Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Doctor of Philosophy




Way, Danielle A.

2nd Supervisor

Longstaffe, Fred J.



For most of the recent evolutionary history of plants, atmospheric CO2 concentrations have been far below modern values. However, little is known about how plants grown in a low CO2 environment balance their carbon demand for growth while meeting their requirement for nitrogen acquisition, the establishment of mycorrhizal fungal associations, and the production of defense compounds. Here, I investigated how low CO2 affects Elymus canadensis and Picea mariana by comparing their growth at low and current CO2 concentrations. I found that reduced N availability exacerbated low CO2 effects on growth, and reduced stomatal index and N isotope composition, indicating that the use of these variables as paleo-indicators can be influenced by N availability. Mycorrhizal association was enhanced in low-CO2 plants, and increased root N but decreased root δ15N, which could skew the interpretation of δ15N when comparing non- and mycorrhizal ancient plant samples. Growth at low CO2 decreased the formation and size of resin ducts and terpene production, suggesting that plants growing at past CO2 conditions had reduced capacity to chemically defend against herbivory.

Summary for Lay Audience

Carbon dioxide (CO2) is an atmospheric gas essential to plant life as plants use CO2 in photosynthesis to produce the energy necessary for growth. In the past, atmospheric CO2 concentrations were far below the levels observed today, which limited plant growth. By studying plants grown at the low CO2 concentrations of the past we can learn about how plants thrived in an environment where carbon resources were limiting, and recreate ancient dynamics between plants and their environment. In my thesis, I compared the performance of Canada wild rye (Elymus canadensis) and black spruce (Picea mariana) grown at modern CO2 concentrations and CO2 conditions that occurred 20,000 years ago, when CO2 concentrations were less than half the levels observed today. I investigated how plants growing under such past conditions were affected by the availability of nitrogen, a soil nutrient essential for plants. I also compared the association between plants and fungi when grown at low and ambient CO2 conditions. Lastly, I examined whether plants growing at low CO2 conditions would be able to form compounds that are used as defense against herbivory. I found that nitrogen availability and association with fungi alter certain plant traits that are used in the study of ancient plant samples, and thus the present use of these traits may require reconsideration. I also found that plants growing at the low CO2 conditions of the past associated more closely with fungi, which helped increase plant access of nitrogen. Finally, I observed that low CO2-grown plants not only had reduced growth but also had a decreased ability to form defensive compounds called terpenes. In other words, plants in the past had a lower capacity to defend against parasites and herbivores compared to modern plants.