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Ecosystem responses to atmospheric CO2 enrichment, as to other climate change drivers, depend on plant community responses to CO2 (community response) and feedbacks from community change on ecosystem processes (community effect).  We used data from two multi-year experiments in central Texas, USA to assess the role of plant communities in the response of grassland productivity to CO2 enrichment.  In experiments with C3-C4 pasture and C4-dominated tallgrass prairie assemblages, increasing CO2 from pre-Industrial to elevated concentrations increased aboveground productivity (ANPP) by 80-120 g/m2 per 100 ppmv rise in CO2.  Feedbacks from species change on the ANPP response to CO2 differed in these experiments.  CO2-mediated change in the C4 composition of communities increased ANPP slightly in C3-C4 pasture, but accounted for 14-38% of the ANPP increase in C4-dominated prairie.  Community responses to CO2 were linked to plant traits associated with a positive response to greater water availability.  By contrast, community effects on ANPP were linked to species differences in water-use efficiency.  Our results indicate that the species traits favored by climate change drivers may differ from the traits that determine feedbacks of community change on ANPP.