2 g1
H2O. The diurnal courses of WUE in the mixed plantation were
similar to that in Quercus ilex L. forest obtained by Reichstein
et al. (2002), but different to those in kataba forest found by
Scanlon and Albertson (2004) and the irrigated olive orchard
reported by Testi et al. (2008).