Abstract: A water-controlled experiment with four treatments (no rain, half raining, natural raining and double raining) was carried out in a Platycladus orientalis forest. The factors including soil water content (SWC), precipitation, sap flow density (J_s), leaf area index (LAI), vapor pressure deficit (VPD) were monitored during August 2016 to August 2017. We further analyzed the response of canopy stomatal conductance (g_s) to changes of SWC. The results showed that the SWC of plots (half, natural and double raining) showed a positive correlation with precipitation, and the range of SWC was 4.9%-16.0%, 7.2%-22.9%, 7.4%-29.6%, respectively. The SWC in the plot with no rain decreased by 50% from August to October. The daily g_s reached a peak of 166.64 mmol·m^-2·s^-1 at 14:00 in July, which was significantly higher than other months. A bimodal phenomenon occurred. The daily g_s reached a peak of 54.1 mmol·m^-2·s^-1 at 12:00 in January. Under the three rain plots, diurnal variation of g_s and SWC showed a negative quadratic correlation. The SWC corresponding to the peak of g_s was 8.5%, 12.5% and 18.5%, respectively, close to the annual average SWC. Sensitivity (δ) of g_s to VPD /reference canopy stomatal conductance (g_sref) was more than or equal to 0.6 in different water-controlled plots, indicating that soil water condition was more suitable for water demand of P. orientalis. When SWC was between 3.7% and 7.5%, the δ and g_sref increased rapidly, indicating that stomata had better regulation ability, and that plant stomata was more sensitive to VPD. When SWC increased to 11%, SWC alteration did not affect the response sensitivity of g_srefand g_s to VPD. There might be a SWC threshold value for the adaptation of P. orientalis. By closing or reducing stomatal aperture, leaf water potential decreased, P. orientalis could adapt to excessive VPD and avoid excessive transpiration, which was more effective in regulating transpiration.

Key words: canopy stomatal conductance, soil water content, sap flow density, vapor pressure deficit.