Thesis Format

Integrated Article

Effects of elevated temperature and elevated CO2 on leaf carbon fluxes in boreal conifers: lab and field studies

Author

Mirindi Eric Dusenge, The University of Western OntarioFollow

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Way, Danielle A.

Abstract

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 (g_s) tended to increase with warming in tamarack seedlings, while g_sdeclined with warming in spruce. In both species, CO₂ had weak effect on g_s. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V_cmax and of electron transport, J_max) 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 g_swith warming in tamarack, while it decreased in spruce. However, g_s 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 V_cmaxand J_maxin spruce. In both seedlings and mature trees, the temperature sensitivity parameters of V_cmaxand J_max responded strongly to warming, with few CO₂ effects. Similarly, thermal optimum of A (T_optA) increased with warming with little CO₂ effect. Therefore, T_optA was largely correlated with temperature sensitivity parameters of V_cmax and J_max. In seedlings, leaf respiration (R_d) measured at a common temperature decreased with warming. In contrast, in mature trees, R_d 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.

Recommended Citation

Dusenge, Mirindi Eric, "Effects of elevated temperature and elevated CO2 on leaf carbon fluxes in boreal conifers: lab and field studies" (2019). Electronic Thesis and Dissertation Repository. 6607.
https://ir.lib.uwo.ca/etd/6607

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