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 N 2 -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-N 2 -fixers were significantly higher in spring, summer or autumn, and never significantly lower in mixture than in monoculture. All three non-N 2 -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.