Study On The Evolution And Mechanism Of Species Spatial Pattern In The Process Of Natural Grassland Succession On The Loess Plateau

The spatial distribution information of species comprehensively reflects the characteristics of species,the relationship between species distribution and environmental factors,and the interaction among species.It is an effective tool for ecologists to study population dynamics,community building mechanism,the impact of climate change,the prediction of species invasion and the estimation of species abundance.Most of the existing studies focused on the spatial distribution of single species,but there was a lack of systematic studies on the spatial pattern and changes of multiple species in grassland succession.In this study,four typical grasslands with different flocking years in Yunwu Mountain of Ningxia on the Loess Plateau were taken as the research object,and the spatial distribution of species was studied by using the method of space instead of time and the association between plant species and the spatial point pattern of species.By 2 x 2 variance ratio method of contingency table(VR),χ~2test,species coupling coefficients(AC)and correlation between coupling degree(JI)plants,and based on Ripley’s K function of point pattern analysis and calculation of single variable species point pattern and species of bivariate point pattern,At the same time,the indices of species richness,Shannon-Wiener diversity index,evenness index and dominance index were quantified,so as to reveal the change law of interspecies association,species and interspecies spatial pattern of natural grassland succession process in loess region.From the perspective of species spatial pattern,this study revealed the intra-species and inter-species interactions and community building mechanisms,which provided data support for the theoretical study of natural grassland succession and theoretical guidance for the formulation of grassland ecological protection,restoration and sustainable management strategies in the semi-arid region.The main results are as follows:(1)The species composition and diversity of natural grassland changed significantly during the succession process.In the succession process,a total of 50 plant species,including 20 families,39 genera and 26 species,41 species,30 species and 31 species,appeared in different succession stages,and the species were significantly different.15species of plants appeared in all four stages of succession,including Artemisia sacrorum,Oxytropis ochrantha and Aster altaicus.11 species appeared in three stages of succession,including Androsace mariae,Thymus mongolicus and Poa sphondylodes.13 species appeared in two successive stages,including Scutellaria scordifolia,Melilotus officinalis and Viola striatella.8 species only appeared in a certain succession stage,including Elsholtzia densa,Cleistogenes songorica and Saposhnikovia divaricata.The species abundance,Shannon-wienner diversity index,Pielou evenness index and Simpson dominance index were the maximum values in the early stage of succession.The mid-succession species richness index,Shannon-Wiener diversity index,Pielou evenness index and Simpson dominance index were all the minimum values.In general,species diversity increased in the early period of succession,decreased in the middle period of succession,and increased again in the late period of succession,showing a trend of first rising,then declining and then rising.(2)The change of spatial pattern of natural grassland species depends on succession stage,spatial scale and species characteristics.The spatial pattern of Aster altaicus species at a large scale(0～10m)was as follows:random distribution--random distribution--aggregation distribution--aggregation distribution,and the aggregation degree showed an increasing trend.In the small scale(0～1m),the distribution pattern is mainly clustered.The species spatial pattern of Artemisia sacrorum in large scale was also shown as follows:Random distribution--random distribution--aggregation distribution--aggregation distribution,the aggregation degree decreased in the early stage of succession,and increased in the middle and late stages of succession,showing a general trend of first decreasing and then increasing.In the small scale(0～0.4m),the spatial pattern of Artemisia sacrorum showed random distribution--aggregation distribution--random distribution--random distribution.The aggregation distribution of Stipa grandis was mainly concentrated in the large scale,and the aggregation degree increased in the early stage of succession and decreased in the middle stage of succession.The aggregation distribution was as follows:aggregation distribution--random distribution--aggregation distribution in the small scale(0-1.1 m).In the large scale,the distribution of Stellera chamaejasme was always random,and the degree of aggregation showed a gradual decline.In the small scale(0-0.3 m),the distribution was always random.In the large scale,Stipa przewalskyi showed random distribution-aggregation distribution,and the aggregation degree showed a gradually increasing trend.In the small scale(0-2.3 m),the spatial pattern always showed aggregation distribution.(3)The changes of interspecific spatial pattern in different succession stages of natural grassland were mainly species-dependent.The interspecific association of species was positive in early succession,and negative in other succession stages.With succession,positive species pairs of subshrub and herbaceous increased gradually,and negative species pairs of herbaceous and herbaceous increased first and then decreased,and interspecific species pairs of Artemisia sacrorum and Thymus mongolicus tended to have no correlation.The interspecific association between Artemisia sacrorum and A.gansuensis tended to be negative.The species pairs of positive association between Artemisia sacrorum and herbaceous plants showed a gradually increasing trend with succession.With the progression of succession,the correlation between Artemisia sacrorum and Aster altaicu showed as follows:no correlation--no correlation--positive correlation--no correlation;There was no correlation between Stipa grandis and Aster altaicus.The correlation between Stipa grandis and Artemisia sacrorum was as follows:no correlation--positive correlation--no correlation;The correlation between Artemisia sacrorum and Stipa przewalskyi,Stellera chamaejasme and Aster altaicus,Stipa przewalskyi and Aster altaicus was mainly no correlation.(4)The interaction intensity of promotive,competitive and neutral effects varied with succession stage and spatial scale.The promotion effect increased gradually with succession,and mainly came from intraspecific promotion and the promotion effect of subshrubs on herbaceous species.The results of interspecies association showed that the subshrub had a significant promotion effect on herbs,and the analysis of interspecies point pattern further indicated that the subshrub Artemisia officinalis had a promotion effect on herbs.The competition increased gradually with the succession stage,mainly due to the increasing competition among herbs.The neutral interaction between species in the process of succession is the result of interspecific competition,interspecific mutual benefit and the convergence or differentiation of species to the surrounding environment.