Plant responses to CO2 are a question of time

Rising carbon dioxide (CO2) concentrations in the atmosphere as a result of fossil fuel burning are expected to fertilize plants, resulting in faster growth. However, this change is not expected to be the same for all plants. Rather, scientists believe that differences in photosynthetic mechanism favor one plant group—the C3 plants—over the other, the C4 plants. On page 317 of this issue, Reich et al. (1) show that, although this expectation is met in the first few years of a long-term experiment, the situation reverses after 15 to 20 years, with important implications for future crop production and ecosystems.