Effects of carbon dioxide enrichment and nitrogen supply on growth of boreal tree seedlings

The effects of two levels of atmospheric carbon dioxide (350 mu l l(-1), 750 mu l l(-1)) and three levels of nitrogen (15.5 mM, 1.55 mM, 0.155 mM N) on biomass accumulation and partitioning were examined in aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) seedlings grown in controlled environment rooms for 100 days after germination. Nitrogen supply had pronounced effects on biomass accumulation, height, and leaf area of both species. Root weight ratio (RWR) of white spruce was significantly increased at the lowest level of nitrogen, whereas RWR of aspen did not change much with increasing levels of nitrogen. Carbon dioxide enrichment significantly increased (1) the leaf and total biomass of spruce seedlings grown in the high-N regime, (2) the RWR of seedlings in the medium-N regime, and (3) the root biomass of seedlings in the low-N regime after 100 days. Carbon dioxide enrichment of aspen temporarily increased biomass and height in all three nitrogen regimes. Root, stern, and leaf mass, height, and leaf area of aspen were increased only at the 30-day harvest ill the high-N treatment and at 50 and 60 days in the low-N treatment. Height, stern biomass. and leaf biomass of aspen seedlings were significantly increased by CO, enrichment after 40 days in the medium-N treatment. These effects did not persist, possibly because Of the onset of mineral nutrient supply limitations with increasing plant size.