The Response Of Leaf Traits And Tree Growth To Nitrogen And Phosphorus Additions In A Subtropical Broadleaved Evergreen Forest

The atmosphere deposition of high nitrogen and low phosphorus has become a serious environmental issue in subtropical China,but its influence on plant functional traits and growth remains unclear,especially in the uneven-age subtropical broadleaved evergreen forest with coexistence of arbuscular mycorrhizal(AM)and ectomycorrhizal(ECM)species.We design an N and P addition experiment of four fertilization treatments(control;+N,100 kg N ha^-1 a^-1;+P,50 kg P ha^-1 a^-1;N+P,100 kg N ha^-1 a^-1+50 kg P ha^-1 a^-1)with three replications(12 20 m×20 m plots)in a representative broadleaved evergreen forest in Jiulianshan Forest Ecological station at late 2015.After3-years,six AM trees,and four ECM trees,were selected as our model species and used to intensively study on leaf traits,tree growth and the relationship between them under two different growth stages(saplings and adult trees).The main results are as follows:(1)Leaf structural traitsFertilizations had no significant effect on leaf structural traits(LMA and morphological traits)and suggested that conservative leaf structure in response to N and P addition at stand level,and leaf morphological traits were mainly affected by mycorrhizal association or species characteristics.AM and ECM trees had different structural traits strategies acclimate to nutrients deposition,AM trees tend to increase leaf area and reduce leaf width to length ratio to obtain more light resources in saplings,and increase leaf thickness and LMA in adult trees,while ECM trees tend to increase LMA in both growth stages.The plasticity of leaf structure traits to nutrient additions of AM trees is stronger than that of ECM trees.(2)Leaf chemical traitsFertilization had significant effects on the leaf N,chlorophylls and amino acids,and had no significant influence on leaf organic carbon,non-structural carbohydrates,P and N:P ratios at the stand level.Considering the effect of different growth stages,the results showed that fertilization had significant effects on leaf organic carbon,N,chlorophylls and amino acids in saplings,while the leaf organic carbon of mature trees and leaf N was significantly affected by the interaction between fertilization and mycorrhizal association.The response strategies of leaf chemical traits to N and P addition of AM and ECM trees are different.The nutrients absorption capacity of AM trees is stronger than ECM trees.N+P significantly increased N:P and amino acids of ECM trees,which may lead to nutrient imbalance.Under systems that increasing with inorganic nutrients,AM trees tend to accumulate nutrients rapidly,while ECM trees tend to improve physiological defense capacity.In combination of+N and N+P significantly increased leaf N.We suggested that our studied forest may not reach the status of N saturation in 3-years fertilization,and the negative effect of N deposition on plants was not obvious.(3)Leaf photosynthetic physiological traitsThe response of the maximum rate of carboxylation(Vcmax)and the maximum rate of electron transport(Jmax)to fertilization were different between AM and ECM trees.+N tends to increase ECM trees and reduce the Vmax and Jmax of AM trees.The response of photosynthesis N use efficiency(PNUE)and photosynthesis P use efficiency(PPUE)to fertilization is more sensitive than that of AM trees,while the response of water use efficiency(WUEi)of AM trees to fertilization is more sensitive than that of ECM trees.Fertilization had significant effects on Jmax and PPUE in saplings,while the interaction between fertilization and mycorrhizal association had significant effect on Vcmax in saplings.The sensitivity of photosynthetic physiological traits of ECM trees to fertilization is greater than that of AM trees.It is possible that enhancing PPUE is an important strategy of ECM trees in fertilization acclimation.(4)Response of tree growthFertilization has little effect on the relative growth rate(RGR)of AM trees,but it has an obvious effect on the increasement or inhibition of ECM trees.The interaction between tree species and fertilization had a significant effect on RGR.Mycorrhizal association may not be the key factor that dominates the response of tree growth to fertilization,and species-specific growth response may largely determine the effect of fertilization.In addition,RGR of ECM trees at medium and small size increased by+N and+P,while N+P increased RGR of AM trees at large size.It can be seen that the RGR of large-diameter trees were increased,while the RGR of medium and small-sized trees often decreased,which showed that the nutrient supplement and light availability are two key factors to control the growth of trees in the subtropical broadleaved evergreen forest.(5)The internal and external factors of tree growthThere was no significant correlation between tree RGR and leaf traits at stand level,but there was a significant correlation between leaf traits and maximum photosynthetic rate(Pmax).The trait correlation networks of AM and ECM trees were different.For AM trees,leaf P was the key leaf trait that affects RGR,but ECM trees only found that leaf N:P was negatively correlated with RGR.At the same time,PPUE was the key leaf traits that can affect the Pmax of both AM and ECM trees.The slope of PPUE and LMA in ECM adult trees was higher than that in AM adult trees and ECM saplings,and that in ECM adult trees was higher than that in ECM saplings.AM trees had stronger traits plasticity than ECM tree species,and adult tree species had stronger traits plasticity than saplings.The structural equation model(SEM)analysis showed that fertilization treatment and mycorrhizal association affect tree growth by mediating soil nutrients and leaf traits.It is concluded that the tree growth feedback of N and P addition is lagging behind the leaf traits in the subtropical broadleaved evergreen forest.In conclusion,the response of leaf traits of dominant trees to nutrient supply in the subtropical broadleaved evergreen forest varied with mycorrhizal association and growth stage.AM trees are less responsive,which may be due to the great difference in traits plasticity among trees.ECM trees are more responsive and acclimate to N and P deposition mainly through improving resource utilization efficiency and chemical defense.Young trees were more sensitive in response to fertilization than adult trees,which could be used as the main material to evaluate the acclimation and response of forests to environmental change due to their smaller nutrient sink.3-years of fertilization experiments have not found a significant change in forest growth,but fertilization and mycorrhizal association can indirectly affect tree growth by mediating soil nutrients and leaf traits.Overall,the effects of N and P deposition on tree growth in the broadleaved evergreen forest mainly depend on tree species or mycorrhizal association,and the contrasting acclimation of mycorrhizal association will increase the uncertainty of future community succession.