| Numerous studies have found that nitrogen addition decreases plant diversity and changes community composition in grasslands(often leading to more grasses,fewer legumes,and no clear trend in forbs/sedges).To explain this phenomenon,researchers have proposed three main hypotheses: the asymmetric competition hypothesis,the niche dimensionality hypothesis and the nitrogen detriment hypothesis.Although many studies have tested these hypotheses,they have not directly,precisely and integrally tested the three hypotheses from plant individual traits to population growth and dynamics to species interactions,which has caused difficulties in understanding the mechanisms and debates among the different hypotheses.As a biodiversity hotspot,the Qinghai-Tibet Plateau has unique alpine grassland ecosystems that are sensitive to global change factors such as nitrogen deposition.To better test the three hypotheses,this dissertation combines a nitrogen-addition density gradient experiment and the Lotka-Volterra competition model in an alpine grassland on the Tibetan Plateau within the framework of modern coexistence theory,and quantifies intrinsic growth rates(i.e.,capacity of population growth),niche differences and fitness differences(i.e.,differences in competitive ability)of 20 species representing four functional groups(grass,sedge,forb and legume)under control and nitrogen application treatments to investigate the effect of nitrogen addition on species coexistence and its biological mechanisms(plant traits,light competition,allometric growth).The results showed that: 1)nitrogen application significantly increased intrinsic growth rates of grasses,but decreased intrinsic growth rates of the other three functional groups.Moreover,nitrogen application significantly reduced niche differences between species(supporting the niche dimensionality hypothesis),sharpened fitness differences between species(supporting the asymmetric competition hypothesis),making competition exclusion between species more likely to occur.Overall,the effects of nitrogen application on these three parameters favored grasses but disfavored legumes(partly supporting the nitrogen detriment hypothesis),while the effects on sedges or forbs varied across contexts,consistent with the changes in the relative abundance of different functional groups after nitrogen application found in many previous studies;2)Individual traits,photosynthetically active radiation received,and height scaling exponents significantly determined intrinsic growth rates of species and fitness differences between species(but affected niche differences only marginally),and the effects generally strengthened after nitrogen application.In addition,the changes in individual biological characteristics from the control to nitrogen application generally explained the changes in intrinsic growth rates and fitness differences.Overall,these findings suggest that individual biological characteristics are the biological basis for population growth capacity,species competition and coexistence relationships and their changes from the control to nitrogen application.In summary,this dissertation tests the three hypotheses in a more systematic way,and clearly shows how nitrogen addition affects herbaceous plants’ existence and coexistence from individuals to populations to species interactions,which provides new insights into the mechanisms underlying the loss of plant diversity under nitrogen deposition in natural grasslands. |
