Effects of elevated CO2 and growth temperature on respiration rates in Norway spruce seedlings

Background/Question/Methods

Atmospheric CO₂ concentrations are expected to double by the year 2100, leading to temperature increases of 7-10˚C in high latitude, boreal forest regions. Given the role of these forests in the global carbon cycle, we need better understanding of how these climate variables affect photosynthetic carbon gain and carbon loss through respiration in boreal tree species. While much attention has been paid to CO₂ and temperature acclimation of photosynthesis, acclimation of respiration, to these drivers has received less attention. While leaf respiration in the dark (R_night) is easily measured, this rate is inhibited by light, and rates of light respiration (R_day) are often extrapolated from values of R_night. In our study, we grew Norway spruce (Picea abies) in both ambient and elevated CO₂ combined with ambient, +4°C and +8°C growth temperatures, mimicking future climate change scenarios. We then measured the temperature response of both R_day and R_night from 10-40°C to determine how respiratory fluxes in both the light and the dark acclimate to elevated growth temperatures and CO₂ concentrations, compared to current ambient conditions.

Results/Conclusions

Our results show that the ratio of R_day to R_night increases with rising measurement temperatures. Light inhibition of R_night was greatest at the lowest measured leaf temperature for all treatments. At 40 ˚C, R_day was higher than R_night in trees grown at ambient temperatures. Growth temperature had a significant effect on R_night but not on R_day, with trees grown at higher temperatures showing significantly lower R_night at a given leaf temperature than ambient temperature-grown trees. Elevated growth CO₂ had no strong effect on either R_night or R_day. We conclude that the effect of growth temperature is much stronger than that of growth CO₂ on respiration, and that while we need to account for temperature acclimation of R_night, R_day may not acclimate strongly to future climate change.