Dried Earth Layers of Artificial Forestland in the Loess Plateau of Shaanxi Province

DOI: 10.1007/s11442-007-0001-5

Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province

ZHAO Jingbo1,2, DU Juan1, CHEN Baoqun1, YUE Yingli1, YU Yaochuang1

1. College of Tourism and Environment, Shaanxi Normal University, Xi'an 710062, China; 2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment CAS, Xi’an 710075, China

Abstract: By determining the earth moisture content of artificial forestland between 0 and 6 m deep in the Loess Plateau of Shaanxi province, the vertical change of moisture content, distribution and formation causes of a dried earth layer are researched. The results show that the average moisture content is 9.3%–9.5% between 2 and 4 m under artificial forest of over 10 year’s growth in Guanzhong Plain, and chronic weak dried earth layers are developed which show that the dried earth layers are distributed extensively on the Loess Plateau. The southern boundary of the dried earth layer has reached the northern foot of the Qinling Mountains. When precipitation reaches 600 mm, there are weak dried earth layers between 2 and 4 m under artificial forest of more than 10 years old. When the precipitation is between 400 and 500 mm, there are moderate dried earth layers. When precipitation is above 800 mm, there are no dried earth layers. There are no dried earth layers under meadow land, corn land and less than 5 years old of artificial forestland in central and southern parts of the Loess Plateau. The development of dried earth layers under cypress forest is weaker than broad-leaved forest. Under the same climatic conditions, the development of dried earth layers under the loess tableland is nearly at the same level as the 2nd and 3rd river terrace. Dried earth layers developed in membrane water zone, and the buried depth is small and motion velocity is slow in the Loess Plateau, which is the direct water factor of the formation of the dried earth layer, while differences of tree age and tree species are the plant factors that consumed much moisture. From the depth of the gravity water and the membrane water in Guanzhong Plain, it is clear that the formation cause of dried earth layers is mainly due to natural factors. The dried layers generally develop in middle-aged artificial forestland that consumed too much moisture, which is the general character of earth moisture in subhumid and semiarid zones. The appearance of dried layers doesn’t show that the forest doesn’t develop in this area; this is depended on their development intensity. Artificial forest of Chinese poplar, locust tree and Chinese scholartree consuming less water can be planted in the areas where dried earth layer developed weakly, but can not be planted in the areas where dried earth layer developed intensely. Keywords: Loess Plateau; dried earth layer; effective factors; cause of formation; soil moisture zone

Received: 2006-06-19 Accepted: 2006-10-25 Foundation: National Natural Science Foundation of China, No.40571004; Project of State Key Laboratory of Loess and Quaternary Geology, CAS, No.SKLLQG0606

Author: Zhao Jingbo (1953-), Ph.D. and Professor, specialized in physical geography. E-mail: [email protected] 转载 www.scichina.com www.springerlink.com 中国科技论文在线 http://www.paper.edu.cn 2 Journal of Geographical Sciences

1 Introduction Abroad, a lot of research on soil water removal, seeping and evaporation has been done (Ole, 1998; Rapp, 2000). A seeping model of soil water was suggested (Yuin, 1998) and it was known that water removal is very slow in the unsaturated zone (Bond et al., 1990; Jaynes et al., 1995; Padilla et al., 1999). Although dried earth layers occur in dried areas abroad, not many researches have been done. About the earth moisture content and its change in the Loess Plateau, many research achievements have been obtained. Now it has been found out that young artificial woods less than 5 years old grow well in the Loess Plateau but the woods about 10 years old grow badly, and crooked, short and old trees occur (Yang, 1998; Hou et al., 2000). The research has shown this to be the consequence of the dried earth layer caused by serious lack of earth moisture (Wang, 1995; Yang, 2000). Though a lot of research was carried on the earth moisture of artificial vegetation in the northern Loess Plateau (Fu et al., 2002; Wang et al., 2004), less has been carried out in the central and southern parts. Effect of landform on earth moisture content was conducted in loess hilly area in the past, but difference of earth moisture content between loess tableland and river terraces has not been done, change of earth moisture content from north to south and effect of membrane water on dried earth layer has not been researched. It was not known that if there is a dried earth layer in artificial forestland in Guanzhong Plain. Because natural forest is a little and artificial forest is wide in the Loess Plateau, we research earth moisture content and its change under artificial forest. By determining earth moisture content in Guanzhong Plain and the Loess Plateau in Shaanxi, progress has been made on studies of the characteristics of dried earth layers, their distribution, influencing factors and the relations between the dried earth layer, gravity water and membrane water.

