Effect Of Nitrogen Addition And Grazing Disturbance On The Composition Of Seed Germination Traits In Alpine Meadow

Alpine meadows on the Qinghai-Tibet Plateau are important parts of the terrestrial ecosystem and they can provide important ecosystem services,including species diversity maintaining,climate regulation,prevention of soil erosion,and water conservation.They also provide an important materials basis for the development of animal husbandry.Alpine meadows are being affected by varies of human disturbance,such as nitrogen addition and grazing,to meet the needs of social and economic development.It has caused dramatic changes in plant community structure and species diversity.Seed germination traits are the earliest functional traits expressed in the life history of plants.They are regulated by environmental conditions and to a certain extent,they can determine the type of habitats that plants faced by plants after seed germination.They can also affect the expression of post-germination traits.Therefore,seed germination traits can affect population dynamics,community species composition and ecosystem function.However,there is few research on the influence of human disturbance on the composition of seed germination traits at the community level.The main hypothesis of this study is that human disrurbance（nitrogen addition and grazing）can significantly change the selection of environment on germination traits,and ultimately cause significant changes in the composition of germination traits and in diversity of germination traits of alpine grassland plant communities.In order to test this hypoyhesis,effect of nitrogen addition and grazing on germination traits composition of alpine meadow was studied by combining nitrogen and grazing experiments in alpine meadows and germination experiments in the laboratory.This research can provide a better understanding in the influence mechanism of human disturbance on community species composition and diversity under the perspective of seed germination traits,and can give us a theoretical basis for the restoration of degraded rangelands.The main findings are:1)At the species level,germination percentage（GP）,germination rate（GR）and breadth of temperature niche for germination（BTN）were significantly positive related to the response of species abundance to fertilization(R_af).2)The community-weighted mean of germination percentage(CWM_GP),germination rate(CWM_GR),germination response to alternating temperature(CWM_R5/25)and the breadth of temperature niche for germination(CWM_BTN)increased with the increase of nitrogen addition gradients;while the response of germination to wet-cold storage(CWM_Rwc)and the richness of germination traits（FRic）were decreased accordingly.3)At species level,seed germination percentage（GP）at 15℃is significantly negative related to the response of abundance to grazing(R_ag);GP under other temperature conditions（5,10,20,25℃）,germination rate（GR）at all temperatures,breadth of germination temperature niche（BTN）and R_ag showed negative correlation trend,but it is not statistically significant.The response of germination to alternating temperature(R_5/25)and to wet-cold storage(R_wc)show positive correlatin trend with R_ag,but it is not statistically significant.4)At the community level,the response of germination to alternating temperature(CWM_R5/25)and germination trait richness（FRic）in grazing grassland are significantly higher than those in nongrazing grassland.In contrast,the breadth of temperature niche for germination(CWM_BTN)is significantly lower than that of nongrazing grassland.In conclusion,environmental changes after nitrogen addition and grazing can significantly affect the selection of the environment on seed germination traits and can generate a different set of germination traits in alpine meadows.The diversity of germination traits in alpine meadows has also significantly changed by nitrogen and grazing（nitrogen addition can significantly decrease,however,grazing can increase germination traits diversity in alpine meadow）.To some extent,this research can help us understand the influence mechanism of disturbance on the community structure of alpine meadow from the perspective of seed germination,and provide a theoretical basis for the restoration of degraded grasslands.