The objective of this study was to understand the mechanism underlying nitrogen (N) and water competition between jack pine (Pinus banksiana Lamb.) and four boreal forest species. Large-leaved aster (Aster macrophyllus L.), Canada blue-joint grass (Calamagrostis canadensis (Michx.) Beauv.), trembling aspen (Populus tremuloides (Michx.), and red raspberry (Rubus idaeus L.) were planted at a range of densities (0-8 plants/m2) with jack pine seedlings. Net photosynthesis (Pn), nitrogen-use efficiency (NUE), water-use efficiency (WUE) of each species were monitored over three consecutive growing seasons. Changes in available soil N and water were also measured. Jack pine Pn, NUE, and WUE decreased as competitor density increased, but these effects varied among species (p < 0.001) and over time (p < 0.001). The influence of density on jack pine Pn decreased over time for aster and blue-joint grass and increased over time for aspen and raspberry (p < 0.001). At most sample times, jack pine Pn correlated with available soil N. In contrast, the correlation between jack pine Pn and soil water was rarely significant.