2 Sites, sampling and research method The 13 research sites (Figure 1) were chosen, namely Wujiafen in south suburb of Xi’an, Shangying in Wugong, Pangxicun in Xianyang, Yangcun in Xingping, Tianjia in Gaoling, Yuejiagou in south suburb of Xi’an, 4 km away from Xi’an, and plants are 16 years old apple orchards. Shangyin is 16 km away from Wugong County, and plants are 10 years old poplar forest. Yangcun is about 8 km away from Xingping County, and plants are 18 years old phoenix forest. Pangxicun is 10 km away from Xianyang County, and has 10 years old apple orchards. Tianjiacun is 1.5 km away from Gaoling, where phoenix forest is 14 years old. Yuejiagou lies in the west of Lintong, and has 17 years old poplar forest. The above 4 sites are located on the 2nd and 3rd terraces of Weihe river. Mengcun lies in the loess tableland, about 10 km away from northwest of Lantian County, and plants are 12 years old poplar forest and phoenix forest. The spacing of the plants is between 3 and 4 m. Luochuan is 3 km away to the east of Luochuan County, and plants are 14 years old and 5 years old apple orchards. Jiaodao is 6 km away to the southeast of Fuxian County, and plants are 16 years old apple orchards. Longfang is 15 km away to the northwest of Huangling County, and plants are 14 years old apple orchards. The above 3 sites are located on the loess tableland, and the plants spacing is about 3 m. Sites of Yan’an lie in the south suburb of Yan’an city, and plants are 17 years old locust forest, 17 years old 中国科技论文在线 http://www.paper.edu.cn ZHAO Jingbo et al.: Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province 3

Figure 1 Location of the studied profiles

cypress forest and 20 years old pear forest. The plants’ spacing is about 3 m. Sunjiayuan is 15 km away to the south of Yanchuan County, and plants are 18 years old apple orchards and 10 years old Chinese scholartree. The plants grow badly and spacing of them is about 4 m. Hejiayuan is 6 km away to the southeast of Yanchang County, and plants are 15 years old apple orchards. The plants spacing is about 4 m. The top soil of the above mentioned sites is covered by homogeneous Malan loess. Apple tree, Chinese phoenix and Chinese scholartree grow well with mean annual precipitation of 600–750 mm, Chinese poplar trees grow well with mean annual precipitation of 500–700 mm, they consume less moisture when they are less than 5 years old and consume more moisture when they are 10 years old or over. Cypress and pear (a drought-enduring species) grow well with mean annual precipitation of 400–600 mm, and grow slowly and consume all along less moisture. In order to find out effect of climate, tree age, tree species and landform on earth moisture content, we research earth moisture content in different areas, under different artificial forests and on loess tableland, loess ridge and river terraces covered with thick loess. To ascertain effect of membrane water on earth moisture content and change of earth moisture content from north to south in the Loess Plateau, earth moisture content of representative sites from southern Guangzhong Plain to northern Jingbian area are researched (Figure 1). Samples were collected by utilizing lightweight manpower drill, the hole is 6 m deep, at a sample spacing of 10 cm. Moisture content was determined by the drying method which is considered to be accurate. Drying temperature is 105℃, and drying time is more than 24 hours. Before and after drying, samples were weighed by electronic balance.

