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Doctor of Philosophy




Way, Danielle A.


Rising CO2 may warm northern latitudes up to 10 °C by the end of the century. However, responses of plant physiological processes (such as photosynthesis and respiration) and growth to climate change remain uncertain. Seedlings and mature trees of tamarack (a deciduous species) and black spruce (an evergreen species), North America dominant conifers, were exposed to combined warming (up to +9 ˚C) and elevated CO2 (up to +300 ppm). In seedlings, stomatal conductance (gs) tended to increase with warming in tamarack seedlings, while gsdeclined with warming in spruce. In both species, CO2 had weak effect on gs. Photosynthetic capacity (maximum rates of Rubisco carboxylation, Vcmax and of electron transport, Jmax) was reduced in warm-grown seedlings, while it was not affected by high CO2. As a result, photosynthetic rates (A) remained constant in tamarack while they declined in warm-grown spruce seedlings. In mature trees, there was a slight increase in gswith warming in tamarack, while it decreased in spruce. However, gs was not affected by growth CO2 in both species. A was slightly stimulated by warming in mature tamarack, but similar across warming in spruce trees. A was also increased by elevated CO2in tamarack but not spruce trees, a result that correlated with strong CO2-induced reductions in Vcmaxand Jmaxin spruce. In both seedlings and mature trees, the temperature sensitivity parameters of Vcmaxand Jmax responded strongly to warming, with few CO2 effects. Similarly, thermal optimum of A (ToptA) increased with warming with little CO2 effect. Therefore, ToptA was largely correlated with temperature sensitivity parameters of Vcmax and Jmax. In seedlings, leaf respiration (Rd) measured at a common temperature decreased with warming. In contrast, in mature trees, Rd was constant across warming treatments. Differential responses of these physiological processes to the treatments resulted in different growth between species. In seedlings, moderate warming increased biomass in tamarack, while warming reduced biomass in spruce. However, in mature tamarack, growth was not affected by warming while it decreased in mature spruce. Overall, my findings largely suggest that warming-induced productivity expected in higher latitudes in future climates may be species-dependent.

Summary for Lay Audience

Recent human activities have increased atmospheric CO2 concentrations and global temperatures, with further increases predicted. Plants play a significant role in the global carbon cycle by fixing CO2 through photosynthesis and releasing it back to the atmosphere via respiration. Photosynthesis and respiration are strongly affected by temperature, both via short-term responses (minutes to hours) and longer-term acclimation (weeks to years). Boreal forests store ~ 32 % of the total carbon found in forests, however, our knowledge about the acclimation of photosynthesis and respiration to temperature, and to combined increases in CO2 and temperature, is relatively poor, particularly in boreal trees. This lack of understanding hinders our ability to predict whether this biome will accelerate or exacerbate global warming through climate-vegetation feedbacks. For my doctoral projects, I investigated how photosynthesis and respiration, and their subsequent effects on plant growth, responded to combined warming (up to +9 ˚C above the ambient) and elevated CO2(up to +300 ppm above the ambient) in both seedlings and mature trees of tamarack (a deciduous species) and black spruce (an evergreen species), which are two North America dominant conifers. In both seedlings and mature trees, tamarack and black spruce differed in their responses to the treatments. In tamarack seedlings, moderate warming (+4 ˚C above the ambient) stimulated both plant carbon gain and growth, but it reduced both carbon gain and growth in spruce seedlings. Extreme warming (+8 ˚C) reduced both carbon gain and growth in both species. Elevated CO2stimulated both carbon gain and growth only in spruce seedlings. In mature trees, carbon uptake slightly increased in tamarack, while it was similar in spruce across warming treatments. Leaf carbon losses were increased by warming in both species. In mature trees, elevated CO2 stimulated carbon uptake in tamarack, but not in spruce trees. In addition, in mature tamarack, growth was not affected by warming while it decreased in mature spruce. My doctoral work thus highlights that the temperature- and CO2-sensitivity of boreal conifers differ between species, perhaps reflecting the deciduous and evergreen leaf strategies of tamarack and black spruce.

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