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Article

Age- and Drought-Related Variation in Plant-Available Water of Rain-Fed Jujube Orchards on the Loess Plateau of China

by
Lusheng Li
1,
Lili Zhao
1,*,
Jiankun Ge
1,
Hongchen Li
2 and
Peiwen Yang
1
1
School of Water Conservancy, North China University of Water Resources and Electric Power, Zhengzhou 450046, China
2
School of Resource and Environmental Engineering, Ludong University, Yantai 264025, China
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(17), 10998; https://doi.org/10.3390/su141710998
Submission received: 24 July 2022 / Revised: 28 August 2022 / Accepted: 29 August 2022 / Published: 2 September 2022
(This article belongs to the Section Sustainable Agriculture)
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Abstract

:
As an important part of the large-scale ecological restoration project of “Grain for Green”, the planting area of jujube (Ziziphus jujuba) trees has increased significantly in the hilly region on the Loess Plateau of China, which aims to improve water and soil conservation and develop economic prospects of the region. Understanding the long-term effects of expanding orchards and the responses of soil water dynamics to drought are important for orchard management. Therefore, we use a space-for-time substitution to investigate the variations of plant-available water storage in returning cropland to orchards with different stand ages (2, 6, 10, and 15 years) in a normal year (NY2014, 442.1 mm rainfall) and the next year with low annual precipitation (DY2015, 388 mm rainfall). The results showed that the plant-available water storage in jujube orchards decreased with increasing stand age, and the trend was most obvious in the 60–180 cm layer. The mature stands (10 and 15 years) primarily absorbed soil water from the deep layer (180–300 cm) in DY2015, leading to negative values of plant-available water storage. The whole soil profiles were all subjected to severe water deficits in our study. The findings will help guide rain-fed orchard management in the loess hilly region of China and similar dryland regions.

1. Introduction

Land degradation has been, and continues to be, a major environmental issue affecting agricultural production, food safety, and quality of life [1,2], which is compounded by increased drought equivalents and further climate change [3]. It is expected that drought and climate change will raise evapotranspiration rates and reduce soil water resources in arid and semiarid regions, particularly on the Loess Plateau of China [4], as well as in other parts of the world [5,6,7]. Among the several options for combating land degradation, afforestation has been proven to be an efficient avenue for facilitating land production and restoring soil quality [8,9,10].
The jujube (Ziziphus jujuba) is a deciduous perennial fruit tree native to Central and Eastern Asia’s arid and semiarid regions. Since the Chinese central government launched afforestation programs in 1999, the area of jujube orchards on the Loess Plateau has grown rapidly, primarily to conserve soil and water [11,12,13], and to improve economic income [14,15]. Soil water is a key variable associated with the land degradation process in water-stressed environments because it regulates plant growth and yield [16,17,18]; in turn, plants have a variety of effects on soil water resources, depending on climatic conditions, stand age, and species [19,20,21]. For instance, plant growth should result in an increase in water consumption [22,23], which can lead to soil degradation if soil water resources are not managed properly [24].
Most orchards are grown under rain-fed conditions due to the prohibitive costs of irrigation services in the loess hilly region of China. Fruit trees usually absorb water from shallow layers during the rainy season, while they switch to taking more water from deep layers (groundwater) during the dry seasons in this area [25,26]. Thus, water deficits unavoidably emerge when plant water consumption exceeds soil water sources, and sustained excessive consumption produces soil desiccation, which eventually results in the establishment of a dry soil profile that is difficult to remediate [27,28]. Meanwhile, the fruit trees typically flourished well at first, but began to degenerate once the initial water supply was exhausted. As a result, the widespread planting of jujube trees has altered ecological and hydrological processes, significantly influencing regional sustainable development.
Orchards differ greatly from ecological forests because they are generally heavily managed by landowners (e.g., pruning and fertilizer application). Hence, the soil water contents in orchards were radically different from those in ecological forests. However, detailed assessment of the variations in soil water content in orchards with different stand ages have rarely been studied [29,30]. Understanding the importance of soil water to jujube orchards, as well as the impact of orchards on soil water resources, is critical for designing water management methods to preserve agricultural and other plant productivity in such a water-limited environment.
The purpose of this study was to investigate the variations of plant-available water storage in returning cropland to jujube orchards with a range of stand ages (2, 6, 10, and 15 years), and how drought affects root zone plant-available water storage in jujube orchards. We assumed that: (1) root zone plant-available water storage in jujube orchards decreases with increasing stand age, and (2) mature stands would be more reliant on soil water from deeper layers, and thus be more drought-resistant than young stands. Here, we evaluated the changes in water availability to plants of different stand ages (2, 6, 10, and 15 years) in the 0–300 cm layer of jujube orchards in a normal year (2014) and the following year with a dry year (2015) in the loess hilly region of China.

