| Due to the unreasonable man-made factors superimposed vulnerable natural environmental conditions, which resulting in severe degradation of grassland in northern farming-pastoral ecotone, the grassland desertification is main form of grassland degradation, in order to control sandy grassland, the state implemented step by step returning farmland to forest (pasture), returning grazing land to grassland and other ecological management projects, which have different degrees of ecological restoration to the grassland desertification. One of the important measures for the vegetation restoration of degraded grassland is enclosure. In order to research grassland enclosure management in northern farming-pastoral ecotone, evaluate effects of grassland fencing governance, and study of different fencing types, fencing time on the impact of vegetation restoration of degraded grassland. In this paper, combined with desertification monitoring project of the State Forestry Administration (Yanchi County, Ningxia Hui Autonomous Region), the study area was selected in Liuyangpu enclosure demonstration area, which is a typical desertification grassland management area, which is located in the north of Yanchi County and the southwest margin of Maowusu Sandy Land. In three kinds of fencing types (year-round enclosure, seasonal enclosure, no enclosure) grasslands, the fixed samples are respectively set up, combining line transects and quadrats methods, conventional vegetation, soil surveys and scientific researches such as the spatial heterogeneity, biological crusts etc., were carried out. Based on 2003～2014 continuous monitoring and test data, using theories of vegetation restoration, community ecology, community dynamics, enclosure measure effects on vegetation restoration in degraded grassland was studied. The study elaborates restoration ecology and plant community dynamics mechanisms systematically, and provides references for the degraded grassland restoration and reconstruction, grassland dynamic research and pasture management. Firstly, from the community scale, contrast analysis of the changes and reasons for the changes of vegetation community characteristics of species composition, community succession, vegetation fluctuation, population niche, species diversity, community complexity, community spatial heterogeneity in grasslands of different fencing types and fencing time under grassland vegetation restoration process. Secondly, from the perspective of ecosystem, health conditions of grassland ecosystem were further studied under different enclosure grasslands. And the effects of artificial improvement measures were evaluated along with the degradation problem of the long-term year-round enclosure grassland. Finally comprehensive benefits of fencing measures of degraded (sandy) grassland in Yanchi county were evaluated. The results were mainly concluded as follows:(1) The fluctuation rate of the vegetation was relatively stable in year-round enclosure grassland, but the values were mainly negative, which indicates that the long-term closure does not make the vegetation growth well; The fluctuation rate of the vegetation was also relatively stable in seasonal enclosure grassland, and the values were mainly positive, which indicates that the vegetation is in the recovery stage; However, variation range of the vegetation fluctuation rate was relatively large, which indicates that there is more disturbance in the no enclosure grassland, the fluctuation of the vegetation growth and the stability of the community are poor.(2) The studies of niche width in different enclosure grasslands showed that niche width of annual herb became increase in long-term year-round enclosure grassland, which resulted in competing resources space with perennial herbs and shrubs, increasing difference of species niche breadth, niche differentiation serious, poor community structure and grassland vegetation degradation. Difference of the niche breadth between species is small in seasonal enclosure grassland, the competition of the environment resources is moderate, the effect of vegetation restoration is good; Due to a certain degree of human disturbance, there is poor living environment in no enclosure grassland, the dominant position of the dominant species is obvious, there is fierce competition of survival resources for species, and large difference of the niche breadth between species. In no enclosure grassland, there are fewer species, species niche breadth variation, community structure is relatively simple, community stability is not high. There was a high niche overlap among the species with high niche breadth; however, the maximum niche overlap emerged among species with low niche breadth, which implied that there was no direct linear correlation between the niche breadth and niche overlap.(3) Under the condition of enclosure, due to the reduction of human disturbance, the diversity index and complexity index of vegetation were relatively high, such as year-round enclosure grassland and seasonal enclosure grassland. The diversity index and complexity index in the no enclosure grassland were relatively low due to some degree of disturbance. Long-time fencing would occur regularly index fluctuations to pasture vegetation, and frequency of occurrence of the high and low peak value was recorded about 3 to 5 years/times, which indicates that with the extension of the fencing time, the vegetation growth does not become better, the biological diversity and the complexity of the community do not become higher. (4) The community spatial distribution characteristics of three different types of enclosure areas were analyzed using the power law. Results showed that all three types of grasslands followed the power law and exhibited a strong heterogeneous distribution compared with the random distribution. The index value of spatial heterogeneity for the entire community in seasonal enclosure area was higher than that in year-round enclosure area, data still higher than that in no enclosure area. (5) In order to quantitatively assess the influence of enclosure on the grassland ecosystem health, using ecosystem health index to evaluate desert grassland under different enclosure grasslands. Results revealed that ecosystem health level in enclosure grassland was in a state of alert, which indicated that the ecological environment of degraded grassland had been restored, but was weak. In 2003-2014, the overall level of health are sorted as:seasonal enclosure grassland> year-round enclosure grassland> no enclosure grassland. The research also indicates that the long-term enclosure can’t make health level of grassland ecosystem well.(6) Due to the long-term enclosure, vegetation growth in year-round enclosure grassland is poor. The implementation of artificial improvement measures such as shallow tillage measures etc, can improve the soil moisture conditions and the grassland biodiversity, promote vegetation recovery, but 3-5 years later, biodiversity of artificial improvement grassland can not improve effectively.(7) Fencing project can effectively promote the natural restoration of degraded grassland vegetation and update, improve the grassland ecological environment, promote farmers income increase, agricultural and animal husbandry efficiency and rural development, can also speed up the adjustment and optimization of industrial structure, Government should continue the implementation of returning farmland to forests and grasslands and grazing prohibition policy. It should pay attention to the enclosure time, biological crust problem, grassland resource utilization in the process of effective grassland management, which helps to optimize the ecological, economic and social benefits.To sum up, protective utilizations such as seasonal enclosure or rotation of pasture etc is more reasonable pasture management. And long-term single enclosure measure is not conducive to vegetation recovery of year-round enclosure grassland. According to degree of vegetation restoration, people should consider the periodic (3-5 years) implementation of human disturbance (such as shallow plowing, grazing, cutting, etc.), which maintain stable state of health ecosystem in year-round enclosure grassland. |
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