Effects of branch removal on water use of rain-fed jujube (Ziziphus jujuba Mill.) plantations in Chinese semiarid Loess Plateau region

As a drought tolerant plant, jujube (Ziziphus jujuba Mill.) is commonly planted in Chinese semiarid Loess Plateau Region under rain-fed conditions. To evaluated the effectiveness of branch removal to conserve water resources in rain-fed jujube plantations in the region, two degrees of branch removal treatments (T1–two main branches removed; T2–one main branch removed) were compared with control treatment (none of the three main branches removed) and the effects on water use in Jujube plantations were analyzed. Sap flow was monitored (using the Granier-type thermal dissipation probes) along with meteorological factors, soil water content, vessel diameter, crown projection area and leaf area index, with soil moisture condition divided into water stress and non-water stress. Results showed that branch removal was an efficient way of conserving water resources in jujube plantations. a) It improved soil water content by reducing water loss via total evapotranspiration (ET) during jujube growth period. Total ET values were 452, 463 and 478mm respectively for T1, T2 and the control treatments during jujube growth period in 2012; 408, 426 and 451mm respectively for T1, T2 and the control treatments during jujube growth period in 2013 and 374, 381 and 392mm respectively for T1, T2 and the control treatments during jujube growth period in 2014. b) Branch removal not only decreased transpiration per unit ground area (stand transpiration: Tg), but also increased the sensitivity of Tg to environmental variables. In contrast, the removal of two (out of three) branches increased transpiration per unit leaf area (leaf transpiration: Tl), making it less sensitive to soil water content. c) Treatment T1 significantly increased the sensitivity of canopy conductance (Gc) to vapor pressure deficit (VPD), which effect was not significant under T2. Isohydric regulation of water loss through Gc response to VPD was the same for the three treatments, regardless of the different tree canopy sizes and soil water conditions, but reference canopy conductance (Gcref: canopy conductance at VPD=1kPa) was treatment-specific. d) Branch removal improved the tolerant of jujube tree to negative water potential and xylem cavitation by decreasing average vessel lumen diameters. Compared with the control, both branch removal treatments (T1 and T2) resulted in a significant decrease in average vessel lumen diameter at 15mm below the cambium (40.74, 42.53 and 46.81μm respectively for T1, T2 and the control treatments). Similarly, T1 and T2 reduced mean vessel lumen diameter at 5mm below the cambium (43.20, 47.44 and 48.39μm respectively for T1, T2 and the control treatments). e) Branch removal significantly decreased the fresh fruit yield and water use efficiency except in 2013, during which differences were not significant. Fresh fruit yield values were 8082, 8964 and 11293kgha−1 respectively for T1, T2 and the control treatments in 2012; 5691, 5426 and 6000kgha−1 respectively for T1, T2 and the control treatments in 2013; 6804, 8341 and 10589kgha−1 respectively for T1, T2 and the control treatments in 2014. WUE values were 1.79, 1.94 and 2.36kgm−3 in 2012; 1.40, 1.28 and 1.33kgm−3 in 2013; 1.82, 2.19 and 2.70kgm−3 in 2014. T1 treatment was recommended in terms of improving soil water storage as well as maintaining a certain amount of productivity in jujube plantations in Chinese semiarid Loess Plateau Region. The result of the study is critical for the development of effective water management strategies in jujube plantations ensure that the fragile ecology of the Loess Plateau is sustained.