**1. Introduction**

In semi-arid region, water is the key factor for the survival and succession of plant community [1]. The use of limited water resource concerns not only plant survival, but also interspecific interaction and community dynamics. Since there is generally no stable isotope fractionation during water uptake by root system and water transportation before arriving leaf, water source of a plant could be identified by comparing stable isotope of xylem water and potential water sources [2]. The potential water sources for plant species are shallow soil water recharged by rainwater, deep soil water recharged by rainwater, snow water or groundwater in semiarid region [3–10]. Different life form plants usually used different water source, which related to their root types. Generally, tree and shrub with deep root system could use deep soil water or groundwater [6]. Woody species with dimorphic root system may use different depths of soil water or groundwater simultaneously [9]. And shallow rooted shrub and perennial grass only used shallow or middle soil

water [11]. Water source of plant species in semi-arid region was affected by many environmental factors, like season [6, 7], annual variance of climate [8] and habitat heterogeneity [12].

Ordos Plateau is located in the middle reaches of Yellow River, which is the ecotone between Loess Plateau and Mongolian Plateau, with the total area of 1.30 × 105 km2 . The eastern part is hilly gully area with loess hill and valley, the southern part is Mu Us Sandy Land with fixed and semi-fixed sand dune, and the northern part is Hobq Desert with moving sand dune [13]. The elevation increases from 774 m in southeast to 2148 m in northwest [14]. From southeast to northwest, the annual precipitation decreased from 400 mm to 200 mm; meanwhile, the natural vegetation varies from forest grassland, grassland, and sandy land to desert [13]. In the last decades, land desertification was severely on Ordos Plateau. From 1994 to 2000, land desertification area increased for 1.90 × 104 km2 , focused on west Hobq Desert and south Mu Us Sandy Land [15]. In these years, many measures was taken to increase vegetation coverage, decrease sand storm hazard and improve ecological environment, such as air seeding, fence and afforestation.

In eastern Ordos Plateau, the hilly gully area is one of the most severely soil and water erosion region in the middle reaches of Yellow River, which is sand stone covered by loess and contributed up to 25% of the total course sediment for the lower reaches [16]. Both of engineering measure and vegetation restoration was carried out to control soil and water erosion, such as plant trees and shrubs, including *Hippophae rhamnoides*, *Pinus tabuliformis* and *Armeniaca sibirica*.

Mu Us Sandy Land is one of the four sandy land in China, which located in central Inner Mongolia, Northern Shaanxi and Northeast Ningxia. The total area is 4.22 × 104 km2 , with the elevation varies from 1000 to 1600 m [17]. The main natural vegetation are forest steppe, steppe, and shrub sandy land and desert steppe. Vegetation growth was improved in Mu Us Sandy Land, which were resulted by climate change and human activity [18]. The dominant species of sand control are *Sabina vulgaris*, *Artemisia ordosica*, *Salix psammophila* and *Caragana intermedia* in shrub sandy land [19].

Previous studies in Mu Us Sandy Land showed that *S. vulgaris* mainly used soil water within 1.5 m and groundwater, whereas *A. ordosica* mainly used shallow soil water within 50 cm [5]. Moreover, *A. ordosica* mainly used deep soil water recharged by 65 mm rainstorm in summer, *Cynanchum komarovii* mainly used middle rain of 10–20 mm, whereas *Stipa bungeana* mainly used shallow soil water recharged by small rain [3]. However, we still do not know the seasonal dynamic of water source for these dominant shrubs in Mu Us Sandy Land. Moreover, there is few report about water source of woody species in hilly gully area of Ordos Plateau. Therefore, seasonal dynamic of water source of six woody plants was explored by stable isotope technology in two different habitats of Ordos Plateau. The purpose of this study was to understand how these trees and shrubs adapt to the semi-arid climate by adjusting water source in the growing season. The results would give theoretical supports for ecological forestry engineering, including Natural Forest Protection, Grain for Green and Three Norths Shelterbelt Program.

