Date of Award


Document Type

Honors Thesis (Colby Access Only)


Colby College. Environmental Studies Program


David H. Firmage

Second Advisor

Robert A. Gastaldo

Third Advisor

F. Russell Cole


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 reach levels unprecedented in 60 my.

This paper is available on-campus only.


Full-text download restricted to Colby College campus only.


stomatal sensitivity, CO2, conifers, atmosphere, climate change