the ecophysiological responses of four boreal tree species, trembling aspen (populus tremuloides michx.), black spruce (picea mariana (mill.) b.s.p.), white spruce (picea glauca (moench) voss), and jack pine (pinus banksiana lamb.) were examined at six different levels (from 25 to 775 ppm) of soil nitrogen (n). at the 50th and 100th day of treatment, two-year-old conifer seedlings and six-month-old trembling aspen seedlings were measured for gas exchange, foliar n concentration, growth and biomass allocation traits. the gas exchange and resource use efficiency responses varied with species. the additional n input had no significant effect on photosynthesis (see document) and positive effect on transpiration (e, mmol/m2/s), and beyond 175 ppm n treatment it produced negative effect on whole seedling photosynthetic capacity (see document) in trembling aspen. a, e and at in conifers responded negatively to n treatment, except for a brief positive response from 25 to 125 ppm n in jack pine. <l>psii revealed relevant relationship with a but only for jack pine and white spruce. we found explicit positive response of foliar n concentration and negative response of pnue to increasing soil n availability. however, the trends of pwue between species varied across six n treatments and possibly due to luxury consumption. the pwue was positively correlated with soil n supply in only white spruce and was negatively correlated in other species. the sufficient soil n availability for optimum gas exchange and nutritional status were at 75 ppm n for black spruce and at 125 for the other three species. overall, the growth response of aspen to n was more pronounced than that of conifers due to its fast growing nature. significant growth response of aspen occurred between the 75 to 175 ppm n treatments. substantial growth reduction occurred when aspen seedlings were induced to excessively high soil n concentrations (375 and 775 ppm), specifically due to the vulnerability of these seedlings to pest damages. this is also related to the significant increase in foliar n concentration at these n treatments. greatest growth was achieved at 25 ppm n addition rate in black spruce and jack pine, and most notables at 125 ppm n in white spruce. however, these n levels are not conclusive when optimum soil n concentration(s) for gas exchange parameters, particularly for at, in these seedlings are taken into consideration. nonetheless, at highest soil n supply (375 and 775 ppm), the growth of conifer seedlings was not as adversely affected as that of aspen seedlings. our results also proved strong significance of n availability on biomass allocation between root and shoot components and height growth in all species. root production was significantly suppressed as soil n availability increased in the conifers. the opposite was found in aspen. in conifers, the proportions of total seedling biomass allocated to roots (or the root-toshoot ratio, r/s), at all n treatments, were highest in white spruce, followed by black spruce then jack pine. the r/s ratio in aspen was comparable with, but followed an opposite pattern to, white spruce. the stem and foliage productions in all studied species showed negligible positive response to increasing soil n treatments. however, with the exception of aspen, the order of the species allocating highest to lowest portions of its total biomass to the foliage followed the exact opposite pattern as that of root allocation percentage. aspen showed lesser biomass allocation to the foliage than conifers. however, it allocated highest percentages of total seedling biomass to the stem, at all n treatments, followed in sequential order by black spruce, white spruce and jack pine. when all ecophysiological parameters are considered, our results indicated that low soil n availability, such as that at 25 ppm, does not adversely affect the boreal seedling growth and survival as much as it does at excessively high soil n availability (e.g., 375 and 775 ppm). within the sufficient n regimes (i.e., 75 - 175 ppm), the most suitable soil n concentration for black spruce is at 75 ppm and at 125 ppm for other species.