Soil temperature is a major environmental factor that influence nutrient availability and uptake and subsequently plant growth. The study of interactive effects of soil temperature and nutrient supply on boreal forest’s most dominant and most economic conifer species is therefore vital, especially in the context of increased atmospheric carbon dioxide concentration. We investigated growth and biomass responses of black spruce (Picea mariana [Mill.] B.S.P.) seedlings to 3 soil temperatures (7, 17 and 27 ^oC) and 3 nutrient levels under ambient and doubled [CO₂] (360 and 720 µmol mol^-1). The three nutrient regimes were: high (N/P/K=160/70/132 mg/l, intermediate (N/P/K= 80/35/66 mg/l and low (N/P/K= 4/1.8/3.3 mg/l). We tested the following hypotheses: (1) Growth and biomass production would be reduced by low soil temperature and low nutrient supply even at doubled [CO₂] and that (2) Increased temperature would compensate for slow growth and poor biomass production attributed to low nutrient supply.

Results/Conclusions CO₂ elevation significantly enhanced seedling height under the high and intermediate nutrient and high and intermediate temperature treatments. Diameter growth after 2.5 months was highest in seedlings at grown at high temperature-high nutrient treatment under elevated [CO₂]. Seedlings grown at 17 and 27 ^oC under elevated [CO₂] had the greatest diameter growth after 4.5 months of treatment. Stem volume generally followed the pattern of seedling height. There were more first order lateral branches (FOL) on seedling grown at high nutrition under elevated [CO₂] than at the other treatment combinations after 2.5 months, while seedlings grown at 27 ^oC had the greatest FOL count after 4.5 months. CO₂ elevation significantly increased total plant biomass and seedlings grown at high and intermediate nutrient regimes had a greater positive response to the nutrient treatment. Soil temperature had no significant effect on total plant biomass but affected organ biomass allocation. CO₂ elevation significantly increased root mass ratio at intermediate soil temperature irrespective of the nutrient regime. The three factors had significant interactive effects on root to shoot ratio. Root to shoot mass ratio was generally high at low nutrient supply. Overall, the increased soil temperature did not fully compensate for the effects of low nutrient supply. In the context of increased global [CO₂], our data indicate that black spruce seedlings on fertile and warmer sites would have greater biomass productivity and faster growth.