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«Effects of terracing on soil water and canopy transpiration of Chinese pine plantation in the Loess Plateau, China Handan Zhang1,2, Wei Wei1*, Liding ...»

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Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-223, 2016

Manuscript under review for journal Hydrol. Earth Syst. Sci.

Published: 18 May 2016

c Author(s) 2016. CC-BY 3.0 License.

Effects of terracing on soil water and canopy

transpiration of Chinese pine plantation in the Loess

Plateau, China

Handan Zhang1,2, Wei Wei1*, Liding Chen1, Lixin Wang3

1

State Key Laboratory of Urban and Regional Ecology, Research Center for

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.

2 University of Chinese Academy of Sciences, Beijing 100049, China.

3 Department of Earth Sciences, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis 46202, United States *Corresponding author: Wei Wei (weiwei@rcees.ac.cn), Tel.: +86-10-6291-8673, Fax: +86-10-6291-3840.

1 Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-223, 2016 Manuscript under review for journal Hydrol. Earth Syst. Sci.

Published: 18 May 2016 c Author(s) 2016. CC-BY 3.0 License.

1 Abstract 2 Terracing has long been considered one of the most effective measures for soil water 3 conservation and site improvement. However, the quantitative effects of terracing on soil water 4 dynamics and vegetation water use have not been reported. To fill these knowledge gaps, in this 5 study, soil water content and canopy transpiration were monitored in both terrace and slope 6 environments in the semiarid Loess Plateau of China in 2014 and 2015. Results showed that 7 terracing increased soil water content of different soil layers. Mean soil water content of the 8 terrace site was 25.4% and 13.7% higher than that in the slope site in 2014 and 2015, and canopy 9 transpiration at the terrace site increased by 9.1% and 4.8%, respectively. Canopy conductance at 10 the terrace site was 3.9% higher than that at the slope site and it decreased logarithmically with 11 vapor pressure deficit. This study highlighted the critical role of terracing in increasing the soil 12 water content and mitigating water stress in semiarid environments. Thus, terracing has the 13 potential to enhance sustainable vegetation restoration in water-limited regions.

14 Keywords: terracing; sap flux density; canopy conductance; water stress; Loess Plateau 15 1 Introduction 16 Terraces constitute a crucial engineering measure to control erosion, raise crop yields, and 17 maintain sustainable agroforestry. By leveling hillslopes, terraces seek to create better planting 18 surfaces for mitigating water loss and conserving soil (LaFevor, 2014; Zhang et al., 2014).

19 Terracing has been established as the main measure for soil and water conservation for fields 20 with gradients under 25 degrees (Li et al., 2011; Li et al., 2012b). It has been determined that 21 terracing in such locations can reduce both flood runoff and the sediment transport modulus (Bai 22 et al., 2015; Li et al., 2014a), and that the soil water conditions can be improved noticeably

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24 Transpiration as an important role part of the soil-plant-atmosphere continuum (Newman et al., 25 2010) has considerable implications regarding forest management and water yields (Bosch et al., 26 2014; Brito et al., 2015; Chang et al., 2014b), especially in regions where transpiration is a 27 fundamental datum for understanding the ecophysiology of planted forests (Wang et al., 2012a).

28 It is also central to the construction of an ecosystem-level water balance (Yang et al., 2009). Sap 29 flow measurement can provide insights on environmental limitations and it yields results 30 comparable with the estimates of water use for entire forest ecosystems (Chen et al., 2014b;

31 Chirino et al., 2011; Du et al., 2011; Kim et al., 2014). Previous studies have shown that sap flow 32 characteristics vary with species and growth status, as well as with meteorological, 33 environmental, and edaphic features (Brito et al., 2015; Du et al., 2011). In areas with 34 insufficient water, soil water conditions can restrict many physiological processes (Li et al., 35 2014b). Plants in these areas tend to deepen and extend their root systems to exploit substantial 36 quantities of soil water for transpiration (Chen et al., 2014c; Limousin et al., 2009). Stomatal 37 closure as an important physiological process was employed by plants to regulate water use and 38 to prevent their hydraulic system from irreversible damage (Chirino et al., 2011). Sap flow 39 reduction caused by stomatal closure is considered to be the preliminary response of canopy 40 transpiration to water stress. Under water-sufficient conditions, differences in vapor pressure 41 deficit (VPD) determine the transpiration amount (Chen et al., 2014b). However, transpiration is

