| Global change affects the growth,development,reproduction and distribution of plants.As an important component of global change factors,plant invasion is also affected by other global change factors,such as climate warming and nitrogen deposition by changing the resource dynamics,plant and soil feedbacks,and species interactions.Invasive plants can alter the community structure and function of indigenous species,thus leading to the decrease of biodiversity and the weakening of the ecosystem function.Some researchers have studied the effects of warming and nitrogen enrichment on the growth of invasive species,but how these two factors can affect plant invasion via changing the competition between invasive and native species is still unknown.The increasing nitrogen deposition has resulted in a series of problems such as the nitrogen eutrophication,imbalance between nitrogen and phosphorus.Most researchers just examined the effects of changes in absolute nutrient availability on plants and ecosystem functions,but how the nutrients balance(e.g.,altered proportion of available nitrogen and phosphorus in the soil)may influence the plant functions and traits received less attention,especially lack of relevant research about invasive plants.To answer above questions,we chose an invasive plant P.virginica and a native P.asiatica,and made comparison studies in a common garden experiment.The seeds of the native population of P.virginica were collected from Stateboro,George,US,and the seeds of the invasive population of P.virginica and native P.asiatica were collected from Xiamafang Park in Nanjing,Jiangsu province.We used open top chamber to increase temperature and applied nitrogen to simulate nitrogen deposition.The effects of warming and nitrogen addition on the growth and competitive ability of P.virginica and P.asiatica were examined by measuring and calculating biomass,root-shoot ratios,competitive effects and competitive responses and other characteristics.In addition,we also set an experiment with three different N:P ratios and two different nutrient levels to explore the effects of nutrients balance.The major results are shown below:(1)Nitrogen addition significantly enhanced the aboveground biomass,belowground biomass and total biomass of P.virginica populations from home and introduced ranges and P.asiatica.Meanwhile,nitrogen addition significantly increased the mean spike length,mean spike number and plant height of P.virginica populations from home and introduced ranges,indicating that nitrogen was a limiting factor for the plant growth.(2)Plants in both native and invasive populations of P.virginica had greater aboveground biomass,but less belowground biomass than native species P.asiatica,showing different strategies of biomass allocation between invasive and native species.Under warming and its interaction with nitrogen addition,the plants originating from the native population of P.virginica increased more belowground biomass whereas those from the invasive population increased more aboveground biomass,indicating the responses of the two populations to temperature and nutrient were different and showed different biomass allocation strategies between two populations of P.virginica.(3)Interactive treatments of warming and nitrogen addition significantly increased the SLA of P.virginica from home range,and under this condition,the SLA of P.virginica from introduced range was highest among all populations,indicating the interactive treatments of warming and nitrogen addition was beneficial to improve the photosynthetic ability of P.virginica,thereby enhancing the competition for light.(4)Under controlled condition,the native population of P.virginica received more suppressive effects than the invasive population,so the competitive advantage of invasive population may contribute to the invasion success of P.virginica in China.Under warming treatment,the competitive response of plants from native population of P.virginica was relatively stable,whereas the competitive response of plants from invasive population of P.virginica was significantly reduced.Nitrogen addition,W × N treatments significantly reduced the competitive response of invasive population of P.virginica in belowground biomass,but increased the competitive response of native population of P.virginica in aboveground biomass and total biomass,which indicated that under different treatments,the two populations of P.virginica also had different competitive abilities in addition to the differences in biomass allocation strategies.(5)High nutrient supply level and low N:P supply ratios were beneficial to the biomass accumulation of P.virginica populations from home and introduced range and native species P.asiatica.At the reproductive stage,both the native and invasive populations of P.virginica and native species P.asiatica had larger SLA under high N:P supply ratios.At the vegetative stage,plants of all populations absorbed more nitrogen,the invasive population of P.virginica and native species P.asiatica absorbed more phosphorus under high nutrient supply level and low N:P supply ratios,the higher N:P ratios supplied in experiment,the larger ratios of nitrogen and phosphorus in plant tissues.(6)Under all nutrient treatments,the root-shoot ratios of both native and invasive populations of P.virginica were significantly lower than native species P.asiatica,indicating P.virginica allocated more biomass to aboveground part to increase the competition for space and light.This phenomenon was similar to that of warming and nitrogen addition experiment.(7)The aboveground biomass,belowground biomass,total biomass,root-shoot ratios,nitrogen concentration,ratios of nitrogen and phosphorus of P.virginica to different nutrient treatments showed higher phenotypic advantage than native species.This study revealed the responses of invasive plants P.virginica to warming and nutrient additions,and can help to predict the invasion trend of P.virginica in the context of global change and to formulate effective invasion prevention strategies and managements. |
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