During the last ice age, atmospheric CO₂ concentrations were approximately 180 ppm and have currently risen to a modern value of 385 ppm as a result of fossil fuel combustion and deforestation. In order to understand how changing CO₂ concentrations influenced the growth of trees over evolutionary time scales, we studied juniper wood specimens from the La Brea tar pits in southern California and kauri wood specimens from peat bogs in New Zealand. We hypothesized that both ice age (low CO₂ evolved) and modern trees (high CO₂ evolved) would show positive linear relationships between growth and inter-cellular CO₂ concentrations (c_i) at low c_i levels and reduced correlations between growth and c_i at higher c­_i levels where CO₂ is less limiting. However, we hypothesize that the overall curvilinear response would shift towards lower c_i values in ice age trees that may have adapted to lower atmospheric CO₂ concentrations. 
Results/Conclusions

As predicted, we have shown that modern and ice age trees show positive curvilinear relationships between c_i and growth across their respective c_i gradients. This shows that at low CO₂ concentrations of the last ice age, carbon is the limiting factor affecting growth in trees. These results have significant implications for the evolutionary responses of plants to CO₂ and predict major consequences of continued increases in atmospheric CO₂ concentrations. By understanding plant responses to a range of CO₂ values over geologic time, we will better understand the evolutionary responses of plants to changing environments.