Effects of Elevated Temperature, Elevated CO2 and Photoperiod on Conifer Carbon Fluxes

Increasing temperatures due to rising atmospheric CO2 concentrations will have direct effects on plant physiology, specifically photosynthetic carbon uptake. Changes in photosynthetic carbon uptake will alter feedbacks between vegetation and atmospheric CO2, and changes in forest carbon dynamics will be important in determining whether vegetation amplifies or attenuates the effects of anthropogenic CO2 emissions on climate. Coniferous trees, which are a large component of the boreal forest, are understudied in relation to thermal acclimation of photosynthesis and temperature effects on growth. In the present work, I assess the impact of rising temperatures on carbon fluxes in coniferous trees, using meta-analysis, manipulative experimentation, and in silico modeling. I found that photosynthetic capacity is strongly regulated by temperature in white spruce seedlings, but growth is strongly regulated by photoperiod, desynchronizing growth and carbon uptake. I found that boreal tree carbon uptake is likely to respond positively to moderate warming, particularly during autumn and at high latitudes. However, day length may restrict how much of this carbon uptake is allocated to longer-term carbon stores such as woody biomass, which could enhance the release of CO2 from boreal forests between growing seasons. As well, thermal acclimation of photosynthesis in conifers may reduce carbon uptake, reducing the increase in carbon uptake expected with warming in conifers at high latitudes. However, modeling thermal acclimation of photosynthesis by adjusting multiple parameters of the photosynthetic temperature response equations provides diminishing returns in model performance for increased complexity. Therefore, I recommend that multifactor thermal acclimation of photosynthesis not be used in large scale modeling efforts until the underlying physiology is better understood. Overall, my data suggest that climate change will enhance the seasonality of carbon uptake in conifers, increasing the magnitude of peak carbon uptake and possibly peak carbon efflux, and may decouple photosynthetic carbon uptake and growth during autumn. However, physiological variability between boreal tree species may be introducing uncertainties in modelled boreal tree responses to climate that may propagate into unrealistic predictions of tree net carbon gain in the future. Furthermore, my work demonstrates that there is a large gap in understanding photosynthetic thermal acclimation, both on a fundamental level and in terms of the biological diversity of measured temperature responses