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3 The determined result of earth moisture content of artificial forestland 3.1 Earth moisture content of artificial forestland under different climates

To find out the influence of different climates on the dried earth layer, the earth moisture content under artificial forestland in Jingbian County, Yan’an, Hejiayuan and Xi’an was determined. The precipitation of Jingbian is about 400 mm, and of Yan’an is about 500 mm, and of Xi’an is 600 mm in normal year and is 880 mm in rainy year. In each site, samples of 2 to 4 profiles are determined. The results of 3 profiles under poplar forest in Jingbian show that the change of earth moisture content of hole a can be divided into 3 layers. The 1st layer is between 0 and 190 cm where the moisture content is relatively high, ranging between 7.2% and 12.45% and with an average of 8.8%. The 2nd layer is between 200 cm and 400 cm, the moisture content is low and changes between 5.3% and 7.0%, averaging 6.2%. In the 3rd layer, between 410 cm and 600 cm, the moisture content increases appreciably, variation range is between 6.3% and 7.6% with an average of 6.9% (Figure 2a). The moisture content of hole b (Figure 2b) is basically the same as hole a. The results of 3 profiles under poplar forest in Yan’an show that the change of moisture content of hole a can be divided into 3 layers too. The 1st layer is between 0 and 190 cm, the moisture content is relatively high, variation range is great, being 7.8%–13.2% and the average is 9.2%; the 2nd layer is between 200 cm and 400 cm, the moisture content is low, variation range is small, between 7.3% and 9.2%, averaging 8.0%; the 3rd layer is between 410 cm and 600 cm, the moisture content is between 7.9% and 11.2% and the average is 9.6% (Figure 2c). The moisture content of holes b and c is basically the same as hole a. The moisture content of 3 profiles under poplar forest of Hejiayuan in Yanchang is basically the same as in Yan’an, where the precipitation is basically the same, and the average moisture content between 200 cm and 400 cm is 7.4%. The average moisture content between 0 cm and 200 cm, 200 cm and 400 cm, and 410 cm and 600 cm is 6.3%, 6.3% and 8.1% respectively of Sunjiayuan in Yanchuan County, where the precipitation is about 500 mm, and the development of dried earth layer is stronger than Yan’an. The results of 2 profiles under poplar forest in 2002 (normal year) in Xi’an show that the change of earth moisture content of hole a can be divided into 3 layers. The 1st layer is between 0 and 190 cm, the moisture content is relatively high, variation range is great and between 9.2% and 13.6% and the average is 11.0%; the 2nd layer is between 200 cm and 400 cm, the moisture content is the lowest, which is between 7.5% and 10.7% and the average is 8.9% (Figure 2d); the 3rd layer is between 410 cm and 600 cm, the moisture content increases appreciably, variation range is between 8.6% and 14.8% and the average is 12.1%. The moisture content of hole b is basically the same as hole a. The results of 4 profiles under 15 years old poplar forest in 2003 (rainy year) in Xi’an show that the change of moisture content of hole a can be divided into three layers (Figure 2e). The 1st layer is between 0 and 190 cm, the moisture content is between 15.1% and 20.6% and the average is 17.2%; the 2nd layer is between 200 cm and 400 cm, the moisture content increases and is between 22.3% and 23.5% and the average is 21.7%; the 3rd layer is between 410 cm and 600 cm, the moisture content decreases and is between 16.6% and 19.1% and the average is 18.4%. The average moisture content of the other 3 profiles is above 15%, and the average moisture content between 200 cm and 400 cm of phoenix forest is also above 15%, which indicates 中国科技论文在线 http://www.paper.edu.cn ZHAO Jingbo et al.: Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province 5

that dried earth layer did not develop under artificial forestland in Xi’an, when the precipitation is abundant.

Figure 2 Earth moisture content in artificial forestland under different climate in Shaanxi a, b. Moisture content in two profiles under 16 year old poplar forest in 2005 in northern Jingbian; c. moisture content under 15 year old poplar forest in 2005 in Yan’an; d. earth moisture content under 15 years old poplar forest in 2002 (normal year) in Wujiafen in Xi’an; e. earth moisture content under 15 years old poplar forest in 2003 (rainy year) in Wujiafen in Xi’an; 1. determined moisture content curve; 2. mean moisture content curve in each layer

