| Field experiment was carried out from 2009 to 2010 in the alpine meadow ecosystems from the eastern Qinghai-Tibet plateau of China. We used two experiments to study the interaction between species, which are neighbor removal experiment under natural conditions and fertilization and mowing experiment.Neighbor removal experiment was designed to test that the interactions between species were positive or negative by removing all neighboring species within 10cm of a target individual. Interactions among plants were assessed by comparing the performance of target plants with neighbors removed to that of control target plants around which neighbors were left intact. We calculated Relative Neighbor Effect index (RNE) by observing plant traits such as biomass, number of leaves, height as measures of interaction strength and direction.Neighbor removal experiment's results is:1) Under natural conditions, we found that the performances of species on biomass, number of leaves and the rate of flowering or fruiting were competitive, positive interactions were detected from the performances of species on height and the rate of survival.2) The presence of facilitation was relative to the gradients of disturbance, stress, the spatial and temporal variation. In addition, facilitation is species-specific, trait-specific and regional specificity.3) Whether in harsh environments or in benign environments, facilitation and competition might exist simultaneously. The relationship between positive and negative interactions was on a balance.Fertilization and mowing experiment involved plant community block experiment and one plant control experiment. The experiment took a randomized block experimental design, and set nine treatment-combinations of three levels of fertilization (unfertilized control, once, twice) and three levels of simulated grazing (unmowed control, once, twice). Each treatment had eight replicates. One plant control experiment repeated the operations of neighbor removal experimental treatments, which also set nine treatment-combinations of three levels of fertilization and mowing simultaneously. We used biomass, competitive effect, competitive response to evaluate their responses under the conditions of nine mowing and fertilization treatment combinations.The result of fertilization and cutting experiment indicated that:1) There was no existing evidence of positive interaction, competition still dominated the interactions.2) Productivity of plant communities may be mainly influenced by fertilization in 2008, however, in 2009 and 2010 it was mainly influenced by mowing.3) Fertilization, mowing and their interactions all had great effect on the total biomass of the community during three years. Moreover, fertilization or mowing had great effects not only on the overall productivity of plant communities but also on the relative competitive ability of species, which means that species have different growth response to different treatments.4) Fertilization or mowing had changed the relationships of competitive interactions among species, and then changed the community structure and dynamic.5) With any change of ecological factors, there is some degree of change of plant community. The current composition and structure of plant community are primarily the result of different responses of species to environment factors.The experiments confirmed positive interactions occurred widespread in the alpine meadow community, and further explored the internal mechanisms explained for the positive and negative interactions to provide a theoretical basis for the research on species interactions, which allowed people to a better understanding the significant influence of species interaction on plant community composition, species coexistence, diversity and maintenance of the ecosystem stability and function. This study also provided valuable evidences for the studies on change of plant community responses to ecological factors and prediction for community structure and dynamics.
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