The Dynamic Change Of Dominant Vegetation Soil Moisture In Loess Hilly Region, Northwestern Shanxi

The Loess Hilly Area of Northwest Shanxi province is in the transition zone of the arid and semi-arid environments, which is also of is ecologically fragile area of farming. Because of the dry climate, few precipitation, and strong evaporation, water becomes the key factor of deciding the structure and function of the ecological system, and limits the development of the local forestry. Rainfall is the main source of soil moisture and vegetation growth; however the significant difference of the rainfall distribution in different seasons and the soil desiccation in different seasons caused a low production and efficiency in large areas of the local ecological forest, restricting the process of ecological restoration and construction. Therefore, it is important to research on the dynamic characteristics of different vegetation’s soil moisture in different seasons in Northwest Shanxi Loess Hilly Area.This study takes the different types of vegetation and abandoned land in the Loess Hilly Area of KeLan County, Northwest Shanxi province as the research subject. It intends to measure the moisture of soil within the range of 0-600cm vertical depth, and study the vertical variation of different artificial vegetation’s soil moisture in this area, the seasonal regularity, the degree of soil desiccation, the reasons for the phenomena and the corresponding prevention measures.This study uses fieldwork sampling of artificial drilling, sampling soil depth is 600cm, sampling spacing of 10cm, the number of samples was 60 per drilling, soil moisture content measured by experimental analysis method, the field research and theory basis, qualitative analysis and quantitative analysis concluded that a comprehensive study and understanding of:(1) According to the utilization of moisture in different soil layers, the 0-600cm soil profile of the observed plots are divided into the following four layers by using the standard deviation discrimination law:when standard deviation> 1.5, it is the weak-utilized layer of soil moisture (variable layer); when 1.2< standard deviation< 1.5, it is the utilized layers of soil moisture (active layer);when 0.9< standard deviation< 1.2, it is the transition layer of soil moisture (second active layer); when standard deviation< 0.9,it is the stable layer of soil moisture (adjustment layer).(2) The soil moisture of different vegetation types in different seasons in Northwest Shanxi Loess Hilly Area is different. In the end of Spring and the early of summer (April, May, and June), the soil moisture from high to low is as follows:Uncultivated land> Chinese pine-simonii mixed forest> caragana> Sea buckthorn> Populus simonii; in the late summer (July, August, and September):Uncultivated land> caragana> Chinese pine-simonii mixed forest> Populus simonii> Sea buckthorn; In Autumn (September and October):Uncultivated land> caragana>Chinese pine-simonii mixed forest> Populus simonii> sea buckthorn.The seasonal dynamic changes of different vegetation types in northwest Shanxi Loess Hilly Area is generally divided into four periods:1. The late spring and early summer(April, May, and June) is the beginning of vegetation growth, which is the dissipational period of soil moisture; the late summer and early autumn (July, August, and September) is in the period of vegetation growth, which is the replenishmental period of soil moisture; in the end of autumn and the beginning of winter(October,November and January), vegetation is in the last part of growth which is the saving period of soil moisture; in late winter and early spring(February,March and April),vegetation is in non-growing period and soil moisture is in the stabilization period.(4) Soil dry layer standard of loess hilly region:I mild dry layer with soil moisture from 11 Percent to 30 Percent; II Moderate dry layer, from 8 Percent to 11 Percent; III serious dry layer,7 Percent below. In the growing period of one year, the degree of soil desiccation of different vegetation is very serious. Almost each soil layer has the phenomenon of desiccation, most of which is moderate dry layer and some of which even has severe dry layer. Among them, Populus simonii forest has severe dry layer in the middle and lower part of soil layer throughout the growing period. Sea buckthorn forest has severe dry layer in the middle of soil layer throughout the growing period. Caragana is only in the middle of soil layer in the end of spring and early summer.(4) Prevention measures of soil desiccation are proposed according to the cause:Based on the data of soil moisture, local dominant species should be selected and the density of vegetation should be adjusted; terraces and fish scale pits should be constructed to maintain the soil moisture; covering mats to reduce soil moisture evaporation; enhancing the infiltration of precipitation during the rainy season. So, in the process of vegetation restoration and reconstruction in northwest Shanxi Loess Hilly Area, it is essential to adjust the structure of land utilization, improve the proportion of Arbors, shrubs and grass, diversify the vegetation, establish a sustainable mixed and re-storied forests and a relatively balanced plant communities, and well distribute the community density, ensuring the water supply by the environment and water consumption of the vegetation is in a stable state.