What was done
Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings were maintained for five years at CO2 concentrations of either 350 or 700 ppm.  For the first three years of this period, they were grown in well-watered and fertilized pots placed in open-top chambers, after which they were planted directly into native nutrient-deficient forest soil and maintained for two more years in larger open-top chambers either with or without an extra supply of nitrogen (N).

What was learned
After the first three years of growth in pots, the CO2-enriched trees possessed 11.6% more total biomass than the ambient-treatment trees.  At the end of the next two years of the study, the trees supplied with extra N possessed 15.6% more total biomass than their similarly-treated ambient-air counterparts, while those receiving no extra N had 20.5% more total biomass than their ambient-air counterparts.

What it means
The authors make a point of noting that the CO2-induced increases in growth they observed occurred in spite of a down-regulation of photosynthesis and a reduction of foliar Rubisco activity.  In addition, they report that "visual foliar N-deficiency symptoms (needle yellowing and chlorosis) were obvious on some of the saplings with no added N supply during the final year of the experiment."  However, they further note that "such N deficiency is common in many boreal forest sites, and therefore a growth response to rising atmospheric CO2 can be expected to occur in such forests," that is, in forests growing on nutrient-deficient soils.  In support of this conclusion, they additionally note that "growth responses to elevated CO2 despite nutrient stress have been reported previously in Scots pine (Kellomaki and Wang, 1997), grass (Cannell and Thornley, 1998) and Sitka spruce (Centritto et al., 1999; Murray et al., 2000)."

References
Cannell, M.G.R. and Thornley, H.M.  1998.  N-poor ecosystems may respond more to elevated [CO2] than N-rich ones in the long term, a model analysis of grassland.  Global Change Biology 4: 101-112.

Centritto, M., Lee, H.S.J. and Jarvis, P.G.  1999.  Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis).  I. Plant growth, allocation and ontogeny.  Tree Physiology 19: 799-806.

Kellomaki, S. and Wang, K.Y.  1997.  Photosynthetic response of Scots pine to elevated CO2 and nitrogen supply: results of a branch-in-bag experiment.  Tree Physiology 17: 231-240.

Murray, M.B., Smith, R.I., Friend, A. and Jarvis, P.G.  2000.  Effect of elevated [CO2] and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis).  Tree Physiology 20: 421-434.