2. Materials and Methods

2.1. Study Area

The research was carried out in the Yuanzegou watershed, Qingjian County in Shaanxi Province (37°15′ N, 118°18′ E) (Figure 1). According to Gao et al. [25], the climate of the study area is semiarid continental with a mean annual precipitation of 481 mm, and 70% of precipitation falls between July and September (with high intensity and short-duration storms). The mean annual temperature is 8 °C, and the frost-free season is 160–170 days.
The study area is 15.5 km2 with gradients ranging from 15° to 30°, and the elevation at the study area ranges from 865 to 1105 m. There were three typical land-use types, including jujube orchards, which account for almost half of the uplands, grassland, and cropland. The soil type is representative loess soil (Inceptisols, USDA), the average bulk density in the 0–100 cm soil layer is 1.28 g cm−3, and the field capacity and permanent wilting point are 0.25 cm3 cm−3 and 0.06 cm3 cm−3, respectively. The soil contains 3.35% clay, 67.98% silt, and 28.67% sand.

2.2. Stand Characteristics

The study area includes 2-, 6-, 10-, and 15-year-old stands (designated as S2, S6, S10, and S15, respectively), which were assigned to stands established in 2012, 2008, 2004, and 1999, respectively. The tree spacing in jujube orchards was rather constant, with 3 m be-tween plants in each row and a 2 m row space. The distance between any two adjacent stands was less than 0.4 km to minimize soil heterogeneity, and the acreage of different jujube orchards ranged from 1 to 2.5 hm2. All of the jujube orchards in the study area were rain-fed with no irrigation and managed by local farmers using traditional agricultural practices.
For each stand described above, a temporary plot (20 m × 20 m) was chosen in each jujube orchard. Then, four rain-fed jujube trees were chosen from each plot with a regular distribution, and they had comparable growth indicators, such as height, diameter at breast height (DBH), and crown width (Table 1). Consequently, sixteen rain-fed jujube trees in stands of different ages (four trees in each stand age) were used to evaluate age-related changes in plant-available water storage using the space-for-time substitution method [31,32,33]. To decrease topographic variations, a portable global positioning system (GPS) was used to determine the altitude position, and all of the selected trees grow on the northern upper slopes with a comparable slope gradient (20–24°). Cropland (predominantly fodder species canola (Brassica napus L.)) was utilized as a control to monitor the variations in soil water in a seminatural environment, which was planted on 15 May 2014 and 30 May 2015, respectively. The cropland plot was 8 m × 4 m in size, with three quadrats (2 m × 2 m) randomly placed in the plot.

2.3. Precipitation Data

The historical precipitation data (1960–2012) for the research location were obtained from the China Meteorological Forcing Dataset (CMFD) [34]. The growing season precipitation data were collected by a portable weather station (AR5, Avalon Scientific, Jersey City, NJ, USA) at the research location during the study period (2014–2015). Every 30 min, the data recorder collected the main meteorological variables, such as precipitation, air temperature, wind speed, and solar radiation.

