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Jujube (Ziziphus jujube Mill.) is a traditional economic forest crop and is widely cultivated in hilly areas of the Loess Plateau, China. However, soil desiccation was discovered in jujube plantations. Pruning is recognized as a water-saving method that can reduces soil water consumption. In this study, we monitored the jujube plots with control (CK), light (C1), medium (C2) and high (C3) pruning intensities during the jujube growing period of 2012–2015 to explore the effect of pruning intensity on soil moisture and water use efficiency (WUE) of jujube plantations in the hilly Loess Plateau Region. The results showed that pruning is an effective method for soil water conservation in jujube plantations. Soil moisture increased with increasing pruning intensity during the jujube growing period of 2012–2015. C1, C2 and C3 pruning intensities increased soil water storage by 6.1–18.3, 14.4–40.0 and 24.3–63.3 mm, respectively, compared to CK pruning intensity. Pruning promoted soil moisture infiltration to deeper soil layer. Soil moisture infiltrated to soil depths of 240, 280 and >300 cm under C3 pruning intensity, 220, 260 and 260 cm under C2 pruning intensity, 200, 240 and 220 cm under C1 pruning intensity, and 180, 200 and 160 cm under CK pruning intensity in 2013, 2014 and 2015, respectively. Soil water deficit was alleviated by higher pruning intensity. In 2013–2015, soil water change was positive under C2 (6.4 mm) and C3 (26.8 mm) pruning intensities but negative under C1 (−20.5 mm) and CK (−40.6 mm) pruning intensities. Moreover, pruning significantly improved fresh fruit yield and WUE of jujube plants. Fresh fruit yields were highest under C1 pruning intensity with the values of 6897.1–13,059.3 kg/hm2, which were 2758.4–4712.8, 385.7–1432.1 and 802.8–2331.5 kg/hm2 higher than those under CK, C2, and C3 pruning intensities during the jujube growing period of 2012–2015, respectively. However, C3 pruning intensity had the highest WUE values of 2.92–3.13 kg/m3, which were 1.6–2.0, 1.1–1.2 and 1.0–1.1 times greater than those under CK, C1 and C2 pruning intensities, respectively. Therefore, C3 pruning intensity is recommended to jujube plantations for its economic and ecological benefits. These results provide an alternative strategy to mitigate soil desiccation in jujube plantations in the hilly Loess Plateau Region, which is critical for sustainable cultivation of economic forest trees in this region.

Dew is an important water source for plants in arid and semi-arid regions. However, information on dew is scarce in such regions. In this study, we explored dew formation, amount, and duration of rain-fed jujube (Zizyphus jujube Mill) trees in a semi-arid loess hilly region of China (i.e., Mizhi County). The data included dew intensity and duration, relative humidity, temperature, and wind speed measured from 26 July to 23 October, 2012 and from 24 June to 17 October, 2013 using a micro-climate system (including dielectric leaf wetness sensors, VP-3 Relative Humidity/Temperature Sensor, High Resolution Rain Gauge, and Davis Cup Anemometer). The results show that atmospheric conditions of relative humidity of >78% and dew point temperature of 1°C–3°C are significantly favorable to dew formation. Compared with the rainfall, dew was characterized by high frequency, strong stability, and long duration. Furthermore, heavy dew accounted for a large proportion of the total amount. The empirical models (i.e., relative humidity model (RH model) and dew point depression model (DPD model)) for daily dew duration estimation performed well at 15-min intervals, with low errors ranging between 1.29 and 1.60 h, respectively. But it should be noted that the models should be calibrated firstly by determining the optimal thresholds of relatively humidity for RH model and dew point depression for DPD model. For rain-fed jujube trees in the semi-arid loess hilly regions of China, the optimal threshold of relative humidity was 78%, and the optimal upper and lower thresholds of dew point depression were 1°C and 5°C, respectively. The study further demonstrates that dew is an important water resource that cannot be ignored for rain-fed jujube trees and may affect water balance at regional scales.

The widely adopted practice of clean-cultivation in rainfed orchards in the Loess Plateau has led to serious soil erosion and degradation problems. Agroforestry is a promising management practice for solving these problems, but may reduce available soil water. Thus, reasonable soil and water management strategies should be developed for agroforestry systems in this region. The objective of this study was to determine whether adopting proper mowing management of the understory intercrop limited soil drying in a rainfed jujube (Ziziphus jujuba Mill.) orchard in this region. Various mowing intensities (3-, 6-, 9-cm stubble height and unmowed treatments) were imposed on three intercrop species: bluegrass (Poa pratensis L.), white clover (Trifolium repens L.), and hairy vetch (Vicia villosa Roth) to investigate and compare their root characteristics and soil moisture responses to mowing under greenhouse and field conditions. Based on the results, the most promising intercrop species and mowing intensity to be used in a jujube orchard were selected for field validation. The results showed that mowing effectively regulated intercrop root amount and distribution, as well as associated soil moisture conditions. However, the sensitivities of root characteristics and soil moisture to mowing and growth environment were species-specific. Bluegrass mowed to a 3-cm stubble height produced the minimum root amount, the shallowest root depth, and the greatest soil water content. When this treatment was applied to a rainfed jujube orchard during the 2015 and 2016 growing seasons, average soil moisture in the 0–300 cm soil layer was essentially unaffected, while fruit yield was increased. Consequently, bluegrass mowed to a stubble height of 3 cm when intercropped with jujube trees in the Loess plateau is a feasible practice to increase jujube yield and avoid extra soil drying.