3.2 Earth moisture content of artificial forestland in different landforms

To find out the influence of different landforms on dried earth layer, the earth moisture content of hills in Yan’an, loess tableland in Xi’an, the 2nd terrace in Xi’an and Lintong, and the 3rd terrace in Gaoling were determined. The results of earth moisture content under locust forest on loess ridge in Yan’an show that change of earth moisture content on sunny slope of hole a can be divided into 3 layers. The 1st layer is between 0 and 190 cm, the moisture content is low and variation range is great, which is between 5.9% and 8.0% and the average is 7.5%. The 2nd layer is between 200 cm and 400 cm, moisture content is middling and variation range is small, which is between 6.8% and 7.0% and the average is 6.2% (Figure 3a). The 3rd layer is between 410 cm and 600 cm, the moisture content is the highest, variation range is small and between 8.2% and 10.2% and the average is 9.3%. The change of moisture content on sunny slope of holes b and c is the same as hole a. The change of earth moisture content under 15 years old locust forest on shady slope of hole a in Yan’an can be divided into 3 layers too. The 1st layer is 0 and 200 cm, the moisture content is low and variation range is 7.6%–10.2% and the average is 9.2%; the 2nd layer is between 210 cm and 400 cm, the moisture content is between 8.2% and 10.3% and the average is 9.6% (Figure 3b); the 3rd layer is 410–600 cm, the moisture content is the highest and is between 10.1% and 12.2% and the average is 11.5%. The change of earth moisture content on shady slope of holes b and c is slightly higher than hole a. Samples of 2 holes in Mengcun in Lantian of Bailu tableland was sampled under 12 years old phoenix forest in March 2003 (rainfall before rainy year). The results of 116 samples show that moisture content change of hole a can be divided into 3 layers (Figure 3c). The 1st layer is between 0 and 180 cm, the moisture content is high and is between 9.6% and 13.2% and the average is 10.1%; the 2nd layer is between 180 cm and 360 cm, the moisture content is between 8.3% and 10.8% and the average is 9.5%; the 3rd layer is 370–600 cm, the moisture content increases changes between 11.2% and 15.1% and the average is 13.7%.

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Figure 3 Earth moisture content in artificial forestland under different morphologies in Shaanxi a, b. Earth moisture content on sunny and shady slopes of loess ridge under 15 years old locust forest in Yan’an; c. moisture content of 12 years old phoenix forest on Bailu tableland of loess in Lantian; d. moisture content under 16 years old phoenix forest on second terrace in Lintong; e. moisture content under 12 years old phoenix forest on second terrace in Xianyang; 1. determined moisture content curve; 2. mean moisture content curve in each layer

Samples of 2 holes under 16 years old phoenix forest on the 2nd terrace at Yuejiagou in Lintong was sampled in March 2003. The results of 60 samples show that the moisture content change of hole a can be divided into 3 layers (Figure 3d). The 1st layer is between 0 and 180 cm, the moisture content is 9.2%–13.2%, the average is 10.4%; the 2nd layer is 190–360 cm, the moisture content is lower than the 1st layer, and changes is between 8.4% and 10.7% and the average is 9.4%; the 3rd layer is 370–600 cm, the moisture content increases and changes between 10.2% and 14.7% and the average is 13.2%. Moisture content and its change (Figure 3e) of 2 holes under 12 years old phoenix forest on 2nd terrace in 2002 in Xianyang are similar to that on the 2nd terrace at Yuejiagou in Lintong. Samples of 2 holes under 14 years old phoenix forest on third terrace at Tianjiacun in Gaoling was sampled in February 2002, the results are similar to that show that, from 0 cm to 180 cm, the moisture content is 8.1%–12.8% and the average is 10.4%; from 190 cm to 400 cm, the moisture content is 8.5%–9.7% and the average number is 9.3%; from 410 cm to 600 cm, the moisture content is 10.7%–14.4 % and the average number is 13.3%.

