Shift in community functional composition following nitrogen fertilization in an alpine meadow through intraspecific trait variation and community composition change

In recent years, ecologists have tried to determine the importance of intraspecific trait variation (ITV) versus community composition change (CCC) in shifts in community functional composition. However, results to date have not provided generality. A six-year nitrogen (N) fertilization experiment was conducted in species-rich alpine meadow communities. We focused on five key functional traits (height, leaf dry mass content LDMC, specific leaf area SLA, leaf nitrogen content LNC, and leaf phosphorus content LPC) that belong to three trait categories (whole-plant level trait, leaf morphology trait and leaf chemical trait) and measured biomass of each species in community. A sum of squares decomposition method was used to distinguish the relative contribution of ITV versus CCC to community weighted mean (CWM) traits. Our results showed that N fertilization led to increasing height(CWM) and LNCCWM due to light competition intensification and soil nutrient enrichment. However, the responses of community-wide SLA, LDMC and LPC were highly inconsistent and depended on the balance of different opposing processes. Moreover, during short-term fertilization, ITV played a more important role in mediating functional composition changes in all traits, but the effects of CCC overwhelmed ITV and became more important in determining community-wide whole-plant level trait and leaf morphology traits (height, LDMC and SLA) in subsequent fertilization years. On the other hand, ITV always played a more important role than CCC in determining the community-wide leaf chemical traits (LNC and LPC), but CCC had a greater contribution than ITV in terms of explaining shifts in whole-plant traits (height) and leaf morphology traits (LDMC and SLA). In addition, both positive and negative covariations appeared in our study, indicating that community-wide trait shifts due to ITV and CCC may either reinforce or oppose one another, depending on different trait categories. These findings highlight the importance of ITV, CCC and their covariation in mediating community functional composition, and the relative importance of ITV effects and CCC effects depended on fertilization duration and trait category. This study significantly improve our understanding of the mechanisms governing the shifts in community functional composition under N deposition scenarios.