In chapter 7, we emphasized how the C4 photosynthetic pathway saves water, but some researchers suggest that the greatest advantage of C4 over C3 plants occurs when CO2 concentrations are low. What is the advantage of the C4 pathway when CO2 concentrations are low? As we shall see in chapter 23, the atmospheric concentration of CO2 has been increasing for the last century or so. If this trend continues, and if the interactions between C4 and C3 plants are influenced by atmospheric CO2 concentrations, how might the geographic distributions of C3 and C4 plants change? Discuss.