2.4. Soil Water Data

The soil water contents of the jujube orchards and cropland plots were both observed by a portable time-domain reflectometer (TDR) instrument (TRIME-PICO IPH/T3, IMKO Micromodultechnik GmbH, Ettlingen, Germany). There were 20 experimental locations in total, and soil water contents were monitored once every two weeks using 3-m-long tubes, which were placed 30 cm apart from the representative tree trunk from 31 May to 7 October in 2014 and 8 May to 14 October in 2015. The TRIME-IPH system was calibrated against volumetric water content, and the soil water contents were measured at depths of 0–300 cm at 20 cm intervals using the TDR tool in the vertical profile study [35,36,37]. Additional measurements were taken following rainfall events, of which 25 measurements were taken, in total, in our study. Because all sample sites were obtained within 2 h for each measurement, the temporal fluctuation of soil water could be ignored.

2.5. Evaluation Indices

This paper evaluated the soil water of the study sites from two perspectives. First, plant-available water storage (PAWS, mm) was used to represent the amount of plant-available water storage and released in the plant root zone:
P A W S i = S W C i − P W P i ∆ Z i × 10
where, S W C i and P W P i are the soil water content (cm3·cm−3) and the permanent wilting point (cm3·cm−3) at the ith depth, respectively, and ∆ Z i represents the increment of soil depth (cm). Changes in plant-available water storage (PAWS) were expressed in terms of the differences between the cropland and jujube orchards.
Second, the soil water storage deficit degree (SWSD, %) was applied to determine the viability of ecological restoration [38,39].
S W S D = F C i − S W S i F C i × 100 %
where, S W S i and F C i are the soil water storage (mm) and the field capacity (mm) at the ith depth, respectively. If SWSD ≥ 0, there was a soil water storage deficit at the specified depth, with larger values indicating severe SWS deficits, whereas SWSD < 0 indicated no soil water storage deficit.

2.6. Statistical Analysis

SPSS v20.0 (IBM Corporation, Armonk, NY, USA) and Microsoft Excel 2019 (Microsoft Corporation, Redmond, WA, USA) were used to analyze the PAWS and SWSD data. A one-way ANOVA was used to assess the effects of stand age on PAWS and SWSD in jujube orchards. Following this, the least significant difference (LSD) test with p < 0.05 was performed to test the significance of difference among the treatments. The standard deviation was also calculated for each case.

3. Results

3.1. Weather Conditions during the Study Period

Figure 2 depicts the cumulative frequency of annual total precipitation (TP) and growing season precipitation (GP) at the experimental site (data series 1960–2012), with TP and GP medians of 479.7 and 421.3 mm, respectively. For the cumulative frequency, TP added up to 55.6% in 2014 (442.1 mm) and 84.1% in 2015 (388.0 mm), while GP corresponded to 59.6% in 2014 (386.2 mm) and 93.7% in 2015 (289 mm). Thus, 2014 was classified as a normal year (NY2014), whereas 2015 was considered to be a dry year (DY2015).
The growing season precipitation varied greatly between the two study years (Figure 3). The percentage of days without rain from May to October was approximately 24.6% (mean 2014–2015), and rainfall episodes were unevenly distributed and often to a low extent (below 10 mm day−1) in this period. On the other hand, precipitation events above 10 mm day−1 accounted for approximately 31.5% of rainy days (mean 2014–2015), with some very heavy rains (e.g., a 73.8 mm d−1) occurring in July 2014. During the study period (2014–2015), ET0 was consistently at its peak in June, with an average of 173.7 mm (Figure 3).

