What was done
The authors grew different genotypes of spruce (Picea abies L. Karst.) and beech (Fagus sylvatica L.) seedlings for four years in open-top chambers maintained at atmospheric CO2 concentrations of 370 and 590 ppm in combination with low and high levels of wet nitrogen application on both rich calcareous and poor acidic soils to study the effects of these interacting factors on seedling toughness (fracture characteristics) and rigidity (bending characteristics such as modulus of elasticity).

What was learned
Some individual genotypes of each species were sensitive to elevated CO2, while others were not.  Similarly, some were responsive to elevated nitrogen deposition, while others were not.  Moreover, such responses were often dependent upon soil type.  Averaged across all tested genotypes, however, atmospheric CO2 enrichment increased wood toughness in spruce seedlings grown on acidic soils by 12 and 18% with low and high levels of nitrogen deposition, respectively.  In addition, atmospheric CO2 enrichment increased this same mechanical wood property in spruce seedlings grown on calcareous soils by about 17 and 14% with low and high levels of nitrogen deposition, respectively.  In contrast, elevated CO2 had no significant effects on mechanical wood properties in beech seedlings, regardless of soil type.

What it means
As the atmospheric CO2 concentration increases, spruce seedlings will likely respond by exhibiting increased wood toughness, which can lead to enhanced structural strength and decreased fracturing.  If this response persists in mature trees, the quality of spruce lumber will likely increase with time as a consequence of its ever-increasing CO2-induced structural toughness.