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

Abstract

According to well-known biochemical and biophysical mechanisms, the stimulation of C3 photosynthesis by elevated atmospheric CO 2 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 (An) 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 An (+67 at 20°C), dark respiration (+36) and intercellular to ambient [CO2] ratio (ci/ca; +27) were significantly increased by CO 2 treatment. Stomatal conductance (gs) was not significantly affected. Our results demonstrate that the Farquhar et al. model of photosynthesis has the capability to predict interactions between [CO 2] and seasonal weather variability on An in Norway spruce during the non-frost growing season without accounting for CO 2-induced seasonal and/or thermal photosynthetic acclimation. However, stomatal model assumptions of reduced gs and constant c i/ca under rising atmospheric [CO2] did not hold. © 2012 The Author 2012.