Impacts Of Nitrogen And Phosphorus Addition On Tree Growth Traits In Subtropical Secondary Broadleaved Forest

The intensification of human activities has changed the speed of nitrogen（N）and phosphorus（P）cycle in terrestrial ecosystem,especially increased the rate of N and P deposition,and had a series of effects on tree growth,which may affect forest productivity and ecosystem service function.In the forest soil with relatively rich nitrogen and phosphorus deficiency in the subtropics,the growth of trees may be limited by P,but under the background of serious N and P deposition in the future,the response of subtropical forests is still unknown.This study selected the secondary broadleaved forest in Jiulianshan Nature Reserve as the research object,established 64 100 m² experimental plots of nitrogen and phosphorus addition,including control,nitrogen(100 kg N ha^-1 yr^-1),phosphorus(50 kg P ha^-1 yr^-1)and nitrogen+phosphorus treatments(100 kg N ha^-1 yr^-1+50 kg P ha^-1 yr^-1).Trees in the arborous layer were divided into different functional groups according to dominance（dominant species,non-dominant species）and DBH（small tree,medium tree and large tree）,and discussed the effect and mechanism of N and P addition on the growth of subtropical forest trees from four aspects:forest soil nutrients,underforest light environment,plant leaf functional traits and plant growth status.The main results are as follows:（1）The growth response of trees communities in different functional groups to the addition of nitrogen and phosphorus was different.The N addition promoted the large tree growth of dominant species,while P addition promoted the small tree growth of non-dominant species.The N addition increased the mortality of non-dominant species,while P addition increased the small tree mortality of dominant species.With the progress of fertilization experiment,The N addition gradually reduced the amount of litter in the stands.（2）Compared with the non-dominant species,the leaf community stoichiometry of the dominant species was more sensitive to the addition of nitrogen and phosphorus.The addition of N significantly increased the community leaf community N:P and aggravated the P restriction of tree growth.The effect of nitrogen and phosphorus addition on leaf community stoichiometry of different diameter class communities was not significant.（3）The growth of dominant species was mainly affected by the increase of soil available P and leaf N:P caused by P addition,and the positive effect of leaf N concentration and specific leaf area（SLA）caused by N addition.The growth of non-dominant species was mainly affected by the decrease of leaf N:P and the increase of SLA caused by P addition,and negatively affected by the increase of soil N content,leaf N concentration and leaf P concentration caused by N addition.The death of dominant species was mainly affected by the increase of soil total N and leaf N content and the decrease of leaf N:P content caused by N and P addition,while the mortality of non-dominant species was positively affected by the increase of leaf N content and the decrease of leaf P content caused by N addition.To sum up,in the subtropical secondary broadleaved forest,the addition of P was beneficial to the growth of non-dominant species,while the addition of N was beneficial to the growth of dominant species,and the response of tree growth to the addition of nitrogen and phosphorus was different among diameter classes.The results show that N deposition may be beneficial to maintain the species composition of current forest community,while P deposition can accelerate the succession of subtropical forest community.Therefore,under the background of increasing global N deposition,the succession process of subtropical forests limited by P may become difficult to predict.