Nutrient addition and herbivore exclusion alter plant traits and biomass via distinct mechanisms: intraspecific variability vs species turnover

Soil nutrients and vertebrate herbivory are key ecological factors with opposite and interactive effects on grassland plant traits and biomass. Partitioning trait changes into species turnover and intraspecific change provides a mechanistic linkage between trait shifts and biomass responses. However, their relative contributions in determining plant responses to nutrients and herbivory remain unclear. Based on a long-term experiment in two grasslands differing in productivity, we examined how nutrient addition and herbivore exclusion influenced plant functional composition and biomass, and quantified contributions of inter- and intraspecific trait change. Nutrient addition shifted leaf economics traits to be faster-growing and increased plant height, while herbivore exclusion boosted height and leaf area, both mainly through intraspecific changes. These effects were habitat-dependent: leaf economics traits dominated in the low-productivity grassland, while size-related traits prevailed in the high-productivity grassland. Nutrient addition and herbivore exclusion had weak effects on plant defense traits (tannins). Biomass responses to nutrient addition and herbivore exclusion were, to a greater extent, associated with intraspecific trait variation than species turnover. This study highlights how partitioning traits into different dimensions helps understand the distinct pathways through which nutrients and herbivores shape plant communities, how these vary across environments, and ultimately influence ecosystem functioning.