carbon dioxide concentration and light interactions influence competition among boreal tree species, though little is known about the exact effects. the interactive effects of [co 2 ] and light on growth, biomass production, gas exchange, chlorophyll fluorescence, light response and in-vivo carboxylation of 1-year-old black spruce (sb) ( picea mariana [mill.] b.s.p) and white spruce (sw) ( picea glauca (moench voss) on were investigated. the seedlings were grown under 360 and 720 mol mol -1 [co 2 ] at 30, 50 and 100% light in greenhouses for 4.5 months. it was found that root collar diameter (rcd) of sw decreased with decreasing light while there was no significant difference in sb between 50 and 30% light. height was greater at 100% light than shaded. elevated [co 2 ] increased rcd by 33% and enhanced stem-volume by 67, 98 and 84%, respectively at 100, 50 and 30% light. the co 2 enrichment enhancement of total biomass and net photosynthesis was relatively higher at lower light and greater in sb than in sw. co 2 elevation decreased specific leaf area at 50% light only. co 2 elevation reduced stomatal conductance ( g s ) and transpiration rate ( e ) and subsequently increased water use efficiency. the reduction in g s and e increased with decreasing light and much more in sb than in sw. co 2 elevation significantly reduced dark respiration ( r day ) and the magnitude of reduction was higher in sw than sb and with decreasing light. light compensation point decreased with decreasing light and that of sb was much lower at elevated [co 2 ]. co 2 elevation increased light-saturated electron transport rate ( j max ) and apparent rate of electron transport ( j t ) and the increase was highest at 100% light. after 2.5 months, j max was significantly higher in sb than in white spruce. co 2 elevation reduced light compensation point and increased the light saturation point. these data suggest that in future climates when atmospheric [co 2 ] rises, species competitiveness under low light conditions will increase and the increase will be greater in black spruce than white spruce.