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Four clones of Sitka spruce ( Picea sitchensis  (Bong.) Carr.) from two provenances, at 53.2° N (Skidegate  a  and Skidegate  b ) and at 41.3° N (North Bend  a  and North Bend  b ), were grown for three growing seasons in ambient (∼∼350 μmol mol −1 ) and elevated (∼700 μmol mol −1 ) CO 2  concentrations. The clones were grown in stress-free conditions (adequate nutrition and water) to assess the effect of elevated [CO 2 ] on tree physiology. Growth in elevated [CO 2 ] significantly increased instantaneous photosynthetic rates of the clonal Sitka spruce saplings by about 62%. Downward acclimation of photosynthesis ( A ) was found in all four clones grown in elevated [CO 2 ]. Rubisco activity and total chlorophyll concentration were also significantly reduced in elevated [CO 2 ]. Provenance did not influence photosynthetic capacity. Best-fit estimates of  J  max  (maximum rate of electron transport),  V  cmax  (RuBP-saturated rate of Rubisco) and  A  max  (maximum rate of assimilation) were derived from responses of  A  to intercellular [CO 2 ] by using the model of Farquhar et al. (1980). At any leaf N concentration, the photosynthetic parameters were reduced by growth in elevated [CO 2 ]. However, the ratio between  J  max  and  V  cmax  was unaffected by CO 2  growth concentration, indicating a tight coordination in the allocation of N between thylakoid and soluble proteins. In elevated [CO 2 ], the more southerly clones had a higher initial N use efficiency (more carbon assimilated per unit of leaf N) than the more northerly clones, so that they had more N available for those processes or organs that were most limiting to growth at a particular time. This may explain the initial higher growth stimulation by elevated [CO 2 ] in the North Bend clones than in the Skidegate clones.

tree physiology

Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce ( Picea sitchensis ). II. Photosynthetic capacity and nitrogen use efficiency