Winners always win: growth of a wide range of plant species from low to future high CO2

Evolutionary adaptation to variation in resource supply has resulted in plant strategies that are based on trade-offs in functional traits. Here, we investigate, for the first time across multiple species, whether such trade-offs are also apparent in growth and morphology responses to past low, current ambient, and future high CO2 concentrations. We grew freshly germinated seedlings of up to 28 C-3 species (16 forbs, 6 woody, and 6 grasses) in climate chambers at 160ppm, 450ppm, and 750ppm CO2. We determined biomass, allocation, SLA (specific leaf area), LAR (leaf area ratio), and RGR (relative growth rate), thereby doubling the available data on these plant responses to low CO2. High CO2 increased RGR by 8%; low CO2 decreased RGR by 23%. Fast growers at ambient CO2 had the greatest reduction in RGR at low CO2 as they lost the benefits of a fast-growth morphology (decoupling of RGR and LAR [leaf area ratio]). Despite these shifts species ranking on biomass and RGR was unaffected by CO2, winners continued to win, regardless of CO2. Unlike for other plant resources we found no trade-offs in morphological and growth responses to CO2 variation, changes in morphological traits were unrelated to changes in growth at low or high CO2. Thus, changes in physiology may be more important than morphological changes in response to CO2 variation.