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 Rancho 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

In both kauri and juniper trees, the ratio of c_i to c_a (atmospheric CO₂ concentration) remained fairly constant throughout geologic time (approximately 0.56 for juniper and 0.63 for kauri). Consequently, c_i for both trees decreased into the past as c_a decreased. During the last ice age, these trees operated at c_i values that are unprecedented in modern plants. Even with c_i levels less than half that of modern trees, both species were able to maintain levels of ice age growth that matched growth levels seen today. These results have significant implications for the evolutionary responses of plants to CO₂. 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.