3.3 Earth moisture content in different artificial vegetation land

Samples of 2 holes of 12 years old poplar forest in Mengcun in Lantian were sampled in March 2003.The results of 120 samples show that the change of moisture content of 12 years old poplar forest can be divided into 3 layers The 1st layer is 0–180 cm, the earth moisture content is 9.9%–13.2% and the average is 11.3%; the 2nd layer is 190–360 cm, the moisture content changes between 8.6% and 10.8% and the average is 9.4%; the 3rd layer is 370–600 cm, the moisture content increases with a variation range of 9.6%–14.5% and the average is 13.2%. The difference of moisture content between 12 years old poplar forest and 12 years old phoenix forest (Figure 3c) here is inconspicuous in Mengcun on Bailu tableland. Meadow samples of 2 holes in Pangxicun were sampled in March 2003 and got 58 samples in each profile. The results show that moisture content of hole a is high. From 0 cm to 100 cm, the moisture content is 10.4%–14.2%, the average is 12.3%; from 100 cm to 400 cm, the moisture content increases with a variation range of 10.6%–14.8%, the average is 13.2%; from 400 cm to 600 cm, the moisture content is 13.2%–16.3%, the average is 14.5%. The analyzed results of hole b is basically the same as hole a. In addition, 60 samples under 中国科技论文在线 http://www.paper.edu.cn ZHAO Jingbo et al.: Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province 7

corn field in Dongshicun in Xianyang were determined. The results are basically the same as that of Pangxicun, and there is also no dried soil layer. Samples of 2 holes of 17 years old locust in the south of Yan’an were sampled in March 2004. The results of 60 samples of hole a show that change of earth moisture content of 17 years old locust can be divided into 3 layers (Figure 4a). The 1st layer is 0–190 cm, the moisture content is high with a variation range of 8.9%–33.6% and the average is 15.2%, the moisture content of the upper layer is higher than the nether layer. The 2nd layer is 200–400 cm, the moisture content decreases obviously with a variation range of 5.4%–7.7% and the average is 7.3%. The 3rd layer is between 410 and 600 cm, the moisture content increases slightly with a variation range of 6.2%–10.1% and the average is 7.6%, and the dried earth layer is severe. The moisture content of hole b is basically the same as hole a (Figure 4b).

Figure 4 Earth moisture content under different artificial forestland in southern Yan’an city a, b. Earth moisture content in two sections under 17 years old locust forest; c, d. earth moisture content in two sections under 17 years old cypress forest; e. earth moisture content under 20 years old pear forest; 1. determined moisture content curve; 2. mean moisture content curve in each layer

Samples of 2 holes under 17 years old cypress forest were sampled in March 2004 in Yan’an and 120 samples were obtained. The results of hole a show that the change of moisture content of cypress forest can be obviously divided into 2 layers (Figure 4c). But in order to compare with other profiles, we still divide it into 3 layers. The 1st layer is 0–190 cm, the moisture content is high with a variation range of 9.8%–18.5% and the average is 12.2%, the moisture content of the upper layer is higher than the nether layer. The 2nd layer is 200–400 cm, the moisture content is 5.4%–7.2%, the average is 8.0%, and its variation range is small. The 3rd layer is 410–600 cm, the moisture content is 7.2%–9.3% and the average is 8.3% (Figure 4c). Moisture content of hole b is basically the same as hole a (Figure 4d). Two holes under 20 years old pear forest were sampled the moisture content in 200–400 cm is 5.4%–7.2%, the average is 6.9% (Figure 4e). The difference of moisture content among tree species indicates that moisture content varies with different plants.

3.4 Earth moisture content of trees in different ages

The results of 3 profiles of 5 years old apple orchards in southern Luochuan show that the change of moisture content of hole a can be divided into 3 layers (Figure 5a). The 1st layer is 0–190 cm, the earth moisture content is 9.6%–14.7%, the average is 13.3%. The 2nd layer is 200–400 cm, the moisture content is slightly higher than that of the 1st layer, with a variation range of 11.8%–14.9% and the average is 13.5%. The 3rd layer is 410–600 cm, the

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moisture content is 13.7%–16.6% and the average is 15.3%. The moisture content of hole b is basically the same as hole a (Figure 5b). The results of 3 profiles of 10 years old apple orchards in Luochuan show that change of moisture content of hole a can be divided into 3 layers (Figure 5c). The 1st layer is 0–190 cm, the moisture content is high with a great variation range of 9.6%–14.7% and the average is 12.9%. The 2nd layer is 200–400 cm, the difference and variation of moisture content between the 1st and 2nd layers is small, and variation range is 11.8%–14.9%, the average is 12.2%. The 3rd layer is 410–600 cm, the moisture content increases with a range of 13.7%–16.6% and the average is 14.7%. The moisture content of hole b and hole c is basically the same as hole a.

