Vegetation responses to high [CO2] include both direct photosynthetic effects and indirect effects associated with various plant and soil feedbacks. Synthesis of these direct and indirect effects requires ecosystem process models describing the cycling of carbon and essential mineral nutrients through plants and soils. Here we use the ecosystem model G'DAY to investigate responses to an instantaneous doubling of [CO2]. The analysis indicates that the magnitude and even direction of the growth response to high [CO2] can vary widely on different timescales, because responses on different timescales are determined by different ecosystem-level feedbacks and hence by different sets of key model parameters. Of particular importance are parameters describing the flexibility of plant and soil nitrogen to carbon (N:C) ratios; large responses occur if N:C ratios decline significantly at high [CO2], with little or no response if N:C ratios are inflexible. According to G'DAY, the CO2-response changes over time because responses on longer timescales are dictated by the N:C ratios of less rapidly cycled organic matter.