#### **2. Water source of three woody species on loess slope**

#### **2.1 Three soil and water conservation woody species on loess slope**

This study was conducted in Soil and Water Conservation Park of Jungar Banner, Ordos City, Inner Mongolia. The banner has a temperate continental climate, which mean air temperature varies from 6.2°C to 8.7°C, mean annual *Water Source of Six Woody Plants in Different Habitats on Desertified Land of Ordos Plateau… DOI: http://dx.doi.org/10.5772/intechopen.99658*

precipitation is 400 mm, potential evapotranspiration is 2093 mm and forest free days are 145 d [20]. The natural vegetation is steppe dominated by *Stipa bungeana*, whereas few *P. tabuliformis*, *Juniperus rigida* and *Platycladus orientalis* are distributed on hill slope.

*Hippophae rhamnoides* (sea buckthorn) is a small tree or shrub with the height of 1–5 m. It often inhabits in hill ridge, valley, dry river bed or slope with rock, sandy loam or loam, which distributes in Hebei, Inner Mongolia, Shanxi, Shaanxi, Gansu, Qinghai and West Sichuan. This shrub was widely used on Loess Plateau as a soil and water conservation species [21]. It grows fast, resistant to drought and could fix nitrogen.

*Pinus tabuliformis* (Chinese pine) is an evergreen tree with the height of 25 m and the diameter at breast height of 1 m. It is the dominant species in coniferous forest with elevation varies from 100 to 2600 m, which distributes in south Jilin, Liaoning, Hebei, Henan, Shandong, Shanxi, Inner Mongolia, Shaanxi, Ningxia, Qinghai and Sichuan. It is used as a sand binding and soil and water conservation trees in north China [22].

*Armeniaca sibirica* (wild apricot) is a small tree or shrub with the height of 2–5 m. It mainly inhabits on dry slope, hill steppe or mixed with deciduous forest, which distributes in Heilongjiang, Jilin, Liaoning, Inner Mongolia, Gansu, Hebei and Shanxi, and also in East and Southeast Mongolia, Far East and Siberia of Russia [23]. It is cold-resistant and was used as a soil and water conservation species in Northeast and North China.

In the study site, three woody species was planted in 2013 with density of 1000 plants per hectare. The mean height of *H. rhamnoides*, *P. tabuliformis* and *A. sibirica* were 2.10, 2.14 and 2.71 m, respectively. The main water source of three woody species was measured by comparing their xylem water with different water sources, e.g. rainwater and soil water in 10, 25, 50, 75 and 100 cm. Lignified, two years old twigs of shrubs were collected and the bark was removed with scissor, then the xylem was sampled. Soil and xylem were placed in 8 mL glass vial, sealed with Parafilm, and stored in a medical cool box. Water in soil and xylem was vacuum-extracted and their δ 18O value was measured with a Flash 2000HT elemental analyzer and a Finnigan MAT 253 mass spectrometer. Meteorological data was obtained from Meteorology Bureau of Dongsheng District, Ordos City, which is 50 km west to the study site. The contribution of different water sources to their total water use was analyzed by Iso-source 1.3.1 software [24]. The input data of the model were δ 18O value of xylem water and potential water source, e.g. soil water in different depths or rain water. The results of water use ratio to different sources were expressed as mean ± SD.

#### **2.2 Precipitation of study site in the growing season of 2018**

The total precipitation of Dongsheng District was 385.00 mm in the growing season of 2018 (**Figure 1**), which was slightly lower than annual mean precipitation of 400 mm. The maximal daily precipitation was 52.0 mm and occurred on May 19, the next value was 48.8 mm on July 19. The monthly precipitation were 32.0, 77.9, 15.5, 118.3, 85.0 and 54.3 mm from April to September.

