Effects Of Nitrogen Addition On Reproductive Strategy Of Leymus Chinensis And Carex Duriuscula In Different Communities

Reproductive strategy is an important strategy for plants population to adapt to environmental changes.In recent years,human production and life have increased nitrogen deposition year by year,which will inevitably affect growth and reproduction of grassland plants.Leymus chinensis and Carex duriuscula are typical perennial clonal plants in the Songnen Plain of northeast China.They have drought resistance,salt-alkali tolerance and other ecological characteristics.They can form typical communities of restoration succession series.In this study,we learned the total ramet number dynamic,vegetative reproductive characteristics,sexual reproductive characteristics,vegetative growth characteristics and trade-off between growth and reproduction of L.chinensis and C.duriuscula by artificial transplanting the juvenile tillers,and treated their populations under monodominant community and mixed community with nitrogen addition respectively.The study aims to reveal the regulation mechanism of clonal plant population’s reproductive strategy under the condition of nitrogen addition,enrich the growth and reproductive adaptation mechanism of clonal plants,and provide valuable reference for the restoration of degraded grassland vegetation.The main results are as follows:（1）For ramet number dynamic,nitrogen addition significantly accelerated the change of ramets of L.chinensis and C.duriuscula populations in monodominant and mixed communities.In the first year of transplanting,the populations of L.chinensis and C.duriuscula increased exponentially,and the growth rate increased with the increase of nitrogen concentration.In the second year of transplantation,the actual population growth rate of L.chinensis and C.duriuscula was significantly lower than the previous year.At the moment,the populations of L.chinensis increased by exponential function,the populations of C.duriuscula increased by logarithmic function.In mixed communities,the proportion of L.chinensis populations increased with the increase of nitrogen application concentration,while the proportion of C.duriuscula populations decreased gradually.（2）For vegetative reproductive characteristics,after two growing seasons,nitrogen addition significantly promoted the produce of vegetative ramet and rhizome of L.chinensis populations in monodominant and mixed communities and C.duriuscula in monodominant communities.For C.duriuscula in mixed community,the biomass of rhizome only increased when N concentration reached 20 g N m^-2yr^-1.Nitrogen addition was beneficial to formulate the potential population of L.chinensis in monodominant and mixed communities.The potential population of rhizome of L.chinensis was more dominant in number,and rhizome buds accounted for more than 90%.However,it had no significant effect on the potential population quantity of C.duriuscula in monodominant and mixed communities.In mixed community,the potential population of L.chinensis was more 10 times than C.duriuscula,which has more competitive advantage.Meanwhile,the ramet number,rhizome length and potential population of age spectrum composition of L.chinensis and C.duriuscula remained stable under nitrogen addition,and showed increasing or stable age structure.In two years,nitrogen addition had no significant effect on vegetative propagation ability of ramet and rhizome of different age class in L.chinensis and C.duriuscula populations.（3）The response of sexual reproductive characteristics to nitrogen addition was different between L.chinensis and C.duriuscula in monodominant and mixed communities.For sexual reproductive characteristics,nitrogen addition increased significantly the hight,inflorescence length,number,seed number,floret number,seed biomass and inflorescence biomass of reproductive ramets of L.chinensis in monodominant and mixed communities,and increased significantly with the increase of nitrogen gradient.But there was no significant effect on thousand-grain weight,seed-setting rate and heading rate of L.chinensis populations.For C.duriuscula populations,nitrogen addition only increased the number of reproductive ramets,and decreased RAⅢsignificantly in monodominant communities,and there was no significant difference between 10 g N m^-2yr^-1and 20 g N m^-2yr^-1.The floret number,seed number,seed biomass and inflorescence biomass of C.duriuscula increased with the increase of nitrogen application concentration in monodominant communities.Under the same nitrogen concentration,not only the reproductive ramet number,seed number,floret number,seed biomass and inflorescence biomass of C.duriuscula in mixed community were significantly lower than monodominant community,but the seed-setting rate,heading rate and reproduction reallocation were also lower than monodominant community.（4）The effects of nitrogen addition on vegetative growth characteristics of L.chinensis and C.duriuscula were different in monodominant and mixed communities.In two years,nitrogen addition significantly increased vegetative modules biomass and total biomass of L.chinensis populations.After the second growing season,for the L.chinensis populations in monodominant community,nitrogen addition significantly increased the biomass allocation of leaves and stems,while significantly decreased the biomass allocation to roots.For C.duriuscula populations,nitrogen addition only increased significantly vegetative modules biomass and total biomass of the first growing season.（5）Under different nitrogen addition conditions,the biomass for growth and reproduction was:vegetative growth>vegetative reproduction>sexual reproduction in L.chinensis and C.duriuscula populations.The resources for vegetative growth,vegetative reproduction and sexual reproduction of L.chinensis population increased significantly with the increase of nitrogen concentration,and the allocation to vegetative growth of L.chinensis population in monodominant community increased significantly when nitrogen concentration is 20 g N m^-2yr^-1.Nitrogen addition significantly increased the vegetative reproduction biomass of C.duriuscula populations.For C.duriuscula in mixed communities,nitrogen addition significantly reduced the allocation to vegetative growth and increased the allocation to vegetative reproduction.In mixed community,when nitrogen was not applied,the L.chinensis population adopted a reproductive strategy of positive correlation between sexual reproduction and vegetative reproduction,while the C.duriuscula population adopted a trade-off strategy between vegetative reproduction and sexual reproduction.When nitrogen concentration is 20 g N m^-2yr^-1,the L.chinensis population adopted a trade-off strategy between vegetative reproduction and sexual reproduction,it may be due to the combined effects of limited growth space,increased intraspecific competition and interspecific competition.In this study,two years of nitrogen addition had different effects on the populations of L.chinensis and C.duriuscula in monodominant and mixed communities.For L.chinensis populations,nitrogen addition promoted the vegetative growth and vegetative reproduction in communities,and affected the sexual reproduction of L.chinensis populations mainly by increasing the number of reproductive ramets.For C.duriuscula populations,nitrogen addition mainly promoted vegetative reproduction,and only increase the reproductive ramets in monodominant community,which had no significant effects on the sexual reproduction and vegetative growth of mixed communities.The difference of nitrogen concentration would lead to different reproductive strategies of L.chinensis and C.duriuscula.Therefore,short-term nitrogen addition accelerated the succession from mixed community to monodominant community.This study revealed the plasticity changes of reproductive strategies of L.chinensis and C.duriuscula populations in different communities under nitrogen addition,providing valuable reference for the restoration of degraded grassland.