| Biological soil crusts(BSCs) are a living ground cover widely found in arid and semi-arid regions, and they provide important ecological functions in arid and semi-arid ecosystems. To study its distribution regularity is the theoretical basis of scientific management and utilization of this resource.Wind-water erosion crisscross region of Northern Shaanxi located in the northern Loess Plateau, where wind erosion and water erosion are coexisted, and The weather is very dry, and the ecological environment is very fragile. Thus, The role and status of BSCs in this area is particularly important and special. To study the spatial distribution character, explore the impact factors of its development, and establish the relationship model of crust development characteristics and environmental factors, is the basis and premise of scientifically evaluation and rationally use of the biological soil crust resources and effectively play its ecological function and soil and water conservation benefits.In view of this, this paper chose a typical slope in Liudaogou small watershed of Shenmu county of Northern Shaanxi, investigated the coverage, thickness and shear strength of BSCs in this slope, as well as topography, soil, vegetation indicators ect. By using GIS software to mapping and resampling and extracting of the data, we got 1342 samples and 41 attribute index of samples. By using geo-statistics software(GS+ for Windows), we analyzed the autocorrelation of spatial distribution of BSCs, and by using biostatistics software(CANOCO), we studied the correlation and correspondence relationship between BSCs’ coverage, thickness, shear strength and vegetation community, terrain, soil and other environmental factors. According to these relationships, we chose some main relevant environmental factors to establish their standard stepwise regression equation with BSCs’ coverage and thickness. Through the path analysis, we studied the effect path and effect degree of these environmental variables to coverage and thickness of BSCs. On the other hand, through factor analysis of sample data matrix which is constitute by development index of BSC and some main environmental factors, we extracted the potential common factors. According to the results of factor analysis, we eventually build a structure equation model between development index of BSC and potential environmental factors. In summary, through a series of investigation drawing and statistical analysis, we studied the spatial distribution characteristics of BSCs’ coverage, thickness and shear strength on the slope scale, and discussed the key factors that could influence the development of BSCs and their effect path and internal mechanism. The main conclusions are as follows:1) In sand area, the distribution of BSCs show a large piece of continuous distribution, which have obvious spatial autocorrelation, and the range is 90-100 m, and the mean coverage of BSCs is over 30%. Moreover, BSCs have obvious places of origin, and their coverage can reach over 90%. While in loess area, the BSCs distribute fragmentally and discretely, and mainly distribute in the edge and end of slope, and their coverage are mainly below 20%. The spatial variation of BSCs’ thickness and shear strength is not large. The mean thickness of BSCs in sand and loess area are 13.1 mm and 12.8 mm respectively, and their shear strength are 4.76 kg/cm2 and 4.96 kg/cm2 respectively. In origin place, BSCs’ thickness can reach 20 mm, and the shear strength of BSC can reach 0.7 kg/cm2.2) Canonical correspondence analysis(CCA) showed that, BSCs have obvious selectivity of topography, soil and vegetation communities, tend to grow in the sandy plant communities where slope is relatively slow, soil is moist and Populus simonii Carr woodland where the canopy density is relatively high. In some piece of moist sand part in loess area, they often appear together with Enneapogon borealis. In secondary slash of sand-loess area, they also coexist with Heteropappus hispidus(Thunb.), Setaria viridis(L.) Beauv., Artemisia capillaries, etc.3) Path analysis showed that, sand, soil water content, elevation, and Artemisia coverage are the main factors affecting BSCs’ coverage. Among them, the effects of sand are mainly direct effects, and the direct path coefficient is 0.720. While Artemisia coverage mainly shows indirect effects through sand, and the indirect path coefficient is 0.422. The indirect negative effects of soil water content is a little larger than its direct positive effects, and the effect coefficients are 0.526 and-0.612 respectively. The direct and indirect effects of elevation are both negative, and the path coefficients are-0.358 and-0.244 respectively. Except the development time of BSCs, Artemisia coverage, slope aspect, elevation and coverage of Populus simonii Carr. are the environmental factors affecting the BSCs’ thickness significantly. In which, the coverage of Artemisia and Populus simonii Carr. mainly have direct effects, and the effect path are 0.329 and 0.233. While effects of aspect are mainly indirect, and the indirect path coefficient is-0.311. The indirect effect of elevation is a little larger than its direct effect, and both of them are negative, the effect coefficients are-0.216 and-0.196 respectively.4) In the final structure equation model of this study, solar radiation, erosion, soil texture and secondary vegetation are the main potential factors that affect the development of BSCs. Among them, the effect of soil texture is largest, of which the direct effect coefficient is-1. Followed by secondary vegetation, of which the direct effect coefficient is 0.62, and has a relatively strong correlation with soil texture(the correlation coefficient is 0.74). The negative effect of solar radiation and the erosion are almost, and their direct effect coefficients are-0.31 and-0.27 respectively. In general, the development of BSCs is better in sand area and secondary land where the solar radiation and erosion is weak. |
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