Our data demonstrate substantial among family intraspecific variation in response to elevated CO2. This variation is a necessary precondition to any evolutionary response to rising CO2 and could lead to large shifts in average population-level responses under elevated CO2 conditions. Evolutionary changes within populations could thus alter both ecosystem-level processes such as carbon storage and community-level interactions among species. It is important, therefore, to understand the magnitude and distribution of this variation as we refine our predictions of the ecological consequences of global climate change.