3.2. Distribution of PAWS in Jujube Orchards with Stand Age

The time series for PAWS with four stand ages in densely planted jujube orchards are shown in Figure 4, and the soil profile PAWS for all jujube orchards followed similar trends to the cropland during the two different study periods. Overall, the extreme drought in DY2015 drastically reduced PAWS at all soil layers in jujube orchards, with a clear lag in the 60–180 cm layer for the dry spell. In addition, the PAWS of mature stands (S10 and S15) in the 180–300 cm layer were negative, meaning that the soil water fell below the permanent wilting point in this layer. Generally, the cropland and young stands (S2 and S6) had higher PAWS than the mature stands. Specifically, cropland had the highest PAWS value in the 0–60 cm layer (85.70 mm), whereas the highest PAWS value was recorded from 60–300 cm in S2 (147.14 mm) during the study period.
The vertical distribution of annual average PAWS in jujube orchards with different stand ages is presented in Figure 5. The influences of stand age on the annual average PAWS did not appear in the surface layer (0–40 cm), which was influenced by effective rainfall (rainfall minus interception evaporation) and soil evaporation. In the 40–300 cm layer, NY2014 had a higher annual average PAWS than DY2015, being 43.5%, 83.9%, 104%, 185.2%, and 44.2% higher in S2, S6, S10, S15, and cropland, respectively. Furthermore, the values of annual mean PAWS in S6 and mature stands (S10 and S15) were significantly lower than those in S2 in the 80–300 cm layer (p < 0.05).

3.3. Changes of PAWS in Jujube Orchards with Stand Age

According to stand age and soil profile, the changes in PAWS at the study site are presented in Figure 6. Compared with the cropland, the reduction of PAWS in the 60–300 cm was higher in mature stands (S10 and S15) than in young stands (S2 and S6), especially in DY2015 (p < 0.05). PAWS was consumed across the whole soil profile in S6 and mature stands (S10 and S15), but it was consumed in the shallow soil profile in S2. Take the changes in PAWS in S2 as an example, PAWS in the 0–20 cm and 20–60 cm soil profile in 2014 decreased to 6.93 mm and 4.94 mm, respectively, but increased to 33.88 mm and 16.41 mm in 60–180 cm and 180–300 cm, respectively. Furthermore, the general profile variations in PAWS followed a similar trend between the normal year and dry year.

3.4. SWSD Variation in the Soil Profile

The SWSD in the soil profiles of jujube orchards are shown in Figure 7. In general, the SWSD was observed across the whole soil profile in our study, and the SWSD of all jujube orchards followed similar trends to the cropland with the highest SWSD recorded in the 180–300 cm layer, and the lowest SWSD values were recorded from 20 to 60 cm in both study years. Furthermore, there was no significant difference in SWSD among jujube orchards in the 0–20 cm layer (p > 0.05) in NY2014, and no significant difference was found in the 0–60 cm layer (p > 0.05) in DY2015. The SWSD in S10 and S15 were considerably (p < 0.05) higher than those in S2 in the 60–300 cm layer.

4. Discussion

Space-for-time substitution, in its broadest sense, employs the current geographical distribution of biological properties to explain temporal processes that are otherwise unobservable [32]. Generally, the changes in soil water storage in jujube orchards with a range of stand ages were believed to represent soil water variation in jujube orchards throughout time. The largest uncertainty in space-for-time substitution is that spatial variability is minimal enough that changes in soil water content are caused by stand age differences rather than geographical differences. In this study, the jujube orchards across stand age were within 0.4 km of one another, with similar climatic and agronomic conditions, implying that spatial variability is rather limited. As a result, the space-for-time substitution design can be used to explore the impact of stand age and drought on soil water variation and jujube tree consumption.

