Physiological responses of black spruce (Picea mariana (Mill.) B.S.P.) to climate change parameters: Elevated temperature, drought and carbon dioxide

Black spruce is a dominant North American boreal tree species that has large impacts on high latitude climate and carbon cycling. Growth of black spruce is strongly inhibited in warm years, and climate change may eliminate this species from much of its current range. This thesis addresses how black spruce responds to high growth temperatures, drought stress and CO2 enrichment using greenhouses and growth chambers to vary growth conditions.;Elevated temperatures reduced seedling growth and decreased the root to shoot ratio. Despite increased shoot allocation, warm-grown seedlings had smaller canopies and thinner needles with lower nitrogen content than cool-grown seedlings. Elevated growth temperatures reduced day and dark respiration, and photosynthesis. On a leaf nitrogen basis, photosynthesis showed little thermal acclimation, and the lower carbon gain of warm-grown seedlings was largely due to lower nitrogen content. The ratio of photosynthesis to respiration declined linearly with increasing temperature, implying that reduced carbon gain at elevated temperatures limits black spruce growth. Co-limitation of photosynthesis by Rubisco and RuBP regeneration predominated from 10--30°C, with no evidence for Pi regeneration limitations. At high temperatures, photosynthesis appeared to be limited by Rubisco in warm-grown seedlings, but the limitation was unclear in cool-grown seedlings. High growth temperatures increased seedling thermotolerance; heat damage to needles did not occur below 50°C.;In response to prolonged drought, declines in the maximum efficiency of photosystem II occurred at a water potential of -2 to -3 MPa, indicating damage to photosynthetic processes. Elevated growing temperatures reduced the water potential that damaged photosystem II from -2 MPa to -2.5 MPa, but caused seedlings to reach a damaging level of drought stress two to three days earlier than cool-grown seedlings.;Photosynthetic acclimation to elevated CO2 can occur within 16 days in black spruce seedlings following high CO2 exposure. An exponentially supplied high nutrient treatment did not affect the degree of down-regulation of photosynthesis to high CO2. Exposure to elevated CO2 during the first year of growth did not enhance growth the following year and had little effect on biomass over a five-year field study.