| With the continuous intensification of human activities,the emission of reactive nitrogen（N）compounds has increased sharply,resulting in great increase in the atmospheric nitrogen deposition.High N deposition can have profound impacts on plantation ecosystems.Understory herbs are an important part of plantation ecosystems,leaf functional traits of understory herbs can reflect the mechanism and strategy of plant adaptation to the environment,which is one of the important indicators to predict ecosystem structure and functional stability.It is an important aspect to understand and predict the future changes of plantation ecosystems under increasing N deposition by using long-term N addition experiment to clarify the mechanism of anthropogenic N deposition affecting the functional traits and growth of understory herbs.In this study,the understory herbs of young and mature larch plantations were selected as the research objects.Using the randomized block design method,three levels of N addition treatments were randomly set in nine plots,including control（no N addition,Control）,low N addition(20 kg N ha-1 year-1,N20),and high N addition(50 kg N ha-1 year-1,N50).The relevant indicators of photosynthesis（net photosynthetic rate,stomatal conductance,transpiration rate,chlorophyll content,and specific leaf areas）,hydraulics（predawn water potential,mid-day water potential,and hydraulic conductivity）,anatomy（vascular bundle diameter,palisade mesophyll thickness,spongy mesophyll thickness,upper epidermis thickness,lower epidermis thickness,and leaf thickness）,nutrients（N concentration,phosphorus concentration,and N/P ratio）and growth（nonstructural carbohydrates concentrations and biomass）traits of understory herbs in long-term（12 year）N addition control experiments were measured to study the response of understory herbs in plantations of different stands to long-term N addition.The main results are as follows:1.Nitrogen addition had no significant impact on the biomass of understory herbs in mature stand,but significantly increased leaf N concentration and N/P ratio,and significantly decreased leaf phosphorus concentration.In the young stand,N addition only decreased leaf phosphorus concentration of understory herbs,but had no significant impact on N concentration and N/P ratio.This indicates that understory herbs in mature stand are more severely phosphorus limited than that in young stand.2.In most cases,N addition had no significant impact on leaf functional traits of understory herbs.It is possible that the increase of canopy closure caused by nitrogen addition restricts the light resources obtained by understory vegetation.This indicates that the light limitation may be more severe than the nutrient limitation in the study area.3.Nitrogen addition significantly increased the aboveground biomass of understory herbs in the young stand,but had no impact on the aboveground biomass of understory herbs in the mature stand.The increase of understory herbs biomass in the young stand may be due to the change of species composition of understory herbs.After12 years of N addition,the species capable of rapid N conversion were screened out,and the species with slow N conversion were eliminated.4.In the young stand,there was a significant positive correlation between hydraulic traits,N/P ratio and photosynthetic traits,and a significant negative correlation between the concentration of non-structural carbohydrates and their components and leaf anatomical traits.In the mature stand,there was a significant positive correlation between hydraulic traits and photosynthetic traits,and a significant positive correlation between the concentration of non-structural carbohydrates and leaf anatomical traits.The results of this study can provide a theoretical basis for more accurate prediction of the impact of N deposition on understory herbs in temperate forest ecosystems,and also provide guidance for better nutrient management and vegetation restoration in plantation ecosystems in the future. |
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