What was done
The authors grew young spruce (Picea abies) seedlings for three years in growth chambers receiving atmospheric CO2 concentrations of 280, 420 and 560 ppm.  In addition, seedlings received low, medium and high levels of nitrogen fertilization.  Thus, the authors studied the effects of elevated CO2 and nitrogen on needle carbohydrate concentrations.

What was learned
Nitrogen fertilization did not affect the concentrations of any sugars within mature needles of spruce seedlings.  In contrast, atmospheric CO2 enrichment significantly enhanced needle glucose contents in a season-dependent manner.  In the highly productive growing phases characteristic of spring and early summer, for example, glucose contents in mature needles of CO2-enriched trees were not significantly different from those observed in needles of trees exposed to atmospheric CO2 concentrations of 280 and 420 ppm.  In late summer, fall and winter, however, glucose concentrations in needles on the CO2-enriched trees were 40 to 50% higher than those of needles on trees subjected to ambient and sub-ambient CO2 concentrations (420 and 280 ppm, respectively).  These seasonal fluctuations in needle glucose concentrations suggest that glucose levels may be mediating a seasonal photosynthetic down regulation in spruce needles, as previously noted in these trees by other authors (Hattenschwiler and Korner, 1996).

What it means
As the air's CO2 content rises, spruce trees will likely increase their photosynthetic rates, which should result in greater needle concentrations of soluble carbohydrates, including glucose.  During favorable growing conditions associated with spring and early summer, the additional glucose produced by elevated CO2 will likely be mobilized and sent to active sinks to support their growth and development.  As growing conditions become less favorable in late summer, however, it is likely that glucose will not be mobilized from needles as rapidly as it was during the spring and early summer, which in turn may signal a seasonal photosynthetic down regulation in this species.  Despite this temporal down regulation, it is still likely that spruce seedlings will exhibit greater biomass production at higher atmospheric CO2 concentrations.