Figure 5 Earth moisture content in different artificial forestland in Shaanxi a, b. Earth moisture content in two sections under 5 years old apple orchards in Luochuan; c. earth moisture content under 10 years old apple orchards in Luochuan; d. earth moisture content under 14 years old apple orchards in Luochuan; e. earth moisture content under 16 years old apple orchards in Fuxian; 1. determined moisture content curve; 2. mean moisture content curve in each layer

Samples of 3 holes of 14 years old apple orchards in Luochuan were sampled in February 2003. The results show that the change of earth moisture content of 14 years old apple orchards can be divided into 3 layers. The 1st layer is 0–180 cm, the moisture content is high with a variation range of 10.0%–13.0% and the average number is 11.7%. The 2nd layer is 190–400 cm, the moisture content is the lowest with a variation range of 10.2%–12.2% and the average is 10.9%. The 3rd layer is 410–600 cm, the moisture content is the highest, but there is definite fluctuation, variation range is 12.8%–14.6% and the average is 13.7% (Figure 5d). The moisture content results of 3 holes under 16 years old apple orchards in Jiaodao in Fuxian show that the moisture content and layers are nearly the same as 14 years old apple orchards in Luochuan (Figure 5e). Three profiles under 14 years old apple orchards in Longfang in Huangling are sampled, the results show that, from 0 cm to 180 cm, 190 cm to 400 cm, and 410 cm to 600 cm, the average moisture content is 11.9%, 10.8% and 13.9% respectively, which is close to the moisture content of apple orchards in Luochuan.

4 Discussion

4.1 The dried earth layer in the Guanzhong Plain

Many researches (Wang et al., 2000; Fu et al., 2002; Wang et al. 2003; Zhang et al., 2003) have been carried out about dried earth layer in the Loess Plateau. It is generally acknowledged that the dried earth layer would develop when its moisture content is lower 中国科技论文在线 http://www.paper.edu.cn ZHAO Jingbo et al.: Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province 9

than 12% (Wang et al., 2000), and strongly dried earth layer has a moisture content less than 6%, moderately dried earth layer is of a moisture content between 9% and 12%, and lightly dried earth layer has a moisture content less than 6% (Wang et al., 2000). As top 2 m soil layer is apt to get the supplement of atmosphere precipitation in a rainy season, the top earth layers becoming dry is temporary and usually not named as dried earth layer. The earth moisture under 2 m is likely to be less influenced by evaporation and is supplied very slowly (Wang et al., 2003) and hard to be replenished. This is the real dried earth layer. According to the above analysis and combining the moisture content of artificial forestland of seven sites (Figures 2 and 3), it is known that the moisture content of artificial forestland of 10 years old and over at 190–400 cm deep changes between 9.2% and 9.5%. It is clear that the layer has become a dried earth layer. As dried earth layers have developed in all of the seven studied sites, it is believed that the dried earth layer is ubiquitous in the artificial broad-leaved forestland in Guanzhong Plain. The establishment of dried earth layer in this area is very important to understand the formation of dried earth layer. As the artificial forest of Chinese poplar, locust and Chinese scholartree grows well in Xi’an, it is known that the trees consuming less water can be planted in the areas where dried layer developed lightly.

4.2 Control function of climate on the development of dried earth layer

Dried earth layers, by our definition, have a moisture content of less than 12% and precipitation is the decisive factor. On the basis of the standard of the dried earth layer and the above determined results of moisture content, it is known that the moisture content between 2 m and 4 m under poplar forest is about 6.2% in Jingbian where the annual precipitation is only 400 mm, so the dried earth layers are strong and middling; the moisture content under poplar forest is about 8% in Yan’an area where the annual precipitation is 500 mm, so the dried earth layer is middling; the moisture content under poplar forest is slightly over 9.2% in Xi’an where the normal annual precipitation is 600 mm, so the dried earth layer is weak (Figures 2 and 6). In the rainy year of 2003, the annual precipitation was 880 mm in Xi’an and 800 mm in Xianyang, the dried earth layer under poplar forest and phoenix tree disappeared. This shows that no dried earth layer develops when the precipitation exceeds 800 mm; the dried earth layer develops when the precipitation is less than 600 mm.

