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.
