Response Of Soil Seed Bank And Vegetation Recruitment To The Desert Grassland-Shrubland State Anthropogenic Transition

Caragana caragana(Caragana caraganaii)was introduced for the vegetation recovery of degraded desert steppe in the eastern of Ningxia area,however the steppe gradually was changed into shrubland with the frequency and intensities of introduction,accompanied by a series of degradation in the structure and function of grassland ecosystem,such as soil drought,degradation,and landscape fragmentation.Therefore,based on the situation of desert grassland with introduced shrub and the ecological boundary theory,the desert grassland(DG)?grassland edge(GE)?shrubland edge(SE)and shrubland(SL)in the typical anthropogenic grassland-shrubland mosaics were selected to be as the main anthropogenic transtion stages(sites)and further analysed soil moisture,vegetation,soil seed bank and the competition characteristics of native species(Agropyron mongolicum)and introduced species(C.caraganaii)during the anthropogenic transition from desert grassland to shrubland by using field investigation,indoor incubation method and data simulation method,with an aim to understand the characteristics of aboveground and underground biological communities and the trendency of vegetation recruitment in this transition.The results were as follows:(1)The soil moisture content in 0-100 cm was 5.38%-14.01%in four tansition sites,which was significantly lower than that in deep soil(8.97%-17.38%)(P<0.05).Soil moisture in 0-200 cm of each site decreased with the transition from desert grassland into shrubland(P<0.05)and were 13.92%-19.09%in grassland?13.03%?13.19%in grassland edge?10.87%-12.24%in shrubland edge?6.28%-8.85%in shrubland,respectively.Shannon-wiener index,Pielou index,Margalef index and Simposon index of above ground vegetation in grassland and shrubland edge had the increase edge effect,and these indexes in shrubland were lower than those of desert grassland significantly.The dominant species such as Agropyron mongolicum,Stipa breviflora and other perennial herbs were replaced by annual species,Dysphania aristata and Artemisia scoparia in the transition.(2)The density of soil seed bank species germnated mainly in 0-5 cm soil layer increased in four transition sites with the transition,shrubland had the highest deIsity 1930 seeds·m-2 and were dominated by annual herbs accounting for 97.5%,while desert grassland had the lowest density 750 seeds·m-2 and was coMsist of perennial gramineous herbs mainlycon centrated in 0-5 cm soil depth.Similarity coefficient(SC)between above ground and undergroud vegetation were from 0.14 to 0.35 in four transition sties,but the SC of desert grassland and shrubland were significantly lower than both edge sites(P<0.05),indicating communities stability of vegetation succession in desert grassland and shrubland were more stable than them and bistability of vegetation characteristics in the transition process.,the soil seed bank germination density and the number of species increased firstly(-60%?20%)and then decreased(40%),and there was significant difference under different precipitation gradients(P<0.05).The number of soil seed bank of perennial herbs in desert grassland was the highest under embient rainfall,while the annual species,Artemisia scoparia and Dysphania aristata could germinate under different precipitation gradients and adapted to soil drought and rainfall changes.(3)The average initial germination time of Caragana korshinskii was earlier14 hours than that of Agropyron mongolicum,and the water absorption rate was 201.21%,lower than that of Agropyron mongolicum(293.43%).the germination rate and speed of germination of Caragana korshinskii and Agropyron mongolicum all increased firsty and then decreased with the osmotic potential increase,and the germination rate was the highest at-0.02 MPa,and the lowest at-1.2 MPa.And the initial germination day of Caragana korshinskii are less 1-4 days than that of Agropyron mongolicum.the relationships between germination water potential and germination rate were simulated by implication of the S-shaped growth model and linear model,and the threshold range of germination water potential of Caragana korshinskii and Agropyron mongolicum were calculated-0.3?0.86 Mpa,-0.3?0.65 Mpa respectively,i.e.the threshold of germination water potential of Caragana korshinskii was lower than perennial grasses.The chlorophyll content of them increased firstly and then decreased,while the morality rate of them increased with the drought stress increase.According to the model of soil water characteristic curve in each sites and he threshold of germination water potential of Caragana korshinskii and Agropyron mongolicum,the average soil water thresholds of them in four transiton sites were 5.58%?6.63%and 6.21?6.98%,respectively.The average soil water thresholds of Caragana korshinskii and Agropyron mongolicum during seedling period were 6.94%?12.86%and 7.14%?13.69%.respectively.Therefore,it was concluded that under the decrease of precipitation and the soil aridity acceleration caused by shrub encroachment,Caragana korshinskii would have more advantaged than perennial herb(Agropyron mongolicum)in vegetation regeneration.