Community Specises Of Desert Grassland Experiencing Shrub Encroachment And The Distribution Mechanism Of A. Pedunculata

Shrub encroachment is a very common phenomenon in desert grassland and has various influences on the communities. It can increase the biomass of a grassland ecosystem, underground and aboveground, but without changing its net primary productivity significantly. Its influence on biodiversity is uncertain. Shrub encroachment-induced "fertile island" can cause spatially heterogeneous intensification of light, heat, water and nutrients, disrupting the original even distribution of these resources and changing the resources available on shrubby patches and grassy patches. A. pedunculata is a photophilous, mesoxerophilous, and deciduous shrub. In China, it is mainly distributed in the mountainous areas of the Yin Mountains, Inner Mongolia, and the desert area along the Great Wall stretching from Etuoke Baner and Wushen Banner in YIKEZhao, Inner Mongolia, to the northern region of Shaanxi. A. pedunculata, which is normally distributed in mountains and deserts in arid regions, now grows in the semi-arid steppe in the southwest of the Sunite Right Banner, forming a shrubby landscape covering an area of 50km2. Since there are no favorable topographic and climatic conditions for the growth of the plant in this steppe, soil should be the factor that determines the distribution of A. pedunculata in some particular areas. Based on existing data and the A. pedunculata’s distribution diagram drawn through a field survey, the plant community structure and soil mechanical composition of the areas invaded and not invaded by A. pedunculata were investigated and analyzed. Results are as follows:1. Distribution and Ecological Significance of A. pedunculata.A. pedunculatawhich is normally distributed in arid mountains, has entered the semi-arid sandy and desert area in the highland and occupied some space and resources in the community structure of the steppe, forming a shrub landscape on the steppe. The collection and accumulation of water and nutrients by shrubby plants can enhance the spatially heterogeneous distribution of the resources in soil, resulting in the "fertile island" effect characterized by higher concentration of water and nutrients in shrubby areas. There is a positive feedback effect between the expansion of A. pedunculata in the desert grassland and the growing heterogeneity of soil below the canopy of shrubby plants. This positive feedback effect can increases shrubby plants’ resistance to environmental disturbance and thus make their existence more stable in the ecosystem of the steppe experiencing shrub invasion.2. Community Structure CharacteristicsLocated in the southwest of Sunite Right Banner, Sunite grassland is a temperate steppe constructed by the stipa klemenzii, but it is now widely invaded by shrubs. The areas where A. pedunculata grows have a large number of species, including shrubs and herbs. A. pedunculata is distributed in the stipa klemenzii steppe, forming a local landscape dominated by shrubs. In the area invaded by A. pedunculata, the frequency of stipa klemenzii was 0.54, smaller than 0.94 in the area not invaded by A. pedunculata; the frequencies of allium polyrrhizum and cleistogenes squarrosa were 0.4 and 0.37, respectively, compared to 0.37 and 0.3 in the area not invaded by A. pedunculata. The community in area invaded has a larger number of species whose frequencies exceeded 0.1, but its community stability was worse than the area free from shrub encroachment.3. Factors Governing the Distribution of A. pedunculata in the Sandy Area on the desert grasslandSand blown by wind can be held back by shrubs and thus usually form sand dunes under the A. pedunculata, which can be supplied by the water retained in the sand dunes. The area invaded by A. pedunculata was normally covered by several centimeters or a decimeter of sandy soil containing 97.01% of sand. This layer of sandy soil can facilitate filtration, and reduce the soil evaporation, of surface water. The middle and bottom soil in the area not invaded by the shrub contained more silt and clay and a great deal of gravel and was heavily compacted, obstructing the filtration of water. Therefore, the rainwater in this area often formed small-scale surface runoff. Moreover, the root systems of plants were mostly distributed in soil at 0-30cm deep and seldom extended to deeper soil. The difference in soil mechanical composition caused the difference in water content in soil, which leads to the distribution of A. pedunculata to particular regions.