jack pine, black spruce and white spruce are three vital species of the ontario boreal mixedwood forest. the purpose of this study is to compare and contrast the ecophysiological characteristics of each species and to examine changes that occur in these characteristics over a growing season. ecophysiological measurements (photosynthesis, stomatal conductance and transpiration) were taken on six trees each of jack pine, black spruce and white spruce from three blocks, at three times during the growing season (july, august and september). foliar n concentration was also determined for each measured seedling in july and september. for all three species, photosynthetic rates were highest in september and lowest in july. overall, rates were consistently highest for jack pine, even in partially shaded conditions. white spruce had its best performance in the lower light conditions, likely due to its shade tolerance. black spruce performed in between the jack pine and white spruce. jack pine had the lowest specific leaf areas, nitrogen use efficiencies and the highest water use efficiencies. for all three species, water use efficiency was lowest in july. foliar nitrogen concentration and photosynthesis were only weakly correlated in this study.