Response of plant first flowering and biomass allocation to nitrogen addition in an alpine meadow

In this study, we analyzed the response of flowering phenology and reproductive allocation to nitrogen addition in 31 common species in an alpine meadow community and its effect on the relationship between the first flowering day and biomass allocation. The results showed that nitrogen addition had a significant effect on both flowering phenology and reproductive allocation of the studied species. The first flowering day of graminoid species was significantly delayed by 3～8 days due to nitrogen addition (P < 0.05). The response of first flowering day of forbs to nitrogen addition varied among species, with high-level nitrogen addition significantly delaying the first flowering day of two Compositae species by 4 days (Saussurea hieracioides and Saussurea nigrescens) (P < 0.05); and advancing the first flowering day of Ranunculaceae, Gentianaceae and Scrophulariaceae species (Veronica eriogyne, Pedicularis kansuensis) by 3～4 days. The first flowering day of other forb species remained unchanged with nitrogen addition, while the effect of the high-level nitrogen addition on the first flowering day was greater in comparison to the effect of low-level nitrogen addition. All graminoid species tended to reduce reproductive allocation in response to nitrogen treatment, particularly with high-level nitrogen addition (P < 0.05). For forb species, reproductive allocation increased in most species, with high-level nitrogen addition having a greater effect compared to low-level nitrogen addition. There was a significant positive correlation between the first flowering day and reproductive allocation in the control (P < 0.05), however, this relationship was not observed in nitrogen addition treatments (P > 0.05). The first flowering day showed a significant positive correlation with stem allocation (P < 0.001) and a significant negative correlation with leaf allocation (P < 0.05), with these trends increasing with the increasing amount of nitrogen addition. These results suggest that the different responses of flowering phenology and biomass allocation to nitrogen addition could alter the balance of plant resource allocation and competition coexistence in community, potentially leading to changes in community composition, structure and function.