The concentrations of carbon dioxide (CO₂) and nitrogen dioxide (NO₂) have been rising since the industrial revolution and have the potential to impact the productivity and allocation patterns of plants in natural ecosystems.  In general, elevated CO₂ has been found to increase plant growth, but the magnitude can be altered by nitrogen availability.  Elevated CO₂ tends to increase root:shoot ratio and the C:N of plant tissues, suggesting that nitrogen is limiting the CO₂ growth effect.  NO₂ enters plant leaves through the stomata and is converted to nitrate in the apoplast, making it a potential source of nitrogen for the plant.  However, most laboratory studies have suggested that the oxidative damage caused by NO₂ outweighs any benefit of extra nitrogen.  This study examined the single and combined effects of elevated CO₂ and NO₂ with and without elevated soil nitrate on sugar maple and eastern hemlock seedlings fumigated for two years.  Hemlocks increased total biomass under elevated CO₂ regardless of soil nitrate status while increased soil nitrate caused all three gas treatments to increase biomass.  Interestingly, there was no corresponding change in allocation pattern (root:shoot), although needle C:N went up slightly under elevated CO₂.  Maples showed no change in total biomass or root:shoot under any of the treatments.  However, elevated CO₂ decreased the specific leaf area and increased the C:N of the leaves and elevated NO₂ decreased the total above-ground woody growth.  These findings suggest that CO₂ and NO₂ effects are highly species-specific and dependent on soil nitrogen availability.