Effects of Growth Temperatures and Elevated CO2 on Respiration Rates in Norway Spruce

Projected increase in growth temperatures and CO2 may affect carbon balance in Norway spruce (Picea abies), a dominant coniferous species of the boreal forest ecosystem. To examine this, I exposed three-year-old Norway spruce seedlings to six treatments: ambient (400 ppm) and elevated (750 ppm) CO2 concentrations combined with three growth temperatures: ambient, ambient +4 oC, and ambient +8 oC. I found that while net growth was generally not affected by growth CO2 or temperature, leaf nitrogen concentrations were reduced, mortality rates were higher, and needles were shorter and thinner in +8 oC treatments, compared to cooler treatments. I found that net CO2 assimilation rates and dark respiration acclimated to temperature but not CO2, while patterns of acclimation of light respiration in the light varied between years. The highest net CO2 assimilation rates were found in trees grown at +4 oC combined with elevated CO2, which could indicate that a slight increase in growth temperature with elevated CO2 may benefit the carbon balance of Norway spruce. However, further warming had negative effects on carbon uptake, with trees from the +8 oC treatments showing the lowest CO2 assimilation and dark respiration rates. The Q10 of light respiration was 35% higher than the Q10 of dark respiration, so that the ratio of light respiration to dark respiration increased as leaf temperature increased. I conclude that light respiration is not a constant fraction of dark respiration, although both parameters are tightly correlated, and this relationship can be used to improve models of terrestrial vegetation