Abstract: Nutrient enrichment influences grassland ecosystem productivity through plant functional trait-mediated processes. However, how responses across hierarchical levels-ranging from intraspecific responses to species reordering and turnover-interact to jointly shape aboveground net primary productivity (ANPP) via functional traits is not well understood. Our study aimed to quantify these interrelationships and their relative contributions to predicting ANPP changes. We conducted a five-year experiment involving nitrogen (N) and phosphorus (P) addition and quantified the hierarchical community responses driving ANPP by systematically investigating plant communities and measuring functional traits. Our findings indicated that individual species response was the main driving force for ANPP changes (path coefficient: 0.91), whereas species reordering and turnover contributions were not significant (-0.29 and -0.27). Plant coverage was the key trait linking community processes to ANPP. N addition promoted the growth of plants and reordered the functional groups by increasing the coverage of sedges and grasses and reducing the coverage of legumes and forbs. Species turnover had a weak influence on ANPP due to the low ANPP of both the lost and gained species. These results indicated that N and NP addition significantly increased ANPP by enhancing the coverage of persistent species. Conversely, P addition did not significantly increase ANPP, but was linked to it through plant N:P ratios-mediated individual species responses. This offers a new perspective on ANPP responses to nutrient enrichment through functional traits-ecosystem function relationships.

Key words: alpine meadow, ANPP, individual species response, plant coverage, species reordering, species turnover