Trait plasticity of intensive pasture species due to growth in mixture across seasons and nutrient addition levels

Questions: How do the traits of pastoral species respond to growth in mixture, nitrogen addition and season? What are the impacts of trait plasticity on community aggregate trait values? Study site: A large-scale field experiment on intensively managed dairy pastures in New Zealand. Methods: We measured traits linked to rate of return on investment in leaves – leaf nitrogen content (leaf N) and specific leaf area (SLA) – and biomass investment in leaf area – leaf area ratio (LAR). We collected trait data for 5 pasture species (one grass, two forbs, and two N2-fixing legumes) grown in monoculture or a five-species mixture across three levels of nitrogen (N) addition in four seasons. For each species in each season we tested for significant effects of growth in mixture, N addition, and their interaction. We calculated community-weighted mean (CWM) values in mixture plots using traits collected either from mixtures or monocultures. We tested for significant mixture and N addition effects on CWM, and for significant interactions between mixture and N addition. Results: SLA and LAR for all non-N2-fixers were significantly higher in spring, summer or autumn, and never significantly lower in mixture than in monoculture. All three non-N2-fixers experienced higher leaf N in mixture during summer, but two species had significantly lower leaf N in either winter or autumn. Mixture effects on CWM values for all three traits were negative in winter and positive in either spring or summer. Conclusions: The direction of trait plasticity effects on community level trait means was highly seasonally dependent.