Using Variable Stomatal Sensitivity to CO₂ in Relict Conifers to Reconstruct Ancient Atmospheres and Predict Future Implications of Climate Change

Kerry A. Whittaker, Colby College

Document Type Honors Thesis (Open Access)


With accumulating evidence linking a rise in atmospheric carbon dioxide (CO₂) since the industrial revolution to global climate change, the link between the partial pressure of CO₂ (pCO₂) and Earth's temperature underpins much of paleoclimatology and our predictions of future global warming. This study investigates the linear and non-linear relationships between conifer stomata and atmospheric CO₂ concentration using three extant species of Pinus and four genera of relic conifers. The results of this study show varied response of stomatal frequency (SF) between the relic species Athrotaxis, Callitris, Araucaria, and Agathis on both the species and generic levels in relation to fluctuating levels of CO₂ over the past 200 years. Agathis australis and Callitris columellaris display significant inverse linear relationships to rising atmospheric CO₂. Athrotaxis species display a non-linear response to pCO₂ concentrations above 370ppm, in which the plants become more reactive to CO₂ increase, and decrease stomatal number above this concentration. Maine conifers display insignificant stomatal reduction in higher pCO₂. Pinus banksiana demonstrates an increase in stomata with rising CO2, providing evidence for other ecological selection pressures that may elicit stomatal response. Many previous studies examine the response of angiosperms, which decrease stomata in elevated pCO2. Knowledge of stomatal responses in conifers is less clear. Conifers are more prolific in time and space than angiosperms; therefore, our understanding of stomatal responses in extant fossil analogues and modern species may elucidate the relationship between the fossil stomata record and paleo-atmospheric conditions. Expanding our knowledge of conifer stomatal frequency sensitivity allows for a broader understanding of future shifts in biotic efficiency in an age where concentrations of atmospheric CO₂ reaches unprecedented levels.