Figure 6 Earth moisture content under different climate and vegetation conditions on the Loess Plateau

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a. Mean moisture content under 20 years old pear forest in southern Yan’an; b. mean moisture content under 18 years old apple orchard in Sunjiayuan in Yanchuan; c. mean earth moisture content in 2 profiles under 17 years old locust forest in southern Yan’an; d. mean earth moisture content in 2 profiles under 17 years old cypress forest in southern Yan’an; e. mean moisture content in maize land in Sunjiayuan in Yanchuan; f. mean moisture content in 3 profiles under 16 years old poplar forest in northern Jingbian; g. mean moisture content in 3 profiles under 15 years old poplar forest in southern Yan’an; h. mean moisture content in 3 profiles under 15 years old poplar forest in normally raining year in Wujiafen in Xi’an; i. mean moisture content in 3 sections under 15 years old poplar trees in rainy year in Wujiafen; j. mean moisture content in 3 profiles under 15 years old phoenix forest in Wujiafen in Xi’an; 1. moisture content in 2–4 m deep; 2. moisture content in 4–6 m deep

Climate affects not only the development strength, but also the depth of dried earth layer. The depth of the dried earth layer is 6 m or more in Jingbian and north of Yan’an, where precipitation is 400–550 mm. The depth of dried earth layer is less than 4 m in Guanzhong Plain, where precipitation is about 600 mm. So the vertical extent of the dried layer is inversely related to precipitation.

4.3 Control function of arbor species and tree ages on dried earth layer

As earth moisture varies between tree species, the tree species play an important part in the growth and formation of the dried earth layer in the same area. It is generally agreed that the order that the tree consumed moisture content of artificial forest in this area is: cypress

4.4 Influence of landform on the development of dried earth layer

Landform influences the infiltration and evaporation of precipitation, thus influencing dried earth layers. In the loess hilly area of Yan’an, the difference of earth moisture content between sunny slopes and shady sides in a loess ridge is significant. Moisture content under 中国科技论文在线 http://www.paper.edu.cn ZHAO Jingbo et al.: Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province 11

silver chain in shady sides is 2% higher than sunny slope, this is because landform influences evaporation. In the loess tableland, on the 2nd and 3rd terraces with thick loess, the landform is wide and flat, and their infiltration conditions are similar, so the difference of moisture content between them is small, and the dried soil layer is weak.

4.5 Influence of gravity water and membrane water on dried earth layer

Many factors cause the formation of the dried earth layer (Zhang et al., 2003; Wang et al., 2004), including natural factors and human activity. The groundwater is divided generally into four zones (Table 1). The uppermost is a gravity water zone, and the next is capillary water zone, and then is membrane water zone and the bottommost is groundwater zone (Li et al., 1983; Xiong et al., 1987; Zhao, 1995, 2002). The depth of capillary water zone is small, and exists with gravity water zone, so they can be put together. From research and field observations of chemical composition, it is known that the deposition and accumulation

bottom of CaCO3 is also the bottom of the capillary water-gravity water zone (Hou et al., 2000; Wang et al., 2004). We learn from table 1 that the dried earth layer is caused by natural factor of the small depth of gravity water zone, which is determined by low rainfall. The high density of artificial forest, fast growing plant species, and small influent seepage of rainfall can also help cause the dried earth layer.