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43 that can be obtained from the soil. Soil water influences stand transpiration through the water 44 fluxes within the root zone and the percolation of soil profile caused by different rainfall regimes 45 (Chen et al., 2014c). Based on pot experiments, Cui (2012) concluded that sap flow rates 46 dropped 84.7% under severe water stress (5.33%) compared with that under non-stress (19.78%)

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49 The semiarid Loess Plateau region of China has experienced long-term serious soil erosion, 50 vegetation degradation, and water loss (Zhang et al., 2008). Intense soil erosion has resulted in 51 the decline of land productivity (under traditional agriculture) and environmental degradation 52 (Wang et al., 2010). Because of the depletion of soil moisture and water shortages, there are

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54 controlling erosion and conserving water resources, many investigations have been conducted 55 into a wide range of soil management practices, including structural, agronomic, and biological 56 measures (Jin et al., 2014; Yuan et al., 2016). Among these, terraces are a well-developed 57 structural practice. Unlike native plants, many introduced species of vegetation usually have 58 higher water demands (Chen et al., 2008; Yang et al., 2009). Thus, local soils have become 59 extremely dry in both deep and shallow layers, diminishing the expected positive effects of 60 afforestation in controlling soil erosion and improving the regional environment (Wang et al., 61 2012b; Yang et al., 2012). By analyzing four introduced plant species (Pinus tabulaeformis, 62 Robinia pseudoacacia, Caragana korshinskill and Hippophae rhamnoides), Jian et al. (2015) 63 drew the conclusion that in semiarid loess hilly areas, precipitation cannot meet the water loss 64 caused by evapotranspiration in slope-scale. However, few studies have considered the effects of 65 terracing on plant growth, nor its implications for regional ecological restoration.

66 This paired-site study focused on a small catchment in the western Loess Plateau of China to 67 examine the effects of terracing on the soil water content and canopy transpiration. Similarly 68 aged specimens of Chinese pine (P. tabulaeformis), being one of the main artificial plants in the 69 area, were planted in both terrace and slope plots. The specific aims of this study were to (1)

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74 The study area was located in Anjiapo catchment in Dingxi County of Gansu Province, in the western part of the Loess Plateau in China (35°33′–35°35′N, 104°38′–104°41′E). This region has 75 76 a continental arid temperate climate with mean annual temperature and mean annual rainfall of

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78 months in the form of thunderstorms. The mean annual pan evaporation reaches 1515 mm. The 79 soil type belongs to calcic Cambisol (FAO, 1990), developed from loess material, with the 80 average soil depth varying from 40 to 60 m. In this area, deep percolation can be neglected and 81 groundwater is unavailable for vegetation growth and restoration. Therefore, rainfall is the only 82 water source available for plants. The predominant vegetation types in the study area are native 83 grasses and introduced plants. In this study, two adjacent stands were chosen for the experiment:

84 one with natural sloping topography and the other that has been terraced for over 30 years (Fig.

85 1). Both sites were planted with specimens of P. tabulaeformis, a planted tree species typical of

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90 weather station (Davis Company, USA) located in an open space about 500 m from the site.

91 Vapor pressure deficit (VPD, kPa) was calculated based on the air temperature and relative

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94 Soil water content was monitored continuously using a HOBO U30 (Onset Computer 95 Corporation, Bourne, USA) from 2014 to 2015 within the upper 100 cm of the soil profile. There 96 were five probes in each instrument set to depths of 10, 30, 50, 70, and 90 cm, respectively.

