Optimization of plant coverage in relation to water balance in the Loess Plateau of China
Highlights
► The relationship between plant coverage and soil water was studied for two shrubs. ► Soil water storage in the root zone decreased with increasing plant coverage. ► The soil desiccation occurred in the root zone for different coverages. ► A numerical model was calibrated and verified using the field measurements. ► The optimal plant coverage was determined for each shrub at a 10% risk probability.
Introduction
The wind and water erosion transitional belt of China's Loess Plateau experiences intensive soil erosion, vegetation degradation, and soil desertification (Cha and Tang, 2000, Tang, 2000). The Chinese Government has implemented vegetation restoration practices, e.g. planting perennial shrubs and grasses, to improve the environmental quality and to reduce water and soil losses in the area. In this semi-arid region, however, an incompatibility exists between the limited soil water availability and the extensive plant coverage required for protecting the land (Xia and Shao, 2008).
Increasing plant coverage can significantly reduce the sediment yield and effectively control soil erosion (Cerdá, 1999, Elwell and Stocking, 1976, Morgan et al., 1997). Snelder and Bryan (1995) investigated the relationship between plant coverage and soil loss under simulated rainstorms. They found that soil loss was the maximum for plant coverages less than 25%, and that a minimum of 55% plant coverage was required to achieve satisfactory soil erosion control. García-Ruiz et al. (1995) and Molinillo et al. (1997) showed that in the mountainous northern Spain, soil loss increased rapidly when reducing plant coverage, and that little soil loss occurred for shrub coverages greater than 40%–60%. In the northern China Loess Plateau, Guo and Shao (2009) showed that sediment yield was inversely proportional to plant coverage of C. korshinkii, and that this inverse relationship could be expressed by a sigmoid function.
However important plant coverage is for soil erosion control, the maximum plant coverage is determined by soil water availability. In China's Loess Plateau, extensive plant coverages were achieved during vegetation restoration efforts at the cost of severe soil desiccation and formation of dry layers. Soil desiccation can have a detrimental effect on the environmental and hydrological processes (Chen et al., 2008, Wang et al., 2010). For example, soil desiccation promotes the development of drought-resistant, deep-rooted trees between 3 and 5 m of height , which are not efficient in reducing soil erosion (Chen et al., 2008, Hou et al., 1999). An optimal plant coverage is vital to balance soil water consumption and the eco-environmental service performances of a vegetation restoration program.
Optimal plant coverage is defined as the maximum leaf area index (LAI) which can be achieved without soil water content reaching the permanent wilting point. For plant coverages greater than the optimal plant coverage, there is a series of consequences that not only constrain plant growth but also aggravate water scarcity and deep soil desiccation. Guo and Shao (2004) presented a semi-empirical model to estimate the water balance between water supply and plant water consumption using two years of contiguous climatic data, and determined that the optimal coverage for 16- to 17-year old C. korshinkii was 8155 plants ha− 1 in the semi-arid area of the loess hilly region. Xia and Shao (2008) developed a process-based model to calculate the optimal plant coverage using three years of contiguous climatic data and found the optimum plant density of C. korshinkii to be 2250 trees ha− 1 compared to the observed density of 6500 trees ha− 1; whereas for Salix psammophila, the optimum and observed densities were 2800 and 4800 trees ha− 1, respectively.
A sustainable plant coverage must survive under climatic conditions prevailing during dry years, in which the annual amount of precipitation is less than 30% normal (Yang and Shao, 2000). Previous studies on the viability of plant coverages are limited because they used only two to three years of contiguous climatic data which may or may not include a dry year. In semi-arid regions, the annual and inter-annual climatic variations are important, with large differences in precipitation timing and amount. In the Loess Plateau, the frequency of occurrence of a dry year is about 10%, i.e. a recurrence of ten years; therefore, experiments lasting two to three years may be too short to represent the critical climate of this area (Tang, 2000). An alternative investigative approach is to select a representative dry year from historical climatic records, and to use a model to predict the effect of plant coverage on soil water. Optimal plant coverage can be determined by varying the plant density and leaf area index (LAI) until the soil water content approaches but does not reach the permanent wilting point during the growing season (Kochendorfer and Ramírez, 2008). Such an approach requires the validation of a model before it can simulate the response of soil water to different plant densities and LAI values.
The Simultaneous Heat and Water (SHAW) model can simulate heat and water movement within a one-dimensional profile (Flerchinger and Saxton, 1989a, Flerchinger and Saxton, 1989b). This detailed physical process model has been widely tested (Flerchinger et al., 1996, Flerchinger et al., 1998, Preston and McBride, 2004). The SHAW model has been applied in China's Loess Plateau to accurately simulate water movement (Huang and Gallichand, 2006); it can take into account variations in the plant–snow–residue–soil system.
The objective of this study was to investigate the relationship between plant coverage and soil water content to determine the optimal plant coverage for the two dominant shrubs (Caragana korshinkii Kom and Salix psammophila) in China's Loess Plateau by a combination of mathematical modeling and field experiments.
Section snippets
Experimental site and design
The field experiment was conducted from June 2008 to August 2010 at the Shenmu Erosion and Environmental Experimental Station, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, in the Shaanxi Province, China. The experimental site is located in the wind and water erosion transitional belt of the northern Loess Plateau (110° 21' E, 38° 47' N) where soil erosion and land desertification are serious problems. The climate is semi-arid temperate
Maximum leaf area index
During the 2008–2010 experimental period, plant coverage was correlated to the maximum LAI (MaxLAI), which is the maximum seasonal LAI observed during the three experimental years. For the two shrubs, MaxLAI increased with plant coverage (Fig. 3). For both shrubs, the same initial plant coverages result in an increased LAI with the passing years due to plants establishment. The values of MaxLAI were the largest in 2010 for the two shrubs. Also, MaxLAI values of C. korshinkii were always greater
Plant coverage, MaxLAI, and soil water storage
The relationships between plant coverage and MaxLAI found for the two shrub species were similar to those of other species, as reported by Retuerto et al. (1996) and Bullard et al. (2002). The MaxLAI values were largest in 2010 (Fig. 3) when the total rainfall during the growing season was the lowest, which shows that soil water storage in the root zone at the end of the previous year is important to shrub growth in this area (Wu and Yang, 1998).
For C. korshinkii, there were no significant
Conclusions
We investigated the effect of plant coverage on soil water content variations for two shrub species in the northern China's Loess Plateau. Soil water content is a function of meteorological and soil conditions, together with the influence of plant extraction. In order to investigate the effect of shrub species and coverage and climate on soil water dynamics, the SHAW model was calibrated with meteorological and soil water content data from the 2008–2010 period. The SHAW model provided soil
Acknowledgements
This work was financed by the CAS/SAFEA International Partnership Program for Creative Research Teams — Process simulation of soil water in a watershed, the Chinese National Natural Science Foundation (No. 41171186) and the Chinese Academy of Sciences Visiting Professorship for Senior International Scientists, Grant number 2009Z2-37. Special thanks are given to Prof. Robert Horton for his help in improving the language of the paper and the two reviewers for their comments and suggestions.
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