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 478 mm respectively for T1, T2 and the control treatments during jujube growth period in 2012; 408, 426 and 451 mm respectively for T1, T2 and the control treatments during jujube growth period in 2013 and 374, 381 and 392 mm 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 = 1 kPa) 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 15 mm below the cambium (40.74, 42.53 and 46.81 μm respectively for T1, T2 and the control treatments). Similarly, T1 and T2 reduced mean vessel lumen diameter at 5 mm below the cambium (43.20, 47.44 and 48.39 μm 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 11293 kg ha−1 respectively for T1, T2 and the control treatments in 2012; 5691, 5426 and 6000 kg ha−1 respectively for T1, T2 and the control treatments in 2013; 6804, 8341 and 10589 kg ha−1 respectively for T1, T2 and the control treatments in 2014. WUE values were 1.79, 1.94 and 2.36 kg m−3 in 2012; 1.40, 1.28 and 1.33 kg m−3 in 2013; 1.82, 2.19 and 2.70 kg m−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.

The Jarvis canopy resistance sub-model is commonly used in transpiration modelling, although its optimal structure is rarely studied. It is still not fully clear that to what extent transpiration modeling may differ if using different constraint function forms for key environmental variable in Jarvis sub-model. In this study, various Jarvis canopy resistance sub-model configurations were embedded in the Penman-Monteith model to compare their ability to model daily transpiration of rain-fed jujube (Ziziphus jujuba Mill.) plantations where soil moisture is a key factor of tree water use. Parameters were calibrated using the Bayesian Markov Chain Monte Carlo (MCMC) simulation technique and model comparison was quantified using Deviance Information Criterion (DIC). The results showed significant differences in model performance between the constraint function forms of soil water content. The difference in DIC between the model with the best constraint function form and the other two forms reached 37.66–50.94, much higher than the evaluation criteria for significance (larger than 7). When the best constraint function form was used, the performance of the transpiration model improved. The model performance worsened when the other constraint function forms were used, even worse than those without consideration for soil water content. However, only slight differences in model performance were detected for the constraint function forms of temperature, vapor pressure deficit and photosynthetically active radiation. Using the best configuration of Jarvis canopy resistance sub-model, daily transpiration of jujube plantation was well estimated with overall good accuracy and acceptable uncertainty. The predictions and observations were highly correlated (R2 = 0.87 for calibration and R2 = 0.80 for validation). The results suggested that different constraint function forms of an environmental factor contributed differently to transpiration model performance, and the situation was different for different environmental factors. Including the key environmental factor in the Jarvis canopy resistance sub-model will not always improve the performance of transpiration models.

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) 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

International audience; In order to select suitable evapotranspiration (ET) models for rainfed and sparsely cultivated jujube (Ziziphus jujuba Mill.) plantations in the Loess Plateau, this study used Bayesian analysis to compare the performances (accuracies and uncertainties) of the Shuttleworth-Wallace (SW) model, the Penman-Monteith (PM) model and the Penman-Monteith equation with a variable surface canopy resistance (PMv) model in terms of daily and hourly ET. To determine seasonal variations and soil water dependence, probabilistic analyses were independently used to calibrate the SW and PM model parameters at every growth stages and the PMv model parameters for different soil water conditions in the growth season of rainfed jujube plantations in 2013. Finally, data for the growth season in 2012 were used to validate the calibrated models. The results showed that the posterior distributions of the parameters of the three models narrowed down at both daily and hourly scales, suggesting decreasing uncertainties in the parameters. Some model parameters varied with season (increasing for the last two growth seasons) or soil water condition (decreasing with increasing soil moisture). The performances of the ET models also varied with time scale, generally less precise at hourly than at daily time scale. Irrespectively, the three ET models met the evaluation criteria (mean relative error (MAE) < 20% and coefficient of determination (R-2) > 0.8 and Willmott index of agreement (D) > 0.8) with acceptable accuracy and uncertainty at the daily time scale in both calibration (with R-2, MAE and D values of 0.94, 6.78% and 0.94 for the SW model; 0.94, 7.52% and 0.94 for the PM model; and 0.89, 8.49% and 0.83 for the PMV model, respectively) and validation periods (with R-2, MAE and D values of 0.94, 5.77% and 0.92 for the SW model; 0.93, 5.15% and 0.91 for the PM model; and 0.89, 10.26% and 0.86 for the PMV model, respectively). Where only climate data were available, the PMV model was recommended for estimation of daily ET in the study area due to less complexity, low data requirements, and high accuracy. Where sufficient monitoring data were available, the PM model was preferred for estimation of the daily ET in the study area due to less complexity and small differences in performance with the SW model. All three models met the evaluation criteria for calibration period(with R-2, MAE and D values of 0.92, 16.04% and 0.92 for the SW model; 0.96,14.37% and 0.94 for the PM model; and 0.89, 18.56% and 0.91 for the PMV model, respectively) while only the SW model met the evaluation criteria for validation period (with R-2, MAE and D values of 0.87, 16.33% and 0.88 for the SW model; 0.85, 20.70% and 0.85 for the PM model; and 0.85, 23.95% and 0.83 for the PMV model, respectively) at the hourly time scale. Based on model accuracy and uncertainty at the hourly time scale, the SW model was recommended for use in estimating ET in jujube plantations in the Loess Plateau