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104 Sap flux was monitored continuously from June 5, 2014 to October 10, 2015. At each studied 105 site, six individuals of P. tabulaeformis with different diameters at breast height (DBH, cm) were 106 selected, which represent the size classes within the site (Table 1). Sap flow was measured with

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126 VPD (kPa) is calculated from the air temperature and relative humidity. The value of gc was 127 assumed as approximate average stomatal conductance and considered to reflect the

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130 DBH, sapwood area, and crown projected area were compared using the student t test. For the 131 comparison of soil water content and canopy transpiration dynamics, non-parametric tests of 132 significance were used because of the autocorrelations in the time series data. The Wilcoxon 133 rank sum test, also known as the Mann-Whitney U test, was used to test the differences in soil 134 water content and canopy transpiration between the terrace and slope sites. Curve fitting was

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135 performed using the OriginPro Version 8.0 software (OriginLab Corporation, USA) to establish 136 the relationship between canopy transpiration and soil water content, and between canopy 137 conductance and VPD. Statistical analyses were run using the SPSS version 17.0 software (SPSS 138 Inc., Chicago, IL, USA), for which, the significance level was set at 0.05.

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141 Under the same climatic conditions, soil water content showed differences between the natural 142 slope and terraces (Fig. 2). Data from the shallow layer (0–20 cm) were not analyzed because of 143 anthropogenic disturbance. In both years, statistically significant (p 0.05) higher soil water

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145 Depth-averaged soil water content of the terrace site was approximately 25.4% and 13.7% higher 146 than that at the slope site in 2014 and 2015, respectively. Moreover, the mean soil water contents 147 at both sites were higher in 2015 than that in 2014. Temporal variations of REW between 20– 148 100 cm (Fig. 2 c and d) indicated that soil water conditions were stressed (REW 0.4) in both 149 sites during the two consecutive growing seasons. However, REW was 113.1% more at the 150 terrace site compared with that at the slope site during the two years. It was noted that soil water 151 was severely stressed (REW 0.1) in the slope site, whereas terracing improved the conditions

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154 The diurnal variations of sap flux density (SFd) are shown in Fig. 3. In the growing season, P.

155 tabulaeformis had similar trends of variation at both sites, i.e., high flux density in the daytime

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158 maximum sap flux density at the terrace site compared with that at the slope site. Canopy 159 transpiration was found to be 9.1% and 4.8% higher (p 0.05) at the terrace site than that at the 160 slope site in 2014 and 2015, respectively. Annual variation analysis showed that the cumulative 161 canopy transpiration at both sites was higher in 2014 than that in 2015 (Fig. 4). In the naturally 162 sloping site, the cumulative canopy transpiration was 138.6 mm (32.9% of potential 163 evapotranspiration (PET)) in 2014 and 107.6 mm (24.9% of PET) in 2015. The corresponding 164 proportions at the terrace site were 35.7% and 26.0% in 2014 and 2015, respectively. Variation 165 in canopy transpiration between the slope and terrace sites increased with soil water content

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168 We classified canopy conductance into two levels based on soil water conditions: REW 0.1 169 (Fig. 6 a and b) and REW 0.1 (Fig. 6 c and d). The relationships between canopy conductance 170 and solar radiation, between canopy conductance and VPD under corresponding soil water 171 conditions are shown in Figure 6. It exhibits that canopy conductance declined logarithmically 172 with VPD (Fig. 6 b and d), and there is no significant relationship between canopy conductance 173 and solar radiation (Fig. 6 a and c). When soil water conditions changed from wet (REW 0.1) 174 to dry (REW 0.1), canopy conductance reduced by 12.3% and 24.7% at the slope and terrace 175 sites, respectively. Meanwhile, canopy conductance of P. tabulaeformis at the terrace site was up 176 to 3.9% higher than that at the slope site. The frequency of the SFd peak time suggested that P.

177 tabulaeformis suppressed SFd under high VPD conditions at both slope and terrace sites (Fig. 7).

178 The maximum SFd (SFd, max) was relatively similarly distributed before 14:00 local time (LT), i.e., 179 61.1% at the slope site and 59.2% at the terrace site. However, around 16:00 LT, closer to the 180 most frequent peak time of VPD, the proportion of SFd, max at the slope site was 33.3% less than

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181 that at the terrace site. Therefore, under the same conditions, terracing was found to alleviate the 182 sensitivity of stomatal response to ambient air humidity.

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