4.1. Response of PAWS Dynamic to Stand Age and Annual Precipitation

The jujube orchard region has reached 122·376 ha in the loess hilly region of north Shaanxi Province over the past decade due to both economic and ecological reasons [40]. However, compared with cropland, PAWS (plant-available water storage) was expected to decrease dramatically in S6 and mature rain-fed jujube orchards, especially in the dry year (Figure 4). The natural factors (e.g., geography and weather) and agronomic practices (e.g., pruning and fertilization) of the representative orchards were similar. As a result, changes in stand age and precipitation in each study period affected the rates of both root uptake water and photosynthesis, as well as affected the variation and distribution of soil water in jujube orchards.
Our findings emphasize the importance of variation in PAWS to explore critical pathways for orchard management in the loess hilly region of China. In our study, S6 and mature stands caused severe water consumption (compared to that of cropland). PAWS in the jujube orchards decreased with increasing stand age, and the mature stands mainly consumed deep soil water (180–300 cm) during prolonged drought. Previous research works indicated that the impact of economic forests on surface soil water (<20 cm) could be beneficial [41], negative [42], or insignificant [43]. We found that S2 kept PAWS at a high level across the whole soil profile during the study period, and presumable causes of the initial soil water could keep the younger stand (S2) growing well and precipitation could supply soil water during the early stages of afforestation. However, jujube trees are supposed to present an increase in water consumption when they grow up [24], which could be associated with reductions in PAWS (Figure 5). A similar phenomenon has also been reported by Song et al. [31]. Moreover, compared with cropland, PAWS was severely depleted and obvious soil desiccation occurred after 6 years of afforestation (Figure 4).
Previous studies have shown that the soil water contents are vulnerable to precipitation and root absorption in the 0–180 cm layer [44,45]. As a result, greater changes in the distribution of PAWS were exhibited in the 0–180 cm layer, as found in both study periods, particularly in DY2015, probably because of high evapotranspiration demands, as well as a lack of precipitation in the dry year to replenish the soil water content. Other research has shown that the soil water changed with precipitation in the shallow soil layer, but remained constant in the deep soil layer throughout the study period [46]. Similarly, PAWS in the 0–180 cm layer fluctuated with rainfall events in our study, whereas it stayed stable in the deep layer (>180 cm) in NY2014, with a slight decline in PAWS in DY2015 (Figure 4). Consequently, PAWS in deep layers could play an important part in satisfying the water consumption of jujube orchards, particularly during dry years, and understanding its distribution is critical for establishing efficient soil water management strategies to improve orchard development and production.

4.2. SWSD Variations with Stand Age

Because of the intense interception of precipitation by tree canopies, huge water suction by roots, and significant reductions in water losses via evapotranspiration [47,48,49], afforestation could reduce soil water when compared to the pre-afforestation levels, especially if the preceding land use was cropland (Figure 6). Large volumes of soil water are consumed by transpiration through tree roots when croplands are transformed into orchards. Therefore, converting croplands into orchards usually resulted in a significant water deficit. Deng et al. [42] examined 1740 observations and found that the mean soil water content in northern China declined by 20% when croplands were converted to trees. Because the water table usually remains below 30 m on the Loess Plateau, limited rainfall is primarily responsible for replenishing the soil water content [21]. In our study, the soil profile average SWSD with different stand ages were all under water deficit conditions because the initial soil water and rainfall supplements were insufficient for the development of jujube trees. Specifically, water shortages were slightly observed in the 20–60 cm layer, whereas the soil water contents were almost exhausted in the 60–300 cm layer, and long-term plant water deficits occurred (Figure 7).
Even though afforestation could enhance soil infiltration rates and increase soil water retention, soil water from these supplements is unevenly distributed and unsustainable, making it difficult for plants to develop continuously. Although these replenishments are important in soil water preservation, they are insufficient to compensate for the substantial soil water depletion caused by plant transpiration. As the age of afforestation increased, the imbalance between water availability and demand finally led to water deficit conditions, which, in turn, had an impact on plant development and contributed to land degradation. The entire process is a vicious circle, as evidenced by the “small old trees” generated by soil desiccation on the Loess Plateau of China.

