Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta‐analytic test of current theories and perceptions

C4 plants contribute ≈ 20% of global gross primary productivity, and uncertainties regarding their responses to rising atmospheric CO2 concentrations may limit predictions of future global change impacts on C4‐dominated ecosystems. These uncertainties have not yet been considered rigorously due to expectations of C4 low responsiveness based on photosynthetic theory and early experiments. We carried out a literature review (1980–97) and meta‐analysis in order to identify emerging patterns of C4 grass responses to elevated CO2, as compared with those of C3 grasses. The focus was on nondomesticated Poaceae alone, to the exclusion of C4 dicotyledonous and C4 crop species. This provides a clear test, controlled for genotypic variability at family level, of differences between the CO2‐responsiveness of these functional types. Eleven responses were considered, ranging from physiological behaviour at the leaf level to carbon allocation patterns at the whole plant level. Results were also assessed in the context of environmental stress conditions (light, temperature, water and nutrient stress), and experimental growing conditions (pot size, experimental duration and fumigation method).