Research On The Relationship Between Species Diversity And Aboveground Productivity At Temporal Scale In Typical Grassland Of Loess Plateau

The relationship between species diversity and above-ground productivity has historically been the front and core of community ecology research.Previous studies have shown that the"hump"relationship is the most common model on the spatial scale,but little attention has been given to the relationship between the two on the time scale.Possible mechanism.Furthermore,theoretical and experimental studies only use species richness as a surrogate indicator of species diversity,ignoring other indicators of diversity.Based on this,this study began in 1982 to take the typical grassland ecosystem of Yunwu Mountain on the Loess Plateau as the research object,based on 40 years of positioning observation experiments,using species richness,plant abundance,Shannon Wiener diversity index,evenness index and dominance index,etc.As a multi-dimensional measure of species diversity,above-ground biomass is used as a measure of above-ground productivity to examine changes in species diversity and above-ground productivity on long-term scales through locally weighted regression（Loess）、general linear model（GLM）and detrended correspondence analysis（DCA）etc.Patterns and the relationships between them.We have the following main results:（1）40 years of location monitoring showed that the composition,community species have changed a great deal.Species richness ranged from 3 to 28 species·m^﹣2,with an average of 18 species·m^﹣2;species Shannon-Wiener diversity index ranged from 0.06 to4.12,with an average of 2.78;The range is 0.03～1.40,with an average of 0.95;the range of Simpson index changes is 0.21～4.12,with an average of 0.86.The variation range of plant abundance was 10～860 plants·m^﹣2,with an average of 129 plants·m^﹣2.The variation range of aboveground biomass was 30-690 g·m^﹣2,with an average of 351 g·m^﹣2.In the 40-year succession process,a total of 115 species of plants have appeared,which belong to 29families and 83 genera.The top 5 families are:Gramineae（19 in total）,Compositae（19 in total）,Leguminosae（11 in total）,Rosaceae（10 in total）and Lamiaceae（9 in total）,the total number of plants in the above five families accounts for about 60%of the total number of plants.（2）Based on the changing trends of species diversity and aboveground productivity,we divided the entire succession stage into three stages:pre-succession（1982-1993）,mid-succession（1994-2011）and post-succession（2012-2021）stage.The changes of species richness over time are as follows:sharp rise→relatively stable→sharp decline（“hump”shape）;Shannon-Wiener diversity index:slow decline→sharp decline;Pielou index:slow decline→Relative stability→sharp decline;Simpson index showed:relatively stable→slow decline;plant abundance showed a gradual upward trend;the change of aboveground biomass with time showed:sharp increase→gentle decline（“hump”shape）.In general,from 1982 to the present,species diversity has declined,and each diversity index has declined at different rates and degrees,while species abundance has shown an increasing trend,illustrating that species diversity and productivity are dynamic on time scales.（3）The in-and out-migration of species changed significantly during the succession process.In the early stage of succession（1982-1993）,the species mainly migrated in,among which Compositae has the most,up to 9 species,followed by Leguminosae,which is 4species,and the in-migration rate of species is 3 species/year;succession In the mid-term（1994-2011）,the in-migration of species slowed down,with the largest number of legumes immigrating to 2 species,and the in-migration rate of species was 1 species/year;in the late-succession period（2012-2021）,the species mainly migrated out as Mainly,Compositae has the highest emigration rate of 9 species,half of the species that migrated in the early stage all migrated out in the later stage,and the species emigration rate was 3 species/year.In the early stage of succession,annual plants with strong spreading ability and short-lived perennial weeds（r-strategy plants）were dominant,and in the later stage of succession,perennial grasses with high resistance（K-strategy plants）were dominant.According to the changing trend of the important values of the key species in the grassland community,we can determine the dominant grass species in the early,middle and late stages of succession:Stipa bungeana,Stipa grandis,Stipa przewalskyi.（4）The relationship between species diversity and aboveground biomass on the time scale showed a"hump"relationship as a whole,but it varied with different succession stages.The relationship between species richness,plant abundance,Shannon-Wiener index,Simpson index and aboveground biomass all showed a"hump"relationship;the relationship between species richness and plant abundance,and Pielou index and aboveground biomass showed as follows:"Inverted hump"relationship,the Simpson index and aboveground biomass showed a linear positive correlation.There was no significant correlation between mean annual precipitation and species diversity and above-ground biomass in this region.The mean annual temperature was significantly negatively correlated with Pielou index and Simpson index,it was significantly positively correlated with above-ground biomass.From the above results,the following conclusions can be drawn:（1）The succession process of grassland ecosystem is dynamic.Species richness and aboveground biomass increased rapidly in the early succession（1982-1993）,peaked and was remained relatively stable in the mid-succession（1994-2011）,and decreased rapidly in the late-succession（2012-2021）.The Shannon-Wiener diversity index,evenness index and dominance index showed a downward trend with the progress of succession.（2）During the succession process,the dominant species of the community were replaced,and the specific performance was from Stipa bungeana in the early succession to Stipa grandis in the middle succession to Stipa przewalskyi in the later succession.In the early stage of succession,species migration was the main factor,the middle stage of succession was relatively stable,and the latter stage was dominated by species migration.（3）The relationship between species diversity and aboveground biomass will change with different successive stages,showing a"hump"relationship on the entire time scale,which is consistent with the conclusion on the spatial scale.The trade-off between positive interactions（promoting）and competition among species may be the main cause of changes in species diversity-above-ground biomass relationships on time scales.