The Effects Of Nitrogen Addition On The Population Dynamics Of Dominant Plants In A Semi-arid Grassland

Global climate change is one of the greatest challenges in the 21st century.The accumulation of reactive nitrogen in ecosystems caused by atmospheric nitrogen deposition is a serious threat to global biodiversity.As an important part of terrestrial ecosystem,grassland plays a pivotal role in global biodiversity conservation.Due to low rainfall in grasslands in arid and semi-arid areas,atmospheric nitrogen deposition aggravates salt stress and further affects biodiversity.Grasslands in the semi-arid area of Northern China are relatively fragile,and they are especially sensitive to atmospheric nitrogen deposition.Therefore,examining the responses of dominant plants to nitrogen deposition is essential to the biodiversity conservation and sustainable development of grasslands a in Northern China.This study investigated the effects of different nitrogen addition treatments on the population dynamics of two dominant plant species,Stipa bungeana Trin and Leymus secalinus（Georgi）Tzvel,in a semi-arid grassland of northern China.The demographic censuses were conducted under control（no nitrogen addition treatment）and five nitrogen addition levels(i.e.,1.15 g N·m^-2·yr^-1,2.30 g N·m^-2·yr^-1,4.60 g N·m^-2·yr^-1,9.20 g N·m^-2·yr^-1and 13.80 g N·m^-2·yr^-1).The survival,growth,reproduction data of all individuals in the investigated populations of both study species were collected during two consecutive years,and Integral Projection Models（IPMs）were constructed to simulate the population dynamics of the two study species under different nitrogen addition treatments.The inherent mechanism of population dynamics under different nitrogen addition treatments was examined.The main results are as followed:（1）Nitrogen addition significantly affected the survival and growth of S.bungeana and L.secalinus.It also had a significant effect on the reproduction of L.secalinus,the survival rate of both species first increased and then decreased with the increase of nitrogen addition.The average plant height of both species increased under nitrogen addition treatments.（2）Under all nitrogen addition levels,the population growth rates of S.bungeana and L.secalinus were lower than 1,indicating that both species were declining.The population growth rates of both species under different nitrogen addition treatments were most sensitive to individual survival,followed by annual individual growth.However,the population growth rates of both species were insensitive to the changes in reproduction.（3）The results of Life Table Response Experiment（LTRE）analyses showed that the changes in population growth rates of both species under different treatments were mainly driven by the changes in the survival rates of large individuals.The above results demonstrate that it is important to consider the influence of management actions on the survival rates of large individuals when designing the population management strategies in semi-arid grasslands.The results of this study may provide scientific basis for grassland protection and management in semi-arid region under the background of nitrogen deposition.By improving the survival rate of large individuals through appropriate management measures,it is possible to reverse the declining status of the two study populations,so as to achieve stable population growth in the long run.