Gemini, a mechanistic model linking plant functional traits, plant populations, community dynamics, and ecosystem scale fluxes in grasslands has been reported in a companion paper (Soussana et al., 2012). For monocultures and six species mixtures of perennial grass species, this model has been successfully evaluated against experimental data of above-ground net primary productivity (ANPP) and plant community structure across nitrogen and disturbance (cutting frequency) gradients. The Gemini model combines two categories of processes: (i) C and N fluxes, (ii) morphogenesis and architecture of roots and shoots and demography of clonal plant axes. These two process categories constrain the form and function of the simulated clonal plants within plastic limits. We show here that the plasticity of the simulated plant populations accounts for well-established empirical laws: (i) root:shoot ratio, (ii) self-thinning, (iii) critical shoot N content, and (iv) role of plant traits (specific leaf area and plant height) for population response to environmental gradients (nitrogen and disturbance). Moreover, we show that model versions for which plasticity simulation has been partly or fully suppressed have a reduced ANPP in monocultures and in binary mixtures and do not capture anymore productivity and dominance changes across environmental gradients. We conclude that, along environmental and competition gradients, the plasticity of plant form and function is required to maintain the coordination of multiple resource capture and, hence, to sustain productivity and dominance.