#### **2.3 Stable oxygen isotope of xylem water of three woody species, soil water and rainwater on loess slope**

On May 12, stable oxygen isotope ratio of *H. rhamnoides* and *P. tabuliformis* xylem water were closer to soil water in 10 cm, whereas stable oxygen isotope ratio of *A. sibirica* xylem water was closer to soil water in 10–25 cm. On July 15, stable oxygen isotope ratio of 10–25 cm soil water was closer to rainwater. Stable oxygen isotope

#### **Figure 1.**

*Daily precipitation of study site from April to September of 2018.*

ratio of *H. rhamnoides*, *P. tabuliformis* and *A. sibirica* xylem water were closer to soil water in 10–100 cm, 25–50 cm and 25–100 cm, respectively; moreover, stable oxygen isotope ratio of *P. tabuliformis* and *A. sibirica* xylem water were also closer to rainwater. On September 23, stable oxygen isotope ratio of *H. rhamnoides* xylem water was closer to soil water in 25–100 cm, whereas stable oxygen isotope ratio of xylem water of other two woody species were closer to soil water in 10–100 cm (**Figure 2**).

#### **2.4 Contribution of different depth of soil water and rainwater to the water source of three woody species on loess slope**

Iso-source analysis showed that *H. rhmnoides* mainly used 10 cm soil water on May 12, which accounted for 88.5% of its total water source. On July 15, it mainly used 10–25 cm soil water and rainwater, which accounted for 44.6% and 35.4% of its total water source. On September 23, it mainly used 25 cm and 75–100 cm soil water, which accounted for 88.9% of its total water source (**Table 1**).

Iso-source analysis showed that *P. tabuliformis* mainly used 10 cm soil water on May 12, which accounted for 94.0% of its total water source. On July 15, it mainly used rainwater, which accounted for 93.7% of its total water source. On September 23, it mainly used 10 cm and 50–75 cm soil water, which accounted for 84.5% of its total water source (**Table 2**).

Iso-source analysis showed that *A. sibirica* mainly used 10 cm soil water on May 12, which accounted for 91.6% of its total water source. On July 15, it mainly used 25–100 cm soil water and rainwater, which accounted for 55.9% and 36.8% of its total water source. On September 23, it evenly used 10–100 cm soil water (**Table 3**).

Three woody species on loess slope selected different water source in the growing season, which is an adaptive strategy to semi-arid environment. They mainly used shallow soil water recharged by spring rain. However, there are interspecific difference of water source in summer. For *H. rhmnoides*, it mainly used shallow soil water and rainwater. For *P. tabuliformis*, it mainly used rainwater. However, for *A. sibirica*, it mainly used middle and deep soil water and rainwater. In autumn, *H. rhmnoides* mainly used shallow and deep soil water, *P. tabuliformis* mainly used shallow and middle soil water. However, *A. sibirica* evenly used different depths soil water. The interspecific difference in water source of three woody species is related

*Water Source of Six Woody Plants in Different Habitats on Desertified Land of Ordos Plateau… DOI: http://dx.doi.org/10.5772/intechopen.99658*

#### **Figure 2.**

*Stable oxygen isotope ratio of xylem water, soil water and rainwater of Hippophae rhamnoides (A), Pinus tabuliformis (B) and Armeniaca sibirica (C) in hilly gully area. Dark symbol is soil water, white symbol is xylem water of three woody species.*

to their life form and root distribution. Other shrubs on Loess Plateau also changed their water source. For example, *Caragana korshinskii* and *Salix psammophila* used 40–80 cm soil water in the drought season. However, they mainly used 0–40 cm soil water in the rain season [25]. *Armeniaca sibirica* in the mixed plantation used more shallow soil water than that in the pure plantation. However, *Robinia pseudoacacia* always used shallow and middle soil water in different plantation types [26].


#### **Table 1.**

*Contribution of different depths of soil water and rainwater to the water source of Hippophae rhamnoides in hilly gully area (%, mean ± SD).*


#### **Table 2.**

*Contribution of different depths of soil water and rainwater to the water source of Pinus tabuliformis in hilly gully area (%, mean ± SD).*


#### **Table 3.**

*Contribution of different depths of soil water and rainwater to the water source of Armeniaca sibirica in hilly gully area (%, mean ± SD).*

Moreover, the 18-yr *Robinia pseudoacacia* were sensitive to precipitation variation and used more deep soil water in a drier year, whereas the 30-yr *R. pseudoacacia* always used middle and deep soil water in wetter or drier year in the central region of Loess Plateau [27]. Therefore, woody species which could use deep soil water as a reliable water source during drought will have advantage in semi-arid region.
