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Thesis Format



Master of Science




Way, Danielle A.

2nd Supervisor

Henry, Hugh A. L.


Anthropogenic activities are increasing atmospheric CO2 concentrations, leading to global warming. Boreal forests absorb and store CO2 through photosynthesis and growth, yet we have relatively little data on the photosynthetic and growth responses of boreal trees to elevated CO2 (EC) and warming. I grew seedlings of five North American boreal tree species for one growing season under different CO2 (410 and 750 ppm) and temperature (ambient, +4, and +8 ºC) conditions, then measured photosynthetic traits and whole-plant biomass. The results were generally consistent across species: photosynthetic capacity (Vcmax and Jmax) was unaffected by EC but strongly reduced by warming. Accordingly, net photosynthesis (Agrowth) and biomass were stimulated by EC and suppressed by warming. Interactions between growth CO2 and warming were rare. These findings improve our Earth system models and suggest that photosynthetic CO2 uptake and growth in well-watered, well-fertilised boreal tree seedlings will be similar under current and future climates.

Summary for Lay Audience

Atmospheric carbon dioxide (CO2) concentrations are increasing with fossil fuel consumption and deforestation. Because CO2 is a heat-absorbing greenhouse gas, high concentrations of CO2 in the atmosphere lead to global warming. Plants absorb large amounts of atmospheric CO2 through photosynthesis, where much of this carbon is stored as plant biomass. However, photosynthesis and growth are sensitive to changes in CO2 concentration and temperature. Boreal forests represent one of the largest CO2 sinks on Earth, yet we have relatively little data on how boreal trees adjust their photosynthesis and growth in response to changes in CO2 concentration and temperature. This knowledge gap reduces the accuracy of our climate models, which need good estimates of global photosynthetic CO2 uptake under different environmental conditions to predict future atmospheric CO2 levels and their climate feedbacks. I grew seedlings of five widespread North American boreal tree species under ambient (AC, 410 parts per million) and elevated CO2 concentrations (EC, 750 parts per million), and three warming treatments (ambient temperature, ambient +4 ºC, and ambient +8 ºC) for one growing season. I then measured net photosynthesis (CO2 uptake), photosynthetic capacity (the maximum potential for photosynthesis), and growth (whole-plant biomass) at the end of the season. Across species, photosynthetic capacity was unaffected by but suppressed by warming. This was associated with higher rates of net photosynthesis and growth in EC-grown plants, but lower rates of net photosynthesis and growth in warmer-grown plants. These results imply that most boreal tree seedlings, when given ample water and nutrients, will exhibit similar rates of photosynthesis and growth under current and future climates. My results improve our understanding of photosynthetic and growth responses to climate change in boreal trees and strengthen our model predictions for photosynthetic CO2 uptake in boreal forests as atmospheric CO2 levels and air temperatures rise.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Available for download on Wednesday, March 26, 2025

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