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Master of Science




Way, Danielle A.


Climate warming is increasing the frequency of climate-induced tree mortality events. While drought combined with heat is considered the primary cause of this tree mortality, little is known about whether high temperatures alone can induce mortality, or whether rising CO2 will increase survival. I grew tamarack in two experiments combining warming (0-8 ˚C above ambient) and CO2 (400-750 ppm) to investigate whether high growth temperatures led to carbon limitations and mortality. Using glasshouses, +8 ˚C warming with ambient CO2 (8TAC) led to 40% mortality despite thermal acclimation of respiration. Dying 8TAC seedlings had lower needle carbon concentrations and lower ratios of photosynthesis to respiration, indicating carbon limitation. Using growth chambers, no seedlings died, and carbon flux results contradicted those of the glasshouses. Overall, environmental conditions in the glasshouses were more representative of the field than growth chamber conditions, and my work highlights that warming can directly induce mortality.

Summary for Lay Audience

Trees will be negatively impacted by warming caused by climate change and may be less able to fix enough carbon from the atmosphere to maintain growth or even survive. Previous studies have found that plants can adjust their physiology (i.e. acclimation) to respond to long-term changes in temperature and CO2. Under ideal circumstances, acclimation helps plants deal with climate stress by maximizing carbon gain and minimizing carbon loss, thereby maintaining growth and tree health. However, the number of climate-induced tree mortality events has been increasing as the climate warms. Tree die-offs have been linked to a combination of drought and heat stress, but whether heat stress alone can result in mortality has received little attention. I investigated whether high growth temperatures would cause carbon stress and mortality in tamarack, a common tree in Canada’s northern forests. I grew tamarack in two experiments (using either glasshouses or growth chambers) with warming (of up to 8 ˚C) and high CO2 (up to 750 ppm) to simulate future climate scenarios. In the glasshouses, seedlings reduced carbon losses through acclimation, but carbon gain was unresponsive to warming. The +8 ˚C warming with ambient CO2 led to 40% mortality, which correlated with low needle carbon concentrations and low ratios of carbon gain to carbon loss. The growth chamber experiment was designed as a follow-up to measure a greater number of seedlings, but surprisingly there was no mortality in this study. As well, growth chamber seedlings increased carbon gain with warming, but carbon losses were unaffected, the opposite of what I saw in the glasshouse. For both experiments, high CO2 stimulated carbon gain, which offset mortality in the glasshouses. The glasshouse experiment was more similar to conditions experienced in the field (e.g. natural light and daily temperature changes). I therefore argue that tamarack will have strong acclimation to warming (resulting in lower carbon loss) paired with stimulated carbon gain under high CO2. While warming alone induced carbon limitations and subsequent mortality in seedlings, carbon gain associated with high CO2 will likely offset carbon stress in the future.

Creative Commons License

Creative Commons Attribution-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-No Derivative Works 4.0 License.

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