Photosynthesis, nitrogen-use efficiency, and water-use efficiency of jack pine seedlings in competition with four boreal forest plant species

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. Largeleaved 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.