Interacting effects of elevated CO2 and weather variability on photosynthesis of mature boreal Norway spruce agree with biochemical model predictions

According to well-known biochemical and biophysical mechanisms, the stimulation of C-3 photosynthesis by elevated atmospheric CO2 concentration ([CO2]) is strongly modified by changes in temperature and radiation. In order to investigate whether a static parameterization of the commonly used Farquhar et al. model of photosynthesis (i.e., without CO2-induced seasonal or thermal acclimation of photosynthetic capacity) can accurately predict these interactions in mature boreal Norway spruce (Picea abies (L.) Karst.) during the frost-free part of the growing season, shoot gas exchange was continuously measured on trees during their second/third year of exposure to ambient or doubled [CO2] inside whole-tree chambers. The relative CO2-induced enhancement of net photosynthesis (A(n)) at a given temperature remained stable over the study period, but increased strongly with temperature and radiation, in agreement with predictions by the model. Light-saturated A(n) (+67% at 20 degrees C), dark respiration (+36%) and intercellular to ambient [CO2] ratio (c(i)/c(a); +27%) were significantly increased by CO2 treatment. Stomatal conductance (g(s)) was not significantly affected. Our results demonstrate that the Farquhar et al. model of photosynthesis has the capability to predict interactions between [CO2] and seasonal weather variability on A(n) in Norway spruce during the non-frost growing season without accounting for CO2-induced seasonal and/or thermal photosynthetic acclimation. However, stomatal model assumptions of reduced g(s) and constant c(i)/c(a) under rising atmospheric [CO2] did not hold.