Response of relative sap flow to meteorological factors under different soil moisture conditions in rainfed jujube (Ziziphus jujuba Mill.) plantations in semiarid Northwest China
As a drought tolerant plant, jujube (Ziziphus jujuba Mill.) is commonly planted in semiarid Northwest China under rainfed conditions. To clarify the response of water use to climatic conditions, sap flow (SF) in 8 non-irrigated jujube trees was monitored using thermal dissipation probes. Also meteorological variables, soil water content, crown projection area and leaf area index were measured during the 2012 growth season. Based on relative extractable water (θe), soil moisture conditions were classified into water stress (θe<0.4) and non-water stress (θe>0.4) conditions. Since leaf area (LA) significantly influences SF, relative sap flow (defined as SFR=SF/LA) was used to analyze its relationships with selected meteorological factors. Obvious differences were noted between responses of SFR and its sensitivity to meteorological factors under the two different soil moisture conditions. Daily SFR was linearly related with solar radiation (Rs) but logarithmically related with vapor pressure deficit (VPD), variable of transpiration (VT) and reference crop evapotranspiration under both soil moisture conditions. Hourly SFR was linearly related with Rs, VPD and VT at θe>0.4, while, at θe<0.4, it was lineary related with these variables to a threshold value, but it tracked a horizontal curve thereafter. On the average, the change in SFR with meteorological factors was classified into two on the basis of the time of day. The first characteristic change was during 00:00–15:00h and the second during the rest of the day. There were significant differences in SFR dynamics between the two periods of the day under every soil moisture conditions, while, significant differences in the first periods but few differences in the second periods of SFR dynamics between two soil moisture conditions. The results of the study could be used to fine-tune transpiration modeling for efficient soil water management in rainfed jujube plantations in semiarid regions.
