Nitrogen and water use by jack pine and competing boreal forest plant species

Nitrogen and water use by jack pine (Pinus banksiana Lamb.) and four boreal forest competitors, 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.) at densities ranging from 0 to 8 plants m--2 were examined under field conditions. Nitrogen uptake and nitrogen use efficiency (NUE), or the rate of CO2 assimilated per gram of nitrogen allocated to the foliage, of jack pine seedlings competing with large-leaved aster and Canada blue-joint grass at three nitrogen levels were measured to test two hypotheses of competition.; Jack pine photosynthesis decreased as plant density increased from 0 to 2 plants m--2 for each plant species (p < 0.001). The influence of density on jack pine photosynthesis decreased over time for large-leaved aster and Canada blue-joint grass, and increased over time for trembling aspen and red raspberry. Jack pine NUE decreased as plant density increased (p < 0.001). The influence of increasing plant density on jack pine NUE decreased with time for all species except trembling aspen. Plant densities between 0.5 and 2 plants m --2 reduced jack pine water-use efficiency (WUE: p < 0.001), or the ratio of CO2 assimilation to water transpiration. The correlation between jack pine photosynthesis and total available soil nitrogen and volumetric soil moisture were mutually exclusive, indicating a trade-off between nitrogen and moisture conservation.; Photosynthetic rate was correlated with foliar nitrogen content, but this relation varied among competitor species (p < 0.001). The relation was greatest for red raspberry (1997: p < 0.001, and 1998: p < 0.001) and trembling aspen (1997: p < 0.001, and 1998: p < 0.001). Photosynthetic rates were correlated with the transpiration rates of large-leaved aster ( p < 0.001) and Canada blue-joint grass (p < 0.001) in the month of August in 1997 and 1998. High NUE was concluded to be an important competitive trait for all of the species studied. It is hypothesized that a trade-off exists between NUE as a trait related to competitive outcome, and WUE as a trait related drought tolerance.; At limiting nitrogen levels, jack pine photosynthesis decreased as competitor NUE increased (p < 0.001). Competition for the limiting resource was related to the ability to use that resource efficiently, when all other environmental conditions and resource levels are optimal for photosynthesis. Jack pine photosynthesis decreased as nitrogen uptake by large-leaved aster (p < 0.001) and Canada blue-joint grass (p < 0.001) increased at non-limiting nitrogen levels. Competitive outcome in non-limiting conditions was related to uptake capacity when all other environmental conditions and resource levels are optimal.