4.3. Implications

Since the initiation of the restoration project “Grain for Green” in 1999, extensive orchards have been established on the Loess Plateau. These commercial plants are mainly managed by smallholder farmers, and most of these plants are rain-fed because of the complex geography and expensive cost of irrigation. In our study, extremely low plant-available water levels were observed in the deep layer (>180 cm) in a relatively dry year (2015, Figure 4 and Figure 5). The major cause was the failed rains, coupled with recurrent drought throughout the growing season in DY2015, which unavoidably depleted deep soil water [50]. Furthermore, the aboveground biomass of jujube trees increased with increasing stand age (Table 1), and thus required a higher consumption of water due to increased evaporation, which further aggravated the situation of soil water resource shortage under prolonged drought (Figure 6 and Figure 7).
Precipitation is the main source of ecological recovery and agricultural production in the loess hilly region of China. Reasonable regulation and utilization of precipitation can alleviate the key driving factors of soil erosion, alleviate the problem of drought and water shortage, and realize the efficient use of soil and water resources [51,52]. Water regulatory approaches ranging from ‘open source’ to ‘reduce expenditure’ have been developed and implemented [53]. In this region, engineering measures (e.g., level terraces, fish-scale pits, and RWCI systems) and agronomic approaches (e.g., mulching with branches) are typically successful strategies for water resource management of long-term economic forest growth [54,55,56,57]. Furthermore, studies on water-saving pruning technology showed that an appropriate pruning strategy may greatly reduce the transpiration and water consumption of jujube trees [58], and agroforestry systems were also introduced in orchards to increase infiltration during rain storms [59]. According to the changes in plant-available water storage in mature stands (S10 and S15), one or more measures (such as mulching with branches, structural measures, alley cropping, and long-shoot pruning) are implemented for jujube orchards upwards of 6-years-old to improve soil water retention and reduce excessive water usage to realize the sustainable development of jujube orchards in arid and semiarid regions.

5. Conclusions

In our study, the available soil water storage capacities in the jujube orchards were significantly lower than those in the abandoned cropland (control), except in S2 in the 60–300 cm layer. The results indicated that PAWS decreased gradually with increasing stand age, and the trend was most obvious in the 60–180 cm layer. The mature stands (10 and 15 years) mainly consumed deep soil water (180–300 cm) in the dry year, leading to negative values of PAWS and soil water deficit status occurring in most of the studied soil layers. Therefore, one or more technologies should be considered in the management of jujube orchards upwards of 6-years-old. The next steps will include knowing how jujube orchards use deep soil water (>3 m), as well as how these orchards affect deep soil water resources. Our findings will contribute to the development of effective water resource management strategies and combat the deterioration of jujube orchards in the loess hilly region of China and other similar regions.

Author Contributions

Conceptualization, L.L. and L.Z.; methodology, L.L.; software, L.L. and P.Y.; validation, L.L., L.Z. and H.L.; data curation, L.L.; writing—original draft preparation, L.L.; writing—review and editing, L.L. and L.Z.; visualization, J.G.; funding acquisition, J.G., H.L. and L.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China [grant no. 41901032, 51709110], the Key Technologies R&D and Promotion program of Henan (202102110128) and Foundation for University Young Key Scholar by Henan province (2020GGJS100).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author, L.Z., upon reasonable request.

Acknowledgments

The authors thank Yibo Ding, Qiang Lin, and Wenhao Sun for their valuable assistance in the experimental work.

Conflicts of Interest

The authors declare no conflict interest.

