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 approximate to 20% of global gross primary productivit y, and uncertainties regarding their responses to rising atmospheric CO2 co ncentrations may limit predictions of future global change impacts on C4-do minated ecosystems. These uncertainties have not yet been considered rigoro usly due to expectations of C4 low responsiveness based on photosynthetic t heory and early experiments. We carried out a literature review (1980-97) a nd meta-analysis in order to identify emerging patterns of C4 grass respons es 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 variabi lity at family level, of differences between the CO2-responsiveness of thes e functional types. Eleven responses were considered, ranging from physiolo gical behaviour at the leaf level to carbon allocation patterns at the whol e plant level. Results were also assessed in the context of environmental s tress conditions (light, temperature, water and nutrient stress), and exper imental growing conditions (pot size, experimental duration and fumigation method). Both C4 and C3 species increased total biomass significantly in elevated CO 2, by 33% and 44%, respectively. Differing tendencies between types in shoo t structural response were revealed: C3 species showed a greater increase i n tillering, whereas C4 species showed a greater increase in leaf area in e levated CO2. At the leaf level, significant stomatal closure and increased leaf water use efficiency were confirmed in both types, and higher carbon a ssimilation rates were found in both C3 and C4 species (33% and 25%, respec tively). Environmental stress did not alter the C4 CO2-response, except for the loss of a significant positive CO2-response for above-ground biomass a nd leaf area under water stress. In C3 species, stimulation of carbon assim ilation rate was reduced by stress (overall), and nutrient stress tended to reduce the mean biomass response to elevated CO2. Leaf carbohydrate status increased and leaf nitrogen concentration decreased significantly in eleva ted CO2 only in C3 species. We conclude that the relative responses of the C4 and C3 photosynthetic typ es to elevated CO2 concur only to some extent with expectations based on ph otosynthetic theory. The significant positive responses of C4 grass species at both the leaf and the whole plant level demand a re-evaluation of the a ssumption of low responsiveness in C4 plants at both levels, and not only w ith regard to water relations. The combined shoot structural and water use efficiency responses of these functional types will have consequential impl ications for the water balance of important catchments and range lands thro ughout the world, especially in semiarid subtropical and temperate regions. It may be premature to predict that C4 grass species will lose their compe titive advantage over C3 grass species in elevated CO2.