2.6. Ecophysiological data
Sap flow rate of stems was used as a proxy for the canopy transpiration rate of jujube trees. For each treatment, three jujube trees were randomly selected from three different plots for sap flow measurements. Therefore, a total of 12 jujube trees were used for this experiment. A FLGS-TDP XM1000 system (Dynamax Co., USA) was used for measuring sap flow rate; this includes 12 thermal dissipation probes (TDPs) with two needles (30 mm in length and 2 mm in diameter) and one CR1000 data logger. The installation of the equipment was completed in early March of 2014. On the north side of the trunk and at 20 cm above the ground on each of the 12 selected trees, one TDP sensor was uniformly and horizontally inserted into the 0–30 mm sapwood layer after peeling off the bark layers; it was then packed using silver membrane. From early March 2014 to the end of 2016, sap flow rate was recorded continuously for all 12 jujube trees. The data from the first two weeks were discarded. Sap flow density (Js ) was calculated based on the equation introduced by Granier (1987). The sap flow rate (SF ) was then derived using the following equation, with an assumption of a constant Js across the sapwood profile.
(2)
where K denotes a unit conversion coefficient, equal to 0.00864;AS denotes sapwood area, cm2.
Mid-day leaf water potential (ΨL) was measured on 18 June, 18 July, 18 August, and 18 September in 2015 and on 19 June, 17 July, 19 August, and 19 September in 2016, using a PMS Model 1000 pressure chamber (PMS Instrument, USA).