a new soil temperature control system was developed for greenhouse experiments. the system controlled soil temperature by circulating temperature-controlled water around seedling containers. the system was tested for 5, 20 and 30 °c soil temperatures. tests showed that the system was very efficient in controlling soil temperature in the greenhouse. the coefficient of determination between water and soil temperatures was 0.99. average soil temperatures of all 112 pots (13.5 cm deep, 11 cm top diameter) in each system were within ± 0.4 °c of the set values. the maximum vertical variation in soil temperature was less than 1 °c. to investigate soil temperature effects on ecophysiological traits, a total 1568 (392 per species) of one-year old aspen, white spruce, black spruce and jack pine seedlings were grown at 5, 10, 15, 20, 25, 30 and 35 °c soil temperatures in two greenhouses. at the beginning o f the third month, net photosynthesis, stomatal conductance, mesophyll conductance, transpiration, midday xylem water potential, water use efficiency and specific leaf area were measured. soil temperature significantly affected all the traits in the four species. all the traits had a strong non-linear relationship with soil temperature modeled using a polynomial function. the derived model showed that the traits were the most sensitive to changes in soil temperature at 6.2, 8.9, 12.7 and 12.3 °c, and performed the best at 19.6, 21.8, 22.4 and 26.1 °c, respectively, for aspen, white spruce, black spruce and jack pine. all the aspen seedlings, about 40 % of jack pine, 20 % white and black spruce seedlings survived in the 35 °c treatment. in general, aspen was the most sensitive to low soil temperatures while black spruce was the most sensitive to high soil temperatures. stepwise regression analysis showed that net photosynthesis of aspen was significantly correlated to mesophyll conductance stomatal conductance and water use efficiency. in white spruce photosynthesis was significantly correlated to mesophyll conductance stomatal conductance, transpiration and midday xylem water potential. for black spruce photosynthesis was significantly correlated to mesophyll conductance stomatal conductance, transpiration and water use efficiency. in jack pine net photosynthesis was significantly correlated to mesophyll conductance stomatal conductance and transpiration. net photosynthesis had a strongly linear relationship with mesophyll conductance when data from all the species were combined.