Unexpected reversal of C ₃ versus C ₄ grass response to elevated CO ₂ during a 20-year field experiment

A short-term trend reversed Theory and empirical data both support the paradigm that C 4 plant species (in which the first product of carbon fixation is a four-carbon molecule) benefit less from rising carbon dioxide (CO 2 ) concentrations than C 3 species (in which the first product is a three-carbon molecule). This is because their different photosynthetic physiologies respond differently to atmospheric CO 2 concentrations. Reich et al. document a reversal of this pattern in a 20-year CO 2 enrichment experiment using grassland plots with each type of plant (see the Perspective by Hovenden and Newton). Over the first 12 years, biomass increased with elevated CO 2 in C 3 plots but not C 4 plots, as expected. But over the next 8 years, the pattern reversed: Biomass increased in C 4 plots but not C 3 plots. Thus, even the best-supported short-term drivers of plant response to global change might not predict long-term results. Science , this issue p. 317 ; see also p. 263