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Figure 1. (a) Location of the study area and (b) a general view of the jujube trees.
Figure 1. (a) Location of the study area and (b) a general view of the jujube trees.
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Figure 2. The cumulative frequency of annual total precipitation (TP) and growing-season precipitation (GP) from 1960 to 2012 at the experimental site.
Figure 2. The cumulative frequency of annual total precipitation (TP) and growing-season precipitation (GP) from 1960 to 2012 at the experimental site.
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Figure 3. Crop reference evapotranspiration and precipitation during the growth period in 2014–2015.
Figure 3. Crop reference evapotranspiration and precipitation during the growth period in 2014–2015.
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Figure 4. Seasonal variation in PAWS (plant-available water storage) in (a) 0 to 20 cm; (b) 20 to 60 cm; (c) 60 to 180 cm; and (d) 180 to 300 cm in a normal year (2014), and a year with low precipitation (2015), in jujube orchards with different stand ages, designated S2, S6, S10, and S15. The error bar represents the standard deviation.
Figure 4. Seasonal variation in PAWS (plant-available water storage) in (a) 0 to 20 cm; (b) 20 to 60 cm; (c) 60 to 180 cm; and (d) 180 to 300 cm in a normal year (2014), and a year with low precipitation (2015), in jujube orchards with different stand ages, designated S2, S6, S10, and S15. The error bar represents the standard deviation.
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Figure 5. Variation in PAWS (plant-available water storage) at different soil depths at the study site in NY2014 (a) and DY2015 (b). The data are means ± SDs, n = 3; the error bars indicate standard deviation; the different letters indicate significant differences among the treatments in the study area at p < 0.05.
Figure 5. Variation in PAWS (plant-available water storage) at different soil depths at the study site in NY2014 (a) and DY2015 (b). The data are means ± SDs, n = 3; the error bars indicate standard deviation; the different letters indicate significant differences among the treatments in the study area at p < 0.05.
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Figure 6. Changes of PAWS (plant-available water storage) in different soil layers in the jujube orchards with stand age compared with the cropland in NY2014 (a) and DY2015 (b). The data are means ± SDs, n = 3. The different lowercase letters indicate significant differences among the changes of PAWS in jujube orchards with stand age.
Figure 6. Changes of PAWS (plant-available water storage) in different soil layers in the jujube orchards with stand age compared with the cropland in NY2014 (a) and DY2015 (b). The data are means ± SDs, n = 3. The different lowercase letters indicate significant differences among the changes of PAWS in jujube orchards with stand age.
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Figure 7. Variations of SWSD (soil water storage deficit degree) in different soil layers over stand age at the study site in NY2014 (a) and DY2015 (b). All data are presented as the means ± SDs. The error bars indicate standard deviations; the different lowercase letters indicate significant differences among soil profile SWSD in treatments in the study area at p < 0.05.
Figure 7. Variations of SWSD (soil water storage deficit degree) in different soil layers over stand age at the study site in NY2014 (a) and DY2015 (b). All data are presented as the means ± SDs. The error bars indicate standard deviations; the different lowercase letters indicate significant differences among soil profile SWSD in treatments in the study area at p < 0.05.
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Table
Table 1. Tree characteristics of the jujube orchards.
Table 1. Tree characteristics of the jujube orchards.
CharacteristicS2S6S10S15
Stand age (year)261015
Average height (m)0.87 ± 0.15 c1.92 ± 0.21 b2.25 ± 0.28 a2.27 ± 0.37 a
Average DBH (mm)31.66 ± 6.8 d64.35 ± 10.5 c72.84 ± 11.3 b91.78 ± 11.6 a
Average crown width (m)1.14 ± 0.27 d1.81 ± 0.31 c1.98 ± 0.38 b2.47 ± 0.32 a
The data are the means ± SDs for 12 trees with four stand ages (three per replicate plot). The same letter denotes no significant difference among stands (p > 0.05).
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Li, L.; Zhao, L.; Ge, J.; Li, H.; Yang, P. Age- and Drought-Related Variation in Plant-Available Water of Rain-Fed Jujube Orchards on the Loess Plateau of China. Sustainability 2022, 14, 10998. https://doi.org/10.3390/su141710998

AMA Style

Li L, Zhao L, Ge J, Li H, Yang P. Age- and Drought-Related Variation in Plant-Available Water of Rain-Fed Jujube Orchards on the Loess Plateau of China. Sustainability. 2022; 14(17):10998. https://doi.org/10.3390/su141710998

Chicago/Turabian Style

Li, Lusheng, Lili Zhao, Jiankun Ge, Hongchen Li, and Peiwen Yang. 2022. "Age- and Drought-Related Variation in Plant-Available Water of Rain-Fed Jujube Orchards on the Loess Plateau of China" Sustainability 14, no. 17: 10998. https://doi.org/10.3390/su141710998

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