Table 1 Zones of groundwater on the Loess Plateau and distribution of dried earth layer Division zone of Moving speed of moisture and Name of each zone Moving direction Depth/m groundwater content Water moves fast and its Gravity water zone Ⅰ Down content change is great Loss zone of membrane 2.0 Down in the upper and Water moves slowly and its water (dried earth Ⅱ 1 up in the lower content is less than 10% layer zone) Normal membrane Waters moves slowly and its Ⅱ Down water zone 2 content usually is under 15% 4.0 Capillary zone moving Moisture moves fast and its Ⅲ Up upward content is more Groundwater zone Ⅳ Fluctuating change 60.0 Water moves fast and is saturated

However, these are not the determinant factors. The water in the gravity water zone is characterized by high moisture content and fast movement. Rainwater can reach the bottom of the zone in one year. Under different rainfall conditions, the depth of this zone is different. With the increasing rainfall, the range of it increases. The depth of this zone in the Loess Plateau is usually less than 2 m because of low rainfall, and cannot provide gravity water to the dried layer distributed below 2 m, so this layer looses the important fast-supply water source, and water content is low, which is the main reason for the formation of dried earth layer in the Loess Plateau. Tree roots usually reach below 2 m, so the dried earth layer appears generally when the artificial forest consumes lots of water in 2–4 m. Natural forests are no different. For example, the dried earth layer appears under natural Liaodong robur forest in Ziwu Mountains and Huanglong mountain area (Yang et al., 2000). Depth of 2–4 m is a part of membrane water zone (Table 1), which is connect with the formation of dried earth layer directly. Membrane water, the liquid state water that adheres

中国科技论文在线 http://www.paper.edu.cn 12 Journal of Geographical Sciences

to the earth grains, has two characteristics. One is the low moisture content in the earth layer, the other is the very slow movement. Because the membrane water adheres to the earth grains, it moves from thick water membrane to thin water membrane with very slow speed (Li et al., 1983). The membrane water is available for the vegetation to grow. However, the supplying speed of membrane water is too slow and its content is too low to meet the growth of trees. When the concentrated tree roots get to the membrane water zone and absorb the moisture, the lost moisture can’t be replenished because the membrane water cannot re-supply fast enough and the gravity water does not reach to the layer. So the dried earth layer appears. The membrane water zone shows that the natural factor is the ultimate cause of the formation of the dried earth layer in the Loess Plateau. It is important to realize that natural factors are the dominant causes of the dried earth layer on the Loess Plateau. The appearance of dried earth layer does not show that the forest cannot develop in this area, but its growth is restricted and depends on the development intensity of dried layers. Trees can be planted in areas where dried layer developed slightly, but unsuitable to be planted in areas where dried layer developed intensely.

5 Conclusions The following conclusions can be drawn from the above discussion: (1) Climate has an important control function on dried earth layer. When the precipitation reaches 600 mm, there are weak dried earth layers between 2 and 4 m under artificial forests of 10 or more years. When the precipitation is between 400 and 500 mm, there are moderate dried earth layers. When the precipitation is above 800 mm, there are no dried earth layers. (2) Evidence indicates that the dried earth layer has reached the northern foot of Qinling Mountains and is ubiquity in this area. (3) There are no dried earth layers under meadow land, corn land and young artificial forestland in central and southern parts of the Loess Plateau and its development under cypress land is weaker than under broad-leaved forest. Dried earth layers develop as forests mature. (4) Under the same climatic conditions, the development of dried earth layers under loess tableland is the same on the 2nd and 3rd river terraces; on shady slope of loess ridge, the development of dried earth layers is weaker than sunny slope because of weak evaporation. (5) The development of dried earth layer on the Loess Plateau is mainly caused by natural factor that the buried depth of gravity water zone is small, which is mainly determined by less precipitation. The dried earth layer develops in membrane water zone, buried depth of which is small, motion velocity is slow, and this is the direct water factor of the formation of dried earth layer. The age of the forests and tree species is the plant factor of it. (6) The appearance of dried earth layer doesn’t show that the forest cannot develop in this area, but that the development of forest is not good. Trees consuming less water can be planted in the areas where the dried layer is poorly developed, but not suitable to be planted in the areas where dried layer developed intensely. Artificial forest of Chinese poplar, locust tree and Chinese scholartree consuming less water would be developed. 中国科技论文在线 http://www.paper.edu.cn ZHAO Jingbo et al.: Dried earth layers of artificial forestland in the Loess Plateau of Shaanxi Province 13

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